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A study of sensitivity gradients and spatial summation in the normal retina by static chromatic perimetry… Dunn, Patrice Mary 1979

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A STUDY OF SENSITIVITY GRADIENTS AND SPATIAL SUMMATION IN THE NORMAL RETINA BY STATIC CHROMATIC PERIMETRY WITH PHOTOMETRICALLY-EQUATED STIMULI UNDER FULLY-PHOTOPIC•AND FULLY-SCOTOPIC CONDITIONS by PATRICE MARY DUNN B.A., Un i v e r s i t y of B r i t i s h Columbia, 1972 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF ARTS i n THE FACULTY OF GRADUATE STUDIES Department of Psychology We accept t h i s t hesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA March, 1979 © P a t r i c e Mary Dunn, 1979 I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an a d v a n c e d d e g r e e a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e a n d s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e Head o f my D e p a r t m e n t o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . D e p a r t m e n t The U n i v e r s i t y o f B r i t i s h C o l u m b i a 2075 W e s b r o o k P l a c e V a n c o u v e r , C a n a d a V6T 1W5 E-6 B P 75-51 1 E Abstract A study was conducted to investigate the s e n s i t i v i t y and s p a t i a l summation properties of the normal r e t i n a under f u l l y -photopic and f u l l y - s c o t o p i c conditions, using photometrically-equated chromatic s t i m u l i of four s i z e s . Fully-photopic adaptation yielded equivalent s e n s i t i v i t y gradients e x t r a - f o v e a l l y but d i f f e r e n t foveal thresholds f o r the red, green and blue s t i m u l i . The extra-foveal scotopic s e n s i t i v i t y gradients were s i m i l a r i n form but d i f f e r e n t i n height f o r the chromatic s t i m u l i , while a l l s t i m u l i excepting the smaller red ones yielded " r e l a t i v e scotomata" at the fovea. S p a t i a l summation was shown to increase with e c c e n t r i c i t y and decrease with increasing stimulus s i z e under ful l y - p h o t o p i c but not under f u l l y - s c o t o p i c conditions, but was found i n general to be greater under scotopic adaptation. i i Table of Contents Page Abstract • • • i L i s t of Tables i v L i s t of Figures v i Acknowledgements v i i i Introduction . • 1 Determination of r e t i n a l s e n s i t i v i t y gradients by perimetric methods. S t a t i c perimetry 1 Chromatic perimetry 2 Adaptation 7 S p e c i f i c review of relevant reasearch l . V e r r i e s t and Kandemir 10 2.Sloan 12 3. Wentworth 18 4. Nolte 22 25 Subject v a r i a b l e s 28 S p a t i a l summation and s t a t i c perimetry S p e c i f i c review of relevant research 31 Proposal 35 Method 38 Apparatus Perimeter 38 Photometer 44 Subjects 44 Experimental parameters Adaptation f i e l d 47 Sti m u l i 47 (a) R e t i n a l l o c a t i o n 48 (b) Size 48 (c) Chromaticity 48 Experimental design 53 Procedure 54 T r i a l procedure 55 Results 59 Retin a l s e n s i t i v i t y gradients 59 Fully-photopic adaptation 59 Fu l l y - s c o t o p i c adaptation 64 Fully-photopic and f u l l y - s c o t o p i c adaptation.. 77 S p a t i a l summation 79 Fully-photopic adaptation 79 Fu l l y - s c o t o p i c adaptation 84 i i i F ully-photopic and f u l l y - s c o t o p i c adaptation. 100 Discussion 102 Retinal s e n s i t i v i t y gradients 102 Fully-photopic 102 F u l l y - s c o t o p i c 105 S p a t i a l summation 111 Fully-photopic 112 F u l l y - s c o t o p i c 113 Summary and Concluding Remarks 116 Reference notes 120 References 121 Appendix I i Counterbalancing of subjects 127 Appendix I I Means and standard deviations for ful l y - p h o t o p i c thresholds 129 Appendix I I I Scotopic foveal thresholds 130 Appendix IV Means and standard deviations f o r f u l l y - s c o t o p i c thresholds 135 Appendix V Means and standard deviations f o r ful l y - p h o t o p i c summation exponents 136 Appendix V I Means and standard deviations for f u l l y - s c o t o p i c summation exponents 137 Appendix V I I C a l c u l a t i o n of c o r r e c t i o n factors for photometric equating of chromatic f i l t e r s . 138 i v L i s t of Tables Table Page 1. Gougnard's means and standard deviations for summation c o e f f i c i e n t s f o r Goldmann stimulus sizes 0-1 36 2. Subject data 45 3. Comparison of o r i g i n a l with cinemoid f i l t e r s 50 4. Analysis of variance f o r photopic thresholds 61 5. Analysis of variance for photopic foyeal thresholds.... 63 6. Average standard deviations f o r photopic thresholds obtained at each r e t i n a l p o s i t i o n 65 7. Average standard deviations for a l l photopic thresholds obtained with each col o u r - s i z e combination... 66 8. Analysis of variance for scotopic thresholds 69 9. Analysis of variance f o r scotopic foveal thresholds 72 10. Average standard deviations for scotopic thresholds obtained at each r e t i n a l p o s i t i o n . . . . . ... 75 11. Average standard deviations f o r a l l scotopic thresholds obtained with each col o u r - s i z e combination... 76 12. Slopes of photopic gradient segments 83 13. Mean photopic k values at selected e c c e n t r i c i t i e s . 86 14 Average standard deviations for photopic k values obtained at each r e t i n a l p o s i t i o n 88 15. Average standard deviations for a l l photopic k values obtained with each c o l o u r - s i z e combination 89 V Table Page 16. Slopes of scotopic gradient segments 93 17. Mean scotopic k values at selected e c c e n t r i c t i e s 96 18. Average standard deviations for scotopic k values obtained at each e c c e n t r i c i t y 98 19. Average standard deviations for a l l scotopic k values obtained with each co l o u r - s i z e combination 99 20. K values obtained i n various i n v e s t i g a t i o n s with an achromatic stimulus 114 v i L i s t of Figures Figure Page 1. Nolte's s e n s i t i v i t y gradients from 10° nasal to 10° temporal f o r monochromatic s t i m u l i 5 2. Sloan's s e n s i t i v i t y gradients obtained using dark adaptation methods 9 3. V e r r i e s t and Kandemir's foveal thresholds ...... 11 4. Sloan's scotopic thresholds 13 5. V a r i a b i l i t y i n Sloan's s e n s i t i v i t y gradients 14 6. Hypothetical graph of r e t i n a l s e n s i t i v i t y versus e c c e n t r i c i t y under f u l l y - s c o t o p i c conditions 17 7. Wentworth's scotopic s e n s i t i v i t y gradients to sp e c t r a l l i g h t s 21 8. Nolte's f u l l y - s c o t o p i c s e n s i t i v i t y gradients, 10° nasal to 10° temporal . 23 9. A comparison of Wentworth's and Nolte's f u l l y - s c o t o p i c s e n s i t i v i t y gradients 24 10. Sloan's average summation exponents 33 11. The modified Goldman Perimeter... 39 12. Relative s p e c t r a l energy of C.I.E. illuminant C and Xenon-arc 40 13. Spectral transmission curves f o r chromatic f i l t e r s 49 14 Mean fully-p h o t o p i c s e n s i t i v i t y gradients f o r achromatic, blue, green, and red s t i m u l i . . . 60 15. Mean f u l l y - s c o t o p i c s e n s i t i v i t y gradients for achromatic, blue, green, and red s t i m u l i 68 v i i Figure Page 16. Means and ranges of scotopic foveal thresholds 73 17. Mean ful l y - p h o t o p i c and f u l l y - s c o t o p i c s e n s i t i v i t y gradients 78 18. Mean ful l y - p h o t o p i c s e n s i t i v i t y gradients f o r stimulus s i z e s 1 to 4 81 19 Mean ful l y - p h o t o p i c s e n s i t i v i t y gradients, selected e c c e n t r i c i t i e s , f o r stimulus sizes 1 to 4 82 20. Mean ful l y - p h o t o p i c summation exponents f o r stimulus sizes 1-2, 2-3, and 3-4 85 21. Mean ful l y - p h o t o p i c summation exponents at selected e c c e n t r i c i t i e s f o r stimulus sizes 1-2, 2-3, and 3-4.... 87 22. Mean f u l l y - s c o t o p i c s e n s i t i v i t y gradients f o r stimulus sizes 1 to 4 90 23. Mean f u l l y - s c o t o p i c s e n s i t i v i t y gradients f o r selected e c c e n t r i c i t i e s , f o r stimulus sizes 1 to 4 92 24. Mean f u l l y - s c o t o p i c summation exponents f o r stimulus sizes 1-2, 2-3, and 3-4 95 25. Mean f u l l y - s c o t o p i c summation exponents at selected e c c e n t r i c i t i e s f o r stimulus sizes 1-2, 2-3, and.3-4.... 97 26. D i s t r i b u t i o n of rods and cones i n the human r e t i n a 109 v i i i Acknowledgements The author would l i k e to extend her thanks to the people who contributed so much to the development of t h i s thesis : Dr. R. Lakowski, for h i s continuing support and crea t i v e assistance throughout t h i s endeavoiur. Keith Humphrey, Leonora Lakowski, Rob MacKenzie, John Lenehan, and Ann Milton, f o r t h e i r many hours of t o i l at the perimeter. INTRODUCTION Determination of R e t i n a l S e n s i t i v i t y Gradients by Perimetric Methods S t a t i c Perimetry. Light-sense perimetry, the i n v e s t i g a t i o n of the s e n s i t i v i t y of the v i s u a l system to l i g h t i n s p e c i f i c locations i n the v i s u a l f i e l d , can be done using s t a t i c or k i n e t i c presentation of s t i m u l i . According to Traquair's (1927) representation of the v i s u a l f i e l d as a three-dimensional ' h i l l ' , k i n e t i c perimetry ( i n which a constant-luminance target i s moved c e n t r a l l y u n t i l i t i s seen) repre-sents the process of obtaining h o r i z o n t a l bearings ( i s o p t e r s ) , while s t a t i c perimetry ( i n which a stimulus i s presented at one r e t i n a l point and increased i n luminance u n t i l i t i s seen) represents the pro-cess of obtaining v e r t i c a l soundings ( p r o f i l e s ) (Aulhorn & Harms, 1972). S t a t i c perimetry y i e l d s more precise, definable information i n a shorter period of time than does the k i n e t i c method. For an experimental i n v e s t i g a t i o n , i t i s important that factors which could influence the outcome v a r i a b l e be c o n t r o l l e d or at l e a s t p r e c i s e l y s p e c i f i e d . In k i n e t i c perimetry, the moving stimulus complicates i n t e r p r e t a t i o n of the data obtained as thresholds, because temporal'and s p a t i a l summation are implicated i n a complex, i n t e r a c t i v e manner. In s t a t i c perimetry, invariant stimulus duration controls for temporal summation e f f e c t s (Aulhorn & Harms, 1972), so that s p a t i a l summation can be studied independently using a range of stimulus s i z e s . K i n e t i c perimetry i s also l i m i t e d i n the kind of information obtainable. For example, the 'depth' or r e l a t i v e loss of a scotoma could not be p r e c i s e l y s p e c i f i e d (Sloan, 1961) nor could the ' r e l a t i v e c e n t r a l scotoma 1 reported for 2 short-wavelength s t i m u l i (Verriest & I s r a e l , 1956a, 1956b) be demon-strated, with k i n e t i c perimetry. S t a t i c perimetry i s , by contrast, a precise method y i e l d i n g o b j e c t i v e l y - s p e c i f i e d threshold values f o r any part of the v i s u a l f i e l d . The threshold values so obtained can be com-pared among s t i m u l i of d i f f e r e n t c h r o m a t i c i t i e s , s i z e s , and e c c e n t r i c i t i e s . Chromatic Perimetry. Most perimetric research has been done with achromatic s t i m u l i , where objective s p e c i f i c a t i o n of stimulus s i z e , duration, colour-temperature, and luminance are required. Chromatic perimetry did receive some early research a t t e n t i o n (for example, Ferree & Rand, 1919; Wentworth, 1930). However, i n much of the early work objective s p e c i f i c a t i o n of stimulus conditions was d e f i c i e n t , so that the c r i t i c i s m was made that chromatic perimetry did not give a d d i t i o n a l information to that already y i e l d e d by achromatic perimetry (Dubois-Poulsen, 1952). With the advent of the Goldmann (Goldmann, 1945a, 1945b) and Tiibinger (Harms, 1960) hemispheric perimeters, p e r i -metry i n general began to develop as a precise method i n which experi-mental conditions could be o b j e c t i v e l y s p e c i f i e d and thus meaningful psychophysical data could be obtained. As a r e s u l t chromatic perimetry regained r e s p e c t a b i l i t y as a us e f u l , precise method, and has been investigated extensively by V e r r i e s t and his associates (Verriest & I s r a e l , 1965a, 1965b; Francois, V e r r i e s t , & I s r a e l , 1966; V e r r i e s t & Kandemir, 1974; V e r r i e s t & U v i j l s , 1977a, 1977b) as w e l l as by others (e.g., Nolte, 1962; Hansen, 1974; Carlow, Flynn,.& Shipley, 1976). S t a t i c perimetry with chromatic s t i m u l i can y i e l d two types of thresholds, the achromatic and the chromatic. The achromatic threshold is. defined by the AL required for the subject to detect the stimulus, 3 whether o r not he p e r c e i v e s i t s c o l o u r . The c h r o m a t i c t h r e s h o l d , which i s g e n e r a l l y h i g h e r t h a n the a c h r o m a t i c a t e x t r a - f o v e a l l o c a t i o n s , i s d e f i n e d by the AL r e q u i r e d f o r the s u b j e c t to p e r c e i v e (and r e p o r t ) the colour, o f the s t i m u l u s . F o r a number of r e a s o n s , i t was the a c h r o m a t i c t h r e s h o l d which was of i n t e r e s t i n the p r e s e n t i n v e s t i g a t i o n . The c h r o m a t i c t h r e s h o l d shows g r e a t e r v a r i a b i l i t y than does the a c h r o -m a t i c ( A u l h o r n & Harms, 1972). T h i s i s to be expected, as i t depends on p s y c h o l o g i c a l v a r i a b l e s , such as how l o n g the s u b j e c t w a i t s t o be c e r t a i n o f the hue and how many c h o i c e s he has ( i . e . , how many p o s s i b l e c o l o u r s ) . The c o m p l e x i t y of the s u b j e c t ' s t a s k i n c h r o m a t i c t h r e s h o l d d e t e r m i n a t i o n would thus n e c e s s i t a t e an extended t r a i n i n g p e r i o d . A l s o , because hue d i f f e r e n c e s can be d e t e c t e d w i t h no luminance d i f -f e r e n c e , d e t e r m i n i n g a c h r o m a t i c t h r e s h o l d a g a i n s t a background of d i f f e r e n t c h r o m a t i c i t y may a c t u a l l y r e p r e s e n t h u e - d i f f e r e n c e t h r e s h o l d d e t e r m i n a t i o n ( A u l h o r n & Harms, 1972). S i n c e the a c h r o m a t i c t h r e s h o l d d e t e r m i n a t i o n p r e s e n t s a s i m p l e t a s k t o the s u b j e c t , and r e t i n a l g r a d i e n t s of a c h r o m a t i c t h r e s h o l d s i n d i c a t e the r e l a t i v e l i g h t - s e n s i -t i v i t y a c r o s s the r e t i n a , a c h r o m a t i c t h r e s h o l d s were det e r m i n e d i n the p r e s e n t i n v e s t i g a t i o n . O b j e c t i v e s p e c i f i c a t i o n o f s t i m u l i becomes a f a r more complex problem, b o t h t h e o r e t i c a l l y and p r a c t i c a l l y , w i t h the change from a c h r o m a t i c t o c h r o m a t i c p e r i m e t r y . The s p e c i f i c a t i o n s n e c e s s a r y f o r a c h r o m a t i c s t i m u l i — s i z e , c o l o u r - t e m p e r a t u r e , and d u r a t i o n — m u s t be s t a n d a r d i z e d i f increment t h r e s h o l d s o b t a i n e d a t d i f f e r e n t r e t i n a l l o c a t i o n s f o r a c h r o m a t i c s t i m u l i a r e to be d i r e c t l y compared. Such p r e c i s e s p e c i f i c a t i o n i s r e l a t i v e l y easy to o b t a i n , and much i n f o r m a t i o n 4 has been gained concerning the s e n s i t i v i t y of the v i s u a l system to achromatic s t i m u l i as a function of r e t i n a l location, stimulus s i z e , background luminance, and subject age (e.g., Lakowski & A s p i n a l l , 1969; Verriest & U v i j l s , 1977a; Aulhorn & Harms, 1972). However, i f s t i m u l i are to d i f f e r i n chromaticity, a decision must be made on how such s t i m u l i are to be equated. Two methods are possible, each implying different assumptions about what the obtained thresholds represent. Increment thresholds to s t i m u l i of varied chromaticity can be considered equal i n terms of radiant energy or of luminance. Radiometric equivalence of chromatic s t i m u l i i n perimetry has been advocated as the appropriate method by many researchers (e.g., Ferree & Rand, 1919; Dubois-Poulsen, 1952; Aulhorn & Harms, 1972). I t i s suggested that such physical energy s p e c i f i c a t i o n of. s t i m u l i i s more appropriate than photometric s p e c i f i c a t i o n because the spectral sensi-t i v i t y of the fovea ( i . e . , V^) i s not representative of the spectral s e n s i t i v i t y over the entire retina (Aulhorn & Harms, 1972). However, with radiometric equalization, the one standard with which perimetric data can be compared i s l o s t : the invariant foveal threshold. Radio-metric equivalence of chromatic s t i m u l i results i n high foveal thresholds for blue and red s t i m u l i r e l a t i v e to that for green (and 'white' — achromatic) s t i m u l i ; this merely r e f l e c t s V^. Once outside the fovea, different thresholds for the chromatic s t i m u l i then r e f l e c t not only differences between foveal and extra-foveal s e n s i t i v i t y , but also the r e l a t i v e luminous ef f i c i e n c y which i s known to characterize the fovea. This confounding influence can be i l l u s t r a t e d by referring to Figure 1, which shows Nolte's results using monochromatic s t i m u l i specified 5 loyWutt/cm' W - I 2 -io-" fo-10 10 ~9 w "7 10 ' 6 background - luminance H) a:!, OJa:.h 0 asb 453,0 nm 479,0 nm 499,7nm 523,3nm 577,5nm 599,0 nm G5d,0nm 10 io-10 10° 0° 10' 0" 10° 0° 10° 0° 10° 0° 10° 0° 10° 0° • 10 10-9 10-8 io-7 10'6 10~5 10° At-lu'onia I ici I In cs lmlds in 11 MI rcnl ral rim I |»;ir;ic-riil nil i v » i n n s i i f II" inrrii li.iu of r r l i n a . Ooordinalcs: an in l''ig. K i . I'j iprr cui'vcs obtained v i l l i rumpled ' dark ada]ilat ion. middle curves v i l l i adaptation In l i . l asli background luminance anil lover curves v i l l i adapta l ion to JOasb background luminance (according lo N O I . T K ) Figure 1 (from Aulhorn and Harms, 1972) 6 r a d i o m e t r i c a l l y . A curve could be drawn through the foveal threshold points for the seven selected wavelengths; t h i s curve i n general form r e f l e c t s V^. Since i s w e l l established, i f i t s influence were removed (by photometrically equating the s t i m u l i ) , the foveal thresholds would then coincide, and perimetry would delineate the changes i n sen-s i t i v i t y as a function of r e t i n a l p o s i t i o n for s t i m u l i of v a r i e d chromaticity. A number of investigators have s p e c i f i e d t h e i r s t i m u l i and made th e i r measurements photometrically (e.g., V e r r i e s t & I s r a e l , 1965a, 1965b; Ronchi, 1972), but photometric equivalence was not employed. With the i n i t i a l modifications of the Goldmann perimeter used i n the present i n v e s t i g a t i o n , photometric-equivalence was achieved and the f i r s t chromatic perimetric r e s u l t s based on photometrically-equated s t i m u l i were reported (Lakowski, Wright, & O l i v e r , 1976, 1977). With such equivalence, the s t i m u l i are equated at the fovea, so that a standard i s established to which extra-foveal s e n s i t i v i t y can be com-pared. Once photometric s p e c i f i c a t i o n has been decided upon, i t remains to be decided how the s t i m u l i to be presented under scotopic conditions are to be equated: i n terms of or (the scotopic luminous e f f i -ciency curve) . The use of V-^  to equate a l l s t i m u l i at the fovea has the major advantage that photopic and scotopic adaptation r e s u l t s can be more d i r e c t l y compared i f both are based on the same luminous e f f i c i e n c y curve. The fa c t that a l l equating has been done i n terms of and therefore of the fovea must be considered when i n t e r p r e t i n g r e s u l t s obtained i n t h i s manner. 7 Adaptation. Increment thresholds are influenced by the ambient luminance i n a complex manner, presumably due to d i f f e r e n t i a l c o n t r i -butions of the rod and cone systems at scotopic, mesopic, and photopic l e v e l s . Photopic v i s i o n has generally been considered to begin at -2 10 cd.m. (LeGrand, 1957), although t h i s value depends on the stimulus -2 s i z e . Adaptation luminance of 10 cd.m. has been widely used i n p e r i -metric studies (e.g., V e r r i e s t & I s r a e l , 1965a, 1965b; Lakowski & A s p i n a l l , 1969; Carlow et a l . , 1976). However, the o r i g i n a l work of Goldmann (1945c), on which the s p e c i f i c a t i o n s for the widely-used Goldmann perimeter are based, proposed the use of 40-45 asb (12.7 -14.3 cd.m ) as the adaptation luminance. This value was chosen to f a l l i n the middle of the range of luminances found i n doctors' examining rooms (Goldmann, 1945c). At these adaptation luminances -2 ' (10, 12.7, or 14.3 cd.m. ), both rod and cone systems are a c t i v e (Aguilar & S t i l e s , 1954), so that i t i s d i f f i c u l t to draw conclusions concerning the function of e i t h e r . If an attempt i s made to study these two systems separately, i t i s necessary to consider whether the two systems are to be dealt with as completely separate or as a continuum from scotopic to photopic. There i s no clear-cut s o l u t i o n to t h i s problem. S u f f i c i e n t l y high adaptation luminance can achieve rod saturation (Aguilar & S t i l e s , 1945) so that f u l l y photopic thresholds can be assumed to be rod-free thresholds. However, f u l l y scotopic conditions do not guarantee that thresholds obtained under such conditions are cone-free thresholds. " F u l l y scotopic" r e f e r s to the condition wherein the t h r e s h o l d — t h a t i s , the absolute threshold—changes minimally over time. This occurs only when the r e t i n a i s f u l l y dark-adapted. Thresholds obtained under such conditions would presumably be cone thresholds i n any r e t i n a l l o c a t i o n which had no r o d s — t h a t i s , i n the foveola. One would expect then that beyond the 54-minute-of-arc extent of the foveola (Moses, 1975), any f u l l y scotopic thresholds would r e f l e c t rod s e n s i t i v i t y . Data reported by Sloan (1950) support t h i s (see Figure 2). Using dark-adaptation methods, she found that for an achromatic (Illuminant 'C') stimulus subtending one degree of v i s u a l angle, rod thresholds were lower than cone thresholds at a l l points tested except the fovea. The e c c e n t r i c i t i e s investigated ranged from 50° nasal and 90° temporal to 3.5° from the fovea, i n the h o r i z o n t a l meridian. As was expected, no rod component was seen i n the dark-adaptation curve obtained at the fovea. These r e s u l t s would seem to i n d i c a t e , then, that for an achromatic stimulus of 1° (or, presumably, smaller), thresholds obtained under f u l l y - s c o t o p i c conditions may be assumed to be cone-free thres-holds to at l e a s t within 3.5° and probably closer to the foveal centre. Foveal thresholds obtained under these conditions would appear to be cone thresholds. Whether these r e s u l t s would be duplicated with s t i m u l i of selected chromaticity i s not known. This d i s c u s s i o n w i l l , therefore, be r e s t r i c t e d to references to f u l l y photopic and f u l l y scotopic conditions; the former can be considered to r e f e r s p e c i f i c a l l y to rod-free functioning, while the l a t t e r must be interpreted i n a more r e s t r i c t e d sense. (a) S p e c i f i c review of relevant research. Fully-photopic perimetry requires saturation of the rod mechanism, which occurs with a r e t i n a l i l l u m i n a t i o n of 2,000 to 5,000 scotopic trolands, corresponding 9 Figure 2 From Sloan, 1950, p. 1081. 10 to 120 to 300 cd.m. with a natural p u p i l (Aguilar & S t i l e s 1954). 1. V e r r i e s t and Kandemir There has been no data reported for s e n s i t i v i t y gradients estab-l i s h e d under conditions of complete rod saturation. V e r r i e s t and Kandemir (1974) did obtain foveal thresholds for monochromatic s t i m u l i -2 against a background of 132 cd.m. (Illuminant 'A'), j u s t within the lower l i m i t of rod-saturation found by Aguilar and S t i l e s (1954). They used f i v e 116' s t i m u l i ranging from 500 to 600 nm, and thus did not obtain any r e s u l t s with.a stimulus i n the blue portion of the spec-trum. Their r e s u l t s are shown i n Figure 3. They found that the foveal threshold value, expressed i n radiometric u n i t s , was lowest for green s t i m u l i (A. = 528, 553 nm), higher for red (A = 600 nm), and s t i l l higher for blue-green (A = 500 nm). The r e l a t i v e values of these thresholds r e f l e c t the r e l a t i v e s p e c t r a l luminous e f f i c i e n c y of the fovea. According to the CIE standard photopic r e l a t i v e s p e c t r a l luminous e f f i c i e n c y function (V^), the foveal s e n s i t i v i t y to these wavelengths follows the same r e l a t i v e pattern: highest s e n s i t i v i t y to the green, lower to the red, and s t i l l lower to blue-green. This would seem to i n d i c a t e that the r e l a t i v e s p e c t r a l s e n s i t i v i t y of the fovea under f u l l y - p h o t o p i c conditions i s s i m i l a r i n form to that s p e c i f i e d by the CIE V-^  function. This i s i n t e r e s t i n g i n view of the fa c t that the V-^  curve was obtained under conditions i n which the r e t i n a l i l l u m i n a t i o n never reached 100 trolands (LeGrand, 1968), i n d i --2 eating an adaptation luminance of about 10 cd.m. . The small s i z e of stimulus used to determine V-^  (2°; Wyzecki & S t i l e s , 1967) was presumed to r e s t r i c t the measurements to cones. This i s supported by 11 V W«J mi on ooe -J5.0/ (00 500 600 700 Figure 3 Mean foveal thresholds (log s e n s i t i v i t y vs. wavelength). Adaptation= 132 cd.m. , stimulus s i z e = 116', achromatic stimulus ( C L E . Illuminant A) From V e r r i e s t and Kandemir, 1974, p. 5. 5-12 the s i m i l a r i t y of the r e l a t i v e values of V e r r i e s t and Kandemir's thresholds obtained with s u f f i c i e n t luminance to approach rod satura-t i o n . If the r e l a t i v e s p e c t r a l s e n s i t i v i t y of the fovea i s t r u l y independent of adaptation, one would expect that (provided a l l other variables are held constant) foveal thresholds for a given wavelength would not vary with adaptation luminance. Unfortunately, the above proviso i s seldom met, so that comparison, for example, between V e r r i e s t and Kandemir's data and the V^ date can only be i n r e l a t i v e , d e s c r i p t i v e terms. Nonetheless, i f V^ does hold under ' f u l l y 1 photopic adaptation, i t would be expected that photometric-equating of chromatic s t i m u l i i n terms of would y i e l d equivalent foveal thresholds at f u l l y photopic conditions. One further aspect of V e r r i e s t and Kandemir's i n v e s t i g a t i o n war-rants mention: they used a l i g h t source with the s p e c t r a l d i s t r i b u -- 2 t i o n of Illuminant 'A' to achieve the adaptation of 132 cd.m. The low energy of t h i s source i n the shorter wavelengths may have meant that not a l l response systems of the eye were under fu l l y - p h o t o p i c conditions. 2. Sloan S e n s i t i v i t y gradients for achromatic s t i m u l i under f u l l y scotopic conditions (dark adaptation) have been reported by Sloan (1939, 1947, 1950). She used a 1° square, CIE Illuminant 'C stimulus which was presented for one second. Her i n v e s t i g a t i o n took the form of dark adaptation curve determinations, and thus involved ascending and des-cending stimulus presentations. Figure 4 shows the s e n s i t i v i t y gradient for a normal eye determined i n t h i s way; Figure 5 shows curves for the 13 Figure 4 From Sloan, 1939, p. 240. 15 mean gradient (N = 101) plus or minus 2 standard deviations. Several points can be made about these r e s u l t s . F i r s t , i t i s apparent that the scotopic s e n s i t i v i t y to t h i s stimulus i s highest i n the mid-periphery, f a l l i n g o ff r a p i d l y at the fovea and more slowly i n the periphery. This r e l a t i v e drop at the fovea has been r e f e r r e d to as a r e l a t i v e c e n t r a l scotoma. When using t h i s term, i t i s important to keep i n mind that i s does not imply a loss of s e n s i t i v i t y at the fovea; the fovea simply does not show as large a gain i n s e n s i t i v i t y as other r e t i n a l locations under scotopic conditions. The second notable aspect of Sloan's data i s the wide v a r i a b i l i t y i n s e n s i t i v i t y indicated by Figure 5. The reasons for t h i s wide v a r i a b i l i t y are not e a s i l y i d e n t i f i e d . The fact that the data i s based on subjects from 14 to 70 years of age i s l i k e l y a contributing f a c t o r , as age has been shown to influence r e t i n a l s e n s i t i v i t y (Lakowski & AspinaH, 1969; V e r r i e s t & U v i j l s , 1977a). As well as the age f a c t o r , whether or not the v a r i a b i l i t y r e f l e c t s a c t u a l v a r i a t i o n i n the normal l i g h t sense i s obscured by the lack of precise c o n t r o l of f i x a t i o n under f u l l y scotopic conditions. Provided there i s some l i g h t i l l u m i n a t i n g the subject's eye, the experimenter can monitor f i x a t i o n continuously throughout the t e s t i n g and disregard responses made when f i x a t i o n was not maintained. I f , however, conditions of complete dark-adaptation p r e v a i l , unless an i n f r a - r e d l i g h t - s e n s i t i v e f i x a t i o n monitor i s used, the experimenter must r e l y on the subject's subjectiveiimpression that he i s f i x a t i n g p r e c i s e l y . Sloan, as w e l l as others who have reported scotopic s e n s i t i v i t y gradients, did not monitor f i x a t i o n , but r e l i e d on the subject's report 16 that he was f i x a t i n g . Thus, i t i s not possible to t e l l whether the v a r i a t i o n she observed was due e n t i r e l y to variance i n the l i g h t -sense or was p a r t i a l l y the r e s u l t of losses of f i x a t i o n . The large foveal v a r i a t i o n i s p a r t i c u l a r l y suspect i n t h i s regard. I f , indeed, the foveal s e n s i t i v i t y i s greatly and sharply reduced from that of adjacent areas, any small s h i f t of f i x a t i o n would y i e l d a much higher s e n s i t i v i t y . This may be seen i n Figure 6. If the foveal threshold i s a c t u a l l y at the value a_, a f i x a t i o n s h i f t of 2° would y i e l d a 'foveal' threshold of b_, which might be as much as 0.5 to 1.0 log unit lower (more s e n s i t i v e ) than a.. As the l i t e r a t u r e to date indicates that such a foveal " r e l a t i v e scotoma" does e x i s t (Sloan, 1939, 1947, 1950; Nolte, 1962; Wentworth, 1930), co n t r o l of foveal f i x a t i o n would appear to be an exceedingly important aspect of scotopic threshold determination. Returning to Sloan's work, i t i s important to note that for foveal f i x a t i o n , she used a 6 ° diameter pattern of four radium-painted dots, into the centre of which the 1° square stimulus was projected. The p o s s i b i l i t y of f i x a t i o n s h i f t s cannot be ruled out as a factor i n the wide v a r i a b i l i t y i n the.foveal thresholds determined. More importantly, the actual value of the foveal threshold cannot be s p e c i f i e d with any c e r t a i n t y . There appears to be a natural tendency under scotopic conditions, to s h i f t f i x a t i o n u n t i l the stimulus image f a l l s on a more s e n s i t i v e paracentral area of the r e t i n a . It i s obvious that the only s o l u t i o n to t h i s problem would be to d i r e c t l y monitor f i x a t i o n through-out scotopic threshold determination. Unfortunately, t h i s presents considerable methodological problems, and has not been done i n any of 17 Figure 6 Hypothetical graph of r e t i n a l s e n s i t i v i t y vs. e c c e n t r i c i t y under f u l l y - s c o t o p i c conditions. Refer to text for explanation. 18 the scotopic perimetric studies reported thus f a r . Aside from the highly v a r i a b l e foveal thresholds, Sloan's data also i n d i c a t e considerable v a r i a t i o n i n the threshold obtained r i g h t across the 0°-180° meridian, t h i s v a r i a b i l i t y being greatest i n the.far periphery, and decreasing as e c c e n t r i c i t y decreases. Aulhorn and Harms (1972) report s i m i l a r r e s u l t s , except that t h e i r minimum v a r i a -t i o n was found at 0° as opposed to the large v a r i a t i o n at 0° found by Sloan. As was pointed out, the foveal thresholds under scotopic conditions are very u n r e l i a b l e , so that t h i s d i f f e r e n c e between the. two studies i s d i f f i c u l t to i n t e r p r e t . Aulhorn and Harms found t h i s same v a r i a b i l i t y pattern under other adaptation conditions (wherein f i x a t i o n was presumably monitored), except that the magnitude of the va r i a t i o n s tended to decrease as adaptation luminance was increased from 0 to 100 a p o s t i l b s . Using dark adaptation methods, Lakowski, Drance, & Goldthwaite . (1976) also found smaller v a r i a t i o n s i n foveal as opposed to perip h e r a l thresholds. 3. Wentworth Wentworth (1930) obtained scotopic s e n s i t i v i t y gradients on the 0°-180° meridian f o r various 1°16' monochromatic s t i m u l i (A = 672.5, 581.5, 522, and 468 nm). Her data i s a l l based on one subject, and a foveal f i x a t i o n device f i t t e d p r e c i s e l y to t h i s subject was used. This consisted of a pattern of four radium-painted dots so positioned as to f a l l w ithin the subject's b l i n d spot only when she was f i x a t i n g c o r r e c t l y . Foveal threshold data was c o l l e c t e d only when these four dots disappeared from her view. For other r e t i n a l points, a s i m i l a r pattern of four dots of radium-paint subtending 2° was Used which 19 would disappear (due to the ' r e l a t i v e c e n t r a l scotoma') when f i x a t i o n was correct. Wentworth's r e s u l t s are reported i n radiometric u n i t s . I t would be expected, then, that the foveal s e n s i t i v i t i e s would be i n the order of 581.5 (yellow) > 522 (green) > 468 (blue) > 672.5 (red), as t h i s i s the order of s p e c t r a l e f f i c i e n c i e s for these wavelengths as s p e c i f i e d by ( S t i l e s & Wyszecki, 1967). This was not found; the foveal s e n s i -t i v i t y order was green > blue > yellow > red. This i s rather s u r p r i s i n g , p a r t i c u l a r l y as the blue and green foveal s e n s i t i v i t i e s were almost i d e n t i c a l , i n s p i t e of the f a c t that gives a r a t i o of 1:9 (468:522 nm) for the r e l a t i v e s p e c t r a l e f f i c i e n c i e s of these wavelengths. The fac t that the stimulus used subtended 1°16' of v i s u a l angle implies that the 'foveal' threshold may not be s t r i c t l y a rod-free threshold, and the order of foveal s p e c t r a l s e n s i t i v i t i e s obtained might r e f l e c t Vj^ rather than V^. The r e s u l t s appear to support t h i s : the r e l a t i v e s p e c t r a l e f f i c i e n c i e s of these wavelengths according to V-^ ' are green > blue > yellow > red, the same order as was obtained for s e n s i t i v i t i e s . I t would seem that Wentworth's foveal thresholds can be interpreted as rod thresholds, assuming the rods present within 68' of the foveal centre are more s e n s i t i v e than the cones under f u l l y scotopic condi-t i o n s . I t i s also important to note that the monochromatic s t i m u l i used were produced by a spectroscope with a constant s l i t width (1.05 mm). This would r e s u l t i n d i f f e r e n t amounts of energy reaching the eye for d i f f e r e n t wavelengths, as more short-wavelength energy would be sampled by a given s l i t - w i d t h than long-wavelength energy. This would then represent yet another v a r i a b l e inf l u e n c i n g the thresholds 20 obtained for the d i f f e r e n t chromatic s t i m u l i . Considering now the e n t i r e s e n s i t i v i t y gradients obtained by Wentworth (see Figure 7) wherein s e n s i t i v i t y i s plotted as a function of e c c e n t r i c i t y , shows that at a l l r e t i n a l points tested, the highest s e n s i t i v i t y was to the green and the lowest was to the red stimulus. The foveal r e l a t i v e s e n s i t i v i t y order (green > blue > yellow > red) i s not p r e c i s e l y r e f l e c t e d p e r i p h e r a l l y , where the order i s green > yellow > blue > red. The apparent r e v e r s a l of the yellow and blue i n the order of s e n s i t i v i t i e s between the c e n t r a l and peri p h e r a l f i e l d may not be s i g n i f i c a n t , as the magnitude of the diffe r e n c e at the fovea i s small (.46 log units) as compared to that i n the periphery (Figure 7). For a l l s t i m u l i , a s i m i l a r trend to that found f o r an achromatic stimulus (Sloan, 1939) i s seen: maximal s e n s i t i v i t y i n the mid-periphery, f a l l i n g o f f rapidly at the fovea, and more slowly i n the far periphery. The separation of the gradients for the d i f f e r e n t chromatic s t i m u l i presumably r e f l e c t s d ifferences i n the dark-adapted retina's s e n s i t i v i t y to s t i m u l i of d i f f e r e n t s p e c t r a l composition. Whether these peripheral s p e c t r a l s e n s i t i v i t y differences are the same as those of the fovea i s obscured by the fact that the s t i m u l i were not photometrically equated. As with Sloan's data, i t must be kept i n mind that the v a l i d i t y of Wentworth's r e s u l t s depends on whether f i x a t i o n was c o r r e c t l y maintained during a l l measurements. Although her techniques f o r ensuring f i x a t i o n would seem more accurate than any others thus f ar reported, Wentworth s t i l l r e l i e d on a subjective impression of stable f i x a t i o n rather than an objective measure. 21 Figure 7 Achromatic s e n s i t i v i t y to spectrum l i g h t s under dark adaptation. From Wentworth, 1930, p. 21. 22 4. Nolte In 1962 Nolte used the Tiibinger perimeter to obtain scotopic s e n s i t i v i t y gradients on the 0°-180° meridian to monochromatic s t i m u l i subtending 30' of v i s u a l angle. His data are based on three subjects, using s t i m u l i of seven wavelengths from 453 to 658 nm. Figure 8 shows the 10° nasal-0°-10° temporal segments of the gradients he obtained. Once again, radiometric s p e c i f i c a t i o n of s t i m u l i has been used, and the r e l a t i v e values of the foveal thresholds r e f l e c t the form of the curve, with the foveal s e n s i t i v i t y to A = 523.5 nm being highest and that to A = 658 nm the lowest. Nolte compared his r e s u l t s to Wentworth's (Figure 9). The major discrepancies between the two sets of data are as follows. F i r s t , Wentworth's s e n s i t i v i t y gradients are a l l higher than Nolte's and t h i s increased s e n s i t i v i t y i s not uniform across the r e t i n a , being greatest i n the periphery and smallest i n the c e n t r a l and paracentral areas. This presumably r e f l e c t s the d i f f e r e n c e i n s i z e of s t i m u l i used; Wentworth's s t i m u l i were approximately 2% (2.53) times as large as Nolte's. The increased s e n s i t i v i t y to the larger s t i m u l i r e f l e c t s the s p a t i a l summation capacity of the r e t i n a , and the greater increase seen i n the periphery indicates a higher capacity for s p a t i a l summa-tion,there. The second major d i f f e r e n c e between Wentworth's and Nolte's r e s u l t s i s i n the foveal s e n s i t i v i t y to long wavelengths. While Wentworth found a r e l a t i v e l y lower s e n s i t i v i t y i n the fovea to a l l s t i m u l i including the red one (A = 672.5 nm), Nolte did not f i n d t h i s foveal 'dip' at the longest wavelength used (A = 658 nm), though he did f i n d one for A = 599 nm. It seems u n l i k e l y that the stimulus Figure 8 F u l l y - s c o t o p i c s e n s i t i v i t y gradients from 10° nasal to 10° temporal, for monochromatic s t i m u l i . From Nolte, 1962. Figure 9 F u l l y - s c o t o p i c s e n s i t i v i t y gradients obtained by Wentworth (1930) arid Nolte (1962). From Nolte, 1962. 4> 25 s i z e was an important factor here. Wentworth's stimulus (1°I6') most l i k e l y stimulated rods as w e l l as cones, so that, i f anything, one would expect greater s e n s i t i v i t y than i f cones alone were stimulated. However, the low scotopic s p e c t r a l luminous e f f i c i e n c y of t h i s wave-length (.0001) makes i t u n l i k e l y that the rods would contribute much i n any case. I t i s quite possible that c o n t r o l of foveal f i x a t i o n i s an important factor i n t h i s discrepancy. Nolte used an achromatic pattern c o n s i s t i n g of a c i r c l e having four p a r t i a l r a d i i pointing to the centre, wherein the stimulus was presented. With such a f i x a t i o n devise, s h i f t s of f i x a t i o n are quite possible and cannot be monitored. The p o s s i b i l i t y , therefore, e x i s t s that foveal s e n s i t i v i t y to t h i s stimulus was not assessed accurately. Subject Variables. Luminance thresholds determined by s t a t i c perimetry have been shown to be influenced by a number of subject-r e l a t e d v a r i a b l e s . Refraction abnormalities are known to r a i s e thres-holds within 25° of the fovea (Aulhorn & Harms, 1972). It i s necessary, then, that normal s e n s i t i v i t y gradients be obtained with emmetropes. There i s , of course, a p r a c t i c a l l i m i t to t h i s c r i t e r i o n . An emme-tro p i c eye i s defined by Adler as "one i n which the r e t i n a coincides with the posterior p r i n c i p a l focus of the o p t i c a l system when the muscular a c t i v i t y c o n t r o l l i n g focusing i s at a physiologic minimum" ( Moses, 1975, p. 298). The p r e c i s e l y emmetropic eye according to this d e f i n i t i o n i s a non-existant phenomenon c l i n i c a l l y (Newell & Ernest, 1974); for p r a c t i c a l purposes an eye may be considered emmetropic i f i t i s assessed by objective r e f r a c t i v e methods at 20/20 Snellen acuity with a c o r r e c t i o n of less than ±.5 diopters (Drance, Note 1). 26 Colour v i s i o n d e f i c i e n c i e s influence increment thresholds for some chromatic s t i m u l i . V e r r i e s t and I s r a e l (1965b) and V e r r i e s t and U v i j l s (1977b) have shown that protan, deutan, and t r i t a n defects are associated with increment s e n s i t i v i t y losses for the long, middle (around 500 nm), and short wavelengths, r e s p e c t i v e l y . In addition, for a l l colour v i s i o n defects, i t was found that foveal s e n s i t i v i t y was reduced for a l l s t i m u l i and the ' r e l a t i v e c e n t r a l scotoma' for the short wavelengths was also reduced (Verriest & I s r a e l , 1965b; V e r r i e s t & U v i j l s , 1977b). Lakowski et a l . (1977) presented contra-d i c t o r y evidence: a protanope showed, i n addition to reduced s e n s i -t i v i t y to a red stimulus, increased ( r e l a t i v e to the normal) sen s i -t i v i t y to a blue stimulus. The e f f e c t of age on perimetric s e n s i t i v i t y gradients has been investigated by Lakowski and A s p i n a l l (1969) and by V e r r i e s t and U v i j l s (1977a). While the l a t t e r used monochromatic s t i m u l i , Lakowski and A s p i n a l l used an achromatic target. In both cases, age was shown to influence s e n s i t i v i t y , but d i f f e r e n t age categories were used i n each case. Lakowksi and A s p i n a l l ' s age groups of 13-15 and 17-25 years of age showed the highest s e n s i t i v i t y both c e n t r a l l y and p e r i -p herally, s e n s i t i v i t y being lower i n both younger and older age groups. It i s important to note that these subjects were not c l a s s i f i e d as emmetropic. Though the 13-15 and 17-25 year age groups had a mean v i s u a l acuity of 20/20 Snellen, the older age groups had lower mean a c u i t i e s . However, when only subjects with acuity of 20/22 Snellen or better were considered, the older age groups (except perhaps the 26-35 years group) s t i l l showed reduced foveal s e n s i t i v i t y . V e r r i e s t 27 and U v i j l s ' r e s u l t s indicated that 'younger' subjects (10-15 years) were les s s e n s i t i v e i n the fovea, but more s e n s i t i v e p e r i p h e r a l l y than a 'medium' group (16-41 years). This was a general f i n d i n g over a l l wavelengths used excepting 480 and 553 nm, for which the young group was more s e n s i t i v e at a l l points tested (180° meridian, 45° nasal to 0°). The wide range of t h e i r 'medium' group makes i t d i f f i -c u l t to compare t h i s to Lakowski'and A s p i n a l l ' s r e s u l t s . In general, both studies i n d i c a t e that for large age d i f f e r e n c e s , age exerts a s i g n i f i c a n t e f f e c t on luminance d i f f e r e n c e thresholds. P r a c t i c e has been shown to influence increment thresholds, but the e f f e c t i s not the same for a l l subjects (Aulhorn & Harms, 1972). Pre - s e l e c t i o n of subjects showing consistent responses to the task of threshold determination would be expected to reduce t h i s p r a c t i c e e f f e c t . There i s some question concerning the importance of p r i o r know-ledge of stimulus l o c a t i o n . Grindly and Townsend (1968) and Mertens (1956) found that foreknowledge of stimulus l o c a t i o n did not s i g n i f i -cantly a f f e c t the p r o b a b i l i t y of detection. However, Engel (1971) found that such foreknowledge did increase the 'conspicuity a r e a 1 , the r e t i n a l area within which a 75 m i l l i s e c o n d peripheral stimulus i s detected. I f , i n f a c t , foreknowledge of the l o c a t i o n influences detection, i t i s important i n perimetry that either random presenta-t i o n be used or that the subject know p r i o r to t e s t i n g the sequence of test, locations to be used. P u p i l s i z e i s a subject v a r i a b l e which influences increment thresholds by l i m i t i n g the amount of l i g h t reaching the r e t i n a (Sloan, 28 1940)-. Experimental c o n t r o l of p u p i l s i z e can be attained through use of an a r t i f i c i a l p u p i l or drugs to f i x p u p i l s i z e . Neither of these methods are appropriate for a perimetric examination (Sloan, 1940). Use of an a r t i f i c i a l p u p i l requires r i g i d s t a b i l i z a t i o n of the subject using a f u l l dental-bite apparatus, as any small movement of the eye may change the r e l a t i v e positions of the a r t i f i c a l and natural p u p i l s . This would present p r a c t i c a l d i f f i c u l t i e s not n e c e s s a r i l y j u s t i f i e d by. the gain i n p r e c i s i o n . The use of drugs to f i x p u p i l s i z e i s not advisable for p r a c t i c a l and t h e o r e t i c a l reasons. Not only would i t necessitate the presence of a medical a s s i s t a n t at a l l test sessions, but the use of drugs i s only possible on the assumption that they a f f e c t only the autonomic and not the c e n t r a l nervous system, an assumption which seems unwarranted considering the involvement of parts of the CNS i n some autonomic reflexes (such as the accommodation r e f l e x , Barr, 1974). S p a t i a l Summation and S t a t i c Perimetry The e f f e c t s of stimulus area (A) and stimulus luminance (L) on absolute threshold are generally accepted to be inversely r e l a t e d for small targets, but the exact r e l a t i o n s h i p appears to depend on many fa c t o r s . Ricco's law of complete s p a t i a l summation (L • A = constant) seems to hold i n the fovea, but only for very small targets (less than 10') (Baumgardt, 1972); Piper's law of p a r t i a l summation (L • A 2 = constant) has been shown to hold i n the periphery for s t i m u l i up to 1° (Baumgardt, 1972). However, factors such as stimulus duration and chromaticity can influence the r e l a t i o n s h i p between A and L at threshold. 29 Assuming that no simple law exi s t s r e l a t i n g the two under a l l condi-tion s , i t would be us e f u l to have some measure of th i s r e l a t i o n s h i p which could be compared among conditions varying along one dimension only: for example, chromaticity. Goldmann (1945a, 1945b) suggested 'k', the 'exponent of summation', as a measure of the area-luminance r e l a t i o n s h i p at thresholds obtained v i a k i n e t i c perimetry. K was defined by the following equation: • "1/ where $ i s the transmittance of the neutral-density f i l t e r required to maintain the f i e l d s i z e obtained with a stimulus of s i z e F q , using a stimulus of s i z e F. This formula gives an objective measure of the area-luminance r e l a t i o n s h i p based on thresholds obtained v i a k i n e t i c perimetry, with k = 1 representing complete s p a t i a l summation and k = 0 representing no summation. Goldmann found that k = 0.84 f i t h i s data w e l l . K i n e t i c perimetry gives d i f f e r e n t data than does s t a t i c perimetry, and by i t s nature, y i e l d s data i n which s p a t i a l and temporal summation i n t e r a c t . Even i f one adapted Goldmann's formula for use with s t a t i c data, the r e s u l t i n g k values could not be d i r e c t l y compared to those obtained with k i n e t i c methods. From the proposed inverse r e l a t i o n s h i p between A and L, s p a t i a l k summation can be defined by the formula L •-A = constant, where k i s the exponent of summation which t h e o r e t i c a l l y can vary from 1 (no summation) to 1 (complete summation) (LeGrand, 1957). From t h i s formula, the value of k for a change i n stimulus s i z e from A, to A„ 30 may be defined by the following formula: l o g L - log L ? k = 1 IT 1 IT (Gougnard, 1961) (2) log A 2 - log A1 where and L 2 are the absolute thresholds obtained with s t i m u l i of areas and A 2, r e s p e c t i v e l y . This formula provides an objective measure of s p a t i a l summation when absolute thresholds are involved, and can therefore by j u s t i f i a b l y used for perimetric data obtained under f u l l y - s c o t o p i c conditions. However, when perimetry involving any adaptation luminance i s done, a problem a r i s e s i n how to define k. S p a t i a l summation has been studied and the laws thereof defined s t r i c t l y i n terms of absolute rather than increment thresholds (Baumgardt, 1972); some measure of s p a t i a l summa-ti o n under photopic or mesopic conditions i s required. What i s i n fact required i s a measure of the differ e n c e between the luminance increments necessary for detection for s t i m u l i of d i f f e r e n t areas. It would seen appropriate then to use AL i n place of L i n the expression L • A = constant. If comparison i s required between data obtained at d i f f e r e n t adaptation luminances, could replace L to take into account the fact that AL i s proportional to adaptation luminance. C a l c u l a t i o n of k v i a formula (2) would thus y i e l d the same k values using e i t h e r AL or i f L was constant. Hence, formula (2) may be replaced by the following forumla f o r c a l c u l a t i n g k under conditions of constant adaptation i l l u m i n a t i o n : log AL - log AL k = - (3) log A 2 - log A 1 31 However, comparison between k values calculated for absolute thresholds (formula 2) and increment thresholds (formula 3) must be done taking into account the fac t they are d i s t i n c t e n t i t i e s : one r e f l e c t s the s p a t i a l summation capacity of r e t i n a when absolute thresholds are involved, while the other r e f l e c t s s p a t i a l summation with respect to increment thresholds. S p e c i f i c Review of Relevant Research. In his o r i g i n a l work, "Grundlagen exakter Perimetric", Goldmann (1945a) studied the summa-ti o n c a pacities of the r e t i n a using k i n e t i c perimetry. He defined the summation exponent k by formula (1), and found that i t had a value of approximately 0.84 i n the normal r e t i n a . This value has been widely quoted i n studies of summation i n v o l v i n g s t a t i c perimetric methods. The fact that t h i s k value was derived using two moving targets, and pertains to a point i n the periphery where equivalent isopters were were obtained using d i f f e r e n t area-luminance s t i m u l i , makes i t questionable whether i t can be compared to s t a t i c data. Fankhauser and Schmidt (1958, 1960) compared the s p a t i a l summation seen i n s t a t i c and k i n e t i c perimetry using the Goldmann perimeter. They did not c a l c u l a t e values of k, but used the slope of and separation between s e n s i t i v i t y gradients as a measure of summation. The summa-ti o n factor determined using k i n e t i c perimetry with d i f f e r e n t speeds of movement (5°/sec and l°/sec) was not constant, being greater f o r the greater target speed. Scatter was also increased f o r a higher speed of movement. However, both the s t a t i c and the two k i n e t i c methods indicated that summation increased from the c e n t r a l to p e r i -pheral f i e l d s (indicated by an increase i n the v e r t i c a l separation 32 between gradients obtained with d i f f e r e n t s i z e s ) , and that summation decreased as stimulus s i z e increased. It would seem then that s t a t i c and k i n e t i c methods lead to s i m i l a r general conclusions concerning s p a t i a l summation, but that the s t a t i c method i s preferable as i t avoids complication by temporal v a r i a b l e s . Fankhauser and Schmidt (1960) also studied summation under d i f --2 ferent l e v e l s of adaptation, from 0.04 to 40 asb (0.013 - 12.7 cd.m. ). They found that summation increased as adaptation decreased, the magnitude of t h i s e f f e c t being s i m i l a r c e n t r a l l y and p e r i p h e r a l l y . Fankhauser and Schmidt also noted that there was considerable v a r i a b i l i t y i n a l l the r e s u l t s , and while no s t a t i s t i c a l l y s i g n i f i c a n t c o r r e l a t i o n between adaptation luminance and scatter was found, the data did i n d i c a t e an increase i n s c a t t e r with decreased adaptation luminance. Sloan (1961), using s t a t i c perimetry, determined values of k _2 under mesopic (L = 10 cd.m. , CIE Illuminant 'A') conditions i n the Goldmann perimeter. She confirmed the r e s u l t s of e a r l i e r researchers: s p a t i a l summation was shown to decrease with increasing s i z e and to increase with e c c e n t r i c i t y . In addition, she was able to quantify these r e s u l t s i n terms of the summation exponent k. Rather than c a l -culate k according to formula (3), Sloan plotted the increment thres-hold as a function of stimulus area, and then took the slope of the b e s t - f i t t i n g l i n e as -k according to the l i n e equation log AL = -k log A + constant (Figure 10) . She found that a s t r a i g h t l i n e could be f i t to the points corresponding to Goldmann sizes 0-3 (.0687 to 2 4.4 mm ), but not to those for the larger two s i z e s . Considering then 33 Figure 10 From Sloan, 1961, p. 33. 34 only sizes 0 to 3 , Sloan obtained values of k ranging from 0.90 (at 45° nasal) to 0.55 (at 0°) which decreased as e c c e n t r i c i t y decreased. From t h i s data one would expect that values of k calculated v i a formula (3) would be very s i m i l a r , at a given e c c e n t r i c i t y and at an adapta--2 t i o n of 10 cd.m. , for sizes 0-3, 1-2, and 2-3. The larger s i z e s would be expected to give generally lower values, as the slopes of l i n e s drawn between sizes 3 and 4 or 4 and 5 i n Figure 10 would be lower than the slopes determined f o r the smaller s i z e s . Gougnard (1961), again using the Goldmann perimeter with an -2 adaptation of 10 cd.m. , CIE Illuminant 'A', obtained values of k between sizes 0 to 1 for thresholds obtained on the nasal meridian for 20 normal subjects. He also determined k for sizes 1-2, 2-3, and 3-4 but only on the lower temporal meridian. He again, confirmed that the value of k ( r e f l e c t i n g summation capacity) increased with e c c e n t r i c i t y and decreased with increasing stimulus s i z e . His k values were c a l -culated for each i n d i v i d u a l e c c e n t r i c i t y measures (using formula 3). For the fovea, he obtained k = 0.49 (sizes 0-1), 0.39 (sizes 1-2), 0.17 (sizes 2-3), and 0.18 (sizes 3-4). These are a l l lower than Sloan's mean value of k, for sizes 1 to 3, of 0.55, Conversely, at 30° nasal, Gougnard's k values of 0.99 (sizes 0-1) and 1.09 (sizes 1-2) are higher than Sloan's (0.88). Both studies were done using the same instrument, Sloan having used an a u x i l i a r y f i x a t i o n device f o r measuring foveal thresholds, but i t i s possible that some unspecified variables such as stimulus duration, age, or r e f r a c t i v e error of sub-j e c t s (Gougnard s p e c i f i e d h i s subjects as emmetropes aged 22-25 years; Sloan s p e c i f i e d her subjects as 'normal') may have been p a r t i a l l y 35 responsible for the d i f f e r e n c e s . However, the large standard devia-tions found by Gougnard (Table 1) i n d i c a t e that considerable i n d i v i d u a l v a r i a t i o n exists i n s p a t i a l summation as r e f l e c t e d by k. The general conclusions which can be drawn from the foregoing investigations may be summarized as follows. F i r s t , s p a t i a l summation i s inversely proportional to stimulus s i z e and adaptation luminance, and d i r e c t l y proportional to e c c e n t r i c i t y . Second, considerable v a r i a t i o n i s seen i n the normal retina's threshold responses upon which measures of s p a t i a l summation are based. Because of the m u l t i -tude of factors which can influence threshold measurement (subject age, acuity, colour v i s i o n ; stimulus s p e c t r a l composition, duration, e t c . ) , i t i s d i f f i c u l t to determine how much v a r i a t i o n i s a c t u a l l y due to v a r i a b i l i t y of the light-sense i t s e l f . Therefore, v a r i a b i l i t y i n summation capacity as measured by k (e.g., Gougnard, 1961, see Table 1) may, i n f a c t , r e f l e c t variance i n only the l i g h t sense rather than i n summation capacity. The exact nature of the p r o p o r t i o n a l i t i e s mentioned i n the f i r s t point have not been defined, l a r g e l y due to t h i s wide v a r i a b i l i t y . Proposal It was apparent from the l i t e r a t u r e that l i t t l e information has been presented on r e t i n a l s e n s i t i v i t y gradients to chromatic s t i m u l i under adaptation conditions capable of separating the photopic and scotopic response systems. Photometric-equivalence of chromatic s t i m u l i had r a r e l y been used, and the s p a t i a l summation of chromatic s t i m u l i had not been investigated. I t was therefore proposed that normal 36 Table 1 Means and Standard Deviations of Summation C o e f f i c i e n t s (k) Determined with Goldmann Targets 0 and 1 on the Nasal Horizontal Meridian (Gougnard, 1961) E c c e n t r i c i t y 0° 0.49 0.20 5° 0.87 0.25 10° 1.02 0.20 15° 0.98 0.22 20° 1.03 0.16 25° 1.00 0.13 30° 0.99 0.13 37 r e t i n a l s e n s i t i v i t y gradients be established f o r photometrically-equated chromatic s t i m u l i of varying s i z e under both f u l l y - p h o t o p i c and f u l l y - s c o t o p i c conditions. In addition, i t was proposed that the s p a t i a l summation ca p a c i t i e s of the normal r e t i n a be explored for such photometrically-equated s t i m u l i using several stimulus sizes under fully- p h o t o p i c and f u l l y - s c o t o p i c conditions. Hence, achromatic thresholds were to be determined at 13 points on the 0-180° meridian, from 40° nasal to 40° temporal. Stimuli sub-tending 6.8, 13.6, 27.2, and 54.3 minutes at the eye (at 30 centimeters) were to be presented, gradients being obtained for each s i z e with an achromatic (T = 6000K) and red (X D= 623 nm), green (X Q= 504.5 nm), and blue (A^460 nm) chromatic s t i m u l i . Such gradients were to be -2 obtained under conditions of f u l l y - p h o t o p i c (L =250 cd.m. ) and f u l l y - s c o t o p i c (L = 0, dark-adaptation) conditions. 38 METHOD Apparatus Perimeter. A l l data was obtained using a modified Goldmann Pro-j e c t i o n Perimeter (Lakowski et a l . , 1977; Lakowski & Dunn, Note 2) (see Figure 11). It i s a hemispheric perimeter allowing the determination of luminance thresholds at s p e c i f i c points i n the v i s u a l f i e l d . The stimulus i s projected on the in s i d e of the hemisphere (the adaptation bowl) along a l i g h t path c o n s i s t i n g of a series of front-surface mirrors,.focusing lenses, and a prism. A pantograph connecting the stimulus projector arm and a marker on the data chart provides precise s p e c i f i c a t i o n of the p o s i t i o n of the stimulus. Both the adaptation f i e l d (for the photopic condition) and the stimulus were provided by xenon-arc lamps, "of correlated colour tem-perature T c = 6000K. The s p e c t r a l d i s t r i b u t i o n of the xenon-arc appro-ximates that of the CIE Illuminant 'C' (see Figure 12). Thus, the photopic adaptation luminance was high quite consistantly across the v i s i b l e spectrum, so that a l l response-systems of the eye were under ful l y - p h o t o p i c conditions. The s p e c t r a l d i s t r i b u t i o n of the stimulus -2 xenon-arc source, together with i t s high luminous output (8000 cd.m. ) provided the required high luminance at a l l v i s i b l e wavelengths including the short (blue) wavelengths. The stimulus source was a L e i t z XE 75, a 75-watt xenon-arc lamp housed i n a Leitz/ Lamphouse 100, mounted behind the hemisphere on the subject's l e f t . The adaptation-f i e l d source was a L e i t z XBO-150, a 150-watt xenon-arc lamp, housed i n a L e i t z Lamphouse 250 mounted on the upper edge of the adaptation 39 Figure 11 The modified Goldmann Perimeter 40 U3 O l i l I I I I I I I i | | I I I I J?0 +00 4lC *flO I " D ° S60 sxo Wo 5*0 MO SOO SIO <1o « 0 «£> T» 7 a ° Wavelength Figure 12 Relative s p e c t r a l energy d i s t r i b u t i o n of C L E . Illuminant C and Xenon-arc. From Wyszecki and s t i l e s , 1967. 41 bowl on the subject's r i g h t . Both lamps were cooled by an extraction fan, which directed some of the heat as w e l l as the ozone produced by the lamps into the building's exhaust duct. The output of the adaptation source was dire c t e d up to the top of the adaptation bowl from which i t was dif f u s e d throughout the bowl. A d i f f u s i o n b a f f l e mounted across the upper edge of the bowl aided t h i s d i f f u s i o n and decreased loss of l i g h t out of the bowl. The high luminance, I l l u -minant 'C' character of the adapting f i e l d was further ensured by the surface of the bowl, which was painted with Kodak Eastman White Reflectance Paint. This provided a surface with 98% refle c t a n c e across -2 the v i s i b l e spectrum. In t h i s way, an adapting f i e l d of 250 cd.m. , T = 6000K was achieved, c The luminance of the stimulus was varied by placing neutral density f i l t e r s i n the l i g h t path. A serie s of such f i l t e r s are mounted i n the instrument, allowing reduction of stimulus luminance by two log un i t s , i n 0.1 log unit steps. Two f i l t e r caps, each of which reduces the stimulus luminance by two log un i t s , can be mounted on the projector arm. In addition, two sets of neutral density f i l t e r s 1 , each set reducing stimulus luminance by 2 log u n i t s , could be. inserted d i r e c t l y i n front of the stimulus source. As a r e s u l t , the stimulus luminance was v a r i a b l e , i n 0.1 log unit steps, over a range of ten log u n i t s . The stimulus s i z e was varied by means of a series of d i a -phragms b u i l t into the instrument. The stimulus sizes used were .275, 2 ' 1.1, 4.4, and 17.6 mm , which subtended v i s u a l angles of 6.8, 13.6, *Wild L e i t z #126131 NG4, diameter = 32 mm. 42 27.2, and 54.3 minutes of angle at the subject's eye (at a distance of 30 cm ). Stimulus presentation was co n t r o l l e d by an automatic shutter mechanism which presented the stimulus for 150 milliseconds, with an interstimulus i n t e r v a l of one second. During a l l t e s t i n g , an a u x i l i a r y mechanism kept a steady ' c l i c k i n g ' sound going at one ' c l i c k ' per second. This served to mask the sound of the stiutter opening, which would replace the a u x i l i a r y sound when the experimenter activated the shutter mechanism. In th i s way, the subject could not use the sound of the shutter as a clue to determine when the stimulus was presented. The eye to be tested was positioned by having the subject s i t with his. chin i n the chin-rest and h i s forehead against a r e s t r a i n i n g band, the non-test eye being covered by a white d i f f u s i n g occluder. The experimenter positioned the eye by moving the chin-rest while the subject f i x a t e d on the c e n t r a l black spot i n the adaptation bowl. This spot i s the base of a telescope through which the experimenter views the eye and centres i t using the cross-wires i n the telescope. These cross-wires have millimeter gradations for measurement of p u p i l d i a -meter. When the eye was centered, the distance from the corneal surface to any point on the bowl, was 30 cm. For scotopic measurements, a red probe l i g h t was used which was inserted i n the mounting cylinder for the perimeter's telescope, the luminance being c o n t r o l l e d by a v a r i a c . This red probe l i g h t was the f i x a t i o n point for a l l e x t r a - f o v e a l . t e s t i n g under scotopic conditions. Foveal threshold measurements were made using a a l i g h t l y d i f f e r e n t procedure for the photopic and scotopic conditions. For the l a t t e r , 43 the a u x i l i a r y f i x a t i o n d e v i c e p r o v i d e d w i t h t h e i n s t r u m e n t was used. T h i s d e v i c e p r o j e c t s a p a t t e r n of f o u r l i g h t s i n a diamond shape, each l i g h t s u b t e n d i n g a p p r o x i m a t e l y seven minutes and the e n t i r e p a t -t e r n s u b t e n d i n g two degrees of v i s u a l a n g l e . T h i s p a t t e r n i s p r o j e c t e d a t a p o i n t c e n t e r e d a t f i v e degrees to t h e s u b j e c t ' s r i g h t of the a d a p t a t i o n bowl's c e n t r e . The s u b j e c t was i n s t r u c t e d t o f i x a t e on the c e n t r e o f t h e p a t t e r n , and i n d i c a t e when he saw a l i g h t . The p r o j e c t o r was f i t t e d w i t h a r e d c i n e m o i d f i l t e r 2 , and i t s luminance c o n t r o l l e d w i t h a v a r i a c . Two s c o p t o p i c f o v e a l t h r e s h o l d s were o b t a i n e d u s i n g t h i s d e v i c e , one w i t h the f i x a t i o n p a t t e r n j u s t b a r e l y v i s i b l e t o t h e s u b j e c t , and one w i t h the p a t t e r n s l i g h t l y h i g h e r i n luminance. Another s i m i l a r f i x a t i o n p a t t e r n which subtended 3.4 degrees of v i s u a l a n g l e was a l s o used t o determine t h e s c o t o p i c f o v e a l t h r e s h o l d , a g a i n a t two luminance l e v e l s . Thus, f o u r d e t e r m i n a t i o n s of the f o v e a l t h r e s h o l d were made d u r i n g each s c o t o p i c t e s t . The a u x i l i a r y f i x a t i o n d e v i c e d e s c r i b e d above c o u l d not be used i n t h e p h o t o p i c c o n d i t i o n , as i t was not b r i g h t enough t o be seen -2 a g a i n s t a background o f 250 cd.m. . I n s t e a d , a s m a l l p l a t e on which were p a i n t e d f o u r b l a c k d o t s was a f f i x e d to the edge o f t h e a d a p t a t i o n bowl on t h e s u b j e c t ' s r i g h t , the p a t t e r n b e i n g c e n t e r e d a t 64 degrees i n the 0-180° m e r i d i a n from t h e c e n t r e o f the bowl. T h i s p a t t e r n sub-tended two degrees o f v i s u a l a n g l e . The s u b j e c t was i n s t r u c t e d t o t u r n h i s eye and f i x a t e on the c e n t r e of t h i s p a t t e r n , and i n d i c a t e when he saw a l i g h t . T h i s was found to y i e l d the same t h r e s h o l d as the 2 P r i m a r y Red //6, S p e c t a c u l a r P r o d u c t i o n s L t d . 44 f o u r - l i g h t pattern method when the two were compared at an adaptation -2 • luminance of 10 cd. m. . The perimeter was surrounded by a framework of black cardboard and c l o t h which prevented extraneous l i g h t from the xenon-arc lamp or the data-chart i l l u m i n a t i o n from reaching the adaptation bowl during scotopic t e s t i n g . Photometer. The Goldmann perimeter comes equipped with an A.G. Metrawatt Luxmeter for measuring i n t e n s i t i e s . Measurements i n the v i s u a l laboratory indicated that t h i s was an imprecise instrument, and i n any case, i t i s not equipped for precise measurement over the range required for t h i s i n v e s t i g a t i o n . Therefore, as i n the e a r l i e r work from t h i s laboratory (Lakowski et a l . , 1976, 1977), a Spectra P r i t c h a r d Photometer (Model 1970-PR) for which c o r r e c t i o n factors are a v a i l a b l e to correct the s p e c t r a l s e n s i t i v i t y to was used for a l l luminance measurements of both targets and background. Subj ects Because t h i s i n v e s t i g a t i o n was intended to study normal photopic and scotopic s e n s i t i v i t y gradients to chromatic s t i m u l i , subject v a r i -ables thought to influence the thresholds were con t r o l l e d where pos-s i b l e . A l l subjects were emmetropes with normal colour v i s i o n , ranging from 16 to 29 years of age (see Table 2). In preliminary investigations p u p i l s i z e was found to be f a i r l y consistent for the subjects of i n t e r e s t , but p u p i l dimater was, i n any case, measured before and a f t e r each photopic test session, and moni-tored during photopic t e s t i n g . This was done using the telescope through which the experimenter observed the tes eye. The telescope has 45 Table 2 Subject Data Subject Age Sex Dominant Eye Colour Visxon Refraction LL 17 F R normal +0.50 to 6/4.5 AM 22 F L normal +0.75 to 6/4.5 JL 23 M R normal +1.25-to 6/4.5 RM 29 M R normal -0.50 to 6/4.5 KH 29 M . L normal +0.25 to 6/6 Colour v i s i o n assessed with Dvorine and Ishihara PIC plates, Farns-worth-Munsell 100-Hue t e s t , and Pickford-Nicholson anomaloscope. A l l subjects assessed as'emmetropic'at the Department of. Ophthalmology, University of B r i t i s h Columbia. 'Emmetropia' i s here defined as being emmetropic within 0.99 diopters, s p h e r i c a l equivalent. 46 cross-wires marked i n millimeters for t h i s purpose. In the scotopic condition, p u p i l diameter was not measured as there was no l i g h t i n the bowl. To minimize any p r a c t i c e e f f e c t s , t r i a l sequences were counter-balanced among subjects. In addition, a standard learning t r i a l (achromatic stimulus subtending 6.8' v i s u a l angle, backround luminance = -2 250 cd.m. ) was given before the experimental t r i a l s began, and was repeated a f t e r the l a s t t e s t i n g for an i n d i c a t i o n of the magnitude of the p r a c t i c e e f f e c t . Subjects were higher selected, not only i n terms of acuity, age, and colour v i s i o n , but also for consistency of response and s t a b i l i t y of f i x a t i o n . This was expected to minimize further p r a c t i c e e f f e c t s . In t h i s i n v e s t i g a t i o n , only one eye of each subject was tested. As i t has been shown that most i n d i v i d u a l s have a dominant eye which i s superior to the non-dominant eye m o t o r i c a l l y and i s favoured i n v i s u a l s i t u a t i o n s r e q u i r i n g choice (Porac & Coren, 1976), only dominant eyes were tested. This was the r i g h t eye i n three cases and the l e f t i n two cases. The possible influence of foreknowledge of stimulus l o c a t i o n on threshold was c o n t r o l l e d as follows. Because of instrumental l i m i t a t i o n s , i t i s impractical to present the s t i m u l i randomly using the Goldmann perimeter; a standard presentation order was necessary. With p r a c t i c e , the subject learns the order, so that foreknowlegde concerning stimulus l o c a t i o n would operate as an uncontrolled v a r i a b l e i f no information on t e s t i n g sequence was given i n i t i a l l y . Thus, subjects were informed before any testing of the stimulus presentation sequence to be followed 47 on a l l t r i a l s . This was expected to decrease the p r a c t i c e e f f e c t . Experimental Parameters Adaptation F i e l d . A f u l l hemispheric f i e l d was provided by the perimeter adapting bowl so that f i e l d s i z e , texture, and distance from the corner (30 cm) were a l l held constant. For the photopic condition, - 2 uniform f i e l d luminance and chromaticity of 250 cd.m. , 6000K were provided by the 150-watt xenon-arc lamp as previously described. There was no adaptation f i e l d i n the scotopic condition. P r e a d a p t a -t i o n was c a r r i e d out for four minutes and twenty-five minutes for photopic and scotopic t e s t i n g , r e s p e c t i v e l y . These periods were found to be adequate to give the same thresholds as longer pre-adaptation periods. Because of the extensive preadaptation required for scotopic t e s t i n g , once the subject was dark-adapted, a number of t r i a l s were run, so that longer adaptation was given for the majority of t r i a l s . These t r i a l s , as w e l l as the photopic ones, were counterbalanced among and within subjects. S t i m u l i . The stimulus c h a r a c t e r i s t i c s manipulated were s i z e , r e t i n a l l o c a t i o n , and chromaticity. The test luminance necessary for detection was the dependent v a r i a b l e , A L . D i f f e r e n t i a l e f f e c t s of other factors on the increment thresholds obtained were eliminated when possible. To t h i s end, stimulus duration was held constant at 150 milliseconds. This i s i n accordance with Enoch (Note 3), who states that perimetric stimulus duration should l i e between 100 milliseconds (below which A L x duration of exposure = constant, and above which A L = constant) and 250 milliseconds (above which saccadic eye move-ments may occur). Stimulus duration was, therefore, held at 150 m i l l i -seconds. 48 (a) R e t i n a l l o c a t i o n . Due to the time-consuming and f a t i g u i n g nature of the experimental task, i t was impractical to obtain thres-holds along more than one r e t i n a l meridian. The h o r i z o n t a l (0-180°) meridian has been used almost e x c l u s i v e l y i n previous experimental s t a t i c perimetry, and seems the l o g i c a l choice. Unfortunately, some diseases characterized by v i s u a l f i e l d losses, notably glaucoma, r e s u l t i n losses to areas not on t h i s meridian. Studies concerned with such pathological f i e l d losses must involve the i n v e s t i g a t i o n of other r e t i n a l meridians i n normal as w e l l as c l i n i c a l populations. The major purpose of t h i s work was to study normal thresholds i n chromatic perimetry, and thus the h o r i z o n t a l meridian was used. Measurements were made at 13 points along t h i s meridian, from 40° i n the temporal to 40° i n the nasal f i e l d . The luminous output of the stimulus pr o j e c t i o n system prevented the determination of thresholds beyond 40° i n the periphery i n the photopic condition. (b) Size. Stimulus s i z e was set at 0.275, .1.1, 4.4, or 17.6 mm, corresponding at a l l r e t i n a l locations to v i s u a l angles of 6.8, 13.6, 27.2, and 54.2 minutes at 30 mm (the distance from the cornea to the adaptation bowl surface). These are not the nominal values on the Goldmann instrument, but are the actual values as s p e c i f i e d by Goldmann i n his early work (Goldmann, 1945a) , ( V e r r i e s t , 1965a). (c) Chromaticity. The Goldmann perimeter uses s e l e c t i v e f i l t e r s . to a t t a i n chromatic s t i m u l i . The o r i g i n a l f i l t e r s accompanying the instrument are compared with a set of cinemoid f i l t e r s i n Figure 13 and Table 3. When choosing chromatic f i l t e r s , one wishes to obtain maximal luminous transmission, maximal e x c i t a t i o n p u r i t y and thus minimal 49 4 0 0 5 0 0 6 0 0 7 0 0 W A V E L E N G T H ( N A N O M E T E R S ) Figure 13 Spectral transmission curves for chromatic f i l t e r s (from Lakowski et a l . , 1977) 50 Table 3 Comparison of O r i g i n a l with Cinemoid F i l t e r s Stimulus F i l t e r S p e c i f i c a t i o n s under CIE Illuminant 'C Y% A D E x c i t a t i o n P u r i t y Goldmann blue Goldmann green Goldmann red .1562 .2331 .729 .0233 2704 1.635 460 nm .6466 24.180 504.5nm 0.913 623 *Cinemoid #32 medium blue *Cinemoid #39 primary green *Conemoid #6 primary red U n f i l t e r e d Stimulus Source: xenon-arc. T = 6000°K c ,1417 .1180 10.21 474 .2145 .6973 14.43 535 .6738 .3134 10.21 617 .3220 .3318 0.86 0.81 0.97 F i l t e r s used i n the present i n v e s t i g a t i o n . (Cinemoid f i l t e r s from Spectacular Productions, Ltd.) 51 overlap among the f i l t e r s , and peak transmissions as close as possible to the peaks of the cone response systems. The f i r s t two requirements are d i a m e t r i c a l l y opposed, so that a compromise must be made such that s u f f i c i e n t luminance can be attained at the expense of some overlap among the three f i l t e r s ' s p e c t r a l transmission curves. Taking into account these c r i t e r i a , the cinemoid f i l t e r s described i n Figure 13 and Table 3 were substituted for the o r i g i n a l Goldmann f i l t e r s i n the e a r l i e r m o d i f i c a t i o n of the Goldmann perimeter (Lakowski et a l . , 1976, 1977). These cinemoid f i l t e r s gave the higher luminance necessary to explore the v i s u a l f i e l d with small s t i m u l i . The reduced luminous transmission of the cinemoid compared to the Goldmann green was not a problem, as s u f f i c i e n t luminance i n the middle wavelengths was e a s i l y obtained. The advantages of the cinemoid green are i t s narrower transmission curve and i t s peak transmission within the 535-555 nm. range found for the peak of the 'green' or middle-wavelength response system ( S t i l e s , 1959; Brown & Wald, 1963; 1964; Rushton, 1963; Marks, Dobelle, & MacNichol, 1964; Baker & Rushton, 1965). The Goldmann 'green' i s a c t u a l l y a blue-green, with = 504.5 nm. While the cinemoid red was substituted f o r the Goldmann red mainly to achieve higher luminance, neither of these f i l t e r s peaks anywhere near the peak of the longest wavelength cone system, at 570 to 590 nm. I t i s not clear why there i s no 'red' cone system, though the t r i v a r i a n c e of normal volour v i s i o n indicates that colour matching requires a long, as w e l l as short and middle, wavelength s t imulus. In comparing the Goldmann and the cinemoid blue f i l t e r s , i t i s 5 2 c l e a r that the cinemoid has f a r greater luminous transmission than does the Goldmann f i l t e r . However, the gain i n Y% i s accompanied by a s h i f t i n peak transmission away from the range found for the peak of the 'blue' response system, from 440 to 450 nm ( S t i l e s , 1 9 5 9 ; Brown & Wald, 1963, 1964; Rushton, 1963; Marks, Dobell, & MacNichol, 1964; Baker. & Rushton, 1965). This s h i f t may be very s i g n i f i c a n t , as the ' r e l a t i v e c e n t r a l scotoma' to short-wavelength s t i m u l i found using the Goldmann blue (Verriest & I s r a e l , 1965a) has not been duplicated with t h i s cinemoid blue (Lakowski & Dunn, 1978 ). In this respect as w e l l the cinemoid i s superior to the Goldmann blue f i l t e r for the present purposes. I t i s proposed here that normal s e n s i t i v i t y gradients be established; these can then be compared with gradients obtained i n pathological eyes. Hence, i t would not be advantageous to have normal p h y s i o l o g i c a l scotomata which could mask or be confused with patho-l o g i c a l scotomata. The Goldmann blue has the added disadvantage that i t shows no d i s c r e t e peak i n the v i s i b l e spectrum, but plateaus down into the u n t r a v i o l e t wavelengths (Figure 13). By contrast, the cine-moid blue shows a well-defined peak transmission. In summary, i t appeared that the cinemoid f i l t e r s used by Lakowski et a l . (1976, 1977) were preferable to those accompanying the Goldmann instrument for the present purposes, p a r t i c u l a r l y as very high luminance l e v e l s were required. For comparative purposes, achromatic stimulus thresholds were also determined, using the xeono-arc output without s e l e c t i v e f i l t e r i n g . These thresholds were determined against an adaptation luminance of the same colour-temperature (6000K) as the target, as both were produced by xenon-arc. 53 Experimental Design I. Subjects: 5 emmetropic normal trichromats, 3 males and 2 females, aged from 16 to 29 years, were selected for s t a b i l i t y of f i x a t i o n . Only the dominant eye of each was tested; t h i s resulted i n data from 3 r i g h t and 2 l e f t eyes (see Table 2). I I . Constant Condition: A. Stimulus duration: 150 mil l i s e c o n d s . B. Interstimulus i n t e r v a l : 1 second. I I I . Independent Varia b l e s : -2 A. Adaptation Luminance: 1. zero cd.m. (pre-adaptation: 25 minutes) -2 2. 250 cd.m. (preadaptation: .4 minutes) B. Stimulus Location: 13 points on the h o r i z o n t a l (0-180°) meridian were tested i n the following order: 0°, 5°, 10°, 15° nasal f i e l d ; 5°, 10°, 20°, 25°, 30°, 40° temporal; 20°, 30°, 40° nasal f i e l d . 2 C. Stimulus Size: 1. 0.275 mm , subtending a v i s u a l angle of 6.8' at 300 mm 2 2. 1.1 nun •, subtending a v i s u a l angle of 13.6' at 300 mm ' ' 2 3. 4.4 mm , subtending a v i s u a l angle of 27.2' at 300 mm 2 4. 17.6 mm , subtending a v i s u a l angle of 54.3' at 300 mm. 54 l D. Stimulus Chromaticity : 1. Achromatic (xenon-arc, T = 6000K) c 2. Blue A^ = 474 3. Red 535 4. Green A^ = 617 IV. Dependent Va r i a b l e : AL, the minimal stimulus luminance re-Procedure Each subject was tested for acuity monocularly on the Bausch and Lomb Orthorater and the Snellen Chart. They were ref r a c t e d at the UBC Department of Ophthalmology to e s t a b l i s h s u b j e c t i v e l y that they were emmetropic. Monocular c o l o u r - v i s i o n assessment was done using the Dvorine and Ishihara PIC plates, the Farnsworth-Munsell 100-Hue t e s t , and the Pickford-Nicholson anomaloscope. Eye dominance was tested using the Asher test (Asher, 1961) and the Miles ABC Test (Miles, 1929, 1930). A learning t r i a l was given (achromatic stimulus -2 subtending 6.8' v i s u a l angle, background luminance = 250 cd.m. ). On the basis of these tests the subjects were selected. Before the learning t r i a l the subject was t o l d the order of r e t i n a l locations to be tested on a l l t r i a l s . The order of test t r i a l s was counterbalanced among and w i t h i n subjects with respect to stimulus s i z e and chromaticity (see Appendix 1). A l l photopic tests were done f i r s t ; the experimenter thus could quired to obtain a threshold response. This was v a r i a b l e i n 0.1 log unit steps. l For further s p e c i f i c a t i o n see Figure 13 and Table 3. 55 monitor f i x a t i o n during the e a r l i e r t r i a l s when the subject was less experienced. The lengthy pre-adaptation period necessary for scotopic t e s t i n g made i t advantageous to run several t r i a l s once the subject was adapted; t h i s resulted i n a d i f f e r e n t counterbalancing order for the scotopic as opposed to the photopic t e s t s . Testing was done for periods of two to three hours, as i t had been found i n previous i n -vestigations that a f t e r t h i s time fatigue began to influence the r e s u l t s . The subject rested between t e s t s : he l e f t the test room between photopic t e s t s , but remained at the instrument between scotopic test s . ' Af t e r a l l 24 test t r i a l s had been completed, the i n i t i a l learning t r i a l was repeated. T r i a l Procedure. 1. A f t e r s e t t i n g up the test conditions, that i s , s e l e c t i n g the appropriate adaptation luminance and stimulus s i z e and chromaticity, the experimenter measured and recorded the luminances of both the stimulus to be presented and the background using the Pr i t c h a r d Photometer. 2. The subject was seated at the perimeter and positioned as comfortably as possible with the eye to be tested centered on the p e r i -meter telescope, through which the experimenter could see the eye. An opaque white occluder was placed over the other eye. The p o s i t i o n i n g was done by means of the adjustable chair and the movable chin-rest, on which the subject was positioned with his forehead against a r e s t r a i n i n g band. In t h i s way, the distance from the cornea to the stimulus was held constant at 30 cm. 56 3. Room i l l u m i n a t i o n was extinguished and the subject was pre-adapted to the adaptation background f o r 25 minutes (scotopic condi-tion) or 4 minutes (photopic condition). This was timed with a stop-watch. In the photopic condition, during t h i s time the subject was instructe d to move his gaze around the bowl, not f i x a t i n g on the dark c e n t r a l f i x a t i o n spot. After-images were thus avoided. 4. When the pre-adaptation was over, i n the photopic condition, the p u p i l diameter was measured with the subject f i x a t i n g on the f i x a -t i o n spot, which was the base of the telescope. The stopwatch was then reset to time the tes t session. 5. A l l threshold measurements were obtained by the following ascending method of l i m i t s . The subject was instruc t e d to tap on the instrument table when he saw a l i g h t f l a s h . The experimenter then increased the stimulus luminance by 0.1. log unit steps, allowing two exposures at each step, u n t i l the subject responded. Stimulus luminance was then decreased to a l e v e l varying from .5 to .1 log units below the threshold j u s t obtained, and again increased u n t i l the subject res-ponded. This was repeated u n t i l the subject's response occurred con-s i s t e n t l y at the same luminance l e v e l two or three times; t h i s generally took only three or four ascending runs with most subjects under most conditions. 6. Using the ascending method of l i m i t s described above the foveal threshold was determined f i r s t , i n both the photopic and the scotopic conditions. This threshold was measured i n s l i g h t l y d i f -ferent ways i n the two adaptation conditions. Under scotopic conditions, the o r i g i n a l instrument's f o u r - l i g h t 57 f i x a t i o n pattern (with a red f i l t e r ) was projected at 5° to the r i g h t of the bowl's centre. The subject was instruc t e d to f i x a t e on the centre of the diamond-shape formed by the four red l i g h t s , and i n -dicate when he saw a l i g h t there. The usual ascending runs were then made and the threshold determined. This was done with the luminance of the four red f i x a t i o n l i g h t s set at two l e v e l s . This procedure was repeated with another, s i m i l a r f o u r - l i g h t f i x a t i o n pat-tern which subtended 3.4° of v i s u a l angle (compared to the o r i g i n a l pattern, which subtended : 2°). Thus four separate foveal threshold determinations were made for each scotopic t r i a l . The small s i z e of these f i x a t i o n l i g h t s (about 6' of. v i s u a l angle) made i t impossible to p r e c i s e l y specify t h e i r luminance; they were (at both settings) not over .15 cd.m. ^ . In the photopic condition, the f o u r - l i g h t f i x a t i o n pattern was -2 not bright enough to be seen against the background of 250 cd.m. Therefore, an alternate method was used to determine foveal thresholds. The subject was t o l d to d i r e c t his gaze away from the f i x a t i o n spot u n t i l any after-image had faded. He was then t o l d to look i n the centre of the four-dot pattern a f f i x e d to the r i g h t side of the adaptation bowl, and tap on the instrument table when he saw a l i g h t . The same ascending method was then followed. 7. The subject was next instru c t e d to f i x a t e on the f i x a t i o n point (central dark spot i n the photopic and c e n t r a l red probe i n the scotopic condition). He was t o l d to maintain f i x a t i o n throughout the remainder of the t e s t i n g . F i x a t i o n was monitored throughout the photopic test sessions by the experimenter v i a the telescope, but 58 t h i s was not possible i n the scotopic condition. In the l a t t e r case, f i x a t i o n was i n d i r e c t l y monitored by questioning the subject occa-s i o n a l l y as to whether the red f i x a t i o n l i g h t seemed to be moving. Because of the r e l i a n c e on such an i n d i r e c t measure of f i x a t i o n i n the scotopic condition, only subjects showing stable f i x a t i o n were used. 8. The subject was t o l d to i n d i c a t e (by tapping on the i n s t r u -ment table) each time he saw a l i g h t . At each r e t i n a l l o c a t i o n tested, the same ascending method of l i m i t s was used as has been described. The subject was encouraged to b l i n k h i s eyes whenever he l i k e d ; t h i s seemed to reduce fatigue. In photopic t r i a l s , a f t e r a l l locations had been tested, the p u p i l diameter was again measured and the subject l e f t the t e s t i n g room while the experimenter again measured and recorded the luminances of the background and stimulus. During scotopic t e s t i n g , a number of t r i a l s were done once the subject had dark-adapted. Thus, the subject rested a few minutes before the next t r i a l began, but stayed at the instrument. 59 RESULTS Re t i n a l S e n s i t i v i t y Gradients S e n s i t i v i t y gradients were obtained under f u l l y - s c o t o p i c and fully-photopic adaptation conditions using exactly the same s t i m u l i and subjects. However, because the f u l l y - p h o t o p i c gradients represent increment thresholds while the f u l l y - s c o t o p i c are absolute thresholds, they w i l l f i r s t be presented separately and then compared. Fully-Photopic Adaptation. For each stimulus s i z e used, the three chromatic and the achromatic s t i m u l i yielded s i m i l a r gradients (see Figure 14). The s i m i l a r i t y among the gradients i s greatest for the smallest stimulus s i z e (6.8'). As stimulus s i z e increased, i t ap-peared that the achromatic stimulus did not show as large an increase i n s e n s i t i v i t y as did the chromatic s t i m u l i , r e s u l t i n g i n a separa-t i o n of the gradient for t h i s stimulus from the others. This was shown p a r t i c u l a r l y w e l l for sizes 3 and 4 (27.2' and 54.3'). Excluding the fovea, the mean thresholds obtained with the chromatic stimulus never d i f f e r e d by more than 0.2 log u n i t , and generally d i f f e r e d by 0.1 log unit or l e s s . The differences did not show any consistent r e l a t i o n s h i p between the colours; that i s , no one colour yielded co n s i s t e n t l y higher thresholds. A 4-way analysis of variance was c a r r i e d out on t h i s data, the summary table of which i s presented i n Table 4. S i g n i f i c a n c e beyond the .01 l e v e l was found for the main e f f e c t s of colour, s i z e , and subject. Newman-Keuls Mu l t i p l e Range Tests indicated that the e f f e c t of colour was contributed s o l e l y by the achromatic stimulus. This 60 I o oo o < 60 o Q 5 -I . O -1.5 Q 5 -I . O -1.5-I . C H I-5H 2 . C H I . O -1.5-2P-2.5' 3 D 4 0 x A 1 © Size. 4 Size 3 x Size 2 Size 1 A v — i — : — i — i — i — i — i — T — r ~ — r — T — r — — i 4 0 ° 3 0 ° 2 0 ° I O ° 0 ° I O ° 2 0 ° 3 0 ° 4 0 ° N T Figure 14 Mean fully-ph o t o p i c s e n s i t i v i t y gradients for blue(o), gr^en(A) red(X), and achromatic(•) s t i m u l i . Adaptation luminance = 250 cd.m. 61 Table 4 Analysis of Variance for Photopic Thresholds Source df SS MS Colour 3 Size 3 Colour x Size 9 Subject 4 Colour x Subject 12 Size x Subject 12 Colour x Size x Subject 36 2.6353 0.8784 160.0284 53.3428 0.3747 6.3019 0.3614 0.6759 0.0416 1.5755 0.3011 0.0563 0.54671 0.0152 20.2748 395.4917 5.9225 15.6055 4.9284 7.1377 2.5747 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 62 c o n f i r m s t h e i m p r e s s i o n g i v e n by the g r a d i e n t s i n F i g u r e 14. The main e f f e c t of s i z e r e s u l t e d from s i g n i f i c a n t d i f f e r e n c e s among a l l f o u r s i z e s . The s u b j e c t v a r i a b l e a l s o was s i g n i f i c a n t , w i t h s u b j e c t LL showing s i g n i f i c a n t l y h i g h e r s e n s i t i v i t y t h a n AM, J L , and RM, a l l o f whom showed s i g n i f i c a n t l y h i g h e r s e n s i t i v i t y t h a n KH. . T h i s i s i n -t e r e s t i n g i n view o f the f a c t t h a t s u b j e c t LL was the youngest (17 y e a r s ) and KH was one o f the o l d e s t (29 y e a r s ) . The main e f f e c t o f s u b j e c t may t h e r e f o r e r e f l e c t the age v a r i a b l e . From T a b l e 4 i t can be seen t h a t t h e r e were many s i g n i f i c a n t i n t e r a c t i o n e f f e c t s i n the d a t a . These were not o n l y f i r s t o r d e r but a l s o second o r d e r s i g n i f i c a n t i n t e r a c t i o n s . Because of the l a c k of independence o f t h e v a r i a b l e s as r e f l e c t e d i n these i n t e r a c t i o n s , the t r u e s i g n i f i c a n c e o f t h e main e f f e c t s i n d i c a t e d by the a n a l y s i s o f v a r i a n c e becomes q u e s t i o n a b l e . I n p a r t i c u l a r , the f a c t , t h a t a l l i n t e r a c t i o n s between the s u b j e c t v a r i a b l e and any o t h e r s were s i g n i -f i c a n t may i n d i c a t e t h a t t h i s v a r i a b l e i s a c c o u n t i n g f o r a g r e a t d e a l of the observed v a r i a n c e . N o n e t h e l e s s , the a n a l y s i s does c o n f i r m the i n t e r p r e t a t i o n of the d a t a made on t h e b a s i s of F i g u r e 14, i n t h a t b o t h i n d i c a t e t h a t the a c h r o m a t i c s t i m u l u s y i e l d s a lower s e n s i t i v i t y g r a d i e n t than do any of the c h r o m a t i c s t i m u l i . The f o v e a l t h r e s h o l d s showed the w i d e s t v a r i a t i o n among the t h r e e c h r o m a t i c and the a c h r o m a t i c s t i m u l i . A 3-way a n a l y s i s o f v a r i a n c e was performed on the f o v e a l t h r e s h o l d s (see T a b l e 5) and t h i s v a r i a t i o n w i t h s t i m u l u s c o l o u r was found to be s t a t i s t i c a l l y s i g n i f i -c a n t (a < .01), w i t h the r e d s t i m u l u s y i e l d i n g s i g n i f i c a n t l y h i g h e r s e n s i t i v i t y than the green, which showed h i g h e r s e n s i t i v i t y t h a n the 63 Table 5 Analysis of Variance for Photopic Foveal Thresholds Source df SS MS Colour Size Colour x Size 3 1.4965 3 3.6215 9 0.0975 0.4988 68.6074 0.0000 1.2072 141.6724 0.0000 0.0108 1.3277 0.2571 64 blue and achromatic s t i m u l i (Newman-Keuls Range Test, a = .05). Again, the s t a t i s t i c a l differences must be interpreted with caution, but the same general pattern i s apparent from the plotted foveal s e n s i t i v i t i e s which i n d i c a t e the decreasing s e n s i t i v i t y to red, green, and blue, and achromatic s t i m u l i . The v a r i a b i l i t y of the increment thresholds obtained as a function of r e t i n a l l o c a t i o n and stimulus s i z e and chromaticity i s indicated by the standard deviations shown i n Tables 6 and 7. (Complete tables of means and standard deviations for a l l conditions may be found i n Appendix I I ) . From Table 6 i t appears that the v a r i a b i l i t y i n thres-holds obtained i s highest i n the periphery, decreasing as one moves toward the fovea. This i s i n agreement with previous i n v e s t i g a t i o n s (Aulhorn & Harms, 1972). A notable exception to t h i s trend for increasing v a r i a t i o n with e c c e n t r i c i t y was found at 20° temporal. The large standard deviation found at t h i s point r e s u l t s from i t s proximity to the b l i n d spot. The r e l a t i v e l y small standard deviation at 40° nasal does not f i t i n with the trend for increasing v a r i a b i l i t y with increa-sing e c c e n t r i c i t y . There i s an i n d i c a t i o n of some asso c i a t i o n between stimulus s i z e and v a r i a b i l i t y i n the thresholds obtained as shown i n Table 7. For each stimulus chromaticity, the standard deviation decreases as stimulus s i z e increases. Ir does .not appear that v a r i a b i l i t y changes as a function of stimulus chromaticity. Fully-Scotopic Adaptation. The close s i m i l a r i t y among gradients obtained using equivalent-sized blue, red, green, and achromatic s t i m u l i under fu l l y - p h o t o p i c adaptation was not duplicated under f u l l y -Table 6 Average Standard Deviations for Photopic Thresholds Obtained at Each R e t i n a l P o s i t i o n P o s i t i o n Standard Deviation Nasal 40° 0.13 30° 0.17 20° 0.14 15° 0.12 10° 0.09 5° 0.09 Fovea 0° 0.10 Temporal 5° 0.11 10° 0.13 20° 0.19 25° 0.14 30° 0.14 40° 0.19 R e t i n a l E c c e n t r i c i t y on 0-180° meridian. Table 7 Average Standard Deviations for a l l Photopic Thresholds Obtained with Each Colour-Size Combination Stimulus Size Stimulus Colour 1 2 3 4 Achromatic 0.16 0.14 0.14 0.12 Blue 0.13 0.14 0.13 0.08 Green 0.16 0.15 0.10 0.12 Red 0.18 0.13 0.13 0.12 67 scotopic conditions. Figure 15 shows the mean s e n s i t i v i t y gradients obtained under these conditions. For a l l stimulus s i z e s , the gradients in d i c a t e that the s e n s i t i v i t y at a l l non-,foveal points (with two exceptions at 40° nasal) was i n the order (highest to lowest s e n s i t i v i t y ) blue, green, achromatic, and red. The s e n s i t i v i t y gradients obtained with the red stimulus were i n each case from one to 1.5 log units lower than the gradients obtained with the achromatic s t i m u l i . A 4-way analysis of variance was c a r r i e d out on the scotopic threshold data; the summary table appears i n Table 8. As with the photopic ANOVA, there were s i g n i f i c a n t f i r s t and second order i n t e r -a ction e f f e c t s , so that the i n t e r p r e t a t i o n of s i g n i f i c a n t e f f e c t s must be made with caution. However, the main e f f e c t of s i z e i s obvious from Figure 15 as w e l l as from the F - r a t i o , and a general main e f f e c t of colour i s expected merely on the basis of the low red gradient. The Newman-Keuls Range Tests i n d i c a t e , as well as s i g n i f i c a n t differences among a l l four s i z e s , a s i g n i f i c a n t d i f f e r e n c e between the red stimulus and a l l the others, between the blue and a l l others, but not between the green and the achromatic. I t i s i n t e r e s t i n g that no main e f f e c t of subject was found i n the scotopic data, though some in t e r a c t i o n s involving t h i s v a r i a b l e were found to be s i g n i f i c a n t . Before discussing the foveal thresholds obtained under f u l l y -scotopic conditions, mention must be made of the l i m i t e d r e l i a b i l i t y of these measures. As has been stated, v e r i f i c a t i o n of f i x a t i o n i s not possible under f u l l y - s c o p t o p i c conditions without the a i d of some objective method of assessing f i x a t i o n such as an i n f r a - r e d f i x a t i o n monitor. I t i s not enough to say that thresholds were only obtained 69 Table 8 Analysis of Variance for Scotopic Thresholds Source df SS MS Colour 3 372.6065 Size 3 362.9890 Colour x Size 9 0.2162 Subject 4 0.8702 Colour x Subject 12 1.4946 Size x Subject 12 0.5398 Colour x Size.x Subject 36 1.8471 124.2022 157.7455 0.0000 120.9963 1721.4514 0.0000 0.0240 0.8002 0.6180 0.2176 2.1105 0.0931 0.1246 3.2275 0.0004 0.0450 1.6320 0.0885 0.0513 2.0641 0.0004 70 when the subject f e l t he was f i x a t i n g . In the present i n v e s t i g a t i o n , i t was not possible to obtain a fixation-monitoring device, so that the foveal thresholds i n p a r t i c u l a r (due to the sharp reduction of s e n s i t i v i t y at the fovea, see section i n the Introduction, "Adaptation") cannot be considered precise measurements. An attempt was made to maximize the v a l i d i t y of these measurements as follows. The foveal threshold i n each scotopic t r i a l was assessed four times, as opposed to once as for the other thresholds, using two f i x a t i o n patterns at s l i g h t l y d i f f e r e n t luminances as described i n 'Method.' An assumption-was then made that the highest threshold obtained (representing the lowest s e n s i t i v i t y ) was the best approximation to the actual foveal threshold. This assumption was based f i r s t on the evidence that scotopic s e n s i t i v i t y gradients do show a sharp decline i n s e n s i t i v i t y at the fovea; second, i t was found that when a wide range of foveal thresholds was found (sometimes on the order of .5 to 1.0 log unit) for one t r i a l , the subject reported that some of the l i g h t s he saw were very bright, while others were of ' s i m i l a r ' brightness to the other threshold values he responded to. The assumption i s then that the 'brighter' l i g h t s he saw were seen with the paracentral r e t i n a , which has a much higher s e n s i t i v i t y . Thus, the values taken for scotopic foveal thresholds were selected according to th i s system. The foveal thresholds shown i n Figure 15 were determined i n the way described above. (A complete table of a l l scotopic foveal thres-sholds obtained i s found i n Appendix III.) I t i s apparent that f o r size s one and 2 the d i f f e r e n t colours and white gave v i r t u a l l y the same mean foveal thresholds, while s i z e s 3 and 4 produced a spread i n 71 these thresholds. For s i z e 3 the order of decreasing s e n s i t i v i t y i s blue > green > red > achromatic, while for s i z e 4 i t i s blue > achromatic > red > green. Because of the indeterminacy of these values one must be c a r e f u l not to a t t r i b u t e too much s i g n i f i c a n c e to these d i f f e r e n c e s . Nonetheless, there does seem to be a consistent d i f f e r e n c e between the blue and red thresholds of 0.5 log units for both sizes 3 and 4, though these thresholds are within 0.1 log unit of each other for s i z e s 1 and 2. A 3-way analysis of variance on the scotopic foveal v thresholds indicated that the main e f f e c t of colour on the scotopic foveal thresholds expected on the basis of the differences seen i n Figure 15 for sizes 3 and 4 was not s i g n i f i c a n t (see Table 9). The main e f f e c t of s i z e was s i g n i f i c a n t (a < .01), but no s i g n i f i c a n t i n t e r a c t i o n between s i z e and colour was found. In Figure 16 t h i s e f f e c t of increased foveal s e n s i t i v i t y with increased stimulus s i z e i s i l l u s t r a t e d . There i s a c l e a r c u r v i l i n e a r r e l a t i o n s h i p evident for the red stimulus. A s i m i l a r r e l a t i o n s h i p appears for the white and green s t i m u l i i f i n several cases one highly i r r e g u l a r threshold i s removed and the mean for that colour and s i z e i s r e c a l c u l a t e d . If this i s done, the only stimulus not showing a smooth curve i s the blue. The i r r e g u l a r thresholds are not a l l ob-tained from one subject, so that i s i t not a case of one subject with completely d i f f e r e n t foveal s e n s i t i v i t y . The red stimulus, besides y i e l d i n g generally lower s e n s i t i v i t y gradients, shows an i n t e r e s t i n g e f f e c t of s i z e on foveal s e n s i t i v i t y : as the s i z e i s increased, a 'dip' of r e l a t i v e l y decreased s e n s i t i v i t y i s seen at the fovea. What i s i n f a c t occurring i s that with the 72 Table 9 Analysis of Variance for Scotopic Foveal Thresholds Source df S_S MS F' £ Colour 3 1.6104 0.5368 2.0319 0.1624 Size 3 15.775 5.2585 35.2473 0.0000 Colour x Size 9 1.5491 0.1721 1.1089 0.3815 r-3 < oo o • •2.5H - 3 D - Achromatic Blue 3 T T 3 Green Red 4 Stimulus Size Figure 16 Means (X) and ranges of foveal scotopic thresholds, sizes 1 to 4. Broken l i n e s i n d i c a t e a range obtained without removing an extremely anomalous threshold. See text for explanation. 74 larger sizes the paracentral s e n s i t i v i t y increases more r a p i d l y than the foveal, r e s u l t i n g i n the so- c a l l e d ' r e l a t i v e c e n t r a l scotoma. 1 The general form of the red gradient for each of the four sizes i s very s i m i l a r except i n the centre. It i s possible that with the more accurate f i x a t i o n c o n t r o l such a 'dip' would also be found f o r sizes 1 and 2. Table 10 gives an i n d i c a t i o n of the v a r i a b i l i t y i n the scotopic thresholds determined, i n the form of the average standard deviation for a l l thresholds obtained at each e c c e n t r i c i t y . (A complete table of a l l means and standard deviations for a l l conditions may be found i n Appendix IV.) The s p e c i a l problems involved i n determining scotopic foveal thresholds are r e f l e c t e d i n the large average standard deviation for the foveal measurements. Excluding the fovea, a general tendency for standard deviation to increase with increasing e c c e n t r i c i t y i s seen, with an exceptionally high value at 20° temporal due to i t s proximity to the b l i n d spot. The r e l a t i v e l y high standard deviation obtained at 5° temporal i s another exception to the tendency f o r small v a r i a b i l i t y to be found near the fovea. This could conceivably r e s u l t from i n d i v i d u a l v a r i a t i o n i n the e c c e n t r i c i t y at which the r e l a t i v e drop i n s e n s i t i v i t y toward the fovea begins. There was no consistant i n d i c a t i o n of a di f f e r e n c e i n v a r i a b i l i t y f o r d i f f e r e n t stimulus sizes i n Table 11, wherein the average standard deviations for a l l thresholds obtained with each stimulus size-colour combination are shown. There appeared to be some tendency f o r larger s i z e s to.be associated with greater v a r i a b i l i t y for the red and green s t i m u l i , but the trend was not very pronounced. Such a trend would be Table 10 Average Standard Deviations for Scotopic Thresholds Obtained at Each R e t i n a l P o s i t i o n P o s i t i o n Standard Deviation Nasal 40° 0.16 30° 0.14 20° 0.11 15° 0.11 10° 0.12 5° 0.10 Fovea 0° 0.37 Temporal 5° 0.15 10° 0.11 20° 0.21 25° 0.13 30° 0.13 40° 0.16 R e t i n a l E c c e n t r i c i t y , 0-180° Meridian Table 11 Average Standard Deviations for a l l Scotopic Thresholds Obtained with Each Colour-Size Combination Stimulus Colour Stimulus Size Achromatic 0.13 0.17 0.18 0.13 Blue 0.18 0.20 0.16 0.20 Green 0.15 0.14 0.18 0.17 Red 0.12 0.12 0.14 0.16 77 the converse of what was found under photopic conditions, where i n -creasing s i z e was accompanied by a decrease i n the standard deviation. There are no in d i c a t i o n s that v a r i a b i l i t y was d i f f e r e n t among the d i f -ferent colours. Fully-Photoplc and Fully-Scotoplc Adaptation. When the thresholds obtained under the two adaptation conditions are compared, the following points can be made. F i r s t , when compared with the ful l y - p h o t o p i c sen-s i t i v i t y gradients, the f u l l y - s c o t o p i c gradients showed an increase i n s e n s i t i v i t y over the photopic s e n s i t i v i t y on the order of 4.5 to 5 log units for a l l but the red stimulus and excluding the fovea (see Figure 17). The red stimulus yielded an increase i n s e n s i t i v i t y of only 3 to 3.5 log units with the change from photopic to scotopic. At the fovea, the s e n s i t i v i t y increase was s i m i l a r for a l l colours and the achromatic: on the order of 2 to 2.5 log un i t s . In general, then, the s e n s i t i v i t y increase was found to be greatest outside the fovea for a l l colours, but of a smaller magnitude for the red stimulus. Second, f u l l y -photopic adaptation yielded s i m i l a r s e n s i t i v i t y gradients for the chromatic s t i m u l i which were s i g n i f i c a n t l y higher than those f o r the achromatic s t i m u l i , while f u l l y - s c o t o p i c adaptation yielded gradients which were highest for the blue, followed by the green and achromatic, and lowest f o r the red stimulus. Third, both adaptation conditions yi e l d e d gradients which were s i g n i f i c a n t l y higher f o r each successive s i z e increase. Fourth, while the photopic foveal s e n s i t i v i t y was the highest obtained across the r e t i n a , and showed s i g n i f i c a n t higher s e n s i t i v i t y to blue than green, and to green than blue or achromatic, the scotopic foveal s e n s i t i v i t y was the lowest across the r e t i n a for 78 •a-<Jx. «3 • O K -< 1 » . 0$ X <J3 4 ao a - T> o u w O 01 P 00 79 a l l but sizes 1-3 of the red stimulus, and did not show any s i g n i f i -cant differences among the colours. The scotopic gradient for the red stimulus did show a foveal 'dip' as did the blue, green, and achromatic, but only f o r the two larger s i z e s . F i f t h , the photopic data showed a s i g n i f i c a n t e f f e c t of subject, with the youngest subject showing s i g -n i f i c a n t l y more and one of the (two) oldest subjects s i g n i f i c a n t l y less s e n s i t i v i t y . The scotopic data showed no s i g n i f i c a n t subject main e f f e c t . F i n a l l y , photopic threshold v a r i a b i l i t y as r e f l e c t e d by stan-dard deviations appeared to increase with e c c e n t r i c i t y and decrease with s i z e , while for scotopic thresholds i t seemed to increase with e c c e n t r i c i t y but was highest at the fovea, and showed no consistent a s s o c i a t i o n with s i z e . The v a r i a b i l i t y i n both photopic and scotopic threshold determination did not appear to vary c o n s i s t e n t l y with stimulus chromaticity. S p a t i a l Summation The summation capacity of the r e t i n a was studied using the same s t i m u l i under fu l l y - p h o t o p i c and f u l l y - s c o t o p i c conditions. The a b i l i t y of the v i s u a l system to summate luminous input s p a t i a l l y may be investigated with the obtained perimetric data i n two ways: by studying the change i n s e n s i t i v i t y gradient slope with a change i n stimulus s i z e , or by c a l c u l a t i n g the summation exponent k at each r e t i n a l l o c a t i o n of i n t e r e s t . Both of these approaches were used to gain in s i g h t into the s p a t i a l summation of the eye under f u l l y - p h o t o p i c and f u l l y - s c o t o p i c adaptation. Fully-Photopic Adaptation. The e f f e c t of an increase i n stimulus s i z e on the s e n s i t i v i t y gradient for each chromatic and achromatic 80 stimulus i s shown generally by Figure 18. As well as the o v e r a l l i n -crease i n s e n s i t i v i t y with increased s i z e , there was a tendency for the gradients to become f l a t t e r . To gain a measure of this change, i t would be useful to determine the slope of the gradient. A l l the photopic gradients had the same general form, with a sharp peak from 5° nasal and temporal to the fovea, and a more gentle slope from 5° out to the periphery on both sides. The gradients were therefore redrawn on the basis of only f i v e r e t i n a l locations — 40°N, 5°N, 0°, 5°T, 40°T — thus d i v i d i n g each gradient into four sections, each of which had a measurable slope (see Figure 19). The peripheral segments r e f l e c t the o r i g i n a l 6-point gradient segments, and thus give an es-timation of the change i n threshold with e c c e n t r i c i t y . The slopes calculated for these gradients are presented i n Table 12. For a l l four segments of each gradient, i t appears that the slope decreases with increasing stimulus s i z e , i n d i c a t i n g that summation i s greater i n the periphery than i n the c e n t r a l f i e l d . The ' f l a t t e n i n g ' of the foveal peak i n the s e n s i t i v i t y gradient with increasing s i z e , due to t h i s d i f f e r e n c e between the foveal and 5° e c c e n t r i c i t y summation capa-c i t y , appears i n Figures 18 and 19 to be more pronounced for the blue and achromatic s t i m u l i than for the green or red. However, the slopes i n Table 12 i n d i c a t e that t h i s i s not the case, i n that the red stimulus 0-5° gradient segments show a slope decrease from s i z e 1 to s i z e 4 which i s as great or greater than that for the blue. I t i s c e r t a i n l y true that the s i z e 4 0-5° segments of the blue and achroma-t i c gradients have much smaller slopes than those of the red. In order to compare the summation ca p a c i t i e s q u a n t i t a t i v e l y , the 8 1 Table 12 Slopes of Photopic Gradient Segments Gradient Segments Stimulus Size 40°N to 5°N Achromatic .023 .017 .012 .010 Blue .023 .,021 .015 .013 Green .022 .016 .010 .019 Red .021 .020 .016 .013 N to 0° Achromatic . 144 .120 .096 .052 Blue .112 .080 .056 .040 Green .144 .112 .108 .092 Red .180 .128 .116 .096 to 5 T Achromatic -.136 -.112 -.084 -.060 Blue -.096 -.076 -.048 -.032 Green -.140 -.112 -.100 -.080 Red -.164 -.120 -.108 -.100 5°T to 40°T Achromatic -.019 -.010 -.012 -.006 Blue -.019 -.016 -.013 -.012 Green -.019 -.016 -.009 -.009 Red -.020 -.020 -. 004 -.012 84 summation exponent k was calculated at a l l r e t i n a l locations according to formula (3) (see Introduction and Figure 20). The mean k values obtained for the locations 40° and 5° nasal, 0°, and 40° and 5° temporal are presented i n Table 13 and Figure 21. (A complete table of a l l mean k values may be found i n Appendix V.) It seems that i n most cases, summation, as r e f l e c t e d by the magnitude of k, increased with e c c e n t r i c i t y . Figure 21 also indicates that summation decreased as s i z e increased. This e f f e c t appeared to be greatest i n the per-phery and generally smallest at the fovea. It i s of i n t e r e s t that the mean value of k for the green stimulus, sizes 1 and 2 at 40° nasal exceeded the t h e o r e t i c a l l i m i t of k = 1. It i s also notable that the mean foveal k value for si z e s 3 and 4, achromatic stimulus, was equal to zero, implying no summation. The extreme values obtained for k (see Figure 20) and the lack of any but the most general r e g u l a r i t y i n th i s quantity are c h a r a c t e r i s -t i c of other determinations of k (Gougnard, 1961). The v a r i a b i l i t y i n k values obtained as a function of r e t i n a l l o c a t i o n , s i z e combination, and colour can be seen i n the standard deviations presented i n Tables 14 and 15. The standard deviations are quite large r i g h t across the r e t i n a , including the fovea, and do not d i f f e r i n a regular way among the stimulus colours or size-comparisons. F u l l y - S c o t o p t i c Adaptation. The increase i n scotopic s e n s i t i v i t y r e s u l t i n g from an increase i n stimulus s i z e i s shown i n Figure 22 for a l l c h r o maticities. The scotopic gradients can not be so neatly divided into four segments as could the photopic gradients. The low foveal s e n s i t i v i t y creates a depression of varying depth i n each Table 13 Mean Photopic k Values at Selected E c c e n t r i c i t i e s Stimulus Retinal Location, 0-180° Meridian Sizes Compared Colour 40°N 5°N 0° 5°T 40°T 1- 2 Achromatic 0.86 0.46 0.28 0.46 0.98 Blue 0.88 0.74 0.46 0.62 0.78 Green 1.06 0.72 0.42 0.66 0.84 Red 0.94 0.86 0.42 0.80 0.80 2- 3 Achromatic 0.84 0.56 0.38 0.60 0.50 Blue 0.86 0.58 0.36 0.60 0.78 Green 0.74 0.40 0.36 0.48 0.86 Red 0.70 0.46 0.38 0.48 . 0.84 3- 4 Achromatic 0.46 0.38 0.00 0.20 0.54 Blue 0.48 0.32 0.22 0.34 0.38 Green 0.36 0.32 0.22 0.36 0.40 Red 0.58 0.36 0.18 0.26 0.86 03 ON C M 1 1 — i — i — i — r — I — I 1 — 1 — r — 1 4 0 ° 3 0 ° 2 0 ° ' | 0 ° 0 ° I O ° 2C> 3 0 ° 4 0 ° N T Figure 21 Mean fully-ph o t o p i c summation exponents (k), at selected e c c e n t r i c i t i e s , f o r Goldmann stimulus sizes 1-2 ( — ) , 2-3 (----), & 3-4 (-88 Table 14 Average Standard Deviations for Photopic k Values Obtained at Each R e t i n a l P o s i t i o n (k „, k „, k .) P o s i t i o n Standard Deviation Nasal 40° 0.23 30° 0.25 20° 0.25 15° 0.17 10° 0.14 5° 0.17 Fovea 0° 0.21 Temporal 5° 0.19 10° 0.15 20° 0.22 25° 0.20 30° 0.17 40° 0.24 R e t i n a l E c c e n t r i c i t y , 0-180 Meridian 89 Table 15 Average Standard Deviations for A l l Photopic k Values Obtained with Each Colour-Size Combination Sizes Compared Colour 1-2 2-3 3-4 Achromatic 0.20 0.18 0.19 Blue 0.20 0.16 0.18 Green 0.23 0.21 0.20 Red 0.20 0.15 0.20 Figure 22 Mean f u l l y - s c o t o p i c s e n s i t i v i t y gradients, Goldmann stimulus sizes 1-4. 91 gradient. The e f f e c t of increasing stimulus s i z e on the foveal sen-s i t i v i t y i s best seen i n Figure 16, where c e r t a i n highly a t y p i c a l thresholds have been removed. As a r e s u l t , each colour shows a regu-l a r increase i n s e n s i t i v i t y with s i z e except the blue, which shows only a small s e n s i t i v i t y change between sizes 1 and 2 and s i z e s 3 and 4, but a large change between sizes 2 and 3. Aside from the fovea, the scotopic gradients can be divided into four segments on the basis of general slope, but the d i v i s i o n at one point i s d i f f e r e n t for the achromatic than for the chromatic s t i m u l i . The slope of the gradients for the chromatic s t i m u l i i s p o s i t i v e from 0° to 15° nasal, negative (or occasionally 0) from 15° to 5° nasal, p o s i t i v e from 5° to 10° temporal, and negative from to 40° tem-poral (Figure 23). The achromatic stimulus shows a s i m i l a r pattern except that the point of i n f l e c t i o n i n the nasal f i e l d i s at 10° rather than 15° i n two cases, and at 15° i n only one (the 10°-5° slope i s zero i n the fourth case) (see Figure 22). The slopes for the chromatic segments are shown i n Table 16. There did not appear to be any consistent trend for the slope of the 40°-15° nasal or 5°-40° temporal segments to be associated with an increase or decrease i n stimulus s i z e . However, the 15°-5 0 and 5°-10° segments showed i n most cases (excepting the blue and green 5°-10° temporal segments) an increase i n slope with an increase i n stimulus s i z e . This would seem to i n d i c a t e that s p a t i a l summation i s greater at 15° nasal than 5° nasal, and at 10° temporal than at 5° temporal. The lack of any clear r e l a t i o n s h i p between the slopes of the more peripheral segments and increasing s i z e would seem to i n d i c a t e that summation i s not i n Figure 23 Mean f u l l y - s c o t o p i c s e n s i t i v i t y gradients, selected e c c e n t r i c i t i e s , Goldmann stimulus s i z e s 1-4. Table 16 Slopes of Scotopic Gradient Segments 93 Gradient Segments Stimulus Size 40°N to 15°N Blue Green Red 15°N to 5°N Blue Green Red 5°N to 10°T Blue Green Red 10°T to 40°T Blue Green Red ,016 .020 .017 .016 .002 .000 .016 .012 .000 .006 .007. .004 ,013 ,016 ,022 ,018 .020 .008 .016 .016 .004 .001 .005 .008 ,019 .022 .024 .018 ,020 .012 .060 .012 .024 .004 ,005 .007 .025 .018 .018 ,030 ,080 ,012 .044 .032 .024 .005 ,006 .003 94 general higher i n the far periphery than i n the near periphery, under scotopic conditions. A q u a n t i t a t i v e assessment of the summation capacities r e f l e c t e d by the change of gradient slope with increasing stimulus s i z e was ob-tained from the c a l c u l a t i o n of the summation exponent k (see Figure 24). The mean values obtained at 40° and 5° nasal, 0°, 5° and 40° temporal are presented i n Table 17 and i n Figure 25. (A complete table of a l l mean k values may be found i n Appemdix VI.) The k values obtained at 40° and 5° confirm the impression gained from Figure 23 that over a l l and within each chromaticity, there i s no clear increase i n summation from the near (5°) to the f a r (40°) periphery. The foveal mean k values show no r e g u l a r i t y at a l l , ranging from the t h e o r e t i c a l l y impossible -0.05 to the other extreme at +1.44, also far beyond the t h e o r e t i c a l l i m i t of 1.0. The decrease i n summation with increasing s i z e found for the photopic data i s not c h a r a c t e r i s t i c of the scotopic data, except perhaps for the red stimulus, which i n the nasal f i e l d s shows an increase i n k as s i z e decreases. The wide v a r i a t i o n s i n k across the r e t i n a , among stimulus s i z e -comparisons and colours (Figure 24) might appear to r e f l e c t the absence of any r e g u l a r i t y i n summation capacity at a l l . However, from the standard deviations shown i n Tables 18 and 19 i t appears that the v a r i a b i l i t y i n the determination of k i s considerable, and as such may e a s i l y be obscuring any r e g u l a r i t y e x i s t i n g . This v a r i a b i l i t y appears to be somewhat greater i n the periphery, with very high values at 20° nasal and temporal, but i s greatest of a l l at the fovea. This l a t t e r e f f e c t i s to be expected considering the s p e c i a l problems inherent Ln Figure 24 Mean f u l l y - s c o t o p i c summation exponents (k) for Goldmann stimulus sizes 1-2 (—) 2-3 (—-) and 3-4 (---). ' 96 Table 17 Mean Scotopic k Values at Selected E c c e n t r i c i t i e s Stimulus E c c e n t r i c i t y , 0-180° Meridian Sizes Compared Colour 40°N 5°N 0° 5°T 40°T 1- 2 Achromatic 0.70 0.82 0.72 ' 0.86 0.94 Blue 1.04 0.86 0.50 0.82 1.06 Green 1.22 0.72 0.78 0.82 1.00 Red 0.66 0.76 0.80 0.86 0.68 2- 3 Achromatic 1.02 0.88 -.50 0.92 1.22 Blue 0.64 0.90 1.44 0.70 0.90 Green 0.62 0.86 0.74 0.96 0.90 Red 0.90 0.90 0.58 0.92 1.14 3- 4 Achromatic 0.94 0.80 1.10 0.78 0.70 Blue 0.78 0.80 0.54 1.10 0.94 Green 1.02 0.84 0.02 0.60 0.86 Red 1.18 0.92 0.58 0.80 0.82 4J c CD c o X (U c o •H 4-1 ni e 4 0 ° 3 0 ° Figure 25 VD 40 u 3CT 20° N 10°' o° ' |b 0r.20 9' 30° 40° Mean f u l l y - s c o t o p i c summation exponents (k) , at selected e c c e n t r i c i t i e s , for Goldmann stimulus sizes 1-2 ( — ) , 2-3 (---),• and 3-4 (---). 98 Table 18 Average Standard Deviations for Scotopic k Values Obtained at Each E c c e n t r i c i t y (k „, k , k ,) Nasal Temporal 40° 30° 20° 15° 10°" 5° 0° 5° 10° 20°~" 25° 30° 40° .28 .27 .46 .21 .21 .21 .80 .23 .20 .46 .27 .24 .32 Table 19 Average Standard Deviations for A l l Scotopic k. Values Obtained with Each Colour-Size Combination Colour Sizes Compared 1-2 2-3 3-4 Achromatic 0.24 0.37 0.35 Blue 0.34 0.38 0.36 Green 0.23 0.21 0,38 Red 0.26 0.40 0.32 100 i n scotopic foveal threshold determination (see Results section on Fully-Scotopic Gradients). No clear a s s o c i a t i o n between the v a r i -a b i l i t y of k values and either stimulus s i z e or colour (Table 19) was-indicated. Fully-Photopic and Fully-Scotopic Adaptation. A comparison of the summation capacity of the r e t i n a under fully- p h o t o p i c and f u l l y -scotopic conditions, as r e f l e c t e d by changes i n gradient slope and values of the summation exponent k, indicated the following points. F i r s t , under photopic conditions summation seemed to increase with e c c e n t r i c i t y , while no such trend was found for the scotopic data. Mean photopic k values ranged from 0.36 to 1.06 at 40° e c c e n t r i c i t y , from 0.20 to 0.86 at 5° e c c e n t r i c i t y , and from 0 to 0.46 at the fovea. Mean scotopic k values ranged from 0.62 to 1.22 at 40° and.from 0.60 to 1.10 at 5° e c c e n t r i c i t y . The range of mean scotopic foveal k values, from -0.05 to 1.44, may e a s i l y r e f l e c t methdological problems rather than v a r i a t i o n s i n summation. Summation, therefore, appeared to be greater under scotopic than photopic conditions, both i n the p e r i -phery and close to the fovea, though the di f f e r e n c e was greater i n the near than i n the far periphery. The second point i s that summation seemed to decrease with an increase i n stimulus s i z e under photopic but not under scotopic con-d i t i o n s . This e f f e c t under photopic adaptation appeared to be greatest at the fovea. F i n a l l y , there was a great deal of v a r i a b i l i t y i n the k values determined; average standard deviations ranged from 0.14 to 0.25 under photopic and from 0.21 to 0.38 under scotopic conditions. It would 101 seem then that the v a r i a b i l i t y i s greater i n the scotopic condition. No a s s o c i a t i o n between stimulus chromaticity, s i z e , or e c c e n t r i c i t y , and the v a r i a b i l i t y observed i n k, seemed to characterize the photopic data. Such v a r i a b i l i t y did seem to increase with e c c e n t r i c i t y under scotopic conditons, but showed i t s highest value at the fovea. 102 DISCUSSION Retina l S e n s i t i v i t y Gradients  Fully-Photopic The s e n s i t i v i t y gradients obtained under fu l l y - p h o t o p i c conditions in d i c a t e that, outside the fovea, the retina's photopic response system (assumed to be rod-free) on the h o r i z o n t a l meridian was equally s e n s i t i v e to chromatic s t i m u l i which had been equated photometrically i n terms of V^. At the fovea t h i s did not seem to be the case; the foveal s e n s i t i v i t y under these conditions seemed to be highest for the red, intermediate f o r the green, and lowest f o r the blue. This presents a rather paradoxical p i c t u r e . V. was established on the •r . A basis of foveal measurements; hence i t might be expected that the process of equating the chromatic s t i m u l i i n terms of would y i e l d the same foveal threshold for a l l three chromatic s t i m u l i . That t h i s was not found to be the case implies that e i t h e r some assumptions made i n in t e r p r e t i n g thedata as photometrically-equated s t i m u l i are i n v a l i d , or that some other v a r i a b l e i s operating. The assumptions made i n the i n t e r p r e t a t i o n of the present data are as follows. 1. The response-curve and c a l i b r a t i o n of the photometer i s assumed highly accurate, and the error introduced through i n t e r p o l a t i o n of sp e c t r a l transmission values used to derive the cinemoid f i l t e r c o r r e c t i o n factors i s assumed n e g l i g i b l e (see Appendix VII for a complete d e s c r i p t i o n of the de r i v a t i o n of the co r r e c t i o n f a c t o r s ) . With these assumptions, the s t i m u l i can be considered photometrically-103 equated i n terms of V^. -2 2. i s assumed applicable under high luminance conditions (L=250 cd.m. with small (6.18-54.3 minutes of v i s u a l angle) s t i m u l i , using ascending method of l i m i t s perimetry. V. was derived under conditions of low luminance — for example, one of the studies contributing to the standard V^, that of Coblentz and Emerson (1918), used adaptation luminances as low as 1.5 cd.m. . The s t i m u l i used were 2 to 3 , and f l i c k e r and step-by-step heterochromatic photometric methods were used (Wyszecki and S t i l e s 1967, LeGrand, 1968). Fovea The data obtained under photopic conditions i n the present study show some i n t e r e s t i n g patterns. F i r s t , the foveal s e n s i t i v i t y i s highest to the red, followed by the green and then the blue stimulus. The achromatic yielded the same foveal s e n s i t i v i t y as the blue stimulus. The r e s u l t i s a sort of "reverse Purkinje e f f e c t " : the opposite to that found under scotopic conditions. A s i m i l a r e f f e c t has been found i n -2 preliminary studies done with these s t i m u l i under mesopic (L=10 cd.m. ) conditions, so that i t i s not purely a r e s u l t of f u l l y -photopic adaptation. This implies that, granting the aforementioned assumptions, d i r e c t a p p l i c a t i o n of to foveal s e n s i t i v i t y as measured by increment-threshold s t a t i c perimetry cannot be made. Periphery In contrast to the d i f f e r e n t i a l e f f e c t of stimulus chromaticity on ful l y - p h o t o p i c foveal s e n s i t i v i t y , outside the fovea a l l three chromatic s t i m u l i y i e l d e d s i m i l a r s e n s i t i v i t i e s . At each extra-foveal l o c a t i o n tested, the red, green, and blue s t i m u l i yielded thresholds within 0.1 to 0.2 log units of each other; no one colour yielded c o n s i s t a n t l y higher or lower thresholds. This contrasts with 104 the foveal thresholds, wherein the thresholds ( i n decreasing order) were i n a l l cases blue, green, and red, with differences as large as 0.6 log units between the thresholds f o r the red and blue s t i m u l i . The fact that photometrically-equivalent chromatic s t i m u l i gave s i m i l a r extra-foveal but d i f f e r e n t foveal thresholds may be interpreted i n terms of the perceptual task required of the subject. The subject was i n a l l cases required to respond when he saw the stimulus — he did not need to i d e n t i f y i t s colour. Although no systematic attempt was made to determine when the subjects could i d e n t i f y the stimulus colour, some subjects reported that the stimulus appeared coloured only when viewed f o v e a l l y . I f only foveal s t i m u l i were perceived as coloured, then foveal and extra-foveal threshold determinations represented d i f f e r e n t tasks to the subject: the former a task of wave-length d i s c r i m i n a t i o n , the l a t t e r a task of luminance-difference d e t e c t i o n . The experimental data support t h i s hypothesis: extra-foveal s e n s i t i v i t y would be expected to be s i m i l a r f o r photometrically-equated chromatic s t i m u l i i f the thresholds were based on luminance-difference detection alone. The d i f f e r e n t i a l foveal chromatic thresholds are expected i f at the fovea the subject i s required to make wavelength-discriminations, as the a b i l i t y to make such discriminations varies with wavelength (Wright and P i t t , 1934). On the basis of wavelength di s c r i m i n a t i o n data, the order of foveal s e n s i t i v i t y expected would be red > blue > green ( i b i d ) , while that found was red > green > blue. The reversed order of the green and blue may be due to the small f i e l d s izes used, as Willmer and Wright (1945) found with a 20' f i e l d d i s c r i m i n a t i o n decreased considerably i n the sp e c t r a l region of the blue stimulus used 105 here. Most wavelength d i s c r i m i n a t i o n data has been obtained with larger f i e l d s on the order of l°-2° (Wyszecki and S t i l e s , 1967). The extra-foveal achromatic s e n s i t i v i t i e s were s i m i l a r to those for the chromatic s t i m u l i f o r the two smaller stimulus s i z e s , as was expected under the i n t e r p r e t a t i o n of extra-foveal thresholds as derived from luminance-difference detection. The lower s e n s i t i v i t y to the achromatic as opposed to the chromatic for the largertwo stimulus sizes indicates some i n t e r a c t i o n between s p a t i a l summation and the perceptual task being performed . I t i s possible that with l a r g e r s t i m u l i the task involves hue-discrimination even outside the fovea . Determination of the precise r o l e played by hue-discrimination as opposed to luminance-difference detection i n the perimetric data presented here could be done by studying the photochromatic i n t e r v a l under equivalent conditions. F u l l y - s c o t o p i c Interpretation of the f u l l y - s c o t o p i c s e n s i t i v i t y gradients rests on s i m i l a r assumptions concerning photometric e q u a l i z a t i o n and as did the i n t e r p r e t a t i o n of the photopic gradients . In addition, the scotopic thresholds are presumed (excluding the foveal thresholds) to represent cone-free thresholds, while was presumed to apply s t r i c t l y to the cone system. Because of the indeterminacy of the scotopic foveal thresholds the amount of r e l i a b l e information to be gained from them i s li m i t e d , though no major differences among them as a function of chromaticity were apparent , at le a s t for the two smaller s i z e s (Figure 15). Periphery The fact that a l l three chromatic s t i m u l i yielded e s s e n t i a l l y the same extra-foveal photopic thresholds may be considered when an i n t e r p r e t a t i o n of the scotopic extra-foveal thresholds i s sought. 106 Dark-adaptation, and with i t the presumed switch to a cone-free response system, y i e l d e d s e n s i t i v i t y gradients which d i f f e r e d s i g n i f i c a n t -l y among the chromatic s t i m u l i . An increase i n s e n s i t i v i t y was found for a l l three colours, but t h i s increase was generally greatest for the blue, s l i g h t l y l e s s f or the green, and considerably less (one to one and one-half log units less) for the red (Figure 17). If the fovea i s disregarded, the gradients for a l l three chromatic (as well as the achromatic) s t i m u l i were of very s i m i l a r form, being r e l a t i v e l y f l a t across the r e t i n a but showing a s l i g h t r i s e i n ;the mid-periphery (5°-15°). It i s only i n absolute s e n s i t i v i t y that they d i f f e r (extra-foveally) . The differences i n s e n s i t i v i t y among the colours i r e f l e c t the (Purkinje) s h i f t from to with the blue y i e l d i n g higher s e n s i t i v i t y than the green, though each yielded s i m i l a r s e n s i t i v i t y gradients under photopic conditions . The very low s e n s i t i v i t y to the red stimulus again seems to r e f l e c t the Purkinje phenomenon. In general form the photopic and scotopic extra-foveal gradients are very s i m i l a r , excepting the r e l a t i v e increase i n s e n s i t i v i t y i n the 5-15° e c c e n t r i c i t y region which characterizes only the scotopic data. The i m p l i c a t i o n i s that the change from fully- p h o t o p i c to f u l l y - s c o t o p i c adaptation y i e l d s an increase i n e x t r a - f o v e a l s e n s i t i v i t y , which i s uniform across the r e t i n a with the exception of a greater increase i n the mid-peripheral, 5-15° eccentric regions. Fovea In turning to the i n t e r p r e t a t i o n of the foveal scotopic thresholds, complications a r i s e due to the uncertain v a l i d i t y of scotopic foveal measurements due to the lack of precise f i x a t i o n c o n t r o l . To estimate the r e l a t i v e depth of the foveal 'dip', i n the various scotopic gradients, use can be made of the slopes of the 15-5° nasal and 5-10° 107 t e m p o r a l g r a d i e n t segments ( T a b l e 16). A h i g h l y s l o p e d g r a d i e n t a d j a c e n t t o the d i p would i n d i c a t e a deep f o v e a l d e p r e s s i o n , as t h e s e segments r e p r e s e n t the b e g i n n i n g s of the f o v e a l 'scotoma 1. On t h i s b a s i s the b l u e and green s t i m u l i b o t h y i e l d e d l a r g e r d i f f e r e n c e between m i d - p e r i p h e r a l and f o v e a l s e n s i t i v i t i e s - t h a t i s , they gave h i g h e r s l o p e s - than d i d the r e d s t i m u l u s . T h i s s e n s i t i v i t y d i f f e r e n c e between the f o v e a and the a d j a c e n t r e t i n a l a r e a s g e n e r a l l y i n c r e a s e d w i t h s t i m u l u s s i z e , f o r a l l s t i m u l i i n c l u d i n g the r e d . The d a t a thus i n d i c a t e t h a t under s c o t o p i c c o n d i t i o n s the f o v e a i s l e s s s e n s i t i v e than the a d j a c e n t r e t i n a to b l u e and green s t i m u l i , and to re d s t i m u l i o n l y f o r l a r g e r s i z e s . T h i s l e n d s s u p p o r t f o r an i n t e r p r e t -a t i o n o f the s c o t o p i c t h r e s h o l d s as r o d t h r e s h o l d s o u t s i d e the f o v e a and c o n e - t h r e s h o l d s w i t h i n i t . The d i f f e r e n c e between 5° and 0° e c c e n t r i c i t y s c o t o p i c t h r e s h o l d s f o r b l u e and green s t i m u l i r e f l e c t the s e n s i t i v i t y d i f f e r e n c e between dark-adapted rods ( a t 5 ) and cones ( a t 0 ). The m i n i m a l d i f f e r e n c e between 0° and 5° t h r e s h o l d s f o r the r e d s t i m u l u s r e f l e c t the m i n i m a l i n c r e a s e i n s e n s i t i v i t y o f dark-adapted rods over d a r k - a d a p t e d cones t o l o n g wavelength s t i m u l i . I t i s not immediately c l e a r why t h e r e d s t i m u l u s y i e l d e d a r e l a t i v e d e c r e a s e i n s e n s i t i v i t y a t the f o v e a f o r o n l y the l a r g e r s t i m u l i , but t h i s r e s u l t a g r e e s w i t h the r e s u l t s o f N o l t e (1962) and Wentworth (1930). N o l t e , w i t h two r e d s t i m u l i (599 nm and 658 nm) s u b t e n d i n g 30' o f v i s u a l a n g l e d i d not f i n d the f o v e a l d e c r e a s e , w h i l e Wentworth w i t h a r e d s t i m u l u s (672.5 nm) s u b t e n d i n g 1°16' of v i s u a l a n g l e d i d show the r e l a t i v e d e c r e a s e a t the f o v e a . In the p r e s e n t i n v e s t i g a t i o n the s t i m u l i s u b t e n d i n g 6.8' and 13.6' d i d not y i e l d the f o v e a l d e c r e a s e , w h i l e those 108 subtending 27.2' and 64.3' did. On the basis of Nolte's data no foveal " r e l a t i v e scotoma" would have been expected with the 27.2' red stimulus. However, the trend for increasing s i z e of the red stimulus to be associated with appearance of the foveal "scotoma" was a highly consistant f i n d i n g i n the present study. Moreover, t h i s trend appears to characterize a l l the chromatic and achromatic scotopic data, as r e f l e c t e d by the increasing slope of 5°-0° gradient segments which g e r e r a l l y accompanies increasing stimulus s i z e . Two processes could explain the observed trend f o r increasing stimulus s i z e ( p a r t i c u l a r l y for the red stimulus) to be associated with increasing d i f f e r e n c e between foveal and 5°-eccentricity s e n s i t i v i t y . E i t h e r the increased s i z e i s associated with a decrease i n the foveal s e n s i t i v i t y , or with an increase i n the para-central s e n s i t i v i t y . Figure 22 indicates that the l a t t e r explanation i s the more l i k e l y , which would seem more l o g i c a l i n any case. This indicates then that the scotopic s e n s i t i v i t y increased p r o p o r t i o n a l l y more i n the para-central than i n the foveal r e t i n a with an increase i n stimulus s i z e . This i s tantamount to saying that s p a t i a l summation was greater i n the mid-periphery than i t was i n the fovea, an i n t e r p r e t a t i o n consistant with previous i n v e s t i g a t o r s (Sloan, 1961; Gougnard,1961). The d i s t r i b u t i o n of r e t i n a l elements may be considered relevent to an i n t e r p r e t a t i o n ofthe photopic and scotopic gradients; f i g u r e 26 shows the r e t i n a l d i s t r i b u t i o n of rods and cones. While there i s no d i r e c t c o r r e l a t i o n between cone d i s t r i b u t i o n and the photopic gradients, i n general the peaked photopic gradient r e f l e c t s very generally the cone density which i s highest f o v e a l l y and tapers o f f p e r i p h e r a l l y . 109 Figure 26 D i s t r i b u t i o n of rods and cones i n human r e t i n a , based on 0 s t e r b e r g , 1935. From Moses, 1975, p. 382. 110 The extra-foveal scotopic gradients r e f l e c t rod d i s t r i b u t i o n to the extent that both the highest rod density and the highest scotopic s e n s i t i v i t y f a l l i n the mid-periphery. The lack of any precise c o r r e l a t i o n between receptor density and s e n s i t i v i t y i s not s u r p r i s i n g , as many other factors such as absorption by the ocular media a f f e c t s e n s i t i v i t y . Throughout t h i s discussion , many int e r p r e t a t i o n s have been made based on the assumption that c e r t a i n threshold measurements represent responses from either the rods or the cones. Hence, a l l foveal thresholds have been presumed to r e f l e c t cone s e n s i t i v i t y , while peripheral thresholds were presumed to represent cone s e n s i t i v i t i e s under photopic and rod s e n s i t i v i t i e s under scotopic adaptation. These assumtions are not held with equal c e r t a i n t y , however. While i t seems quite c e r t a i n that the photopic foveal measurements represent only cone s e n s i t i v i t i e s , i t i s les s c e r t a i n that the peripheral photopic measurements represent only cone s e n s i t i v i t i e s . I nterpretation of the peripheral photopic data as purely cone s e n s i t i v i t i e s rests on the evidence of Aguilar and S t i l e s -2 (1954) that the rods are saturated at an adaptation of 250 cd.m. under scotopic conditions i t seems l i k e l y that the peripheral measure-ments represent only rod s e n s i t i v i t i e s , but i t i s with considerably le s s c e r t a i n t y that one states that scotopic foveal measurements represent only cone s e n s i t i v i t i e s due to the lack of precise f i x a t i o n c o n t r o l . Because of the uncertainty of the r e l a t i o n s h i p s among the various s e n s i t i v i t i e s and the d i s t r i b u t i o n of r e t i n a l elements, conclusions based on these r e l a t i o n s h i p s tend to take the form of tentative g e n e r a l i t i e s rather than precise statements. I l l Before leaving the topic of s e n s i t i v i t y gradients, one further cautionary note i s necessary. An attempt has been made to in t e r p r e t the r e s u l t s obtained i n terms of V^ and of the r e t i n a l d i s t r i b u t i o n of photoreceptors. Although the C L E . 1924 curve i s u n i v e r s a l l y accepted as the standard luminous e f f i c i e n c y curve, i t i s an average of many i n d i v i d u a l curves. Individual luminous e f f i c i e n c y curves can d i f f e r s u b s t a n t i a l l y from t h i s average function (LeGrand, 1968). For example, Coblentz and Emerson (1918) using f l i c k e r photometry found i n 125 normal subjects a range of 549 to 570 nm for the peak of the luminous e f f i c i e n c y curve. These represent departures up to 15 nm from the peak of the C L E . 1924 V^. On the other hand, v i r t u a l l y a l l information on the d i s t r i b u t i o n of the rods and cones Is based on only one human eye (0sterberg, 1935). In the l i g h t of these f a c t s , v a r i a t i o n i n any v i s u a l function based on and interpreted i n terms of these standards may be expected to be considerable. I d e a l l y for the present study, one would l i k e to determine each subject's i n d i v i d u a l luminous e f f i c i e n c y curve and then photometrically equate the s t i m u l i using his own . Were t h i s done, the r e s u l t s might be expected to be more e a s i l y i n t e r p r e t a b l e , and would i n any case eliminate the error due to the departure of each subject's v i s u a l system from the average standard system. S p a t i a l Summation From the slopes of the s e n s i t i v i t y gradients obtained i n t h i s study as w e l l as the derived values of the summation exponent K, some i n d i c a t i o n of the s p a t i a l summation ca p a c i t i e s of the normal r e t i n a can be seen. S p a t i a l summation would appear to vary as a function of r e t i n a l l o c a t i o n , adaptation, and stimulus s i z e , but the exact nature of the 112 i n t e r r e l a t i o n s h i p s among these variables which may influence summation i s f a r from c l e a r . The measures of s p a t i a l summation used did seem to i n d i c a t e a d i s t i n c t d i f f e r e n c e i n t h i s capacity between the f u l l y -photopic and the f u l l y - s c o t o p i c r e t i n a . Fully-Photopic Under fully-p h o t o p i c conditions, summation appeared to increase with distance from the fovea i n a f a i r l y regular fashion (Figure 20) for a l l chromatic and achromatic s t i m u l i . This i s i n agreement with previous i n v e s t i g a t o r s who used achromatic s t i m u l i under mesopic -2 (10 cd.m. ) conditions (Farkhauser and Schmidt, 1958; Sloan, 1961; Gougnard, 1.961). The r e s u l t s of these previous studies might be interpreted to i n d i c a t e the increasing contribution of rods to the thresholds obtained under mesopic conditions as one moves p e r i p h e r a l l y from the fovea. However, i f f u l l y - p h o t o p i c thresholds are assumed to be rod-free t h i s cannot explain the present r e s u l t s . In t h i s case i t would appear that peripheral cones showed a greater capacity for s p a t i a l summation than did the c e n t r a l cones. This i s consistant with the greater convergence of receptors, and the greater numbers of cones connecting with each h o r i z o n t a l c e l l , which characterize the p e r i p h e r a l as opposed to the cenral r e t i n a (Rodieck, 1973). For a l l chromatic and achromatic s t i m u l i under fu l l y - p h o t o p i c conditions i t was also found that s p a t i a l summation varied inversely with stimulus s i z e (Figure 20). This i s also i n agreement with previous i n v e s t i g a t i o n s with achromatic s t i m u l i under mesopic conditions ( i b i d ) . Thus s p a t i a l summation under photopic conditions i s an important determinant of v i s u a l function for small s t i m u l i of achromatic and 1 1 3 chromatic character. Comparison of the present r e s u l t s with previous studies indicates then that s p a t i a l summation i n the f u l l y - p h o t o p i c r e t i n a i s s i m i l a r to that i n the mesopic r e t i n a , increasing with e c c e n t r i c i t y and decreasing with stimulus s i z e . In magnitude, summation as r e f l e c t e d by K seems to be greater i n the mesopic than the fully-photopic condition (Table 20); t h i s presumably r e f l e c t s the contribution of rods to the mesopic thresholds. Fully-Scotopic S p a t i a l summation under f u l l y - s c o t o p i c conditions presents a d i f f e r e n t p i c t u r e . F i r s t , there i s a general increase i n summation with the change from fu l l y - p h o t o p i c to f u l l y - s c o t o p i c adaptation. This o v e r a l l increase i n summation from photopic to scotopic adaptation presumably r e f l e c t s the change from cone to rod functioning. However, the r e l a t i o n s h i p s between summation and stimulus e c c e n t r i c i t y and s i z e found underphotopic conditions were not duplicated under scotopic conditions. There did not appear to be any consistant r e l a t i o n s h i p between s p a t i a l summation i n the scotopic r e t i n a and e i t h e r e c c e n t r i c i t y (excluding the fovea)or stimulus s i z e , over the ranges of e c c e n t r i c i t y and s i z e used. The lack of any consistant change i n summation from the near to the far periphery under scotopic conditions implies that s p a t i a l summation invo l v i n g the rod system does not p r e c i s e l y r e f l e c t rod d i s t r i b u t i o n , despite the r e g u l a r i t y of t h i s d i s t r i b u t i o n (Figure 26). Instead, scotopic extra-foveal summation appears to be independent of rod d i s t r i b u t i o n and hence of e c c e n t r i c i t y , and t h i s independence seems to apply to a l l the chromatic and achromatic s t i m u l i . 114 Table 20 K Values Obtained i n Various Investigations With an Achromatic Stimulus E c c e n t r i c i t y Coldmann Sizes Compared Sloan (1961) K Value Obtained Gougnard (1961) Present Study I II 15°N 30°N 40°N 45°N 0- 1 1- 2 2- 3 3- 4 0- 1 1- 2 2- 3 3- 4 0- 1 1- 2. 2- 3 3- 4 0- 1 1- 2 2- 3 3- 4 0- 1 1- 2 2- 3 3- 4 .55 .75 -.88 .49 .39 . 17 .18 1.02 .99 .28 .38 0 .52 .52 .54 1.02 .64 .42 .86 .84 .46 • 72 -.05 1.10 .96 .94 .76 .72 1.20 .74 .70 1.20 .94 -.90 Adaptation: Sloan: C L E . Illuminant A, 10 cd.mT Gougnard: C L E . Illuminant A, 10 cd.mT Present Study I : C L E . Illuminant C, 250 cd.mT II : L = zero 115 The inverse r e l a t i o n s h i p between s p a t i a l summation and stimulus s i z e found i n the photopic data has no c l e a r p a r a l l e l i n the scotopic r e s u l t s ; scotopic summation appears to vary i n no consistant way with stimulus s i z e for any of the chromatic or achromatic s t i m u l i or s i z e s used. The scotopic summation r e s u l t s imply that summation i n the rod system does not vary i n any predictable way with stimulus s i z e , colour, or (non-foveal) r e t i n a l l o c a t i o n . This contrasts with the s i t u a t i o n under fu l l y - p h o t o p i c adaptation, wherein summation seems to vary d i r e c t l y as a function of e c c e n t r i c i t y and inversely as a.function of stimulus s i z e . This d i f f e r e n c e may be r e l a t e d to the d i f f e r e n c e response systems presumed to be involved, the cones under photopic and the rods under scotopic conditions. However, such a conclusion may not be warranted i n view of the wide v a r i a b i l i t y i n the summation exponents derived, e s p e c i a l l y as t h i s v a r i a b i l i t y was found to be greater under scotopic than photopic conditions. The v a r i a b i l i t y found i n the summation exponent K i s considerable under both photopic and scotopic conditions, though greater i n the l a t t e r . This might seem to imply that the summation capacity of the r e t i n a i s i t s e l f highly v a r i a b l e , even under an invariant set of conditions. However, the p o s s i b i l i t y e x i s t s that the v a r i a b i l i t y i n K r e f l e c t s at l e a s t p a r t i a l l y the variance i n the threshold values from whick K i s derived rather than variance i n summation per se. 116 SUMMARY AND CONCLUDING REMARKS The present i n v e s t i g a t i o n was designed to study the s e n s i t i v i t y and s p a t i a l summation properties of the normal r e t i n a under well-defined, p r e c i s e l y c o n t r o l l e d conditions. The use of three chromatic s t i m u l i (as well as an achromatic) gave information on the d i f f e r e n t i a l s e n s i t i v i t y of the r e t i n a "to s t i m u l i of varied s p e c t r a l composition, while the use of four stimulus sizes yielded data on the a b i l i t y of the r e t i n a to summate luminous input s p a t i a l l y . Thus sixteen stimulus s i z e -chrbmaticity combinations were presented at points on the h o r i z o n t a l meridan to f i v e emmetropic normal trichromatic observers under both fu l l y - p h o t o p i c and f u l l y - s c o t o p i c adaptation conditions . The use of -2 these two extreme adaptation luminances (250 and 0 cd.m. ) allowed the separation of the photopic and scotopic response systems. The r e s u l t s of these i n v e s t i g a t i o n s may be summarized as follows: 1. Under fu l l y - p h o t o p i c conditions, the chromaticity of the stimulus had no e f f e c t on extra-foveal s e n s i t i v i t y : a l l three chromatic s t i m u l i yielded equivalent mean gradients which were s l i g h t l y higher than the gradients yielded by the achromatic s t i m u l i . Conversely, f u l l y - p h o t o p i c adaptation yielded foveal thresholds which varied as a function of stimulus chromaticity, the foveal s e n s i t i v i t i e s being ( i n decreasing order) red, green, and blue. These r e s u l t s were discussed with reference to the a p p l i c a b i l i t y of the standard C L E . 1924 luminosity function ( v^) to these conditions, the perceptual task required of the observer, and the d i s t r i b u t i o n of r e t i n a l receptors. 2. F u l l y - s c o t o p i c adaptation yielded extra-foveal s e n s i t i v i t y gradients 117 which were s i m i l a r i n form for a l l chromatic and achromatic s t i m u l i , being generally f l a t across the r e t i n a but showing a r i s e at 10°-15° nasal and f a l l i n g o f f toward the c e n t r a l f i e l d . The r e l a t i v e heights of these gradients r e f l e c t e d the Purkinye s h i f t from to V^', being i n decreasing order blue, green, and red. Dark-adaptation thus produced the greatest increase i n s e n s i t i v i t y to the blue and the l e a s t to the red stimulus. The scotopic foveal thresholds were not c l e a r l y i n t e r p r e t a b l e due to the lack of precise c o n t r o l of f i x a t i o n under scotopic conditions. However, a major diffe r e n c e was found between the red stimulus, which yielded s l i g h t l y lower foveal s e n s i t i v i t y r e l a t i v e to the para-foveal region only for the larger two stimulus s i z e s , and the other s t i m u l i , a l l of which yielded foveal s e n s i t i v i t i e s markedly reduced r e l a t i v e to the para-foveal areas. These r e s u l t s were discussed with reference to the e a r l i e r work of Wentworth (1930) and Nolte (1962). A notable discrepancy was seen between the red stimulus sizes expected on the basis of t h i s e a r l i e r work to y i e l d the " r e l a t i v e c e n t r a l scotoma", and those which did y i e l d such an e f f e c t . D i s t r i b u t i o n of r e t i n a l elements and the indeterminacy of the scotopic foveal thresholds were also discussed. 3. S p a t i a l summation under ei t h e r adaptation conditions was not found to vary i n any systematic way with stimulus chromaticity. For a l l chromatic and achromatic s t i m u l i summation increased with e c c e n t r i c i t y and decreased with increasing stimulus s i z e under fu l l y - p h o t o p i c condit-ions, i n agreement with previous i n v e s t i g a t i o n s using achromatic -2 s t i m u l i under mesopic (10 cd.m. ) conditions. No clear r e l a t i o n s h i p between summation and stimulus s i z e or e c c e n t r i c i t y was found under scotopic conditions. The change from photopic to scotopic adaptation 118 c o n d i t i o n s was a s s o c i a t e d w i t h a g e n e r a l i n c r e a s e i n s p a t i a l summation. These r e s u l t s were d i s c u s s e d w i t h r e f e r e n c e t o t h e v i s u a l r e s p o n s e systems assumed t o be i n v o l v e d . 4 . V a r i a b i l i t y i n the determined t h r e s h o l d s and i n the d e r i v e d summation exponent k was found t o be c o n s i d e r a b l e under b o t h a d a p t a t i o n c o n d i t i o n s , but was not found t o v a r y as a f u n c t i o n of s t i m u l u s c h r o m a t i c i t y . V a r i a b i l i t y i n t h r e s h o l d s , as r e f l e c t e d by the s t a n d a r d d e v i a t i o n f o r mean t h r e s h o l d s , was found t o i n c r e a s e w i t h e c c e n t r i c i t y under b o t h f u l l y - p h o t o p i c and f u l l y - s c o t o p i c a d a p t a t i o n , but to d e c r e a s e w i t h i n c r e a s i n g s t i m u l u s s i z e o n l y under the former c o n d i t i o n . I n g e n e r a l v a r i a b i l i t y was g r e a t e s t a t t h e f o v e a under s c o t o p i c c o n d i t i o n s (presumably due a t l e a s t i n p a r t t o m e t h o d o l o g i c a l p r o b l e m s ) . V a r i a b i l i t y i n the summation exponent, as r e f l e c t e d by s t a n d a r d d e v i a t i o n s o f mean k v a l u e s , was found t o be g r e a t e r under f u l l y - s c o t o p i c c o n d i t i o n s . The v a r i a b i l i t y i n t h e o b t a i n e d d a t a was d i s c u s s e d w i t h r e f e r e n c e t o method-o l o g i c a l problems. Whether t h e v a r i a b i l i t y i n k r e f l e c t e d t r u e v a r i a n c e i n summation o r merely v a r i a n c e i n the t h r e s h o l d d e t e r m i n a t i o n s on which k i s based c o u l d n ot be det e r m i n e d . These r e s u l t s have been i n t e r p r e t e d as r e p r e s e n t i n g the c h a r a c t e r -i s t i c r e s p o n s e o f the normal r e t i n a under the s p e c i f i c c o n d i t i o n s des-c r i b e d , and as such c o u l d be c o n s i d e r e d norms a g a i n s t which c l i n i c a l d a t a might be compared. Such a p p l i c a b i l i t y must take i n t o account the dependence o f t h i s type o f p s y c h o p h y s i c a l d a t a on the many s t i m u l u s , o b s e r v e r , and surround v a r i a b l e s which t o g e t h e r determine t h e r e s p o n s e . I n view o f t h i s f a c t , s p e c i f i c a t i o n o f t h e s e f a c t o r s i n c r e a s e s the scope of a p p l i c a b i l i t y o f any such d a t a . The r e s u l t s o b t a i n e d from t h e p r e s e n t 119 i n v e s t i g a t i o n might be extended by the following experimental r e v i s i o n s 1. Use of each subject's own empirically-derived V^, to equate the chromatic s t i m u l i photometrically, would aid i n i n t e r p r e t a t i o n of thresholds obtained with such s t i m u l i under various conditions of adaptation and stimulus s i z e . 2. Assessment of whether the subject i s perceiving the stimulus as coloured or achromatic, and thus whether more than luminance-difference detection i s involved, might be determined under s i m i l a r conditions to those used here. 3. Precise control of f i x a t i o n under scotopic conditions i s manditory for the determination of scotopic thresholds, p a r t i c u l a r l y at the fovea This could be done using an i n f r a - r e d camera system. 120 Reference Notes Note 1. Drance, S.M. Personal communication, 1978. Note 2. Lakowksi, R. & Dunn, P.M. In preparation. Note 3; Enoch, J.M. Draft two of the International Perimetric Society Perimetric Standards, 1978. 121 References Aguilar, M. & S t i l e s , W.S. Saturation of the rod mechanism at high l e v e l s of stimulation. Optica Acta, 1954, 1, 59-65. Asher, H. Experiments i n seeing. New York: Basic Books, 1961. Aulhorn, E. & Harms, H. V i s u a l perimetry. In D. Jameson & L.M. Hurvich (Eds.) V i s u a l psychophysics, Volume VII/4. Handbook of  sensory physiology. New York: Springer-Verlag, 1972. Baker, H.D. & Rushton, W.A.H. The red-sensitive pigment i n normal cones. Journal of Physiology (London), 1965, 176, 56-72. Barr, M.L. 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Wright, W.D. & P i t t , F.H.G. Hue discrimination i n normal colour v i s i o n . Proceedings of the Ph y s i c a l Society (London), 1934, 46, 459. Wyszecki, G. & S t i l e s , W.S. Color science. New York: John Wiley & Sons, 1967. 127 APPENDIX I Counterbalancing Subj ect Testing Session T r i a l s Run Per a Testing Session Adaptation LL 1 WI Bl Rl Gl B2 Photopic 2 R2 G2 W2 R3 G3 3 W3 B3 G4 W4 4 B4 R4 5 W2 W3 - W4 WI B4 Scotopic 6 Gl G2 G3 G4 Bl 7 R3 R4 Rl R2 B2 B3 AM 1 WI Bl Rl Photopic 2 Gl B2 R2 G2 W2 3 R3 G3 W3 B3 4 G4 W4 B4 R4 5 Gl G3 G4 G2 Scotopic 6 WI W2 W3 W4 7 R2 R3 R4 Rl B2 8 B3 B4 Bl JL 1 G4 W4 B4 R4 WI Bl Photopic 2 Rl Gl B2 R2 G2 W2 3 R3 G3 W3 B3 4 G4 W4 B4 R 4 Scotopic 5 Rl R2 R3 B2 B3 Bl 6 G3 Gl G2 WI W2 W3 RM 1 R3 G3 W3 B3 Photopic 2 G4 W4 B4 R4 3 WI Bl Rl Gl 4 B2 R2 G2 W2 5 Rl R2 R3 R4 Scotopic 6 B3 B4 Bl B2 7 G.2 G3 G4 Gl .8 W4 WI W2 W3 continued 128 APPENDIX I continued Testing Subject Session T r i a l s Run Per Testing Session Adaptation 1 B2 R2 G2 W2 2. R3 G3 W3 B3 3 G4 W4 B4 R4 4 Wl Bl Rl Gl 5 W3 W4 Wl W2 6 G2 G3 G4 Gl 7 Bl B2 B3 8 R4 Rl R2 R3 B4 Photopic aW = White (T = 6000K); B = blue; R = Red; G = Green. e.g., Bl = Blue stimulus, s i z e 1. 1 2 9 A P P E N D I X II WE^ MS A NO siMbA|U> OEfiAVUNS FULLY-PHOTOF-ic T ^ £ S H O L D 5 M6A)IS ( l o ^ T d T n T T l AO 3f, 2C 1 5 1 0 5 0 =; 10 2 0 2 5 3 0 4 0 1 2 . " . 2 . 7 2 . ? 2 . 2 2 . 2 2 . r 1 . 3 2 . 0 2 . 3 2 . 5 2 . 4 2 . 5 2 . 6 • w •> "* • ^ 2 . 1 l . c 1. c 1 . P 1 . 7 1 . 1 1 . 7 1 . e 2 . 1 1 . 9 2 . T 2 . 0 w 3 1 . ° 1 • 8 1 .6 1 . 6 1 . 5 1 . 4 . 9 1 . 3 1 . 6 1 . 8 1 . 6 1 . 6 1 . 7 w 4 1 . c 1 c 1 . 4 1 . 2 1 . ? 1 . 2 . 5 1 . 2 1 . 3 1 . 5 1 . 3 1 . 4 1 . 4 n 1 2 . 8 2 . ft 2 . 2 2 . 2 2 . 0 1 . 4 1 . 5 2 . 3 2 . 5 2 . 3 2 . u 2 . 5 M 7 7. 7 2 . 1 1 . 8 1 . f i 1 . 7 1 . 5 1 . 1 1 . 5 " T 7 7 2 . 0 i.e 1 . 8 2 . 1 p 3 1 c 7 1 « 7 1 . 5 1 . 4 1 . 3 1 . 2 . 9 1 . 2 1 . 3 1 . 6 1 .5 1 . 5 1 . 6 R u ] . r l c ! . ? 1 . 1 1 . 1 1 . 0 . e 1 . 0 1 . 1 1 . 4 1 . 2 1 . 3 1 . 4 c 1 2 , 7 2 . 6 2 . 2 2 . 3 2 . 1 2 . ' • l . l 1 . 1 2 . 2 2 . 5 2.4 2 . 4 2.6 7 2 . ? 2 . 1 1 . F- 1 . P 1 . 7 1 . 5 . p 1.4 1. e 2 . 1 1 . 8 1 . c 2 . 1 3 1 . 7 1 . f 1 ' 1 . 4 1 . 3 1 . 2 B t. 1 . 1 1 . 4 ! . 7 1 . 5 1 . c . 1 . 6 1 . 4 1 c L 1 . 3 1 . 2 u r 1 . r . . 5 . 1 . 1 " I T ! i ; 4 1 . 3 1 . 3 . 1 . 4 i ?. 7 1.1- 2 . ? 2 . 2 2 . 2 l . c ! . 2 1 . 9 2 . 2 2 . 4 2 . 2 2 . 4 2 . ^ r. 2 2 . 1 2 .1 1 . P 1 . 0 1.6 1 . 5 1 . 0 1 . 5 1 . B 2 . "> 1 . 7 1 . 8 2 . 1 r. 3 1.6 1 . 7 1.4 1.4 1 . 4 " 1 . 3 . 7 1 . 2 1 . 4 1.6 1 . 5 1 . 5 1 . 6 r. 4 1.4 1 1 . 2 1 . 1 1 • 1 1 . 1 . 6 1 . 0 1 . 2 1.4 1 . 2 1 . 2 1 . 3 STANOA'O OFVIATIGN^ I . n . 1 5 . 2 ! . 1 ' . 0 7 . 1 5 . 1 7 . 1 3 . 1 5 . 2 2 15 . 1 5 . 2 4 w •> . 3 ' . 1 7 . 1 1 .<••> . 1C . 1 3 . 0 5 . 1 2 . 1 5 . 2 1 15 . 1 1 . 1 8 w 7 . I S . 1 5 . 1 2 . 1 1 . 0 7 . 1 3 . 1 c . 1 7 15 . 1 5 . 1 3 I*1 • '"9 . 1 4 .n .1 1 . Tc . 1 3 .OTT T O P . 1 2 " m—; "13 "TT'8 . ! " 3 ^ 1 . 11 . 16 . 2 1 .1.2 . n . 0 3 . 1 1 . 0 7 10 . 24 38 . 1 3 . . 1 3 7 . n . 2 2 . 1 3 . 1 0 . OR . 0 5 . " 9 . 1 3 . 0 5 . 1 7 13 . 1 3 . 1 9 a 3 . 1 5 . ! 8 . 0 9 . 1 3 .c . 0 7 . 1 3 . 1 3 . 0 9 . 1 3 0 ° . 1 6 . 2 7 Q 4 . 11 • ° 5 • OP .-.1 = . 0 7 . 1 0 . 1 0 . 0 ? . .1 5 . 0 7 07 . 12 . 1 1 C 1 . I f . 19 . '4 . 2 4 . 1 3 . 1 3 . A 8 o 1 2 » i 9 2 7 19 . 1 5 . 2 2 "3 •5 . °? . 1 ! . 1 7 .",YB . 0 4 . 1 1 . 1 1 : i 6 . 1 6 ' " . 1 9 I P . 1 2 . 2 4 3 . 0 = . ! ( . 1 ' . 1 2 .•""i . 0 7 . I i . 11 . 2 0 15 . 1 1 . 2 3 D 4 . 17 . ] 6 . 1 1 . 1 1 . 0 5 . 1 5 . 0 4 . 0 8 . 1 6 11 . 2 0 . 1 6 r, 1 . 1 ° . 3 9 . I f . 1 3 . 1 5 . 1 1 . 0 4 . OP . 1 1 . 1 6 17 • 2 3 • 27 r 2 . 11 • 2 r . 1 ° . 1 3 . 1 1 .DP . 0 5 . 1 3 . 1 7 . 30 18 . 0 7 . 1 9 c 3 .<"4 . 1 1 . 1 1 .OP . 0 4 . 0 4 . 0 9 . 0 5 .1 3 . 1 6 0 7 . 1 0 . 1 8 4 . o : . I f . 1 1 . 0 5 . 0 5 . 0 4 . 1"8 . 1 3 rrr . 1 9 09" _ _ _ n »c W = Achromatic B = Blue G = Green R = Red APPENDIX III Scotopic Foveal Thresholds Stimulus -2 Log AL (cd.m. ) F i x a t i o n A*5 F i x a t i o n B c Highest Threshold Highest inresno±a Colour S i z e S Subj ect 1 2 3 4 Obtained X' a Achromatic 6.8 LL -1.1 -1.0 -1.6 -1.6 -1.0 -1.0 .22 AM -1.2 . -1.1 -1.0 -1.0 -1.0 JL -1.4 -1.3 -1.5 -1.4 -1.3 RM -1.5 -1.1 -1.1 -1.5 -1.1 KH -1.8 -1.7 -1.7 -1.7 -1.7 13.6 LL -1.5 -1.3 -1.9 -1.8 -1.3 -1.5 .26 AM -1.9 -1.5 -2.2 -2.2 -1.5 -JL -1.9 -1.9 -1.9 -1.9 -1.9 RM -1.6 -1.5 -1.3 -1.4 -1.3 KH -1.6 -1.3 -2.1 -2.2 -1.3 27.2 LL -2.1 -1.7 -2.1 -1.7 -1.7 -1.5 .81 AM -1.9 -1.9 -2.7 -2.5 -1.9 (-1.9) (.17) JL -0.3 -0.1 -0.5 -0.6 -0.1* RM -2.1 -1.8 -2.6 -2.5 - l i 8 KH -2.0 -2.1 -2.4 -2.3 -2.1 continued APPENDIX III continued Log AL (cd.m. ) •  : Highest Stimulus F i x a t i o n A^ F i x a t i o n B c Highest Threshold . Threshold 3 . • — Colour Size Subject 1 2 1 2 Obtained X a Achromatic 54.3 LL -2.4 -2.0 -2.5 -2..6 -2.0 -2.2 .15 AM -2.4 -2.5 -3.2 -3.1 -2.4 JL -2.4 -2.2 -2.3 -2.3 -2.2 RM -2.2 -2.1 -2.2 -2.1 -2.1 KH -3.0 -2.2 -2.9 -2.8 -2.2 Blue 6.8 LL -1.1 -1.0 -2.2 -2.0 -1.0 -1.1 .39 AM -0.7 -0.7 -1.3 -1.1 -0.7 JL -2.2 -1.6 -2.1 -2.2 -1.6 RM -1.2 -0.9 -2.0 -1.3 -0.9 KH -1.9 -1.5 -2.2 -1.8 -1.5 13.6 LL. -1.3 -1.5 -2.2 -2.2 -1.3 -1.4 .34 AM -2.3 -1.9 -2.8 -2.6 -1.9 JL -2.0 -1.7 -2.2 -2.0 -1.7 RM -1.4 -1.2 . -1.5 -1.2 -1.2 KH -1.1 -1.3 -2.8 -2.8 -1.1 27.2 LL -1.8 -1.9 -2.6 -2.7 -1.9 -2.3 .51 AM -2.7 -2.3 -2.8 -3.3 -2.3 (-2.1) (.29) JL -3.1 -2.4 -2.9 -2.9 -2.4 RM -1.8 -3.6 -1.9 -1.9 -1.8 KH -3.2 -3.1 -3.8 -3.8 -3.1* continued APPENDIX III continued Log AL (cd.m. ) Highest Stimulus F i x a t i o n A Fixation B Highest Threshold Threshold Colour S i z e 3 Subj ect 1 2 1 2 Obtained X . 0 Blue 54.3 LL -3.0 -2.8 -3.2 -3.2 -2.8 -2.6 .64 AM -2.4 -2.1 -2.4 -2.3 -2.1 JL -3.3 -3.2 -3.8 -3.5 -3.2 RM -2.4 -1.8 -2.4 -2.3 -1.8 KH -3.3 -3.2 -3.4 -3.6 -3.2 Red 6.8 LL . . -1.1 -1.1 -1.1 -1.1 -1.1 -1.0 .26 AM -1.0 -1.0 -1.0 -0.9 -0.9 JL -1.0 -1.0 -1.0 -1.0 -1.0 RM -1.3 -1.3 -1.2 -1.2 -1.3 KH -0.8 -0.6 -0.8 -0.9 -0.6 13.6 LL -1.7 -1.7 -1.7 -1.7 -1.7 -1.5 .21 AM -1.2 -1.4 -1.3 - i ; 2 -1.2 JL -1.6 -1.7 -1.6 -1.6 -1.6 RM -1.6 -1.6 -1.5 -1.5 -1.5 KH -1.3 -1.3 -1.3 -1.3 -1.3 27.2 LL -1.9 -1.8 -2.0 -2.0 -1.8 -1.8 .07 AM -1.9 -2.1 -1.8 -1.8 -1.8 JL -2.0 -1.9 -1.8 -1.8 -1.8 RM -1.9 -1.9 -1.9 -1.9 -1.9 KH -1.7 -1.8 -1.8 -1.8 -1.7 continued APPENDIX III continued Log AL (cd.m. ) ' — — 5 : c Highest Stimulus F i x a t i o n A Fixation B Highest Threshold :—: Threshold — Colour S i z e 3 Subject 1 2 1 2 Obtained X O Red 54.3 LL -2.2 -2.2 -2.3 -2.2 -2.2 -2.1 .05 AM -2.2 --2.4 -2.1 -2.2 -2.1 JL -2.2 -2.2 -2.2 -2.2 -2.2 RM -2.1 -2.1 -2.2 -2.1 -2.1 KH -2.2 -2.2 -2.3 -2.1 -2.1 LL -1.0 -1.2 -1.2 -1.1 -1.0 -0.9 .37 AM -.07 -0.5 -0.6 -0.3 -0.3* (-1.1) (.12) JL -1.3 -1.2 -1.5 -1.4 -1.2 RM -1.0 -1.0 -1.1 -1.0 -1.0 KH -1.7 -1.2 -1.9 -1.4 -1.2 LL -1.5 -1.5 -2.2 -1.9 -1.5 AM -0.9 ^0.9 -1.0 -0.8 -0.8 JL -2.2 -2.1 -2.5 -2.4 -2.1 RM -1.3 -1.3 -1.4 . -1.4 -1.3 KH -1.7 -1.5 -1.6 -1.4 -1.4 LL -2.5 -1.8 -2.8 -2.7 -1.8 -1.9 .54 AM -1.2 -1.6 -1.9 -1.7 -1.2 (-1.7) (.31) JL -2.7 -2.7 -2.9 -2.9 -2.7* RM -1.8 -1.7 -2.4 -1.9 -1.7 KH -2.6 -1.9 -2.5 -2.0 -1.9 continued APPENDIX III continued Log AL (cd.m. ) - Highest b c Stimulus F i x a t i o n A Fixation B Highest Threshold Threshold Q _ Colour Size Subject 1 2 1 2 Obtained X a Green 54.3 LL -2.2 -2.2 -2.7 -2.6 -2.2 -1.9 .61 AM -2.1 -1.6 -2.4 -2.5 -1.6 JL -2.9 -2.8 -1.0 -1.0 -1.0 RM -2.3 -1.9 -2.4 -2.2 -1.9 KH -2.7 -2.6 -2.9 -2.8 -2.6 Size i s i n minutes of v i s u a l angle -2 o F i x a t i o n Devices, L < .15 cd.m. , 2 v i s u a l angle F i x a t i o n Devices, L < .15 cd.m. ^, 3.4° v i s u a l angle Denotes a highly i r r e g u l a r threshold which was excluded to y i e l d the Mean and Standard Deviation i n brackets. 135 A ° P E N D I X i v f , T « * : ? . . . A N n > S T A N D ^ D D E V I A T I O N S F O R F U L L Y - S c T T O P I C 7 > D r S H " L r > 5 M E A N S (loe. cd.m. ) ' E C C E N T B I C T T Y NASAL T-MOnr / i i . 3 0 2 0 1 5 i n 0 5 1C 2* 2 5 T T « 1 - ? . ' - 2 . •• - 2 . 2 - 2 . ' - ? . 2 - 2 . 2 - l . C - 2 . 1 - 2 . 1 - 1 . 9 - 2 . 1 - 2 . 1 - 2 . 3 u •) - 2 . <• - ^ • r - 2 . 7 - 2 . P - 2 . 7 - 2 . 7 . - 1 . 5 - 2 . 5 - 2 . 7 - 2 . 5 - 2 . 8 - 2 . 6 - 2 . 6 V 3 - 3 . - 2 . 2 — n n - 3 . 4 - 3 . 4 - 3 . 2 - 1 . 5 - 3 . 1 - 3 . 3 - 3 . 1 - 3 . 3 _ •» i - 3 . 3 w - 3 . 6 - 3 . 6 - 3 . 8 - 3 . e - 3 . o - 3 . 7 - 2 . 2 - 3 . 6 - 3 . P - 3 . 6 - 3 . 8 - 3 . P. - 3 . 7 •i 1 — 7 *3 - 2 . 4 - 2 . f - 2 . 7 - 2 . 7 - 2 . 6 - 1 . 1 - 2.5 - 2 T 6 - 2 . 4 - 2 . 5 - 2 . 5 - 2 . 4 a n - ? . 9 - 3 . "> - 3 . 1 3 . 3 - 3 . 2 - 3 . 1 - 1 .4 - 3 . 0 - 3 . 1 - 2 . 7 - 3 . 1 - 3 . 1 - 3 . 0 3 - 3 1 - 3 . 6 - 3 . 7 - 3 . P - 3 . P - 3 . 6 - 2 . 3 - 3 . 4 - 3 . 7 - 3 . 5 - 3 . f , - 3 . 6 - 3 . 6 4 - 3 . P - 3 . 9 - 4 . ; - 4 . 4 - 4 . 2 - 4 . 1 - 2 . 6 - 4 . 1 - 4 . 3 - 4 . 1 - 4 . 1 - 4 . 1 - 4 . 1 r. 1 - . f - . 7 _ t c - 1 .r - 1 . r - 1 .'< - 1 . " - . 9 - • 9 - . 8 - . 8 - . 9 - . 8 2 - 1 . " - 1 . ' - 1 . 5 ' -1 .<• - 1 . 6 - 1 . 5 - 1 . 5 - 1 . 4 - 1 . 2 - 1 . 3 - 1 . 2 - 1 . 2 3 - ! • fc - 2 . 1 - 2 . 1 - 2 . 1 - 2 . ' . l - ! . 8 - 2 . 3 - 2 . 1 - " I T " - 2 T P - 2 . r - l . a ? -2.2 - 2 . 4 - ? . c - 2 . 7 - 2 . 6 - 2 . 6 - 3 . 1 - 2 . 5 - 2 . 6 - 2 . 3 - 2 . 5 - 2 . 5 - 2 . 4 1 - I . e . - ' . 1 - 2 . 4 - 2 . 4 - 2 . f - 2 . 4 - 0 . 5 - 2 . 3 - 2 . 4 - 2 . 7 - 2 . 4 - 2 . 2 - 2 . 2 r. - 2 . 7 - 2 . 7 - 2 . " - 3 . 1 - 2 . 9 - 2 . 1 - 1 . 4 - 2 . 3 - 2 . 9 - 2 . 7 - 2 . 9 - 2 . 8 - 2 . 8 r. 3 - 3 . " - 3 c 2 - 3 . 5 - 3 . 6 - 3 . c - 3 . 4 - l . C - 3 . 4 - 3 . 5 - 3 . 3 - 3 . 4 - 3 . 3 - 3 . 3 4 - 3 . < - 3 - . 0 - 4 . 0 - 4 * 1 - 4 . 0 - 3 . 9 - 1 . 9 - . 3 . .3 - 3 . 9 - 3 . 8 - 4 . A - 3 . c - 3 . 3 S T A N 0 A P n D E V I A T I O N ? w l . 13 . 1 6 . 1 5 . 1 5 . 11 . 1 3 . 22 . 1 5 . 1 1 . 1 5 . 0 8. . 1 3 • K . w ? ( n 1 i . 1 c . 1 9 . 1 1 . 3 4 . 26 . 19 . 1 4 . 1 t . 0 9 . 1 1 . 3 1 . V . 2 ' ' . "'P. . 1 * . 1 1 . 11 . I t . 1 7 . 35 . 1 3 . 1 1 . r c . 11 . 3 r 4 . 15 . 2 2 . 1 0 . 0 5 . '. 6 . 0 7 . 1 5 . 12 . 1 5 . 1 9 . 1 2 . 0 9 . 1 1 • 1 . > 8 . 1 2 . 2 3 . 1 3 . 1 * . 0 9 . 39 . 2 3 . . 14 . 1 3 . 1 4 . 1 9 . . 1 6 p 2 " . 1 c • 1? . 1 2 . 1 5 . 1 6 . . 3 4 . 3 4 . 1 6 . 1 3 . 6 5 . 1 4 . 2 3 . 1 r H 3 . 13 . 0 4 . 11 . 1 2 . ,08 . 1 3 . 5 1 . 2 1 . 1 1 c . . 0 9 . 1 4 . 1 9 H . •>? . 1 P . 0 * .1 2 . 14 . 19 . 64 . 1 P. . 0 7 . 17 . 2 4 . 2 2 , i " i . 21 . : l . 1 ? . - i f . 1 1 . 0 B . 26 . 3 8 . S • "• F T . 1 1 . . 1 3 1 . 1 4 . 1 3 . 1 8 . 1 7 . 1 ° • I s . 21 . 79 . 0 9 . 0 5 . 0 4 . 0 5 . 0 7 ^ 3 . 1 ° . 1 1 . 1 1 . 1 ! . ! 5 . 0 4 . 0 7 . 1 6 . 1 1 . 3 1 . 1 c . 1 5 . 1 5 V . I P . 1 9 . 1 3 . 1 1 . 1 3 . 1 1 • ,rs . 1 3 . 1 3 . 4 3 . 1 ° . 1 3 . 1 3 r. ] . 25 . 1 7 . 2 1 • OB . 1 ? .05 . 37 . 1 7 . 1 2 . n c . O P . ! " 4 . 1 1 r. 2 • C9 . 1 0 . 1 3 . 1 1 . 0 4 . n o . 4 7 . 1 9 . 1 1 . 1 2 . 1 5 . 11 . 1 5 3 . p 5 . 11 . 1 5 . 1 ! . 1 1 . 1 i . 5 4 . 1 9 . 1 3 . . 2 2 . 1 n . 1 5 . ? C T 4 . n ? • .ne . ! ° . 0 4 . n o . 1 9 . 6 1 • 1 6 . 0 9 . 2 7 . 1 6 . 1 6 . 1 1 W = Achromatic B = Blue G = Green R = Red 136 > o c: a <7 Q O O CC CC CO lf-| r- ro 0 • '• • 0 • • o c.i If. vC ro • • i • 0 « • CM CM •4 •4-ICS i n in r -« • • « 0 • • (\J CXI Ov! <)• :;• O ro V IP. • • : • 0 . « • CM -0 vC CO r o • 0 • • CX' a ; rr ex. CU c c 4 • • i » • • • r\i r". I I 14 CM I I r i r * ' • - , <M vD <f vi.' iro ct O 1 CM c o 00 CO O •4- :C0 o r-l 00 vO a - •vt r-< Z' o s 1 CO i r s ro CC' vO (O CC Cvj o r4' ' i— t r- > r-l CM c r\.i CM rH t— i l_J t 0 • • 0 • • • • ! 0 • .• 0 t • O • • ; t • 0 • • 0 • Q X 1 i.ij. LT\ 1 CM CO a : c n vt rvi •4- VI 1 4- vfi CM vO n i r-l a - ' -1- CM r-l rvi 1 r ~ v r c r i m vr a LA o - r o vt (M CM r—1 rvj f \ i rvi r-l C.i 1 o • • O • « • • ' • « « 0 « • 0 • • ; • • • • • a « r- • ;_J 1 <l 1 o : | : c c . | vO v. VO vt vC vt CO •4" vij vt i n O r-l Cxi CC a - vU er- r~ CM 11- n CO r- r- ;vd- vi) o r \ i . fM m r-.i m r s : r-i — i 1 ° • • 0 • « 0 • • • 0 * • 0 • • 0 t • • • 0 t ! l_> i r-« o 1 c v vO <* rM CO o <4- rvi o i O w. i—t f—1 4" Cv.l rvi o a T-l 1 0 - n CT- r - f s vO -0 r- i n r-i r-l c— »—H •—i -I CM C: 1 0 • • 0 • • 0 • i • ' « • 0 • • 0 • • \ 0 • • « • 0 « c ; 1 . I ct >- 1 vD o rvj c: 4 : o SO vO vO vU CM o ir\ PI ro i n a; 0' 1 1- 1 -4' vO ~o CO •4 o •4" r--l r-1 rM rvi r-i CM r—1 rH rH > 1 o • • 0 • 0 • i • • • 0 - ' • 0 * • : o • • • • 0 » o | <T _J j r--l CC! | > u . « a r o 1 CC or.' vO vt< r\l a: ro r>i r\i CM 0 1 ro (X) CM x> r i o-li" t CM <>"i -4 CO <) O i -4" ^c^ rvj a rM CM r-* CM rM *- i H -4 u U ' ( • ' • • 0 • • 0 • • . • 0 • • o • • ! 0 • • • • 0 • o lj> 1 C' l i - o | CI' Ll) | <x e n i n 1 o vf> c c vt a ) vO S o CM c » rM C, a - CO IT. i—i ~o 00 O rH 1 -4 ITl r o i n r o oc r - — i—i fM O r-1 —< r-l rvj b | « • • 0 • • 0 • : « « « 0 < i • • 0 • • • • « • 0 • v; C CO c c vi. (• v.: vij r-- - o - t r - ' -a o o 0^  r-4 ' CM ro I I I CM C -O CM a . ' u 0 • C^  J- rv. ro I I I jrf •  •-. r i c . c s c : IT, C: r-i CM r - t o c o or n -r'l CM IM CM a s CM r-l Tv) r-H r-l" r-l cx, ro rvi I I r-i r ' r - CM m CM r.l •4 rvj 0 t • 0 . • •4- r-i a-r-i r-l (' r~- CM oi p' vi..i rr-r - , r-1 r l H Ol (' C 0 f4' CM IX, H r-: C h-1 C-J C . g • • r-1 r>" O ' f\ fO r-?. c or a IT. 7s 00 c • »—1 'r-i • • i • if'-, ".1 OS • 0 • 0 M r--i • 0 ' • CO 1 4 • 0 1 1 1 CM 1 fl 1 0 y C': 137 1 s t r\r O o •4 C C ' v t C M o CJ o s t 1 cr C \ l r- a- rH C O o- C O 0 0 a s t O' 0 0 v O rH I-l c rH 1 • 0 • • • • 0 t • a • • s t U l C O r-. CJ C s l rH C M Co C l J r—' r-H r-1 0 • • t • 0 « • 0 • • 0 C O o 1 C D C M s t o C M s t cr C M CJ X ) ro 1 co L . ) O i l ' CC C M on o C O C T rH C O , o li- c an s t O J o s t | • .0. • • • • 0 • • 0 • • r-l C M CI s t CI rH C M rH co. C 1 ! J . i . I rH 0 • • m • 0 • • U • • 0 C, CL in 1 s O C M cr. O f\j -C M s t o 0 0 0 0 s t C M I o 0 0 C O C C ( < I r- C C r- C.i c- in s i - C J cr- CXI) C M C O r-l rH CT' L : 1 • o • • • • 0 • • 0 • • r-( C I on re C M ro. rH C.i rH C i C i C O , i »- • r-l r - l O » • • • • • • O t • 0 l C J j »T'i cr C- 1 o C M C M C M C . J CO C M co 0 0 0 J I" O I r C.i in C O 0 0 r~ 0 ' c; •c C l ' • , s t co, a: C I r-l CO C i r-<I i • a • • • • • * • 0 • C M r i re] o r-i s t r-l LC. in r-l C l < t In- 1 rH H 0 « • o • • • • 0 t • 0 5 in j r-| r-l 5 >- j c 1 (VI s t s t o 0 0 DO o ro s t C M rH 1 cr o o rH a: 0 0 C ^ s - m cr ( M -r-l o in C M s t C O C . J 1 • o • • 0 • • • • o » • C M r st rH r-l CJ CI rH o CJ rH j r—1 rH 0 • • 0 • • • • O • • 0 CL 1 r~> 1 t- > 1 C J cc C M o C C M O C M C ' c: t—t O h- 1 cb cr C C r- rH CO CO C O ~4J r-i 0 ' CI a: o m s t CJ c r- rH ;> l_j •~. ' i • 0 • • 0 • • • • • a t . t - r-4 C ! C M C O C M CJ rH CO rH s t in C J 1 r-l < t O • • 0 • • • • 0 • • 0 X 1 L/l rH I >- n :i > O _ J c. | C M in C o st •3" C U CO C O sr C M LU —~ i 1I ' ir- o r-* U 0 -d CO L O , C P . f-- r-- r- \ C O r - If. C D CO CT' C O r-l L.'• 1 * o « • 0 • • • • 0 » m m <r0 o C O C O C I rH rH - 4 J Q . u. C J 1 1 rH rH ' C J 0 • • 0 • • • • 0 • • 0 o_ C J | Ci: r-l r-i r- 1 rH f * 4 L 1. u- | < cy LT, j CM C O o C ' o C M C M u 1 fr. CC a-, 0 0 0^ a: o r-- cr xi • z.' i. •—I C M ( O l O s t m o ' CI s t C l in [ • CI # . • o • • • • 0 • • <r f l r-l r-l 1—1 C M s i CI ( M C M o rH C l C O j . 0 1 • 0 • • t • 0 • • 0 cC J i ci v f ; cr C C ^ C M -JO s t S i s i ' s t 0 1 r* 1 1 CO r-* r- r- C ' •X) a-' cc O 0 - ' . C O in s t o: C O in oo C i r -•CI 1 • 0 • • • 0 m • t • 0 • • oo rH C M O rH C M C M C O C M r-l rH r-i I H 1 r-« r-4 o • • 0 • • • • • • • O > [ O LT\ 1 ' 0 >4J c; C X I o cv C O v O r\i _ J rH 1 (T c- 0 a- r- 0 o' * ' CO a, CXI V 0 " s t r-l »—l CO r-H C T ' CO r-l c: 1 • 0 • • 0 • • • • • C M r~. r- | ,—t C M co C l r-i r-l rH r-l a. | r-l O • • 0 • • 0 • * • • 0 <: i c, j ~'' c. I oo s t s t •0 C M 0 0 a. C O o CJ 1 Qv U- co c- a- a- C O s t C O c. " 1 0 11 U ' I r-l 4 > v L ' CO C M C \ i r—1 s t 1 i • 0 • • o • • 0 • • • • C M 1 r-i C J C vO C M C l ( V . C O i r- i rH O • • a • • a • • • • o o j r-l r-l | C J s t C M c C J CJ s i C C CO 1 r--C M h- o- a- O c C J ' r - rH •4.'' 0 •c< s i J O r-i C M 0 C M 1 * 0 • b • • • • 0 • • t • C .I C V I CI C l C ' C l m C M rH C M .—1 ^ ; j '-' >-H 0 • • a • 0 • • • t 0 < j 1 j - 1 c s t s t <t c: C O c\, CM 0 , 1 2 ' s f 1 r-c c co 1—1 C M C s i r-l r o- rci o s t o C M I*.. 1 • 0 • 0 • • 0 • • • • C C.I CJ CI s| C.i Cv C O s l - CJ 1 rH r-H rH rH • • • 0 • • 0 • • t • 0 i C ' 1 i c\i 1 cr, 1 i C N . ' 1 rri. • s » 1 C M r<o i 4 1 C M cv <!• 1 C I ro . i s i C ! C i 1 s i C l C l i s t . 1 r-1 1 c. i : < . I f—J 1 C M i o> 1 rH l C M 1 fl-i I i C M rf i. 1 1 C - l 1 f<"; 1 r~ • i r-i 1 fC. 1 r-l 1 C.I 1 C O . 1 •—1 1 C l 1 fC : « • 3 CO CO cr Cv- 0 c o _£ :x C J a _jC (1 C Y Ct CO C O L 1 3 138-Appendix VII Cal c u l a t i o n of Correction Factors for Photometric Equating of Chromatic F i l t e r s The s e n s i t i v i t y of the Pritc h a r d photometer used i n the present i n v e s t i g a t i o n deviates from V^, and the difference between the instrument 1 s p e c t r a l response curve (V^) and V^ must be taken into account by correction factors which are applied to the instrumental readings. The formula used to c a l c u l a t e these c o r r e c t i o n factors was the following: C r — ti where CF= the cor r e c t i o n factor which when m u l t i p l i e d by the instrumental reading y i e l d s the corrected Y value. T^= transmission of the f i l t e r at the wavelength i . Y = the value of V, at the wavelength i . i A Y\= the value of V? at the wavelength i . 1 A The values of T. were obtained using the Zeiss RFC-3 Automatic 1 Colorimeter, an automatic colorimeter. Interpolation of some values was required as the RFC-3 makes measurements at 13nm i n t e r v a l s , while the values of T^ at lOnm i n t e r v a l s were required. The values of Y^ were obtained from S t i l e s and Wyszecki (1967). Judd's 1951 correction of V^ i n the blue wavelengths was used. The values of Y^ 3 were obtained from the instrumental s p e c t r a l i response data supplied with the Prit c h a r d photometer. 

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