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Stimulus-driven spatial attention mechanisisms in audition : evidence from an implicit localization task McDonald, John J. 1996

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STIMULUS-DRIVEN SPATIAL ATTENTION MECHANISMS IN AUDITION: EVDDENCE FROM AN IMPLICIT LOCALIZATION TASK by J O H N J. MCDONALD B.A., Simon Fraser University, 1993 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 coriforming to the reqjtffrjau1 standard THE UNIVERSITY OF BRITISH COLUMBIA August 1996 © John J. McDonald, 1996 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of P S Y C H O L O G Y The University of British Columbia Vancouver, Canada Date 2& MQ^-T DE-6 (2/88) Spatial Attention Mechanisms in audition - ii A B S T R A C T Four experiments examined the effects of uninformative spatial auditory precues on auditory detection latencies when the decision to respond was based on either spatial or non-spatial criteria. The first experiment used a new technique, called implicit localization, in which observers responded to peripheral targets and refrained from responding to central targets. Response times were initially faster for targets at the cued location than at a contralateral location, suggesting that attention was captured at the spatial position of the cue. This facilitatory effect diminished and even reversed at longer cue-target onset asynchonies (CTOAs), indicating that inhibition of return (IOR) also occurs in audition. These effects were not observed in later experiments when the go/no-go decision was based on target presence (Experiments 2 and 3) or target frequency (Experiment 4). These data indicate that the facilitatory and inhibitory components of covert spatial orienting occur in audition only when spatial information is relevant to the task. They may also provide the first clear evidence of IOR in audition. These findings suggest that implicit localization provides a powerful technique for studying covert spatial attention. Spatial Attention Mechanisms in audition - iii T A B L E OF CONTENTS Abstract ii Table of Contents iii List of Tables iv List of Figures v Acknowledgements vi Introduction 1 Spatial Attention Orienting in Vision 2 Attention Orienting in Audition 4 Auditory Inhibition of Return 9 Experiment 1 13 Method 13 Results 17 Discussion • 19 Experiment 2 21 Method 21 Results 22 Discussion • 23 Experiment 3 26 Method 27 Results : 27 Discussion 28 Experiment 4 31 Method 31 Results 32 Discussion 32 General Discussion 34 Spatial Relevance and Auditory Spatial Attention 35 Inhibition of Return in Audition 37 Conclusions 40 Notes 42 References 43 Spatial Attention Mechanisms in audition - iv LIST OF T A B L E S Table Page 1 Selected Auditory Spatial Attention Studies. 6 2 Selected Inhibition of Return Studies Using Auditory Cues or 10 Targets. 3 Mean Response Times (Rts; in milliseconds) and Percentages of 18 Errors as a Function of Cue-Target Onset Asynchrony (CTOA) and Cue Type in Experiment 4. Spatial Attention Mechanisms in audition - v LIST OF F IGURES Figure Page 1 Schematic view of the stimulus apparatus used in Experiments 1 to 4. 14 2 Example of stimulus display sequences in Experiments 1 to 4. 16 3 Mean response times (RTs; top) and cue effects (invalid RT - valid 20 RT; bottom) at the three cue-target onset asynchronies (CTOAs) in Experiment 1. 4 Mean response times (RTs; top) and cue effects (invalid RT - valid 24 RT; bottom) at the three cue-target onset asynchronies (CTOAs) in Experiment 2. 5 Mean response times (RTs; top) and cue effects (invalid RT - valid 29 RT; bottom) at the three cue-target onset asynchronies (CTOAs) in Experiment 3. 6 Mean response times (RTs; top) and cue effects (invalid RT - valid 33 RT; bottom) at the three cue-target onset asynchronies (CTOAs) in Experiment 4. Spatial Attention Mechanisms in audition -A C K N O W L E D G E M E N T S this work was supported by a grant from the Natural Sciences and Engineering Research Council of Canada (NSERC) to Lawrence Ward and by a graduate fellowship awarded to John McDonald. I would like to thank Lawrence Ward for invaluable discussions, and Dan Lin and Godwin Chan for research assistance. I would especially like to thank Debby Giassi McDonald for her continued support. Spatial Attention Mechanisms in Audition - 1 STIMULUS-DRIVEN AUDITORY SPATIAL A T T E N T I O N MECHANISMS: E V I D E N C E F R O M A N IMPLICIT L O C A L I Z A T I O N T A S K Recent studies of spatial attention suggest that similar covert orienting mechanisms may operate in both vision and audition (see Spence & Driver, 1994; Ward, McDonald, & Golestani, for recent reviews). However, the study of spatial attention within audition has been complicated by the fact that auditory detection can be relatively insensitive to spatial attention, presumably because detection responses are typically based oh early, non-spatial representations (Rhodes, 1987). In this paper a new paradigm is introduced, called implicit spatial localization, within which components of covert spatial orienting in audition can be readily studied. In this technique, observers must decide whether or not to make a simple reaction time (SRT) response on each trial depending on the spatial location of the target stimulus. Following arguments of Rhodes (1987), Spence & Driver (1994), and Ward (1994), the intent was to invoke spatial auditory representations because it was hypothesized that covert spatial orienting in audition will be most effective when responses to targets involve such representations. The advantage of implicit localization over techniques that require explicit localization responses is that the go/no-go decision makes space relevant for a SRT response without allowing precues to bias the response. The present study is aimed at clearly demonstrating the existence of both the facilitatory and inhibitory components of stimulus-driven covert orienting in audition. The demonstration of these effects is not new but the use of the implicit localization paradigm Spatial Attention Mechanisms in Audition - 2 allows spatial relevance to be established while at the same time maintaining cues and targets at the same locations and disallowing the possibility of response priming by the cues. Moreover, this technique clarifies the conditions necessary for stimulus-driven cue effects to occur and helps to illuminate the mechanisms that control spatial attention in the auditory domain. Before the results of four experiments are presented, however, the existent literature on covert orienting in both vision and audition is first briefly reviewed. Spatial Attention Orienting in Vision Studies of visual attention have commonly used the spatial cueing paradigm, in which a spatial cue is used to direct attention prior to the onset of a target stimulus, to examine the effects of covert orienting on visual processing. In standard cueing tasks, targets appear either at the cued location (valid-cue trials) or at a contralateral location (invalid-cue trials). Under these conditions, subjects typically respond faster and more accurately to targets on valid-cue trials than on invalid-cue trials (e.g., Posner, 1980). Both psychophysical (e.g., Downing, 1988; Miiller & Humphreys, 1991) and electrophysiological (e.g., Mahgun & Hillyard, 1991) studies indicate that these cue effects reflect relatively early changes in perceptual processes. Moreover, these effects occur in the absence of head or eye movements, indicating that spatial cues activate covert visual attention mechanisms. Much of what is known about the mechanisms and characteristics of covert visual attention has been learned using the spatial cueing technique. For instance, a distinction has been made between stimulus-driven (exogenous) and goal-driven (endogenous) Spatial A Mention Mechanisms in A udition - 3 control of visual attention. Direct cues (such as abrupt visual onsets) summon attention reflexively to the cued location (e.g., Jonides, 1981). If a target stimulus subsequently appears at or near that location, detection, discrimination and localization responses are initially facilitated relative to responses to targets presented elsewhere in the display. In these conditions, attention is said to be under stimulus-driven control because the cue effects are observed even when the cues are uninformative (i.e., when they do not predict target location). In contrast, symbolic cues influence target response latencies only when they accurately predict the location of the target. Attention shifts'produced by symbolic Cues are therefore under strategic control. But while stimulus-driven shifts of attention are thought to be more "automatic" than goal-driven shifts, they too can be susceptible to strategic effects. Perhaps the most evocative example of strategic influences on stimulus-driven effects is the failure of abrupt-onset stimuli to capture attention when attention is actively engaged elsewhere in the visual field (e.g., Yantis & Johnson, 1990). Under certain conditions, then, stimulus-driven attention effects in vision can be influenced by task demands. The spatial cueing paradigm has revealed many interesting characteristics and consequences of covert attention orienting. Particularly interesting is the relative performance decrement that can be observed at the ctied location following direct-cue (but not symbolic-cue) facilitation; detection response times (RTs) are actually longer for validly cued targets relative to invalidly cued targets at longer cue-target onset asynchronies (CTOAs; e.g., Maylor, 1985; Maylor & Hockey, 1985; Posner & Cohen, 1984; Tassinari, Aglioti, Chelazzi, Marzi, & Berlucchi, 1987). Posner and Cohen (1984) Spatial Attention Mechanisms in Audition - 4 originally proposed that this effect, called inhibition of return (IOR), is produced when covert attention is summoned to the cued location and then moved to a new location. Covert orienting is not sufficient to generate IOR, however, as symbolic cues fail to produce inhibition unless subjects are instructed to move their eyes to the cued location. Interestingly, RT inhibition is observed under these conditions, even when the actual eye movement is canceled suggesting that oculomotor programming may be sufficient to generate IOR (Rafal, Calabresi, Brennan, & Sciolto, 1989). While the above results suggest that IOR may be closely related to saccadic eye movements, not all data are consistent with a purely oculomotor account of IOR. Both psychophysical (e.g., GibsOn & Egeth, 1994) and psychophysiological (e.g., McDonald, Ward, Kiehl, & Richard, 1996) evidence show that IOR does influence perceptual processes. Moreover, recent eye movement studies also indicate that IOR affects stimulus detection. For instance, both saccades to (pro-saccades) and saccades away from (anti-saccades) peripheral targets are inhibited at the cued location (Rafal, Egly, & Rhodes, 1994). Consistent with an attentional account of IOR, then, these results suggest that at least some of the inhibitory effect can be attributed to suppressed perceptual processes. Attention Orienting in Audition The understandings of attention mechanisms that have been gained through studies of covert visual orienting beg the question of whether similar mechanisms exist in audition. There is growing evidence that attentional alignment within the frequency domain can aid in the processing of both near-threshold (e.g., Scharf, Quigley, Aoki, Peachey, & Reeves, Spatial A ttention Mechanisms in Audition - 5 1987; Hubner & Hafter, 1995) and supra-threshold (e.g., Mondor & Bregman, 1994; Mori & Ward, 1991, 1992; Ward, 1996; Ward & Mori, in press) auditory stimuli. Similarly, a number of observations indicate that alignment of attention to a particular spatial location may also influence auditory processing. Studies of selective listening, for example, have shown that humans are capable of using spatial information (among other physical cues) in order to listen to one auditory message presented within a complex auditory environment (i.e., the "cocktail party phenomenon"; Cherry, 1957; Broadbent, 1958). But despite these early indications, more recent studies of auditory attention have either reported no evidence of covert spatial orienting in audition, or are susceptible to non-attentional explanations. Table 1 summarizes most of the studies investigating auditory spatial attention. In an early and influential study, Posner (1978) reported a failure to find any effects of symbolic visual cues on auditory SRT, even though the same cues were highly effective in orienting attention in visual tasks. Similar null results have been obtained when the location of an auditory target is precued by auditory stimuli: Buchtel and Butter (1988) found that informative direct cues failed to affect auditory SRT while other researchers (e.g., Spence & Driver, 1994; Ward et al., in press) have more recently replicated these null results using uninformative direct cues. While the apparent lack of spatial cueing effects on auditory SRT seemingly indicates that listeners do not covertly orient attention to the spatial location of sounds, inspection of the auditory system helps to clarify why auditory detection may be particularly insensitive to spatial attention. Unlike the visual system, the spatial position of Spatial A Mention Mechanisms in A udition - 6 Table 1. Selected Auditory Spatial Attention Studies. Study Task Cue C T O A Cue Effect INFORMATIVE CUES ~ Bedard et al. (1993) SRT symbolic 350-750 yes Loc symbolic 350-750 yes Buchtel& Butter (1988) SRT direct 50-1000 no Mondor & Zatorre (1995) Disc direct 150-1500 yes Posner (1978) SRT symbolic 1000 no Disc symbolic 1000 no Rhodes(1987) Loc direct 2000 yes Quinlan & Bailey (1995) SRT direct 100 yes Loc direct 100 yes Spence & Driver (1994) Disc direct 100-700 yes UNINFORMATIVE CUES Spence & Driver (1994) SRT direct 100-700 no Disc direct 100-700 no U/D direct 100-700 yes Ward (1994) Loc direct 100-1000 yes Ward et al. (in press) SRT direct 50-500 no Disc direct 50-500 no key to abbreviations: SRT = simple reaction time; Loc = localization; Disc = discrimination; U/D = up/down localization. Spatial Attention Mechanisms in Audition - 7 sound sources is not encoded directly by the receptors of the auditory system. The early stages of the auditory system are instead organized with respect to sound frequency (i.e., organized tortotopically). Consequently, localization of auditory stimuli occurs only at higher stages of the auditory system. This implies that auditory detection may be based on earlier non-spatial representations since very little information is required to generate simple detection responses (Rhodes, 1987). Based on this logic, a number of researchers (e.g., Rhodes, 1987; Spence & Driver, 1994; Ward, 1994) have begun to study spatial attention in audition using various localization tasks that would presumably invoke later spatial representations. Rhodes (1987), for example, found that listeners were faster to verbally localize target sounds when they were presented from expected rather than unexpected locations. Similarly, Ward (1994) observed strong effects of uninformative auditory cues on localization RTs when subjects were required to make left/right responses to peripheral target tones. But while these results are evocative, they may reflect response-level processes rather than attentional processes. In particular, Spence and Driver (1994) have pointed out that the cue effects observed in left/right localization tasks could arise from response priming by the cue stimuli (cf, Simon, Acosta, & Mewaldt, 1975). To circumvent this confound, they adopted a modified localization paradigm in which targets were presented above or below the horizontal meridian, either ipsilateral (valid cue condition) or contralateral (invalid cue condition) to the cue. Their subjects made elevation discrimination responses so that the response dimension (up/down) was orthogonal to the cueing dimension (left/right). Their Spatial A Mention Mechanisms in A udition - 8 results, like Ward's (1994), seem to indicate that auditory localization is under stimulus-driven attentional control. However, the results from up/down localization tasks are also subject to alternative explanations since the cues and targets occur at different locations. Spehce and Driver (1994) experimentally discounted one such explanation involving relative location or apparent motion judgments based on the perceived elevation of the cue (but see Mondor & Zatorre, 1994). Because cue and target positions are so widely separated, however, other strategic factors are likely to influence performance in the up/down paradigm. For instance, subjects may voluntarily "zoom out" from the cued location to encompass same-hemifield target locations (cf, Eriksen & Yey, 1985). Consequently, the cue effects observed in up/down localization tasks may not represent purely stimulus-driven spatial attention mechanisms. In summary, then, previous studies of spatial attention in audition have indicated that auditory detection can be rather insensitive to spatial attention because they do not require the use of higher spatial representations. Localization responses, on the other hand, seem to be influenced by spatial attention, but these conclusions are weakened by a number of non-attentional explanations. Clearly, it would be preferable to adopt a technique that ensures the use of spatial representations but allows for non-attentional explanations to be ruled out. Interestingly, a few researchers have recently reported small but reliable effects of informative spatial cues on auditory detection responses (e.g., Bedard, Massioui, Pillon, & Nandrino, 1993; Mazzucchi, Cattelani, & Umilta, 1983; Quinlan & Bailey, 1995). In all cases, stimuli were delivered through headphones rather than through external speakers. Spatial Attention Mechanisms in Audition - 9 Although localization of external sound sources is rather poor, lateralization judgments of sounds presented to only one ear can be accomplished with little effort. This implies that subjects may strategically utilize spatial information when the location of an auditory cue is made unambiguous. These results suggest that an effective strategy for studying spatial cue effects in audition is to make spatial information relevant without requiring explicit localization responses. This technique is tested directly in Experiment 1. Auditory Inhibition of Return Spatial cueing studies in vision have shown that inhibition of return is a reliable consequence of direct visual cues at long CTOAs (>500 ms). They also indicate that both attentional and oculomotor components of IOR may exist (Abrams & Dobkin, 1994; Rafal et al., 1994). Given the observations that spatial attention can influence auditory processing (e.g., Mondor & Zatorfe, 1994; Spence & Driver, 1994; Ward, 1994) it seems likely that IOR might also be found in audition. Moreover, there is some evidence that peripheral sounds automatically activate the oculomotor system (Jay & Sparks, 1990). Consequently, both attentional (e.g., Reuter-Lorenz, Jha, & Rosenquist, in press) and oculomotor (e.g., Rafal et al., 1989; Berlucchi, Tassinari, Marzi, & Di Stefano, 1989) accounts suggest that IOR may occur in audition as well as vision. It is therefore surprising that several researchers have failed to observe IOR following uninformative auditory cues (e.g., Farah, Wong, Monheit, & Morrow, 1989; Reuter-Lorenz & Rosenquist, in press, Experiment 2; Spence & Driver, 1994 ; Spence & Driver, in press). Spatial Attention Mechanisms in Audition - 10 Table 2. Selected Inhibition of Return Studies Using Auditory Cues or Targets.T Study Task Paradigm Cue* CueEffect § Reuter-Lorenz et al. (in press) A SRT standard cue auditory no V S R T saccade to cue auditory yes Schmidt (in press) A SRT prepare saccade auditory yes Spence & Driver (1994) A U/D standard cue auditory no Spence & Driver (1996) A SRT target - target - no A / V S R T * target - target - yes Tassinari & Berlucchi (1995) A SRT standard cue auditory yes Ward (1994) A Loc standard cue auditory yes V L o c standard cue auditory no key to abbreviations: A = auditory; V = visual; SRT = simple reaction time; Loc = localization; U/D = up/down localization, uninformative direct cues. all reported cue effects are negative, possibily indicating the existence of IOR. auditory and visual targets randomly presented in same experimental block. Spatial Attention Mechanisms in Audition - 11 However, at least five studies have recently reported IOR in a variety of different tasks using either auditory cues or targets. Table 2 summarizes the results from these studies. Ward (1994) was first to demonstrate a biphasic cue effect on auditory response latencies. Like the pattern of commonly found in vision, RTs were initially facilitated and then later inhibited at the cued location. As previously mentioned, Ward's task required left/right localization of the target sounds and so it remains a possibility that both the early facilitation and the subsequent inhibition result from stimulus-response compatibility effects.1 However, the IOR results have recently been replicated in a variety of SRT tasks (Reuter-Lorenz & Rosenquist, in press; Schmidt, in press; Spence & Driver, 1996; Tassinari & Berlucchi, 1995), indicating that Ward's (1994) findings cannot be fully explained in terms of response-based effects. Tassinari and Berlucchi (1995) recently reported small effects of uninformative auditory Cues on auditory SRT. However, the inhibitory effect only reached significance at one CTOA (600 ms) and was not preceded by facilitation at a shorter cue-target interval (200 ms). Furthermore, the magnitude of the effect was smaller than that typically observed in visual cueing tasks. Small inhibitory effects are also found when subjects are required to prepare (Schmidt, in press) or execute (Reuter-Lorenz & Rosenquist, in press) saccadic eye movements to the cued location. Moreover, Reuter-Lorenz and Rosenquist (in press) found that auditory cues failed to inhibit auditory (or visual) detection in the absence of eye movements. These results suggest that oculomotor activation may be sufficient to generate IOR in audition. Spatial Attention Mechanisms in Audition - 12 Oculomotor accounts of auditory IOR, however, do not easily explain why IOR occurs in some but hot all of the auditory cueing studies that do not require oculomotor activation. However, using a continuous responding (i.e., target - target) paradigm in which no cue stimuli intervened between target stimuli, Spence and Driver (1996) provided some indication as to why IOR occurs in some but not all auditory SRT tasks. They found that subjects were slower to detect peripheral tones when they were preceded by ipsilatefal rather than contralateral tones only when target modality was uncertain (i.e., when both auditory and visual targets were presented in the same experimental block). These results were interpreted with respect to the multimodal representations in the deeper layers of the superior colliculus. Spence and Driver (1996) reasoned that modality uncertainty causes detection responses to be based on later multimodal maps rather than earlier tonotopic auditory representations. This explanation is similar to the claim made earlier that spatial auditory representations can be invoked in SRT tasks by making location information relevant to the task. This suggests that it may be both necessary and sufficient to invoke spatial representations to produce facilitatory and inhibitory components of spatial orienting in audition. In summary, recent studies of covert spatial orienting in audition suggest that auditory processing may be influenced by spatial precues. However, given the tonotopic organization of the early stages of audition, it seems likely that spatial cue effects will be evident only when responses are initiated based on later spatial representations. Previous attempts to invoke the activity of these representations have used tasks that require explicit localization of auditory targets (e.g., Rhodes, 1987; Spence & Driver, 1994; Spatial A ttention Mechanisms in A udition - 13 Ward, 1994). However, since localization tasks are vulnerable to a number of perceptual-and response-related effects, the results of these studies remain open to many alternative interpretations. Therefore, the experiments reported here were designed to test whether implicit requirements to localize target stimuli can also generate spatial cue effects in audition. Experiment 1 Method Subjects. Twenty-three University of British Columbia students (14 female, 9 male) were paid for their participation. All subjects reported normal hearing and had normal or corrected to normal vision. Stimuli and Apparatus. Stimulus timing and data recording were controlled by ah HP Vectra ES/12 microcomputer. Subjects sat in a darkened anechoic chamber (background noise constant at 35 dB SPL) and placed their heads in a chinrest that restricted movements of the ears toward auditory cue and target stimuli. Three speakers were aligned horizontally in front of the subjects. The middle speaker was positioned directly in front of the chinrest, at a distance of approximately 105 cm. The two peripheral speakers were positioned 37° to the left and right of the middle speaker, respectively (see Figure 1). A green light-emitting diode (LED), positioned at the center of the middle speaker cone, was used as a fixation point. Green LEDs were also positioned at the two peripheral speakers in order to aid in the localization of the auditory stimuli. Spatial Attention Mechanisms in Audition - 14 • : • • • • • : • • . • • • • • • • 9 • • ^ mw • • • • ^ • • I • • • . ° a • • • • « • • ^ Lm • • • • • • • • • # a • 0 • • O •# • • • •• • • o • mm • • • • • • - • • • . • • • • • Speaker • Green LED Figure I. Schematic view of the stimulus apparatus used in Experiments 1 to 4. Spatial Attention Mechanisms in Audition - 15 Cue and target sounds were produced by a custom sound generator. Cue stimuli consisted of two successive 30 ms broad-band noise bursts, each presented at 70 dB SPL (at the ears), separated by a 10 ms pause. Target stimuli were 75 dB SPL, 1000 Hz pure tones presented for 50 ms. Subjects responded to the onset of the targets by pressing a microswitch placed under the right index finger. Response times (RTs) were measured in milliseconds by a custom interface card with an interval-timer chip. Design and Procedure. The green LEDs positioned in front of each speaker were continuously visible throughout all trials. Subjects were instructed to fixate the central L E D at all times during a block of trials. Each trial began with a 150 ms flicker of the fixation LED. After a 550 ms delay a cue was randomly presented from either the left, right or center speaker. Following a variable CTOA (100, 300, 700 ms), a target tone was then presented with equal probability from one of the same three speakers. Subjects were told to respond to the onset of peripheral targets (GO trials) but to withhold responses to central targets (NO-GO trials). Eight hundred milliseconds after a correct response, the fixation L E D flickered to signal the start of the next trial. If a response was made on NO-GO trials, a 500 ms feedback tone was sounded and the trial was repeated later in the block. The intertrial interval (ITI) was 1500 ms for all trials. Subjects ran in a single recording session consisting of 29 blocks of trials. Nine of twenty-seven trials in each block (33 percent) were NO-GO trials. Among the remaining 18 trials in each block (i.e., peripheral-target trials), those in which the target was preceded by same-location or opposite-location cues were referred to as valid-cue and invalid-cue trials, respectively; trials in which a peripheral target was preceded by a center Spatial Attention Mechanisms in Audition - 16 Figure 2. Example of stimulus display sequences in Experiments 1 to 4. Cue and target stimuli were each presented from one of the three loudspeakers in Experiment 1. In later experiments, target tones occurred only from peripheral loudspeakers. The variable cue-target onset asynchrony (CTOA) is obtained by adding the cue duration (70 ms) with the variable interval that follows. Spatial A Mention Mechanisms in A udition - 17 cue were called center-cue trials (see Figure 2). Two within-subject factors, CTOA (100, 300, and 700 ms) and cue type (valid, center, invalid), yielded 9 equiprobable conditions. Results In all experiments, the first block of trials was treated as practice and was not analyzed. Al l RTs less than 100 ms and greater than 1000 ms were treated as errors and also excluded from analysis. In addition, RTs falling three standard deviations above or below the mean were excluded as errors. Median RTs were calculated from the remaining data for each subject in each of the 9 CTOA x cue type conditions. The means of these median RTs and the corresponding error rates are shown in Table 3. In this and all subsequent experiments, central cue conditions were not analyzed further because of the inherent difficulty of interpreting "neutral" cues (cf, Jonides & Mack, 1984; Wright, Richard & McDonald, 1995). A 3 x 2 repeated-measures analysis of variance (ANOVA) was run on the mean RT data. Within-subject factors were CTOA (100, 300, and 700 ms) and cue type (valid and invalid). For this and all subsequent experiments, Huynh-Feldt corrected degrees of freedom were used to determine the /^-values for all factors with more than two levels when the sphericity assumption was violated. The results show a significant main effect for CTOA, F(2, 44) = 56.2, p < 0.0001, with subjects being slowest to respond at the shortest (100 ms) interval, this pattern has previously been interpreted as reflecting a general alerting effect produced by the cue; response times to targets occurring shortly after the cue are relatively long presumably because temporal uncertainty is highest immediately following cue onset. Spatial A Mention Mechanisms in A udition - 18 Table 3. Mean Response Times (RTs; in milliseconds) and Percentages of Errors (in parenthesis) as a Function of Cue-Target Onset Asynchrony (CTOA) and Cue Type in Experiments 1-4.* Exp CTOA 100 300 700 1100 ~~ v c ~ ~v c T~ ~~ v c i ~~ v c i 1 503 535 534 (4.1) (5.2) (4.6) 2 353 357 357 (1.9) (1.9) (2.6) 3 324 322 323 (1.5) (1.0) (2.0) 4 406 408 404 (3.7) (2.8) (2.4) 444 452 455 (3.2) (3.2) (3.4) 318 326 314 (2.4) (2.0) (2.0) 277 280 273 (1.7) (1.5) (1.5) 364 369 369 (3.4) (2.0) (3.2) 473 438 450 (3.6) (2.9) (3.4) 377 367 363 (1.9) (1.9) (1.8) 297 292 291 (1.3) (2.0) (1.7) 398 391 394 (3.2) (3.3) (2.7) 304 303 297 (2.4) (1.5) (1.9) key to abbreviations: V = valid cue; C = center cue; I = invalid cue; Spatial A Mention Mechanisms in A udition - 19 In contrast, there was no main effect for cue type, F ( l , 22) = 1.0,/? = 0.33. However, a highly significant CTOA x cue type interaction, F(2, 44) = 12.0,/? < 0.0001, suggests that spatial cueing influenced target detection latencies differently at the three cue-target intervals. Figure 3 shows the mean RTs for valid-cue and invalid-cue trials as a function of CTOA. Response times were initially faster for validly cued targets relative to invalidly cued targets. This effect is largest at the 100 ms CTOA but decreases and even reverses at longer cue-target intervals. Planned comparisons between the mean RTs for valid and invalid trials at each of the three CTOAs show that the cue effects at the shortest (+31 ms cue effect) and longest (-23 ms cue effect) intervals are significant.2 The small positive cue effect (+11 ms) for the 300 ms CTOA was not statistically reliable. An equivalent A N O V A run on the error rates revealed no significant effects for CTOA, F(2,44) =1.7,/? = 0.20, cue type, F(l,22) = 0.11, p = 0.74 or their interaction, F(2,44) = 0.38, p = 0.68. These results indicate that a speed-accuracy trade-off did not occur. Discussion Two critical findings are evident from Experiment 1. First, the results confirm recently reported observations that peripheral sounds are able to capture attention and influence subsequent auditory processing (e.g., Mondor & Zatorre, 1995; Spence & Driver, 1994; Ward, 1994). The pattern of results shown in Figure 3 closely resembles the spatial cueing effects observed in vision; when direct visual cues are uninformative, RT facilitation peaks early and is short-lived (e.g., Jonides, 1981; Miiller & Rabbitt, 1989) and is typically followed by inhibition-of-return at longer CTOAs (Maylor & Hockey, 1985; Posner & Cohen, 1984). The similarity between our findings and those in vision strongly Spatial A Mention Mechanisms in A udition - 20 LU a: 425 > • • • •••:••••--•>•• •• i - • • • r 100 300 500 700 CTOA (ms) C/> O 100 300 500 700 CTOA (ms) Figure 3. Mean response times (RTs; top) and cue effects (invalid RT at the three cue-target onset asynchronies (CTOAs) in Experiment 1. - valid RT; bottom) Spatial Attention Mechanisms in Audition - 21 indicates that stimulus-driven orienting mechanisms are responsible for the biphasic pattern of results observed in the present experiment. Furthermore, the observation that listeners are slower to respond to sounds presented at the cued position at longer CTOAs suggests that IOR may also occur in the auditory modality. The theoretical implications of auditory IOR will be more fully discussed in the general discussion. Second, in contrast to Rhodes' (1987) assertion that detection response times are uninfluenced by spatial attention, we found that spatial cues did affect auditory detection latencies when subjects were required to make implicit target localization judgments. However, recent observations (e.g., Spence & Driver, 1994; Ward et al., in press) indicate that auditory detection is not usually under stimulus-driven attentional control. Consequently, the results of Experiment 1 provide preliminary evidence that auditory implicit localization invokes the use of higher spatial representations. An alternative explanation, however, is that the specific stimulus parameters used in the present experiment differed sufficiently from those used in previous studies to influence detection response times. We conducted Experiment 2 to determine if these same auditory cues would influence auditory SRT when subjects were not required to localize targets sounds. Experiment 2 Method The method of Experiment 2 was very similar to that used in the previous experiment except that central target trials were replaced by no-target catch-trials. Spatial A ttention Mechanisms in A udition - 22 Implicit localization of the target sounds was therefore not required, since subjects were required to respond to all target sounds. Subjects. Fourteen students (7 female, 7 male) from the University of British Columbia were paid for participate their participation. One subject had previously participated in Experiment 1. Al l subjects reported normal hearing and had normal or corrected to normal vision. Stimuli and Apparatus. The stimuli and apparatus were identical to those in Experiment 1 except that target tones were now presented from peripheral speakers only. Design and Procedure. Data was collected in one recording session consisting of 28 blocks of trials. On two-thirds (18 of 27) of the trials in each block a broad-band noise cue was presented and was followed by a 1000 Hz pure tone target after a variable CTOA (100, 300, 700 ms). Targets were randomly presented from the two peripheral speakers only, and were preceded by either valid, center, or invalid cues. Subjects were required to respond to the onset of each target by pressing a response key, as fast as possible. Replacing the NO-GO (central target) trials from Experiment 1 were 9 catch-trials (33%) in which no target was presented. These were included to minimize response anticipation errors. Al l other aspects of the design and procedure were identical to those in Experiment 1. Results The mean RTs and error rates for each of the nine conditions in Experiment 2 are shown in Table 3. A 3 x 2 repeated-measures A N O V A (CTOA [100, 300, and 700 ms] x cue type [valid and invalid]) revealed a significant main effect for CTOA, F(2, 26) = 21.4, Spatial Attention Mechanisms in Audition - 23 p < 0.0001. Unlike the previous experiment, however, subjects were slowest to respond at the longest (700 ms) rather than shortest (100 ms) interval. Closer inspection of Table 3 reveals that the mean RTs initially decrease but then increase (by an average of 55 ms) as the CTOA increases from 100 ms to 700 ms. The early RT reduction presumably reflects an alerting effect that is not sustained for targets at the longest CTOA. The CTOA x cue type interaction also reached significance, F(2, 26) = 4.6, p < 0.02, although the main effect for cue type, F ( l , 13) = 3.2,p = 0.10, did not. Planned comparisons revealed a significant cue effect (-14 ms) at the 700 ms CTOA; the mean RT was significantly slower for targets presented at the cued position relative to targets presented in the opposite hemifield. The mean RTs for validly cued and invalidly cued targets are plotted as a function of CTOA in Figure 4. An equivalent 3x2 A N O V A performed on the error rates revealed no significant effects for CTOA, F(2,26) = 0.65, p = 0.53, cue type, F(l,13) = 0.02,p = 0.89, or their interaction, F(2,26) = 1.09, p = 0.35. These results indicate that a speed-accuracy trade-off did not occur. Discussion As Figure 4 shows, we failed to observe any facilitatory cue effects in Experiment 2. This confirms previous reports that auditory simple detection can be relatively insensitive to stimulus-driven spatial attention (e.g., Spence & Driver, 1994; Ward et al., in press). More importantly, this result also indicates that the facilitatory cue effects observed in the previous experiment were most likely due to the requirement that subjects implicitly localize the target sounds rather than arising from some other aspect of our Spatial A Mention Mechanisms in A udition - 24 LU -25 -ID O -50 - L - — i ' I - i , 100 300 500 700 CTOA(ms) Figure 4. Mean response times (RTs; top) and cue effects (invalid RT at the three cue-target onset asynchronies (CTOAs) in Experiment 2. - valid RT; bottom) Spatial Attention Mechanisms in Audition - 25 stimulus display. The most unexpected result of the present experiment is the significant negative effect that can be observed at the 700 ms CTOA. While it is tempting to conclude that this effect reflects genuine IOR, close inspection of Figure 4 shows that the pattern of results differs from the standard stimulus-driven attentional effects found in vision (e.g., Posner & Cohen, 1984). In visual cueing tasks, a temporal alerting function is typically only observed for targets presented at uncued locations. In contrast, RTs for cued targets may actually increase at longer CTOAs, presumably because some spatially selective inhibitory mechanism (IOR) counteracts the temporal alerting effect. The results obtained in Experiment 1 closely resemble these characteristic stimulus-driven patterns (see Figure 3); mean RTs decreased monotonically for uncued targets but began to increased for cued targets. This suggests that the late negative cue effect observed in the implicit auditory localization task may reflect the same (or similar) inhibitory mechanism that operates in vision. However, neither the cued nor the uncued target functions show a sustained alerting effect in the present experiment (see Figure 4). In fact, both RT functions begin to increase rapidly at the longer CTOAs. This indicates that a strategic suppression of responses may have occurred at longer cue-target intervals. One possible explanation for such a strategic effect may be the large portion of no-target catch trials used in Experiment 2. The general assumption with catch trials is that they effect performance (i.e., reduce anticipation) equally for each trial. However, if subjects can distinguish between short and long CTOAs, the likelihood that any specific trial is a catch-trial increases as the time delay following a cue lengthens. Suppose, for Spatial Attention Mechanisms in Audition - 26 example, that subjects in the present experiment distinguished between short (100 ms and 300 ms) and long (700 ms) CTOAs. If any one trial is determined not to be a short CTOA trial (say because, following a cue, 500 ms passes without a target being presented) then the probability that a target will occur is actually less than the probability that a target will not occur (40% vs. 60 %). Given the likelihood of a target not occurring in this situation, it is probable that subjects would strategically suppress any response that had been previously prepared. Because both stimulus-driven and goal-driven factors may have been involved in the present experiment, it is difficult to interpret the negative cue effect at the 700 ms CTOA. Accordingly, we ran a third experiment in an attempt to reduce the strategic effects outlined above while preserving the general paradigm used in Experiment 2. If a negative cue effect still emerges at longer CTOAs, it may well represent the same inhibitory mechanism found in visual cueing studies. However, if the effect is due to goal-driven factors as we hypothesize, then it should be eliminated once strategic effects are minimized. Experiment 3 Experiment 3 was similar to Experiment 2 except that the number of catch trials was reduced from one-third to one-ninth of the total trials in an effort to minimize the strategic factors that may influence responding at longer CTOAs. We also added a fourth CTOA (at 1100 ms) for two reasons. First, in order to make the predicted null results more compelling (if obtained), we decided to include more intervals at which IOR has Spatial Attention Mechanisms in Audition - 27 been shown to occur in vision. Second, by including another long CTOA we would further reduce the likelihood that subjects would strategically suppress a response at longer cue-target intervals. Method Subjects. Fourteen students (7 female, 7 male) attending the University of British Columbia were paid for their participation. Seven subjects had participated in one of the previous experiments. Al l subjects reported normal hearing and had normal or corrected to normal vision. Stimuli and Apparatus. The stimuli and apparatus were identical to those in Experiment 2. Design and Procedure. The design and procedure were similar to those in Experiment 2 except that the reduction in catch trials (from one-third to one-ninth) and the addition of a fourth CTOA (1100 ms). The two within-subject factors, CTOA (100, 300, 700, and 1100 ms) and cue type (valid, center, invalid), now yielded 12 equiprobable conditions. The overall number of trials remained the same as in the previous experiments; subjects ran in 29 blocks of 27 trials producing 783 trials. Results The means RTs and error rates are presented in Table 3. A 4 x 2 repeated-measures A N O V A with CTOA [100, 300, 700 ms, and 1100 ms] and cue type [valid and invalid] as within-subject factors indicated that only the main effect for CTOA was significant, F(3,39) = 14.8,/? < 0.0001, with subjects slowest to respond to targets at the 100 ms CTOA. Although detection latencies did not decrease monotonically as a function Spatial Attention Mechanisms in Audition - 28 of CTOA, the RT increase was much more gradual than in the previous experiment; RTs increased by only 17 ms between the 300 ms and 700 ms CTOAs (compared to 55 ms in Experiment 2; see Figure 4). Neither the main effect for cue type, F(l,13) = 3.9,/? = 0.07, nor the CTOA x cue type interaction, F(3,39) = 0.4, p = 0.78, were statistically reliable. Planned comparisons found no significant cue effects at any CTOA, even when the Familywise error rate was increased to 0.20 (i.e., a = 0.5 for each contrast). Figure 5 displays the mean RTs for targets preceded by valid and invalid cues at each CTOA. An equivalent A N O V A performed on the error rates revealed no significant effects [CTOA, F(3,39) = 2.1,/? = 0.12; Cue Position, F ( l , 13) = 0.13,/? = 0.72; CTOA x Cue Position interaction, F(3, 39) = 0.91,/? = 0.44] indicating that a speed-accuracy trade-off did not occur. Discussion Both the facilitatory and the inhibitory component of covert spatial orienting were absent in the present experiment. The lack of inhibition at longer CTOAs indicates that the negative cue effect observed in Experiment 2 reflects strategic effects rather than genuine IOR. Moreover, these results confirm previously reported observations that uninforhiative cues do not influence auditory SRT when the spatial position of sound sources is irrelevant (e.g., Buchtel & Butter, 1988; Posner, 1978; Spence & Driver, 1994; Ward et al., in press). Taken together, then, the results of Experiments 1 to 3 provide Spatial Attention Mechanisms in Audition - 29 C/3 E LU LU CO z o Q_ CO LU 100 300 500 700 900 1100 CTOA (ms) E o yj LU LU 6 100 300 500 700 900 1100 CTOA (ms) Figure 5. Mean response times (RTs; top) and cue effects (invalid RT at the four cue-target onset asynchronies (CTOAs) in Experiment 3. - valid RT; bottom) Spatial Attention Mechanisms in Audition - 30 strong evidence that spatial attention does influence auditory detection but only when spatial auditory representations are used to initiate a response. This is consistent with the hypothesis that implicit localization requires the use of spatial auditory maps whereas standard detection responses can be based on earlier non-spatial representations. As pointed out earlier, however, spatial cue effects have also been observed in simple detection tasks when auditory stimuli were presented through headphones (e.g., Bedard, et al., 1993; Mazzucchi, et al., 1983; Quinlan & Bailey, 1995). Based on their results, Quinlan and Bailey (1995) argued that spatial attention may operate at peripheral stages of auditory processing. This argument is clearly inconsistent with the conclusions drawn from the present experiment since the peripheral stages of the auditory system are organized tonotopically rather than spatiotopically. However, further support for Quinlan and Bailey's (1995) claim may come from recent studies that show that frequency discrimination can be influenced by informative spatial precues (Mondor & Zatorre, 1995; Spence & Driver, 1994, Experiment 6). Further evidence suggests that pitch discrimination can be influenced by informative (Mondor & Zatorre, 1995; Spence & Driver, 1994, Experiment 6) but not uninformative (Spence & Driver, 1994, Experiment 7) spatial precues. While these observations appear to confirm Quinlan and Bailey's (1995) proposal, at least for purely stimulus-driven mechanisms, an alternative explanation is that later spatial representations may have been used strategically to form responses in each of these tasks. This strategy seems likely in each of these tasks since spatial position was made relevant by using either informative precues or highly localizable stimuli. It is also important to note that subjects may adopt similar spatial strategies when spatial Spatial Attention Mechanisms in Audition - 31 position has little or no task relevance, if non-spatial features also have no task relevance. This may help to explain why small cue effects are observed in some but not all simple detection tasks. Experiment 4 was conducted to determine whether spatial cues would affect auditory detection when a non-spatial target feature (target frequency) is made relevant for the response. Experiment 4 Method Subjects. Fourteen students (7 female, 7 male) attending the University of British Columbia were paid for their participation. Eight subjects had participated in one of the previous experiments. All subjects reported normal hearing and had normal or corrected to normal vision. Stimuli and Apparatus. The stimuli and apparatus were the same as those in Experiment 1, except that central target tones were replaced by 2000 Hz peripheral tones at 75 dB. Design and Procedure. The design and procedure were similar to those in Experiment 1, except that the decision to respond on each trial was based on target frequency rather than target position. Subjects were instructed to respond to 1000 Hz (standard) tones but not to 2000 Hz (high) tones and were informed that the spatial location of the target tones were irrelevant. High tones (i.e., No-Go targets) occurred on one-third of all trials. The overall number of trials remained the same as in the previous experiments; subjects ran in 27 practice trials followed by 14 blocks of 27 data trials. Spatial Attention Mechanisms in Audition - 32 Results The mean RTs and error rates are shown in Table 3. A 3 x 2 repeated measures A N O V A (CTOA [100 ms, 300 ms, and 700 ms] x cue type [valid and invalid]) revealed that only the main effect for CTOA was significant, F(2,26) = 8.4, p < 0.001. Subjects were slowest to respond at the shortest CTOA, indicating that a temporal alerting effect occurred. Neither the effect of cue type, F(l,13) = 0.01,/? = 0.9, nor the CTOA x cue type interaction, F(2,26) = 1.7,/? = 0.2, were statistically reliable. Planned comparisons found no significant cue effects at any CTOA. Figure 6 shows the mean RTs for targets preceded by valid and invalid cues as a function of CTOA. An equivalent A N O V A performed on the error rates revealed no significant effects [CTOA, F(2,26) = 0.3,p = 0.78; cue type, F ( l , 13) = 2.5,p = 0.14; CTOA x cue type interaction, F(2, 26) = 0.4,/? = 0.70] indicating that a speed-accuracy trade-off did not occur. Discussion The results of this experiment replicated the null effects found in Experiment 3: spatial precues failed to affect detection performance at any CTOA when the decision to respond was based on non-spatial target features. These null results are consistent with previous findings (e.g., Experiment 3, this study; Buchtel & Butter, 1988; Spence & Driver, 1994) and provide converging evidence that auditory detection is under stimulus-driven attentional control only when spatial information is made relevant for the response. Moreover, the lack of significant negative cue effect at the 700 ms CTOA indicates that IOR is also absent when SRT responses are based on non-spatial information. Spatial Attention Mechanisms in Audition - 33 350 1 i •-100 300 500 700 CTOA (ms) 50 05 LU -25-O -50 I - i -. ,-100 300 500 700 CTOA(ms) Figure 6. Mean response times (RTs; top) and cue effects (invalid RT - valid RT; bottom) at the three cue-target onset asynchronies (CTOAs) in Experiment 4. Spatial Attention Mechanisms in Audition - 34 General Discussion A vast number of studies have investigated the effects of spatial cues on subsequent behavioral performance (see van der Heijden, 1992 for a review). It is now well established that spatial precues can be used to direct covert visual attention, leading to faster and more accurate detection of targets appearing at the cued location relative to targets appearing at uncued locations (e.g., Muller & Humphreys, 1991; Posner, 1980). Moreover, in vision, the appearance of an uninformative direct cue typically has a biphasic effect on detection RTs (Maylor, 1985; Maylor & Hockey, 1985; Posner & Cohen, 1984). At longer CTOAs, the initial RT advantage for visual targets presented at the cued location is often replaced by a relative performance decrement. This pattern of results is characteristic of stimulus-driven covert orienting, although the later inhibitory effect (IOR) has been attributed to both attentional (e.g., Reuter-Lorenz et al., 1996) and oculomotor (e.g., Rafal et al., 1989; Berlucchi et al., 1989) processes. The present set of experiments was designed to investigate stimulus-driven spatial attention mechanisms within audition. Previous auditory cueing studies have shown convincingly that attention can be aligned to the frequency of a sound source (e.g., Mondor & Bregman, 1994; Mori & Ward, 1992, 1993; Scharf et al., 1987). By contrast, the evidence for attention shifts within auditory space remains somewhat more equivocal because the majority of spatial attention studies have either reported null results (e.g., Buchtel & Butter, 1988; Posner, 1978) or are susceptible to non-attentional interpretations (e.g., Rhodes, 1987; Spence & Driver, 1994; Ward, 1994). Consequently, the experiments reported here were conducted in an attempt to clarify the conditions Spatial Attention Mechanisms in Audition - 35 under which stimulus-driven spatial attention shifts may occur in audition, while at the same time precluding a number of non-attentional explanations. A new spatial cueing technique, called implicit spatial localization, was introduced (Experiment 1) in which the decision whether or not to make a SRT response depends on the spatial position of the target stimulus. In Experiment 1, observers responded to peripheral targets and refrained from responding to central targets, but in principle the critical locations could be anywhere in the perceptual field. Under these conditions, subjects were significantly faster to respond to validly cued targets at the 100 ms CTOA. This cue effect reduced and even reversed at longer intervals, providing strong evidence that facilitation and inhibition of return may both occur in audition. Although each component has been observed separately in previous studies, this is the first clear demonstration of a biphasic effect of uninformative spatial cues on auditory SRT. The results of Experiments 2 and 3 show that it may be critical to consider this overall pattern of results when interpreting spatial cue effects, especially at longer CTOAs. Spatial Relevance and Auditory Spatial Attention The similarity between the results of Experiment 1 and those found in visual attention studies suggests that stimulus-driven attention mechanisms may operate in both vision and audition. The initial facilitatory cue effect confirms previous reports that attention can be directed to the spatial position of sound sources in order to aid auditory processing (e.g., Mondor & Zatorre, 1995; Spence & Driver, 1994; Ward, 1994). However, this facilitation was not observed in later experiments when the go/no-go Spatial Attention Mechanisms in Audition - 36 decision was based on target presence (Experiments 2 and 3) or target frequency (Experiment 4). Together, these results indicate that it may be necessary to establish spatial relevance for spatial attention to influence auditory detection. This proposal is similar to previous claims that explicit localization responses are required to observe spatial cue effects in audition (e.g., Rhodes, 1987; Spence & Driver, 1994; Ward, 1994), but emphasizes that SRT responses may also be affected by spatial attention when the targets are localized implicitly. Following the logic of Rhodes (1987), it is hypothesized that establishing spatial relevance in auditory RT tasks necessarily invokes the use of higher spatial representations in executing SRT responses. The results of the four experiments reported above suggest that implicit spatial localization is a powerful technique for establishing spatial relevance in auditory SRT tasks. As already mentioned, however, small but reliable cue effects on auditory detection have also been observed in situations where the decision to respond did not depend on the spatial position of the target (Bedard et al., 1993; Mazzucchi, et al., 1983; Quinlan & Bailey, 1995). These findings seem to provide direct evidence against the spatial-relevance hypothesis and have previously been interpreted as indicating that spatial attention may operate "at a very peripheral stage of auditory processing" (Quinlan & Bailey, 1995, p 626). But while listeners were not required to localize target sounds in any of these studies, space may have been made relevant by at least two factors. First, informative spatial cues were used in each of these studies. Unlike the uninformative cues used in Experiments 2, 3 and 4, informative cues make space relevant by indicating the most likely target location. Second, auditory stimuli were presented through headphones Spatial Attention Mechanisms in Audition - 37 rather than through external speakers, thus making the location of the sound sources unambiguous. The results of the present experiments suggest that these two factors may have provided subjects with enough incentive to allow higher spatial representations to form their responses. Further support for this interpretation comes from studies that show reliable effects of informative but not uninformative spatial cues on pitch discrimination (Mondor & Zatorre, 1995; Spence & Driver, 1994).3 In sum, the degree of spatial relevance invoked by a given task appears to be a critical factor for the observation of spatial cue effects in audition. The results of the present study show that implicit localization is a powerful technique for establishing spatial relevance in SRT tasks. Previous findings indicate that auditory detection can also be influenced by spatial precues when implicit localization is not required, but only when spatial relevance is established by other task parameters (e.g., informative cues, unambiguous target localization). Consequently, it appears that spatial attention can affect auditory detection and. discrimination latencies when responses are based on higher spatial representations (cf, Rhodes, 1987). Inhibition of Return in Audition The results of the present experiments help to clarify the conditions under which stimulus-driven covert spatial orienting may influence behavioral performance in audition. This is the case for both the initial facilitatory effect of spatial precues as well as the inhibitory effect at longer CTOAs. Until very recently, studies of IOR within audition found no inhibitory effects of uninformative cues (auditory or visual) on auditory response Spatial Attention Mechanisms in Audition - 38 latencies (e.g., Farah, et al., 1989; Spence & Driver, 1994). Ward (1994) was the first to report the possible existence of auditory IOR using a left/right localization task, but these results may partially reflect stimulus-response compatibility effects. Other recent studies, however, have found IOR in a variety of SRT tasks which are not susceptible to this explanation (Reuter-Lorenz & Rosenquist, in press; Schmidt, in press; Tassinari & Berlucchi, 1995; Spence & Driver, 1996). These studies have also made the important observation that IOR is not always a reliable consequence of spatial precues in audition. Two general explanations have been offered to account for the occurrence of IOR within audition. First, auditory IOR has been explained in terms of explicit oculomotor activation (Reuter-Lorenz & Rosenquist, in press). Support for this proposal comes from data that show that uninformative auditory cues produce IOR when subjects are instructed to prepare or execute saccadic eye movements to the cued location but not in the absence of such requirements (Reuter-Lorenz & Rosenquist, in press; Schmidt, in press). These findings indicate that explicit oculomotor activation is sufficient to produce IOR in at least some auditory tasks. The results of the present study, on the other hand, demonstrate that oculomotor activation may not be necessary for IOR to occur in audition. In Experiment 1, strong IOR was observed without requiring any explicit oculomotor preparation or execution. In fact, subjects were instructed to maintain eye position on a central fixation point and to ignore the cue stimuli. Although eye position was not monitored, recent studies have found that eye movements are rare in auditory tasks and that eliminating trials in which the eyes move has no significant effect on the magnitude of IOR (e.g., Reuter-Lorenz et al., 1996; Reuter-Lorenz & Rosenquist, in press). Moreover, in the present Spatial Attention Mechanisms in Audition - 39 study, IOR was only observed when the decision to make an SRT response was based on the spatial position of the target. Explanations of these results in terms of oculomotor programming would have to assert that listeners prepared eye movements in the implicit localization task but not in the remaining tasks. A more parsimonious explanation of the current data appears to be that establishing spatial relevance may be both necessary and sufficient for IOR to occur in audition. Making space relevant in an RT task presumably invokes the use of higher spatial representations which may be necessary to produce auditory spatial cue effects. This proposal is identical to that offered for the earlier facilitatory cue effect, except that it is broadly consistent with both attentional and oculomotor accounts of IOR in audition. The occurrence of IOR in audition has also been explained in terms of modality-uncertainty rather than explicit oculomotor activation (Spence & Driver, 1996). As previously mentioned, Spence and Driver (1996) observed auditory IOR in a target-target paradigm only when auditory and visual stimuli were presented randomly in the same experimental block. They speculated that SRT responses to auditory targets may be based on multimodal representations when target modality is uncertain. Such a representation is known to exist in the deeper layers of the superior colliculus and is inherently spatial (see Stein & Meredith, 1993 for a review). Presenting mixed-modality targets, then, may also activate higher-level spatial representations that are required to inhibit responses. This effect might even arise because the visual targets (which are highly localizable) reduce the spatial uncertainty of the auditory stimuli. In other words, mixed target modality might be another technique to establish spatial relevance in a cross-modal task. Spatial Attention Mechanisms in Audition - 40 Conclusions The results of the present experiments reveal two important findings about auditory spatial attention. First, uninformative spatial cues influenced auditory detection only when the decision to respond was based on target location. This implies that implicit localization is sufficient to generate spatial cue effects in audition, presumably because it establishes spatial relevance. Second, both the initial facilitatory and later inhibitory components of covert spatial orienting were obtained using the implicit localization task. The similarity between these results and those found in visual attention studies suggest that similar covert orienting mechanisms may exist in audition. Moreover, this is perhaps the first clear demonstration of auditory IOR. The fact that IOR occurred only when the decision to respond was based on the spatial position of the target suggests that oculomotor preparation may not be sufficient to generate auditory IOR, unless peripheral sounds activate oculomotor responses only when the task is inherently spatial. Further research is necessary to distinguish between these two accounts. The implicit localization paradigm offers a number of advantages in the study of auditory spatial attention. First, the go/no-go decision allows for the examination of auditory spatial attention effects since it establishes spatial relevance and therefore requires the use of spatial auditory representations (cf, Rhodes, 1987; Spence & Driver, 1994; Ward, 1994). The results of the present study extend Rhodes' (1987) initial proposal by showing that SRT responses can be based on spatial rather than non-spatial auditory maps. Second, implicit localization makes space pertinent for a SRT response without requiring explicit localization judgments, thus precluding a number of non-attentional explanations Spatial Attention Mechanisms in Audition - 41 (such as stimulus-response compatibility and relative judgment effects). While less direct methods (e.g., informative cues, mixed modality, etc.) might also invoke spatial relevance and/or spatial representations, implicit localization offers clear advantages in terms of (i) controlling the listeners' strategies and (ii) studying purely stimulus-driven mechanisms within one modality. Finally, unlike standard SRT tasks, the go/no-go decision also prevents any interpretation of existing cue effects in terms of changes in response criterion (cf, Shaw, 1980). Although designed specifically for investigating auditory spatial attention, similar advantages for implicit spatial localization may also exist in the study of visual attention. Spatial Attention Mechanisms in Audition - 42 N O T E S 1. Note that this interpretation requires the assumption that cued-location responses are first primed and then later inhibited relative to uncued responses (cf, De Jong, Liang, & Lauber, 1994). However, little evidence for this pattern of stimulus-response compatibility effects was obtained in a recent event-related potential (ERP) study using symbolic cues (Eimer, 1994). 2. In all experiments reported in this article, multiple comparisons were planned in advance for cued and uncued target conditions at each CTOA. Bonferroni t-tests were run using the MSe from the experiment's CTOA x target type interaction to calculate the critical difference for the cue effects. All such tests were two-tailed with Familywise error rates set at 0.10 unless otherwise noted. 3. 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