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Retention of three brightness discriminations by rats following posterior cortical lesions Tryggvason , Svavar 1972

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RETENTION OF THREE BRIGHTNESS DISCRIMINATIONS BY RATS FOLLOWING POSTERIOR CORTICAL LESIONS by SVAVAR TRYGGVASON B.A., University of British Columbia, 1966 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF ARTS in the Department of Psychology We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA April, 1972 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 advanced degree 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 ag ree 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 and s t u d y . I f u r t h e r ag ree 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 Depar tment 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 . Depar tment o f Psychology  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 V a n c o u v e r 8 , Canada Date April 21, 1972 i ABSTRACT Rats were trained on one of three brightness discriminations. In one task, the discriminanda differed i n both luminance and luminous flux . In the second task, the discriminanda differed only i n terms of luminous flux . In the t h i r d task, the discriminanda differed only i n terms of luminance. Following acquisition, half of the animals on each task underwent removal of the s t r i a t e cortex. Retention tests indicated that a discrimination based on flux cues was r e l a t i v e l y undisturbed following s t r i a t e removal, whereas a discrimination based on luminance cues appeared to be permanently l o s t . Transfer discrimination tests indicated that d e f i c i t s other than sensory impairments may follow s t r i a t e ablation. Results are discussed i n terms of sensory and attentional d e f i c i t s which occur with s t r i a t e removal. i i TABLE OP CONTENTS page Abstract - i Table of Contents i i L i s t of Tables i i i L i s t of Figures • • i v Acknowledgments v Introduction 1 M e i t h o d 6 Subjects Apparatus Surgery and Histology General Procedures and Tests Luminance-Luminous Flux Task Luminous Flux Task Luminance Task Results , 12 Discussion 23 References 33 i i i LIST OF TABLES page Table I. Median Errors to C r i t e r i o n Preoperatively and Postoperatively for Animals on A l l Three Tasks 13 Table I I . Luminance Transfer Tests on the LF Task 14 Table I I I . Luminous Flux Transfer Tests on the LF Task 16 Table IV. Transfer Test Results on the F Task 18 Table V. Transfer Test Results on the L Task 19 i v LIST OP FIGURES page Figure 1. Diagramatic representation of c o r t i c a l damage and dorsal l a t e r a l geniculate nucleus retrograde degeneration 21 V ACKNOWLEDGEMENTS I would l i k e to express my deepest gratitude to Dr. Richard C. Tees, without whose helpful criticisms and moral and f i n a n c i a l support t h i s work could not have been accomplished. I would also l i k e to extend my thanks to Dr. David J . Albert for h i s help i n the preparation of th i s manuscript, to Mr. Frederick J . Madryga for >his moral encouragement and influence i n my graduate education, and to my wife, S y l v i a , and our two children, who have suffered through my many lengthy absences from home. 1 Numerous experiments have demonstrated that brightness discriminations can be learned without the st r i a t e cortex i n the rat (Lashley, 1935b; Thompson, 1960), cat (Smith, 1937), dog (Maquis, 1934), and monkey (Kluver, 1942). However, i n the normal animal i t i s clear that the st r i a t e cortex i s involved i n such learning since removal of this area results i n complete loss of a preoperatively learned brightness discrimination (Eashleyj 1930, 1935b; Smith, 1937; Bauer and Cooper, 1964; Schilder, 1966; Pasik, Fasik, and Schilder, 1969; Schilder, Pasik, and Pasik, 1971). Some studies, using emitted rather than reflected l i g h t (Bauer and Cooper, 1964) or amphetamine therapy (Braun, Meyer, and Meyer, 1966; Jonason, Lauber, Robbins, Meyer, and Meyer, 1970), have not obtained complete loss; however, operated animals were s t i l l generally found to be i n f e r i o r to intact controls. Two models have evolved to account for the postoperative d e f i c i t s i n performance. Both are based on the finding that normal and operated animals use different cues to solve a brightness discrimination. Although under some special circumstances intact animals may use flux cues (Bauer and Cooper, 1964; Schilder, Pasik, and Pasik, 1967), intact animals normally use some form of pattern v i s i o n and/or luminance cues* to solve such discriminations, while operated animals depend upon luminous flux cues* (Kluver, 1942; Bauer and Cooper, 1964; Goodale and Cooper, 1965; Schilder, 1966; Pasik et a l , 1969; Bland and Cooper, 1970; Schilder et a l . 1971). Cooper and his coworkers have postulated that while normal animals make at least some use of pattern v i s i o n to solve brightness discriminations, operated * Luminance i s defined as the amount of l i g h t per unit area of the stimulus. Luminous flux i s defined as the to t a l amount of l i g h t emitted by or reflected from the stimulus. 2 animals are unable to u t i l i z e pattern cues and essentially have to relearn a discrimination on the basis of a different cue, i . e . , flux (Bauer and Cooper, 1964; Goodale and Cooper, 1965; Bland and Cooper, 1970). The evidence cited for this position i s twofold. F i r s t , the stimulus parameters used i n origi n a l learning can affect postoperative performance (Bauer and Cooper, 1964). Second, the performance of operated animals i s impaired to a greater extent than that of normal animals when stimulus conditions are changed, e.g., with increases i n background illumination (Bauer and Cooper, 1964) or decreases i n the luminosity differences of the two discriminanda (Lashley, 1930). The two lines of evidence suggest that sensory d e f i c i t s may account for the postoperative d i f f i c u l t i e s of operated animals on brightness discriminations. Hamilton and Treichler (1968) u t i l i z e d Lashley 1s (1935b) hypothesis of two visual systems. They postulated that brightness discrimination i n the normal animal involves acuity and intensity mechanisms located i n the st r i a t e cortex. Operated animals have available only a subcortical intensity mechanism i and must use essentially different structures, i . e . , subcortical areas, to solve such discriminations postoperatively. The evidence supporting this position i s twofold. Some researchers (e.g., Fischman and Meikle, 1965) have shown that secondary destruction of some subcortical areas can cause further disruption of a habit previously disturbed by s t r i a t e removal. In addition, Hamilton and Treichler (1968) found that, although a relevant pattern discrimination f a c i l i t a t e d o r i g i n a l acquisition of a brightness discrimination, the use of such a cue preoperatively did not appear to affect postoperative performance. The above, together with their finding of no postoperative savings, lead them to suggest that pre- and post-operative learning are essentially independent processes, an idea also advanced by Horel, Bettinger, Boyce, and Meyer (1966) and by Jonason et_al (1970). These two lines of evidence indicate that pre- and post-operative learning are subserved by different and independent neural mechanisms. The necessity of switching from a c o r t i c a l to a subcortical system following s t r i a t e removal i s therefore presumed to underly postoperative d e f i c i t s i n performance, rather than any sensory d e f i c i t s which occur following s t r i a t e removal. Further research i s needed to elucidate the nature of the d e f i c i t in^per-formanee, p a r t i c u l a r l y ' i n l i g h t of some research which cannot be adequately accounted for by either of the two models. For example, Thompson (1969), among others, has demonstrated that some subcortical lesions can result i n postoperative d e f i c i t s similar to those following s t r i a t e removal. Braun et a l (1966) and Jonason et a l (1970) have shown that amphetamine treatments can f a c i l i t a t e the reestablishment of a brightness discrimination following s t r i a t e removal while having no effect on origi n a l acquisition i n operated animals. They suggested that amphetamine f a c i l i t a t e d access to a secondary engrain based on flux cues that had been l a i d down at the same time as a primary engram, based on edge-related cues. Striate removal prevented i n some fashion access to this secondary engram. These studies imply that both c o r t i c a l and subcortical regions are involved i n or i g i n a l acquisition and argue against both purely sensory d e f i c i t s explanation and an explanation postulating independent vis u a l systems subserving pre- and post-operative learning. Lashley (1931) and Bauer and Cooper (1964) have reported that a discrimination based on l i g h t vs no l i g h t can result i n high postoperative savings i n rats. Bauer and Cooper (1964) also were able to obtain high savings by the sewing of trans-lucent cups over the eyes of experimental animals. Such a procedure not only would eliminate pattern v i s i o n but also probably force an animal to r e l y on the density of l i g h t or flux i n order to solve a brightness discrimination. It seems possible that acquisition of a light-no l i g h t discrimination may also be 4 based largely on flux rather than pattern or luminance cues. If such i s the case, then i t appears that a preoperatively acquired flux discrimination suffers minimal postoperative disruption. Such savings would argue against the notion that normal animals u t i l i z e only c o r t i c a l mechanisms i n learning brightness discriminations. The present experiment i s an attempt to test the predictions of the two models on postoperative retention of two brightness discriminations. One group of rats was trained on a luminous flux discrimination. The Bauer and Cooper (1964) hypothesis would predict that the discrimination would not be disturbed by removal of the s t r i a t e cortex. Since flux i s the cue u t i l i z e d by operated animals, the sensory impairments occurring with s t r i a t e removal would not affect postoperative performance. Hamilton and Treichler (1968) would predict complete loss of the habit, since s t r i a t e ablation would remove the c o r t i c a l mechanisms upon which learning was based. However, they would also predict that the discrimination would be relearned by operated animals i n no less than the ori g i n a l number of t r i a l s . A second group of animals was trained on a luminance discrimination. Both models would predict complete andspermanent loss of the habit postoperatively, since luminance i s a cue that s t r i a t e lesioned animals are unable to u t i l i z e . Since the discriminanda were presented in a manner not employed by previous studies examining the d e f i c i t s following s t r i a t e removal, an additional discrimination was presented to a t h i r d group of ra t s . In this task, luminance and luminous flux covaried and both were available as cues for solving the discrimination. This was done i n an attempt to replicate previous findings with a black-white task (Thompson, 1960; Horel et a l , 1966) and with situations using translucent doorways illuminated from behind (Schilder, 1966). Previous experiments attempting to test flux discrimination i n the rat 5 have used translucent goal box doorways d i f f e r i n g i n area and illuminated from behind. Differences i n flux were obtained by equating luminance values for the two doors (Goodale and Cooper, 1965; Bland and Cooper, 1970; Tees, 1971). The present experiment u t i l i z e s point sources as the discriminanda to avoid pre-senting size as a relevant cue during original learning. Since luminance does not vary with distance while illumination or density of flux f a l l i n g on a surface varies inversely with the square of the distance (Riggs, 1965), placement of two such sources equated for luminance at different distances from the subject would result i n a flux difference for the two sources. Conversely, two sources of unequal luminance placed at predetermined different distances from the subject would result i n a luminance difference but equivalence i n luminous flux. 6 METHOD Subjects The subjects were 60 naive male Long-Evans hooded rats obtained from Canadian Breeding Farms and Laboratories, Ltd., They were individually housed in 8 x 7 x 10 i n . cages and maintained on ad l i b food and water. Apparatus Subjects were trained in a two-choice shock avoidance discrimination box. It consisted of a start box (11 x 6 x 5 i i n . deep) with a g u i l l o t i n e door, a choice chamber (22 x 18 x 5 i i n . deep), and two goal boxes (8 x 8-J x 5 i i n . deep). The floors of a l l chambers were constructed of \ i n . brass rods, spaced | i n . apart, center to center. The walls and top of the star t box and the walls of the choice chamber were constructed of wood and painted a f l a t black. A wire gri d formed the top of the choice chamber. The side walls and hinged top of the goal chambers were constructed of wood, painted a f l a t black, and covered with a black velvet cloth. The end walls were of clear Plexiglas. The wooden p a r t i t i o n separating the goal boxes extended 4 - J i n . into the choice chamber and this extension was also painted a f l a t black. Swinging clear Plexiglas doors provided entrance to the goal boxes. The gri d floors of a l l chambers were independently wired to a shock gen-erator with a scrambler c i r c u i t . Scrambled shock was used i n the choice chamber. Unscrambled shock was used i n the star t box and i n the grids i n front of the goal boxes, over which the p a r t i t i o n extended into the choice chamber. The shock level varied from 1 4 to . 7 ma., depending upon the level needed to e l i c i t avoidance behavior from the particular subject being run. The goal box grids were activated i n such a way that a subject would receive shock auto-matically when he approached the incorrect doorway. To aid recording of responses, a small neon bulb was placed i n series with each of the goal box 7 grids. When a subject stepped onto a wrong g r i d , thus closing the c i r c u i t and receiving a shock, the appropriate bulb would f l a r e . The bulbs were placed below the level of the testing chamber and were out of sight of the subject. A partitioned box (52 x 18 x 5\ i n . deep), similar to that used by Crawford (1935), was constructed to house the stimuli. The side and back walls, f l o o r , dividing p a r t i t i o n , and hinged top were constructed of wood, painted a f l a t black, and covered with a black velvet clot h . The front wall was open. This chamber was placed against the training box so that the open end of the l i g h t chambers faced the Plexiglas wallsof the goal boxes. The discriminanda were clear 7 watt bulbs mounted on wooden blocks that had been painted a f l a t black and covered with a black velvet clot h . A groove was bored down the center of the f l o o r of each l i g h t chamber. A small s t r i p of wood was glued to the bottom of each block, such that, when this s t r i p was f i t t e d into the floo r groove, the blocks holding the lights were always centered within the l i g h t chambers. The cords for the lights were passed through holes bored into the back walls of the chambers. Power sources for the lights consisted of four voltage transformers (two General Radio Variacs and two Superior E l e c t r i c a l Co. of B r i s t o l Powerstats) set at predetermined values. Aside from the two discrimination l i g h t s , the only l i g h t source within the testing room was a dim red bulb, placed well below the level of the testing chamber, which was used to illuminate the scoring sheets. Luminance values were measured i n n i t s with a Pritchard Photometer (Photo Research Labs.). Illumination of density of flux readings i n lumens per square foot were obtained at the doorways to the goal boxes with a Lightmaster Photometer (Evans Electroselenium, Ltd.). Flux values were calcu-lated by multiplying the obtained readings by the areas of the doorways (16 sq. i n . ) . The black velvet covering i n the l i g h t chambers and the goal 8 boxes served to maximally reduce r e f l e c t i o n of l i g h t from the walls of the chambers. This procedure resulted i n illumination readings that followed the inverse square law f a i r l y closely for the f i r s t three feet of the l i g h t chambers. Beyond this point, readings were somewhat lower than would be expected from the inverse square law. The values used beyond t h i s distance were those obtained, where possible, with the meter rather than the calculated values. Surgery and Histology Surgery was performed under sodium pentobarbital anaesthesia. The dorsal surface of the s k u l l was removed from approximately two mm behind bregma to lambda, and from approximately one mm l a t e r a l to the s a g i t t a l suture to the zygoma. Suction was used to remove the exposed c o r t i c a l tissue. After completion of the experiment, operated animals were perfused with 0.9% saline followed by 10% formal-saline. Brains were removed and placed i n 10% formal-saline for several days. Diagrams were made of the extent of the lesions. Brains were then placed i n 15% alcohol for at least 24 hours and then freeze sectioned at 30 microns. Every fourth section was saved, stained with cresyl v i o l e t , and examined for c o r t i c a l and subcortical damage and for retrograde degeneration i n the dorsal l a t e r a l geniculate nucleus. General Procedures and Tests Pretraining procedures were identical for a l l animals. On a l l pretraining days, illumination was provided by the two discriminanda to be used i n the flux discrimination, placed equidistant from the goal chambers and so equated for both luminance and luminous flux . On Day 1, subjects were placed three at a time into the testing apparatus and allowed to explore i t for 15 minutes. On Day 2, subjects were trained to leave the start box within f i v e seconds of the g u i l l o t i n e door being opened and to enter r either of the two goal chambers within 20 seconds of entering the choice chamber. Failures were punished with 9 shock. Cr i t e r i o n was three consecutive non-shocked runs. On Day 3, subjects were again trained to run into the goal boxes. However, the doors to the goal boxes were gradually closed over the f i r s t f i v e t r i a l s , so that the rats began to learn to push aside a door to enter a goal chamber. C r i t e r i o n was three consecutive non-shbcked runs, a l l of which required pushing open a f u l l y closed door. On Day 4, subjects began training on one of the three discriminations. Twenty subjects were run on each task, with no rat being used for more than one task. For a l l tasks, the stimulus having the higher luminance and/or flux value was the correct stimulus. Each rat was dark-adapted for about 20 minutes before being run. The incorrect doorway was then locked and the gri d i n front of i t activated. Subjects were run for 10 t r i a l s a day, with an i n t e r t r i a l interval varying from 90 to 150 seconds. Position of the correct stimulus was alternated according to a GeHerman (1933) series, a t o t a l of seven such series being used over consecutive days. C r i t e r i o n was 18/20 over two days. Within each task, rats were systematically divided into two groups matched for mean learning scores. One group on each task was subjected to removal of the s t r i a t e cortex 24 hours after reaching c r i t e r i o n . Following a 14 day recovery period Ifor operated animals and a 14 day resting period for controls, a l l subjects were rerun on the o r i g i n a l discrimination u n t i l c r i t e r i o n was reattained. Following this retesting period, a l l rats were run for a further four days on transfer tests. Ten t r i a l s were run on each of these days with the or i g i n a l discrimination alternating with a new transfer discrimination. On transfer t r i a l s , both doors were unlocked and no shock was administered for "wrong" responses. The spe c i f i c transfer discrimination used depended upon the or i g i n a l discrimination and these w i l l be detailed below. 10 Luminance-luminous flux discrimination. In t h i s task, luminance and luminous flux covaried and both were available as cues for solving the d i s c r i m i -nation (LF task). The primary purpose of this task was to fin d out i f the use of the particular discriminanda i n this experiment would result i n postopera-tive d e f i c i t s similar to those obtained with other tasks such as a black-white discrimination (Thompson, I960; Bauer and Cooper, 1964) or a discrimination involving emitted l i g h t with luminance and luminous f l u x dependent upon doorway area (Schilder, 1966). The luminance and luminous flux values were 13500 n i t s and .133 lumens respectively for the brighter l i g h t and 370 n i t s and .011 lumens for the dimmer l i g h t . The brighter l i g h t was 17 i n . and the dimmer 9 i n . behind the goal box doorways. Transfer tests were run after subjects had completed the relearning series of t r i a l s to t r y and f i n d out whether subjects had employed luminance or flux or both cues to solve the discrimination. Half of the normal and operated animals had the following sequence of transfer tests. On Days 1 and 2»of the transfer series, the ori g i n a l discrimination was alternated with a new discrimination in which the stimuli were equated for luminance but had the same flux values as the stimuli of the o r i g i n a l discrimination. On days 3 and 4, the o r i g i n a l discrimination was alternated with a new discrimination i n which the stimuli were equated for flux but had the same luminance values as the stimuli of the o r i g i n a l d i s c r i m i -nation. The rest of the subjects had the transfer tests presented i n the reverse order; Luminous flux discrimination. In t h i s task, the luminance values were equated for the two stimuli but the flux values d i f f e r e d (F task). The purpose of this- task was to f i n d out whether a discrimination based on flux differences could be learned by normal rats and whether such a discrimination would be lost following removal of the s t r i a t e cortex. The luminance and luminous flux 11 values were 3400 n i t s and .04 lumens respectively for the higher flux l i g h t and 3400 n i t s and .0042 lumens for the lower flux l i g h t . The higher flux l i g h t was 16 i n . and the lower flux l i g h t 48 i n . behind the goal box doorways. Transfer tests were given following relearning to both groups to t r y and d i s -cover whether the difference i n distance between the two lights had contributed to learning. For this purpose, the origi n a l discrimination was alternated with a new discrimination i n which the stimuli were equated for flux but were unequal i n luminance and distance from the goal box doorways. Luminance discrimination. In this task, luminous flux was equated for the two discriminanda but luminance values differed (L task). The purpose of the task was to find out whether luminance could serve as a cue for the normal rat and whether such a discrimination would be permanently lost following removal of the s t r i a t e cortex. The luminance and luminous flux values were 13500 nits and .011 lumens respectively for the brighter l i g h t and 370 n i t s and .011 lumens for the dimmer l i g h t . The brighter l i g h t was 48 i n . and the dimmer 9 i n . behind the goal box doorways. Transfer tests were again run following relearning i n order to f i n d out i f the difference i n distance of the two stimuli had contributed to learning. The origi n a l discrimination was alternated with a new discrimination i n which the stimuli were equated for luminance but were unequal i n flux and distance from the goal box doorways. 12 RESULTS Discrimination Tests Luminance-luminous flux task. Table I presents median pre- and post-operative error scores for groups on a l l three tasks. On the LP task, operated animals committed significantly more errors than controls on postoperative retention tests (U = 21.5, p < .025, one-tailed). However, operated animals obtained significantly lower error scores postoperatively than preoperatively (U = 7, p < .001, one-tailed), indicating that considerable savings had occurred. Luminous flux task. Again, operated animals were significantly inferior to controls on retest errors (U = 9, p< .001, one-tailed). As with the LP task, operated animals showed significant savings in error scores postoperatively (U = 11, p < .01, one-tailed). Luminance task. A l l but three of the operated animals failed to reattain criterion postoperatively, despite being run for twice the number of trials as originally required to attain criterion. One animal committed fewer and two animals more errors than preoperatively. Transfer Tests Luminance-luminous flux task. Since there were no significant differences due to the order of presentation of the transfer discrimination tests, the results were combined for analysis. Separate analyses were performed on the luminance and on the flux transfer series. Table II presents median errors on the interspaced original discrimination t r i a l s , mean responses to the higher luminance stimulus, and mean position choices on the luminance transfer trials for the operated and control animals. On the luminance transfer series, operated and control animals did not differ in number of errors on those trials on which the original discrimination was presented. Analysis of variance TABLE I Median Errors to C r i t e r i o n Preoperatively (Preop) and Postoperatively (Postop) on the Luminance-Luminous Flux Task (LF), the Luminous Flux Task (F), and the Luminance Task (L) for the Operated (0) and Normal (N) animals. Task Group Preop Postop 0 (n=10) 37.5 7.0 LF N (n=10) 37.5 3.0 0 (n=10) 32.0 7.0 N (n=10) 33.0 0.0 0 (n=10) 44.0 105.5* N (n=10) 51.0 4.5 * Seven animals did not reattain c r i t e r i o n . TABLE II Luminance Transfer Tests on the LF Task: Median Errors on the Original Discrimination T r i a l s (OD), Mean Responses to the Higher Luminance Stimulus (Lu), and Mean Responses to a Preferred Position on the Transfer T r i a l s (P). Group* OD Lu P 0 X (n=5) 2.0 3.0 7.2 0 2 (n=5) 2.0 4.8 9.4 N x (n=2) 1.5 7.5 5.5 N 2 (n=8) 1.5 4.7 6.0 * The operated (0) and normal (N) groups have been subdivided into two groups each on the basis of mean responses to the higher luminance stimulus. The subscript 1 refers to those animals responding to one stimulus on at least 65% of the t r i a l s ; the subscript 2 refers to those animals responding i n a random fashion to the stimu l i . The maximum number of responses i n each column i s ten. 15 indicated that there was no overall d i f f e r e n t i a l response to the discriminanda on the luminance transfer t r i a l s but there was a si g n i f i c a n t interaction effect (P = 6.26, df = 1/18, p < .025). Operated animals tended to choose the lower luminance stimulus more frequently than controls. There was also a significant overall response to position on the luminance transfer t r i a l s (F = 13.43, df = 1/18, p < .005). The interaction effect approached but did not reach significance (F = 3.36, df = 1/18, .10 >p >.05). The mean response to position on the transfer t r i a l s for the operated and control animals (Table II) suggests that operated animals did have stronger position habits than did controls. Control animals tended to respond i n a random manner. Table III presents median errors on the interspaced o r i g i n a l discrimination t r i a l s , mean responses to the higher flux stimulus, and mean position choices on the flux transfer t r i a l s for the operated and control animals. On the flux transfer series, operated and control animals again did not d i f f e r i n number of errors on those t r i a l s on which the orig i n a l discrimination was presented. Analysis of variance indicated that there was a si g n i f i c a n t choice of the higher flux stimulus (F = 15.57, df = 1/18, p < .005), but no interaction e f f e c t . There was also a s i g n i f i c a n t response to a preferred position on these same t r i a l s .'(*]? = 10.15, df = 1/18, p < .01), but again no interaction e f f e c t . A Spearman rank-order correlation between position choice and choice of the higher flux stimulus was negative and si g n i f i c a n t (rho = -sr.66, t = -2.5059, df = 18, p < .025, one-tailed). A Spearman rank-order correlation between number of correct responses on the interspaced o r i g i n a l discrimination t r i a l s and choice of the higher flux stimulus of the transfer discrimination was positive and si g n i f i c a n t (rho = .74, t = 3.1404, df = 18, p < .005, one-tailed). These correlations suggest that a high degree of transfer occurred between the ori g i n a l and the transfer discriminations. Moreover, to the degree that such TABLE III Luminous Flux Transfer Tests on the LF Task: Median Brrors on the Original Discrimination T r i a l s (OD), Mean Responses to the Higher Flux Stimulus ( F l ) , and Mean Responses to a Preferred Position on the Transfer T r i a l s (P). Group* OD F l P 0 X (n=9) 1.0 8.6 5.8 0 2 (n=l) 6.0 5.0 10.0 Nj_ (n=9) 1.0 8.7 6.1 N 2 (n=l) 3.0 6.0 9.0 * The operated (0) and normal (N) groups have been subdivided into two groups each on the basis of mean responses to the higher flux stimulus. The subscript 1 refers to those animals responding to one stimulus on at least 65% of the t r i a l s ; tKe subscript 2 refers to those animals responding i n a random fashion to the stimuli. The maximum number of responses i n each column i s ten. 17 transfer did not occur, animals of both groups responded by position on the transfer discrimination t r i a l s . Luminous flux task. Table IV presents median errors on the interspaced ori g i n a l discrimination t r i a l s , mean responses to the nearer stimulus on the transfer t r i a l s , and mean position choices on the transfer t r i a l s for the operated and control animals. Operated animals were s i g n i f i c a n t l y i n f e r i o r to controls i n number of errors on those t r i a l s on which the o r i g i n a l d i s c r i m i -nation was presented (U = 22.3, p < .025, one-tailed). On the transfer t r i a l s , analysis of variance showed that there was a s i g n i f i c a n t overall choice of the nearer stimulus (F = 10.65, df = 1/18, p< .001), but no interaction e f f e c t . There was also a s i g n i f i c a n t response to a preferred position on these same t r i a l s (F = 18.03, df = l / l 8 , p < .001), but again no interaction effect. A Spearman rank-order correlation between choice of the nearer transfer stimulus and choice of position on the transfer t r i a l s was negative and s i g n i f i c a n t (rho = -.57, t = -1.9637, df = 18, p < .05, one-tailed). A Spearman rank-order correlation between number of correct responses on the interspaced o r i g i n a l discrimination t r i a l s and choice of the nearer stimulus on the transfer t r i a l s was positive and s i g n i f i c a n t (rho = .62, t - 2.2171, df = 18, p <.025, one-tailed). These two correlations indicate that transfer did occur between the two discriminations. Moreover, to the degree that transfer did not occur, animals i n both groups responded to a preferred position on the transfer t r i a l s . Luminance task. Table V presents median errors on the interspaced o r i g i n a l discrimination t r i a l s , o n the transfer tests, mean responses to the further transfer stimulus, anddmean responses to a preferred position on the transfer t r i a l s for the operated and control animals. Operated animals responded just above chance level on those t r i a l s on which the o r i g i n a l d i s -TABLE IV Transfer Test Results on the F Task: Median Errors on the Original Discrimination T r i a l s (OD), Mean Responses to the Nearer Stimulus (NS), and Mean Responses to a Preferred Position on the Transfer T r i a l s (P). Group* OD NS P 0 X (n=4) 1.5 13.5 11.5 0 2 (n-6) 5.0 11.0 15.7 N x (n=5) 0.0 16.2 11.8 N 2 (n=5) 3.0 11.0 15.4 * The operated (0) and normal (N) groups have been subdivided into two groups each on the basis of mean responses to the nearer stimulus. The subscript 1 refers to those animals responding to one stimulus on at least 65% of the t r i a l s ; this subscript 2 refers to those animals responding i n a random fashion to the stimuli. The maximum number of responses i n each column i s twenty. TABLE V Transfer Test Results on the L Task: Median Errors on the Original Discrimination T r i a l s (OD), Mean Responses to the Further Stimulus (FS), and Mean Responses to a Preferred Position on the Transfer T r i a l s (P). Group* OD FS P 0 (n=3)** 7.0 10.7 14.0 N x (n=6) 2.0 16.7 12.0 N 2 (n=4) 3.5 10.8 15.5 * The normal animals (N) have been subdivided into two groups on the basis of mean responses to the further stimulus. The subscript 1 refers to those animals responding to one stimulus on at least 65% of the t r i a l s ; the subscript 2«refers to those animals responding i n a random fashion to the stim u l i . The maximum number of responses i n each column i s twenty. ** Seven animals did not reattain c r i t e r i o n postoperatively and were not given the transfer tests. 20 crimination was presented. On transfer t r i a l s , one operated animal responded randomly and two showed a strong position preference. Normal animals had a median of three errors on the o r i g i n a l discrimination t r i a l s . A correlated t-test showed a s i g n i f i c a n t choice of the further stimulus on the transfer t r i a l s by the intact animals (_t = 3.1845, df = 9, p < .01, one-tailed), as well as a s i g n i f i c a n t position choice on these same t r i a l s (Jb = 4.5435, df = 9, p< .005, one-tailed). A Spearman rank-order correlation between choice of the further stimulus and choice of position on the transfer t r i a l s was negative and s i g n i f i c a n t (rho = -.64, _t as -2.3333, df = 8, p < .025, one-tailed). A Spearman rank-order correlation between number of correct responses on the interspaced o r i g i n a l discrimination t r i a l s and choice of the further transfer stimulus was positive and s i g n i f i c a n t (rho = .55, .t = 1.8553, df =* 8, p <.05, one-tailed) . These correlations indicate that for some of the intact animals transfer did occur between the two discriminations. Those animals not showing transfer tended to respond by position on the transfer t r i a l s . Histology Figure 1 presents diagrams of the extent of c o r t i c a l damage and corresponding retrograde degeneration i n the dorsal l a t e r a l geniculate nucleus. A l l animals sustained damage to c a l l o s a l f ibers underlying the aspirated areas of the cortex, as well as minor invasion of the hippocampus b i l a t e r a l l y . Such subcortical damage did not serve to d i f f e r e n t i a t e between animals on the basis of postoperative performance on any of the three tasks. Most animals suffered subtotal destruction of the s t r i a t e areas with sparing generally confined to the most posterior margin of the s t r i a t e cortex. The extent of sparing was not related to postoperative performance on any of the three tasks. Spearman rank-order correlations between s t r i a t e damage and postoperative performance andi facing page 21 FIGURE 1 Diagramatic representation of c o r t i c a l damage and dorsal l a t e r a l geniculate nucleus retrograde degeneration.for animals on a l l three tasks. The top two rows present animals learning the LF task; the middle two rows present animals learning the F task; and the bottom two rows present animals learning the L task. Regionsothat have been blacked out represent areas of c o r t i c a l damage and geniculate degeneration. FIGURE 1 21 0to f r % ^ < f l ^ « 1 « ^ • M ( i t t (m TO SB 96 22 between geniculate degeneration and postoperative performance were insignificant for a l l three tasks. A Spearman rank-order correlation between extent of striate damage and geniculate degeneration was positive and significant (rho = .41, z = 3.1493, p< .001, one-tailed). Ranks for striate damage were obtained by superimposing diagrams of the corticalilesions over cortical maps based on Kreig's (1946) mapping of the cortical areas. 23 DISCUSSION A potential problem i n interpreting the present results i s the fact that the lesions were generally incomplete, sparing the most posterior portions of the s t r i a t e areas. However, th i s does not appear to be of major concern. There was no correlation between s t r i a t e damage or geniculate degeneration and post-operative performance on any of the three tasks. The three animals who were able to reattain c r i t e r i o n on the L task were among those animals with the greatest amount of damage i n that group. The only animal to show perfect savings on the F task had a v i r t u a l l y complete lesion, showing only minimal sparing i n the dorso-lateral margin of the right geniculate nucleus. Similarly,, animals with minimal sparing i n one or both geniculate nuclei reattained c r i t e r i o n on the LF task with error scores below the median for that group. F i n a l l y , animals on the LF task had generally more complete damage than animals on the L task. Tet, the LF task animals showed consistently high savings, while seven of ten L task animals (including the fiv e with the least damage) were unable to reattain c r i t e r i o n within the postoperative testing period. Generally, the h i s t o l o g i c a l data seems to be consistent with that of Horel et a l (1966) and Braun et a l (1966) who found no correlation between extent of damage and relearning of ahblack^white task. Horel et a l (1966) concluded that the s u f f i c i e n t lesion for disturbing the habit involved destruction of Lashley's (1932) area c r i t i c a l for pattern v i s i o n . Acquisition of the LF task appears to have been on the basis of the flux rather than the luminance cue or both cues. The high postoperative savings of the operated animals suggests that these animals were u t i l i z i n g essentially the same cue postoperatively as preoperatively. On the luminance transfer t r i a l s , the predominant mode of responding by the intact animals was random, while the operated animals tended to show strong position habits. Although 24 t h i s indicates different modes of responding by the intact and operated animals on the luminance transfer t r i a l s , i t also suggests strongly that luminance was not a relevant cue for either group. On the other hand, both groups showed strong transfer from the or i g i n a l discrimination to the higher flux stimulus of the flux transfer discrimination, indicating that flux was a highly relevant dimension for both groups. Use of the LF task did not results i n the type of d e f i c i t s obtained by previous researchers with a black-white habit or with situations using translucent doorways illuminated from behind (Thompson, 1960; Bauer and Cooper, 1964; Horel et a l , 1966; Schilder, 1966). The difference i n luminance was of the order of 36.5 to 1 while the difference i n flux was of the order of 12 to 1. Since the LF task appeared to have been learned on the basis of the flux cue only, this suggests that for the rat flux may be a more salient cue than luminance. This suggestion i s also supported by the faster i n i t i a l learning of the F task animals than the L task animals. The difference i n the flux task was 10 to 1, while the difference i n the L task was 36.5 to 1. The lower retest errors for the intact animals on the F task gives additional support to this notion. The high postoperative savings by operated animals on the F task again supports the notion that a flux discrimination i s not greatly disturbed by s t r i a t e removal. However, the results of the transfer tests are less clear i n establishing that flux was the sole cue u t i l i z e d by the animals i n learning the discrimination. On the transfer discrimination t r i a l s , four operated and f i v e control animals responded p o s i t i v e l y to the nearer stimulus. On the ori g i n a l discrimination, the correct stimulus was also the nearer stimulus. It i s probable that i t was the dimension of depth to which animals were responding on the transfer t r i a l s . Lashley (1937) noted that s t r i a t e rats 25 seem able to respond to the gross position i n space of illuminated objects and Lashley (1931) described three animals with complete s t r i a t e lesions who were able to accurately jump distances of 20 cm or more, although less r e l i a b l y than controls. It seems, then, that depth as well as flux may have been used to learn the discrimination and that i t was the dimension of depth to which animals transferedi their responses on the transfer t r i a l s . As on the F task, rats acquiring the L discrimination may have done so on the basis of depth as well as luminance. On the transfer tests, six intact animals tended to respond strongly to the further stimulus. This was also the location of the correct stimulus on the o r i g i n a l discrimination, suggesting that these animals transfered from the o r i g i n a l discrimination on the basis of depth differences. It i s doubtful that depth was the primary cue i n either task. The L task had the greater distance difference between the stimuli (9 : 4 8 i n . vs 16 : 4 8 i n . for the F task), yet, o r i g i n a l acquisition was much slower than on the F task. The difference i n postoperative retention by operated animals on the two tasks also argues against this p o s s i b i l i t y . The three operated animals who were able to relearn the L task may have done so on the basis of flux cues. Although flux was equated at the doorways to the goal boxes, the situation was such that there was a flux difference of 5 : 1 i n favor of the higher luminance stimulus at the entrance to the choice chamber from the start box. This would be a d i f f i c u l t but not necessarily impossible discrimination for a s t r i a t e animal. As on the F task, i t i s also possible that the depth^difference between the stimuli may have been u t i l i z e d as a cue for relearning. The transfer test results for the operated animals contribute l i t t l e to an understanding of which was the relevant cue during relearning. A l l animals performed poorly on the interspaced o r i g i n a l discrimi-nation t r i a l s as well as on the transfer t r i a l s , responding either randomly or 26 by position on both discriminations. This does not necessarily indicate that the cues present i n the transfer discrimination were not u t i l i z e d by these animals during relearning. Lashley (1930) has shown that the performance of operated animals i s impaired by changes i n stimulus conditions. He has suggested that this may r e f l e c t an impaired a b i l i t y of s t r i a t e lesioned rats to adapt to new visual situations. If the operated animals on the L task relearned using a minimal flux cue, then the change to a very relevant flux cue on the transfer discrimination could result i n a breakdown i n per-formance, both on the original and the transfer discriminations. Likewise, i f they used a depth cue, or both cues, then large changes i n stimulus illumination could also affect performance adversely. Some deterioration i n accuracy on the interspaced o r i g i n a l discrimination t r i a l s during transfer testing occurred for both operated and control animals on a l l three tasks. The LP task groups were the least impaired and these two groups did not d i f f e r i n the degree to which they became impaired (Tables II and I I I ) . This probably re f l e c t s the greater s i m i l a r i t y between the o r i g i n a l and transfer discriminations, p a r t i c u l a r i l y ohfjtBeiBflux transfer tests, for this task than for the F and L tasks. The transfer discriminations on the LF task maintained at least one of the conditions present during the o r i g i n a l discrimination t r i a l s . On the F and L tasks, operated animals were more impaired than controls (Tables IV and V). This i s consistent with Lashley*s (1930) suggestion that operated animals are somewhat impaired i n their a b i l i t y to adapt to new v i s u a l situations. There are two possible reasons that a less severe impairment occurred for the F task operated animals than for the L taskooperated animals. F i r s t , the F discrimination was an easier one for the operated animals and therefore less susceptible to disruption. Such d i f f e r e n t i a l disruption has been shown for postoperative retention of a light-dark habit by Bauer and Cooper (1964). 27 Second, the change i n overall illumination on the P task transfer t r i a l s represented a decrease in overall l i g h t levels rather than an increase as i n the case of the L task transfer t r i a l s . On a l l three tasks, there was a negative correlation between choice of position and choice of a particular stimulus on the transfer t r i a l s , when at least some animals showed transfer from the o r i g i n a l to the new discrimination. There was also a positive correlation between choice of the correct stimulus on the interspaced o r i g i n a l discrimination t r i a l s and choice of a particular stimulus on the transfer t r i a l s when transfer occurred. This suggests that the primary reason why transfer did not occur for a l l animals was that the performance of some animals on the o r i g i n a l discrimination deteriorated because of the introduction of the transfer discrimination. Such deterioration i n performance would prevent transfer to a new discrimination. There i s a p o s s i b i l i t y that the operated animals on the L task were able to u t i l i z e the luminance cue. Pasik et a l (1969) and Schilder et a l (197$) reported that s t r i a t e lesioned monkeys were apparently able to u t i l i z e luminance as a cue. Two possible explanations for their results are that their monkeys may have used either d i f f e r e n t i a l d i s t r i b u t i o n of flux or differences i n the rate of change i n flux to solve the discriminations. However, both of these explanations are weak i n view of the excellent transfer obtained by Schilder et a l (1971) over three discriminations i n which the only constancy was luminance differences. Although this may r e f l e c t species differences, the p o s s i b i l i t y that luminance may serve as a cue for s t r i a t e lesioned rats seems to be i n need of further examination. The present results indicate that brightness habits based on identical cues for operated and intact animals suffer r e l a t i v e l y l i t t l e disruption postoperatively. Such results argue strongly against the position of 28 Hamilton and Treichler (1968) that pre- and post-operative learning are sub-served by different and independent neural mechanisms. The sig n i f i c a n t savings on the LF and F tasks contradicts t h e i r position that flux discrimination i n the normal animal i s subserved only by s t r i a t e acuity and intensity mechanisms, since complete loss should therefore follow s t r i a t e removal. The evidence that subcortical lesions alone can result i n losses similar to those following s t r i a t e removal (e.g., Thompson, 1969) also contradicts their position. Further, Baden, Urbaitis, and Meikle (1965) have shown that removal of the ectosylvian gyri i n cats caused reappearance of the loss of a light-dark habit after i t had been lost through successive ablation of the l a t e r a l , posterolateral, and suprasylvian g y r i . Since removal of the ectosylvian g y r i alone did not disturb the habit, this suggests that recovery of the habit after t o t a l loss i s a function of an interaction between c o r t i c a l and sub-c o r t i c a l areas, rather than a function of the a c t i v i t y of subcortical areas alone. S u p e r f i c i a l l y , the results appear to support Bauer and Cooper&s (1964) contention that the postoperative d e f i c i t s are due to sensory losses alone. The discrimination data i s i n agreement with their finding of greater savings on a light-dark task than on a black-white task. However, their hypothesis would predict no loss of a discrimination based on flux cues, while the results of the present experiment show that some postoperative disturbance of the habit does occur. In addition, their hypothesis cannot account ade-quately for the operates s i g n i f i c a n t l y i n f e r i o r performance to controls on the interspaced o r i g i n a l discrimination t r i a l s during transfer testing on the F and L tasks. Part of the evidence used by Bauer and Cooper (1964) i n support of a sensory d e f i c i t s hypothesis was the finding that progressive increases i n background illumination caused greater impairments i n performance 29 for operates than for normal controls. They interpreted this difference as demonstrating that operated animals were hampered by sensory d e f i c i t s occurring after s t r i a t e removal. However, Cooper, Freeman, and Pinel (1967), i n a Skinner box situation, found consistently i n f e r i o r performance by operated animals even at l i g h t levels well above threshold. They argued that t h i s does not necessarily r e f l e c t sensory d e f i c i t s since this difference occurred well above threshold, the difference did not increase over decreasing l i g h t levels, as would be expected, and operated animals were consistently i n f e r i o r to controls over several days of testing at baseline l e v e l s . Examination of Bauer and Cooper's (1964) Figure 6 suggests that t h i s same arguement might apply i n the situation of increasing background illumination. This i s not to suggest that sensoryydeficits have not played a prominent role i n causing postoperative performance d e f i c i t s , p a r t i c u l a r i l y on black-white tasks and the L task of the present experiment. However, the above analysis suggests that another factor i s involved i n such d e f i c i t s . Krechevsky (1936) has postulated that posterior lesioned rats suffer an attentional d e f i c i t . He found that under food reinforcement conditions, operated rats were i n f e r i o r to controls on o r i g i n a l learning of a light-dark habit, when dark was positive. On retention testing, non-shocked operates were again i n f e r i o r to controls, but this difference decreased considerably for shocked operates. Krechevsky (1936) has suggested that shock heightened attention to the relevant cue i n the situation. Lashley (1935a) commented that i n successive training to the brighter of two l i g h t s , with successive decreases i n the luminosity differences between the discriminanda (Lashley, 1930), the performance of operated animals broke down as the difference approached limen. Performance was above chance but never reached errorless c r i t e r i o n . He suggested (Lashley, 1935a) that the v a r i a b i l i t y i n performance 30 obtained might be due to attentional fluctuations. His subsequent rejection of the p o s s i b i l i t y of attentional d e f i c i t s (Lashley, 1935b) i s based on his assumption that such a d e f i c i t would be reflected i n slower i n i t i a l learning by operated animals. Krechevsky (1936) showed that attentional d e f i c i t s may be minimized under conditions of punishment for errors by shock, a condition which Lashley always used. Krechevsky (1936) also pointed out that normal animals preferred the darker of two stimuli but this preference was reversed i n s t r i a t e lesioned animals. Since Lashley always trained his animals to the brighter stimulus, normal animals would be performing under a handicap which operated animals did not have. Horel et a l (1966) have suggested that i f this switch i n preference i s taken into account, operated animals i n fact wouldi be i n f e r i o r i n o r i g i n a l learning. This has been shown by Krechevsky (1936) under non-shock conditions and by Parker, Erickson, and Treichler (1969) under shock conditions. In addition, Spear and Braun (1969) have shown that the acquisition functions of s t r i a t e lesioned and normal animals do d i f f e r even whenoreaponding to the brighter stimulus, and that the difference increases as the difference between the stimuli decreases. Braun et a l (1966) and Jonason et a l (1970) have been able to f a c i l i t a t e the reestab-lishment of a black-white habit i n rats with amphetamine injections prior to retention testing. They suggested that amphetamine f a c i l i t a t e d access to a secondary engram based on flux cues that had been l a i d down during o r i g i n a l learning along with a primary engram based on edge-related cues. It seems possible that amphetamine f a c i l i t a t e d attentional processes through general increases i n arousal of the operated animals and that this was the basis of access to the secondary engram. F i n a l l y , Butter (1969) has shown that monkeys with l a t e r a l s t r i a t e lesions may have a selective attentional d e f i c i t . They were not i n f e r i o r to controls on learning of a bar length discrimination but 31 were impaired i n test situations when irrelevant features were added to the discriminanda. However, they were impaired only above a c r i t e r i o n of 70% correct, suggesting attentional rather than sensory or learning d e f i c i t s . Similar results can be found on the LF and F tasks of the present experiment; Operated animals i n both tasks did not d i f f e r from controls on retest scores up to a c r i t e r i o n of 16/20. It was only beyond this c r i t e r i o n l e v e l that operates were i n f e r i o r to controls,,suggesting again attentional rather than sensory or learning d e f i c i t s . There was a successive deterioration i n accuracy on the interspaced o r i g i n a l discriminationstrials during the transfer test series over the LF, F, and L tasks of the present experiment. Rather than r e f l e c t i n g increases i n conditions that could accentuate sensory d e f i c i t s , this appeared to r e f l e c t increases' i n thesdifferencetbetween the ori g i n a l and transfer discriminations. Presumably, the greater the difference, the more l i k e l y the overall situation i s to confuse operated animals, par t i c u l a r l y , i f there i s present an attentional d e f i c i t . Such confusion i s l i k e l y to interact with an attentional d e f i c i t to y i e l d greater general impairment. The deterioration i n performance does not seem to r e f l e c t increases i n learning d i f f i c u l t y over the three tasks, since this order was F, LF, and L, both for ori g i n a l learning and for retention scores of intact animals. An alternative hypothesis has been suggested by Cooper et a l (1967). They have suggested that the primary function of v i s u a l cortex may be to impose an organization over other regions of the brain involved i n visual processes. Under such an interpretation, removal of the visual cortex might well be & expected to result i n performance d e f i c i t s such as those found i n the present experiment on retention testing of operated animals on the LF and F tasks, and explained above on the basis of attentional d e f i c i t s . Their explanation 32 might be applied equally well to the other data cited in support of an attentional deficits model. There seems in fact to be no crucial difference between the two hypotheses. 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