Does auditory attention shift in the direction of an upcoming saccade?

\ PERGAMON

Neuropsychologia 26 "0888# 246Ð266

Does auditory attention shift in the direction of an upcoming saccade< Chris Rordena\b\\ Jon Drivera a

Department of Psychology\ Institute of Cognitive Neuroscience\ Alexandra House\ University College London\ Gower St\ London WC0E 5BT\ UK b MRC Cognition and Brain Sciences Unit\ 04 Chaucer Road\ Cambridge CB1 1EF\ UK Received 7 September 0886^ accepted 17 May 0887

Abstract In a series of experiments\ we examined whether auditory attention shifts in the direction of an upcoming saccade\ as recently reported for visual attention[ Normal listeners made speeded discriminations for the elevation "up versus down# of abrupt sounds\ regardless of their laterality[ Each sound was presented around the time that a lateral saccade was made[ Auditory elevation discriminations were reliably faster when the saccade was made towards the side of the auditory probe rather than away[ Furthermore\ an auditory probe on one side speeded up centrally!cued saccades made in that direction\ although sounds did not in~uence saccades to peripheral visual events[ The latter visual events were insu.cient to a}ect hearing unless a saccade was made towards them[ A further study showed that _xating towards or away from sounds\ rather than saccading\ could also a}ect elevation judgements\ with poorer performance when _xating away[ However\ the in~uence of an upcoming saccade upon hearing could not be reduced to this _xation e}ect\ since the saccade in~uence was found even for sounds which terminated before any shift in _xation began[ Taken together\ our results imply that the direction of an upcoming saccade can a}ect hearing\ as can eye!position[ Þ 0888 Elsevier Science Ltd[ All rights reserved[ Keywords] Covert orienting^ Eye movements^ Hearing^ Crossmodal\ Premotor theory

0[ Introduction Many previous studies have examined the possible links between saccades and visual attention "e[g[ Refs[ ð0\ 2\ 4\ 5\ 03\ 12\ 15\ 30\ 49Ł#[ However\ no study has ever considered whether eye!movements have any impli! cations for auditory attention[ Here we test this issue experimentally for the _rst time[ We begin with a necess! arily selective review of the extensive and complex litera! ture on how saccades may relate to visual attention[ We then turn to the auditory domain\ for which the question has not previously been considered[ Existing visual stud! ies raise several methodological issues that must be addressed when designing a study of possible saccadic in~uences on audition\ and it is for this reason that the visual literature is considered at some length below[

 Corresponding author[ Tel[] ¦33!060!280!0014^ Fax] ¦33!060! 805!7406^ E!mail] chris[rordenÝmrc!cbu[cam[ac[uk[

1[ Previous behavioral studies of saccades and visual attention In the natural environment we frequently saccade towards interesting regions of the visual world\ in order to process them in greater detail[ Nobody would dispute that foveated visual stimuli can be resolved more precisely than eccentric stimuli\ for many visual purposes[ However\ the more interesting question is whether an upcoming saccade can a}ect performance even before the retina shifts\ perhaps due to attention shifting towards the saccade target before the physical eye movement begins[ The neural events which underlie saccade pro! gramming often commence some considerable time before the eye actually starts to move\ as revealed by electrophysiological recordings in animals ð51Ł[ Thus\ saccade programming could in principle a}ect sensory coding before the saccade is executed[ Indeed\ numerous single!cell studies in behaving monkeys have now revealed enhancement of the neural response to visual

9917!2821:88:, ! see front matter Þ 0888 Elsevier Science Ltd[ All rights reserved[ PII] S 9 9 1 7 ! 2 8 2 1 " 8 7 # 9 9 9 6 1 ! 3

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events presented close to the saccade destination\ with this enhancement arising even before the eye begins to shift ð09\ 36\ 52\ 54\ 55Ł[ The exact nature of any causal relationship between covert attention and saccade programming in humans has been hotly disputed "e[g[ Refs[ ð13\ 35Ł#[ As Shepherd et al[ ð49Ł note\ this relationship could in principle take three very di}erent forms[ At one extreme\ the two pro! cesses might be completely independent\ with saccades and shifts of covert attention taking place autonomously[ Many previous studies have shown that visual attention can be directed covertly to a precued location ð13\ 30\ 46Ł without a saccade actually being executed[ However\ this observation alone may not entail a strict independence between saccade programming and covert attention\ because saccades might still be programmed towards the cued location in such cases ð34\ 35Ł\ with only their actual execution being prevented[ On the other extreme view\ of complete dependence\ saccade programming and covert attention would be one and the same[ Rizzolatti and colleagues| {{premotor the! ory|| of attention ð34\ 35Ł may come close to such a view\ since it posits that covert spatial attention is entirely subserved by planned but unexecuted movements\ with motor programs acting to bias sensory processing towards the target of the planned action[ Premotor theory actually encompasses several types of movement in addition to saccades\ since it proposes that spatial atten! tion is subserved by several distinct sensorimotor circuits\ each concerned with di}erent types of action and di}erent sectors of space[ Although premotor theory is thus not uniquely concerned with just saccades\ it nevertheless strongly predicts that covert attention should invariably shift in the direction of an upcoming saccade[ The third possibility is an intermediate one\ whereby covert attention and saccade programming might have substantial overlap\ but also some degree of inde! pendence[ For instance\ it might sometimes be possible to shift attention without always fully preparing an eye! movement^ or to form at least the initial plan for a saccade without always shifting attention in a corresponding direction[ Even if this degree of independence applied\ actually releasing a saccade might still invariably trigger a corresponding shift of attention before the eye itself moves[ In sum\ on any view except total independence\ one would expect upcoming saccades to have at least some in~uence on covert visual attention[ Our review below suggests that saccade programs which have been released "and thus are about to be executed\ rather than still being withheld# certainly do in~uence visual per! formance before the eye itself moves[ The earliest studies of saccade!contingent shifts in attention allowed the participant to choose which direc! tion to saccade towards when presented with a brief cir! cular array of visual characters ð0\ 4Ł[ Character identi_cation was more accurate in the direction of the

subsequent saccade even though the display had ter! minated before saccade execution[ However\ participants may have chosen to saccade towards whichever charac! ters they were already attending or perhaps towards those which were already perceived better due to random factors[ Hence the correlation between report accuracy and saccade direction in such studies does not unam! biguously establish the direction of any causal relation! ship[ Enforcing a saccade on the participant and then testing for any in~uence of this on visual judgements\ seems a better methodology[ Posner ð30Ł summarized several studies in which sac! cades were imposed by the experimenter[ Participants were required to saccade rapidly towards a peripheral visual event and also to press a key when they detected a small dot\ which could appear equiprobably at original _xation or at the saccade destination[ This dot could be presented at several intervals before and after saccade execution[ Subjects were faster to detect the dot at the saccade destination than at the saccade origin\ even before the saccade started\ suggesting that covert atten! tion precedes the eye to its destination[ However\ there are possible methodological problems with this study[ In particular\ the peripheral event which triggered the saccade may also have served as a peripheral cue that attracted attention automatically ð11Ł[ If so\ the results might not re~ect saccade preparation at all[ A similar problem applies to all the positive results from Reming! ton ð33Ł\ which have also been frequently cited as sug! gesting saccade!contingent shifts of covert visual attention[ In another well!known study\ Klein ð12Ł argued that mere saccade preparation is insu.cient to produce shifts of covert visual attention "see alsoð13Ł#[ It is important to note that his experiments concerned possible in~uences on vision from saccades that may have been planned but were never actually executed "unlike the studies described above\ which all concerned saccades that were released#[ Participants in Klein|s ð12Ł study were asked to plan a saccade in the same direction throughout a block\ but to prevent themselves from actually executing this saccade until given the appropriate go!signal "a peripheral aster! isk#[ On each trial they either went on to execute the saccade\ or to make a judgement about whether a per! ipheral marker at either of the possible saccade des! tinations "one on each side# had dimmed[ Thus\ on each trial the participant only had to make a single response] either a saccade\ or an unspeeded luminance decrement judgement[ Klein found that perceptual judgements were una}ec! ted by which side the saccades were directed towards during the block\ suggesting that saccade preparation "without subsequent execution# did not shift attention[ However\ Rizzolatti ð35Ł has argued that saccades may not have been fully prepared in this study until the go! signal was presented[ If so\ saccade preparation would

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not have taken place on those trials where only a per! ceptual judgement was performed\ rendering the null e}ect of saccade direction uninformative[ The dispute over whether the mere preparation of a saccade that is not subsequently executed will invariably induce a shift in visual attention continues to this day ð14Ł[ However\ all parties in this dispute now seem agreed that released saccades "i[e[ those which are both prepared and then executed# can in~uence visual performance before the eye moves\ as implied by the more recent studies discussed below[ The present paper is concerned only with the e}ect of such released saccades\ not with saccades that are partially planned but never released[ Methodologically\ released saccades have the virtue that there can be little dispute over whether or not they were programmed\ since they are fully executed on every trial[ Hence\ their latency with respect to any sensory probe can be measured\ in order to assess the likelihood that the saccade had been programmed when the probe was presented[ Shepherd et al[ ð49Ł used a central cue to direct lateral saccades that were always released[ Their subjects made a speeded manual detection response to suprathreshold visual targets\ which appeared either at the saccade des! tination\ or on the opposite side\ at various points in time[ Most targets were presented at intervals where they would occur after a fast saccade\ and could thus bene_t from direct _xation "which should encourage rapid sac! cades#[ The important result was faster detection of tar! gets in the direction of the saccade even for targets that _rst appeared before the eye shifted[ This appears to provide straightforward evidence for covert attention preceding the eye to the destination of a released saccade[ We have only two main concerns with the Shepherd et al[ ð49Ł study[ First\ the targets remained visible until the detection response was made\ and so were potentially available for foveated post!saccadic processing[ Since manual detection responses were always made sub! stantially after the saccade was initiated\ foveation might thus have contributed to performance even for targets which _rst appeared immediately prior to the saccade[ The second potential problem is that speeded detection responses "simple RTs# are well known to be susceptible to criterion shifts ð6\ 38Ł\ so that faster responses may not necessarily imply better perceptual sensitivity[ An e}ect from an upcoming saccade on sensory performance would be more convincing in a speeded choice task\ where accuracy as well as latency could be measured to assess any speed:accuracy tradeo}s[ Our own auditory studies were closely modeled on the Shepherd et al[ visual study\ but sought to remedy both of the potential de_ciencies in their methodology[ More recent studies have since con_rmed Shepherd et al[|s ð49Ł _nding that visual performance is better in the direction of an upcoming "to be released# saccade than elsewhere[ Moreover\ they _nd this even for targets which have completely disappeared by the time the saccade is

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completed[ In Chelazzi et al[|s ð2Ł study\ a central auditory tone cued the subject to execute a lateral saccade\ which was made in the same direction for an entire block[ Per! ipheral visual targets were presented for simple reaction time "RT# at various intervals following the auditory go! signal[ On some trials visual targets were presented and extinguished before the saccade began\ yet detection responses were still faster for visual targets in the direc! tion of the upcoming saccade[ The only possible objection here is that the simple RT measure may be open to criterion shifts[ Finally\ several recent studies "e[g[ ð5\ 03\ 15Ł# have demonstrated better performance in unspeeded dis! crimination of masked visual targets when these are pre! sented at the destination of an upcoming saccade[ These experiments avoid the methodological problem of targets still being present after the saccade "because the target has been masked by this point of time in the critical conditions#^ and also the objection that mere criterion shifts might be responsible for the e}ects "since dis! crimination accuracy is measured#[ These latest results from masking studies thus convincingly demonstrate that upcoming saccades can in~uence visual performance\ leading to better judgements for targets in the vicinity of the saccade destination before the eye shifts[ However\ the recent masking methods for studying visual attention are not immediately adaptable for studying auditory attention\ which was the aim of our own experiments[ For the reasons described below\ we chose to use an auditory RT measure\ in a study that was analogous to Shepherd et al[|s ð49Ł pioneering visual RT study\ but improved and adapted for the study of hearing[

2[ Measuring spatial attention in hearing We considered it prudent to use an established index of auditory attention when testing for the _rst time whether saccades can in~uence auditory attention[ Recent work on the spatial distribution of covert auditory attention ð41Ð43Ł has used an RT measure "rather than accuracy for masked targets#[ Accordingly\ our study also used an auditory RT measure\ and was thus modeled more closely on Shepherd et al[|s ð49Ł seminal visual RT study than on the recent visual masking studies[ However\ unlike Shepherd et al[\ we used a choice RT task rather than simple RT\ in an e}ort to preclude the involvement of criterion!shifts[ Speci_cally\ we adapted an auditory choice RT task that has been successfully developed by Spence and Driver ð41Ð43Ł to measure the spatial distribution of cov! ert auditory attention[ In their task\ participants make speeded manual responses to indicate the elevation "up versus down# of abrupt auditory targets\ regardless of their laterality[ Covert attention can be cued to the left

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or right in various ways during this task "e[g[ by the occurrence of a spatially uninformative cue event on one side\ to pull {{exogenous|| attention there^ or by an expect! ancy about the likely target side\ which should induce participants to push their {{endogenous|| attention there#[ The prediction is that better up:down localization should be found on a covertly attended side than on the other side[ In a series of studies\ Spence and Driver have repeat! edly con_rmed ð41Ð43Ł that auditory elevation judge! ments are indeed faster "and usually more accurate# for sounds falling on the side which is covertly attended\ be this in an exogenous or endogenous manner[ Their method provides a sensitive measure for the spatial dis! tribution of covert attention\ which is not readily sus! ceptible to criterion shifts "since choice RT rather than simple RT is required and accuracy measured#^ nor to response!priming accounts "since the up:down dimension that must be judged is orthogonal to the left:right dimen! sion along which attention is cued#[ However\ none of Spence and Driver|s studies ever directly examined the possible impact that saccades might have on auditory attention[ In most of their experiments\ central _xation was explicitly required and this was often monitored with eye!trackers\ so that saccades were excluded rather than manipulated[ Here we test for the _rst time whether planning and releasing a saccade towards one side produces better auditory elevation judgements on that side compared with the other side[ This should be found if covert auditory attention pre! cedes saccades to their destination\ in the manner recently demonstrated for covert visual attention[

3[ Why might saccades affect hearing< Intuitively\ one might suppose that saccades should have a tighter link with speci_cally visual attention than with any other modality\ since the apparent purpose of saccades is to explore just the visual world\ selecting relevant sights for further processing with foveal resources[ However\ it may be naive to assume that any e}ect of saccades on attention must be restricted to vision\ for several reasons[ First\ the intuition that such a restric! tion should apply stems largely from the fact that sac! cades unavoidably change visual input "by inducing a shift in retinal location# but cannot change the input to the auditory system at the level of receptors in an anal! ogous manner "this would require a head movement#[ However\ note that the presaccadic shifts of visual atten! tion described above cannot themselves be attributed to any changes at the level of receptor input[ They must be due instead to changes in internal representation\ since they arise before the eye itself moves[ It is thus possible\ in principle at least\ that similar changes in internal rep!

resentation prior to a saccade could a}ect other sensory modalities\ such as hearing[ Moreover\ as discussed in full later\ a few previous studies ð00\ 06\ 07\ 10\ 16\ 17\ 20\ 32Ł have suggested that human selective!listening performance "in tasks such as shadowing# can sometimes be better when listeners _xate towards relevant sounds rather than towards distracting sounds\ even though this does not change the input to auditory receptors[ However\ to our knowledge\ no such study has ever looked directly at the possible in~uence of saccades\ having manipulated only the direction of steady _xation[ Thus\ the possible in~uence of saccades on hearing remains entirely unknown\ although there is some precedent for modulation of hearing by gaze direction[ A third reason to suspect that saccades might in~uence auditory attention stems from the premotor theory described earlier ð34\ 35Ł[ Covert spatial attention undoubtedly exists within audition\ as documented by Spence and Driver ð41Ð43Ł and others ð29\ 31\ 47Ł in recent spatial cuing studies[ According to premotor theory\ such spatial shifts in covert attention must be produced by the preparation of spatial motor acts towards a particular location\ as this is the only mechanism for spatially selec! tive perception according to the premotor account[ Motor acts such as saccades would thus certainly be expected to shift auditory attention towards their des! tination according to this theory[ Fourth\ much of the neural machinery involved in generating saccades "e[g[ the superior colliculus\ sub! stantia nigra and parietal lobe# is sensitive to auditory stimuli as well as to visual stimuli[ In some of these structures the modalities are coded in approximate spatial register\ as revealed by single!cell recording studies in animals ð08\ 19\ 44Ł[ To the extent that activity in these neural systems is responsible for the presaccadic shifts of visual attention that have now been documented behavi! orally\ then similar presaccadic attention shifts might also be expected in audition[ Indeed\ as we discuss later\ several neurophysiological studies have already observed that single!cell responses to a sound can be modulated by the direction of an animal|s gaze ð01\ 05\ 44\ 45Ł\ even though this cannot change the auditory input at the receptor level[ Finally\ it is a considerable shortcoming of most atten! tion research that each sensory modality is typically stud! ied in isolation\ with the focus usually being upon just vision in most recent work[ It is clearly important to determine whether any general principles of selective attention apply across the sensory modalities\ especially since attention in everyday life clearly needs to be con! trolled and directed in a multimodal fashion[ For all these reasons\ we considered it important to determine whether upcoming saccades trigger shifts in auditory attention\ analogous to the presaccadic attentional shifts recently documented for vision[

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4[ Experiment 0 Our _rst study closely followed Shepherd et al[|s ð49Ł visual study\ but examined audition for the _rst time[ We adapted their general RT methodology for studying presaccadic shifts of attention\ but used Spence and Dri! ver|s ð41Ð43Ł speci_c RT method for measuring the spatial distribution of auditory attention[ A symbolic central cue instructed participants to make a saccade either left or right as fast as possible on each trial[ At various points of time in relation to this instruction\ a peripheral auditory target was presented on one side or the other for speeded elevation discrimination "up versus down\ as in Spence and Driver|s studies#[ By analogy with Shepherd et al[|s visual study\ covert auditory attention might be expected to shift towards the saccade destination before the eye actually moved[ By analogy with Spence and Driver|s previous studies of auditory attention\ if attention does indeed shift ahead of the eye in hearing as well as in vision\ then auditory elevation judgements should be fas! ter and:or more accurate for a target sound presented on the side that the saccade is made towards\ rather than on the opposite side[ To demonstrate a truly presaccadic shift of auditory attention\ this should be found even for sound targets presented just before the saccade actually took place[ Note that the present auditory elevation task requires a choice RT\ rather than merely a simple detection RT as in the visual studies of Shepherd et al[ ð49Ł and Chelazzi et al[ ð2Ł[ This should be advantageous in two respects[ First\ simple RT to sounds is an insensitive measure for the spatial distribution of auditory attention ð43Ł\ perhaps because sounds "unlike visual events# can be detected on the basis of early nonspatial representations\ as the initial stages of hearing are tonotopic rather than spatiotopic "whereas vision is spatially organized from the retina onwards#[ Unlike simple RT\ the elevation discrimination used here is known to provide a particularly sensitive measure of auditory attention ð41Ð43Ł[ A second advan! tage of the present method is that discrimination tasks are less susceptible to possible criterion!shifting accounts than the simple detection response used by Shepherd et al[ ð49Ł and Chelazzi et al[ ð2Ł\ since accuracy as well as latency can be considered[ A further methodological change from Shepherd et al[|s ð49Ł visual study was that we tracked eye movements throughout each trial\ whereas they had only measured the direction and amplitude of the _rst saccade on each trial\ raising the possibility that subjects might sometimes have saccaded back to the center\ which could complicate any attentional shifts[ Here we examined eye position for the full epoch between saccade instruction and sound discrimination\ discarding any trials where subjects initially saccaded to the speci_ed side but then saccaded back to the central _xation point prior to their auditory judgement[

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Finally\ Shepard et al[ ð49Ł had presented visual targets which remained visible for the duration of the trial[ This was unfortunate\ as perceptual information sampled after the lateral saccade had been made could in principle have a}ected the speeded visual detection response\ which typically emerged only after the saccade had been executed[ We used quite brief peripheral targets for the auditory discrimination task\ so that on many trials these sounds would terminate before the saccade began[ In sum\ the task on each trial in experiment 0 was to make a saccade in the centrally!cued lateral direction and also to make a speeded manual response to the elevation "regardless of laterality# of a peripheral auditory target[ This sound target was just as likely to appear on the side away from the saccade as on the ipsilateral side\ so participants had no strategic motivation to shift their auditory attention in the direction they were saccading towards[ However\ if presaccadic shifts of covert atten! tion arise spontaneously in hearing\ as recently found for vision\ then better elevation judgements should be found for sounds appearing in the direction of the saccade\ even shortly before the eye moves[ 4[0[ Method 4[0[0[ Participants Ten volunteers took part in this experiment and were each paid -09 for their participation in two sessions "the _rst 0!h session was dedicated to eye movement training\ with experimental data collected only in the second 0[4 h session#[ Nine of these participants were right handed and all had normal\ or corrected!to!normal\ vision and normal hearing\ by self report[ Seven were female and three were male\ with ages ranging from 07 to 20 years old and a median of 16[4 years[ Participants were not informed as to the purpose of the experiment until they had completed all trials[ 4[0[1[ Apparatus and materials A schematic of the experimental setup is shown in Fig[ 0[ The experiment took place in a darkened anechoic chamber "067×011×80 cm#[ The participant sat at a table\ facing forward\ with head movement restrained by an adjustable rest[ The height of the seat was adjusted so that the ears were always at an intermediate height between upper and lower loudspeakers "see Fig[ 0#[ The experiment was controlled by a Viglen 2:22 "IBM 275 compatible# computer[ A central tri!colour LED "which could emit red\ green or yellow light# provided a central cue for indicating the required saccade direction on each trial[ Four loudspeakers "each 4×2 inch\ RS 134Ð293# were used\ mounted in columnar pairs "one 04 cm above and one 04 cm below the participant|s ear level#\ with one column at 04> to the left and one 04> to the right and all loudspeakers 46 cm from the participant[ The target sounds for the elevation task each comprised

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trial\ eye velocity was computed for each sample "cal! culating the change between the mean of the two previous samples\ versus the mean of the current and subsequent sample#[ Saccades were identi_ed when velocity exceeded 099>:s in the same direction for two successive samples[ Both saccade direction and latency "i[e[ which had been the _rst sample in the sequence to exceed criterion# were recorded[ The trial was voided if the same saccade cri! terion was subsequently exceeded in a direction opposite to the _rst saccade\ before the auditory discrimination response was executed "this was in order to exclude any trials with eye movements back toward _xation\ although it also excluded some trials with blinks subsequent to the initial saccade#[ For an acceptable eye movement to be recorded\ the eye position at the beginning of the trial had to be within 1[4> of _xation\ and the identi_ed sac! cade had to be in the cued direction\ with no subsequent saccade in the reverse direction[ Fig[ 0[ Schematic view of experiment 0\ with participant cartooned at the left of the _gure\ looking towards the central _xation point\ with columnar pairs of loudspeakers on either side and a saccade marker between the 1 loudspeakers on each side[ A central tri!colour LED indicated that either a left or right saccade was required towards one of the peripheral markers[ A target sound could come from any one of the four loudspeakers[ Participants judged whether the sound came from the upper or lower row\ regardless of which side it was on[

pulsed white!noise\ as previously used by Spence and Driver ð41Ð43Ł[ Pulsed white!noise is usually quite easy to judge in elevation\ because it has energy across the spectrum^ but note that very brief targets were used in the present study "thus making the elevation task harder#\ to ensure that some sounds would terminate just before the saccade was executed[ Each 094 ms target comprised four successive 04 ms bursts from a white noise generator ðat 72 dB"A# as measured at the participant|s ear positionŁ\ with bursts separated by 04 ms gaps[ The target sound on each trial was played from just one of the four loudspeakers\ which were all equally likely throughout[ Participants had to judge whether the sound came from above or below\ regardless of which side it was played from[ There was one peripheral yellow LED at 04> on each side at eye!level serving as a marker for a possible saccade destination^ these two LEDs were constantly illuminated throughout each block[ A feedback loudspeaker "not shown in Fig[ 0# was mounted directly above the _xation light\ and sounded whenever an error was made in audi! tory discrimination[ To allow eye!movement monitoring\ every participant wore an ASL 109 EyeTrac infra!red re~ection eye tracker\ which recorded the horizontal pos! ition of the left eye[ Analog output from the EyeTrac was sampled every _ve milliseconds throughout each trial by a 01!bit analog!to!digital converter[ At the end of the

4[0[2[ Procedure Each trial began with the central _xation LED turning from yellow to either red or green[ This central cue instructed the participant to saccade 04> either to the left or to the right "participants were randomly assigned to two groups\ either saccading left for green and right for red\ or vice versa#[ The central cue was not in any way predictive of the subsequent location for the auditory target and thus did not predict the required manual response either[ Its colour remained for the duration of the trial[ The target sound then appeared at a stimulus onset asynchrony "SOA# of 199 or 429 ms from the cen! tral colour cue[ These SOAs were chosen because pilot data suggested that most saccades would occur within 299Ð399 following the onset of the central cue[ Therefore\ at the short 199 ms SOA\ the sound target would usually appear shortly before the saccade was executed^ whereas at the long 429 ms SOA\ the saccade should already have been completed when the sound was presented[ Thus\ the short SOA data should allow assessment of any pre! saccadic shifts in auditory attention^ while the long SOA data should allow assessment of any e}ects from deviated gaze[ The participants made a speeded judgement of whether the sound came from an upper or lower loudspeaker\ pressing one of two buttons mounted directly in front of them with the index _nger or thumb of their preferred hand "the button furthest from the participant was used to indicate a sound from one of the upper loudspeakers\ while the closer button was used to indicate a lower loudspeaker#[ Upon response "or after a maximum of 1999 ms had elapsed#\ the central LED switched back to yellow[ In the case of an error in the auditory task\ a 1999 Hz square!wave tone was emitted for 199 ms from a centrally mounted speaker\ to provide feedback[ During the 599 ms inter!trial interval\ the central LED remained

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yellow[ It changed colour to indicate the start of the next trial[ Participants were given extensive training in making consistently rapid eye movements in response to the cen! tral colour cue and subsequently in doing so while making elevation judgements for peripheral auditory targets[ During the _rst session\ participants were trained just in using the colour of the central cue to initiate a saccade in the appropriate direction\ with each subject completing 3 blocks of 017 eye movements each "53 to the left and 53 to the right per block\ in a randomly intermingled sequence#[ No auditory targets were presented in this _rst training session[ At the start of the second session\ subjects completed another 39 trials of eye movements only\ with no auditory targets present[ This was followed by 53 trials of training on just the auditory targets\ with no requirement to execute any saccades at the same time[ Sounds were pre! sented and manual responses were required to dis! criminate their elevation[ After completing this training\ subjects then underwent 6 blocks of trials where both saccades and auditory target discriminations were required on each trial\ with the _rst block being discarded as practice[ Each such block contained 017 trials\ with 7 trials in each of 05 equiprobable subconditions "created by crossing auditory target left versus right^ auditory target up versus down^ short versus long SOA and sac! cade direction ipsiversive versus contraversive to the auditory target#[ For analysis we pooled over left:right and upper:lower sounds\ since these were irrelevant to our hypothesis and have been consistently found to exert no in~uence in many previous studies from our lab! oratory with the auditory elevation task "e[g[ Refs[ ð41Ð 43Ł#[ 4[1[ Results and discussion Trials were included in the analyses only if there was an initial saccade in the required direction and no sub! sequent saccade in the opposite direction before the par! ticipant made a manual response to the auditory target[ This stringent eye!movement criterion excluded 04[6) of the total trials "many of these were due to blinks after the initial saccade\ which sometimes triggered our eye! monitoring algorithm#[ In addition\ we eliminated outlier trials in which the manual RT to the auditory target was less than 049 ms or more than 0499 ms "less than 0) of trials#[ The mean RTs and error rates for each condition in the auditory task are shown in Table 0 after these exclusions[ We begin with a preliminary analysis of these overall data\ and then focus on just the subset of trials which is critical for testing any presaccadic shifts of audi! tory attention[ A two!way within!subject ANOVA was conducted on the auditory RT data\ with the factors of SOA "short versus long# and eye!movement direction "ipsiversive ver!

Table 0 199 ms SOA

RT ) EM EM)

429 ms SOA

ipsiversive

contraversive

ipsiversive

contraversive

484 4[3 205 02[7

518 5[6 223 10[9

347 5[6 220 02[9

375 6[8 214 04[0

Experiment 0] mean auditory reaction times "RT#\ auditory errors ")#\ eye!movement latencies "EM# and eye movement errors "EM)#[ The saccades were either ipsiversive or contraversive to the side of the auditory target[

sus contraversive to the auditory target#[ There was a main e}ect of SOA\ F"0\ 8#59[5\ p³9[990\ with faster responses at the long SOA[ In addition\ there was a main e}ect of saccade direction\ F"0\ 8#50[5\ p³9[990\ indicating faster auditory discriminations when the sac! cade was ipsiversive to the auditory target\ just as predicted[ There was no interaction between these factors\ F"0\ 8#9[4\ p³9[41[ An analogous ANOVA was conducted on auditory discrimination errors[ There was a main e}ect of SOA ðF"0\ 8#8[5\ p³9[902Ł with more errors at the longer SOA[ This _nding is opposite to the SOA result in RTs\ suggesting a possible speed!error tradeo} against SOA[ No signi_cant e}ect was found for saccade direction ðF"0\ 8#0[8\ p³9[19Ł\ although here the numerical trend in errors was in agreement with the reliable e}ect found in auditory RT\ ruling out a speed!error tradeo} for our critical result[ Finally\ the interaction term did not approach signi_cance for auditory errors "p9[74#[ The mean data for saccade latency are also shown in Table 0\ along with saccade error!rates "errors were mostly saccades in the uncued direction\ though some blinks may be included#[ Inspection of Table 0 suggests a pattern for faster eye movements and fewer saccade errors in the ipsiversive condition than the contraversive condition at the shorter SOA[ We return to discuss this possible e}ect and to analyze the saccade data later[ For now\ we focus our analyses on auditory performance\ and on just the subset of trials which is most informative as regards any presaccadic shift of auditory attention[ The critical trials for testing strictly presaccadic shifts of attention are those where the sound target both started and terminated immediately prior to an eye movement[ Further analysis was therefore conducted only for trials where the saccade latency was between 294 and 394 ms from the initial central cue "i[e[ the saccade had to fall within a 099 ms time!window starting immediately after the 094 ms target sound had terminated in the 199 ms SOA trials#[ This critical time!window had been selected before the experiment was run[ The lower limit on saccade latency "294 ms# was to ensure that an auditory target

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presented at the short SOA would be extinguished before the fovea started to move\ so that any e}ects must be due to saccade programming\ rather than to an actual shift in eye position[ The upper limit on saccade latency "394 ms# was chosen to increase the likelihood that the saccade had been fully programmed when the auditory target was still present at the short SOA[ "Such pre! programming is obviously very unlikely for saccades that only emerge much later[ Moreover\ our pilot work sug! gested that the bulk of the saccades should fall within the 294Ð394 ms latency range[ Hence\ the time!window restriction should also serve to eliminate outlier trials with unusually slow saccade latencies[# Single!cell recording studies in monkeys have found that neurons in the superior colliculus can show sensory enhancement for events at the saccade destination up to 099 ms before the eye movement is actually executed ð51Ł[ Thus\ there is some precedent for expecting that saccades up to 099 ms after target onset might exert some e}ect on perception of the preceding target\ but no grounds for expecting saccades later than this to have any reliable in~uence\ hence the 099 ms time!window chosen for acceptable saccade latencies[ The main purpose of our study was to test for presaccadic e}ects on hearing\ and our stringent saccade!latency criteria are clearly essential for testing this with the short SOA data[ However\ for completeness\ we also applied exactly the same saccade! latency criterion to the long SOA data\ to eliminate trials with unusually long or short saccade latencies and so produce comparable data!sets for both SOAs[ Only the short SOA data\ within the critical saccade!latency time! window\ can test for presaccadic e}ects[ The long SOA data\ with the equivalent saccade!latency criterion\ test instead for any e}ects of deviated gaze\ since for these data the saccade was always executed before the sound appeared[ Due to our stringent saccade!latency criterion\ all data from four of the ten participants had to be rejected\ due to there being less than 19 trials of correct auditory judgements in at least one condition after the exclusions within each participant "this high exclusion rate was pri! marily due to saccades that were too fast to meet our narrow time!window criterion#[ Of the remaining six par! ticipants\ four were female and two were male[ Even within this group\ 39[1) of trials were excluded because saccades occurred sooner than 294 ms after the central cue and 19[6) of trials were excluded due to the saccade occurring later than 394 ms afterwards "or not occurring at all#[ The remaining subset of trials is the critical one for testing truly presaccadic attentional shifts with the short SOA data[ The results after the time!window exclusions are summarized in Table 1\ for both short and long SOAs[ A two!way within!subject ANOVA\ with the factors of SOA and saccade direction "ipsiversive versus con! traversive#\ was performed on manual RTs in the audi!

Table 1 199 ms SOA

RT ) EM EM)

429 ms SOA

ipsiversive

contraversive

ipsiversive

contraversive

450 3[9 249 4[6

471 6[9 259 19[4

394 6[2 238 5[2

313 5[1 237 5[6

Experiment 0] mean data for trials where a saccade was executed in the critical time!window of 294Ð394 ms following the onset of the central cue[ Mean auditory reaction times "RT#\ auditory errors ")#\ eye! movement latencies "EM# and eye movement errors "EM)#[ The sac! cades were either ipsiversive or contraversive to the side of the auditory target[

tory discrimination task after these time!window exclusions[ There was still a main e}ect of SOA\ F"0\ 4#70[3\ p³9[990\ with slower responses at the short SOA than at the long SOA\ as in the overall data before time!window exclusions[ This SOA in~uence may re~ect the common {{alerting|| e}ect ð39\ 41Ð43Ł\ whereby per! formance is usually faster at longer intervals following a warning event\ such as the present central cue^ and:or a possible di.culty in performing the saccade and the auditory tasks close together in time at the short SOA\ as we discuss later[ More importantly\ there was again a main e}ect of saccade direction\ F"0\ 4#7[5\ p³9[92\ with faster audi! tory judgements when the saccade was ipsiversive to the auditory target "m372 ms# rather than contraversive "m492 ms#[ No interaction between the two factors was observed "p9[74#\ showing that saccade direction had a similar in~uence at short and long SOAs[ Critically\ saccade direction has a signi_cant e}ect on auditory dis! crimination RTs even for the short SOA data considered in isolation^ t"4#1[6\ p³9[91[ This saccade in~uence at the short SOA suggests that auditory discrimination was better near the destination of an upcoming saccade than on the other side\ even for sound targets which had ter! minated before the saccade began "since all the data for this analysis were selected so that no saccade began prior to the auditory target being extinguished at the short SOA#[ An ANOVA of identical design was conducted on the error rates in the auditory task[ No signi_cant main e}ects or interactions were found " for the main e}ect of SOA\ p9[37^ for saccade direction\ p9[44 and for the interaction\ p9[00#[ Note\ however\ that any error trend was in support of the RT e}ect\ favoring ipsiversive sac! cades at the critical short SOA "see Table 1#[ These results appear to demonstrate a presaccadic shift of auditory attention in the direction of the upcoming saccade\ producing faster elevation discrimination for a target sound on the side of the saccade destination rather

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than on the opposite side[ This pattern was found even among just those trials which met our stringent saccade! latency criteria\ so that auditory targets at the short SOA had terminated before the saccade commenced[ This result apparently con_rms the truly presaccadic nature of the phenomenon[ However\ an alternative explanation must also be con! sidered[ Looking at Table 1\ it appears that not only were manual responses in the auditory elevation task faster with ipsiversive saccades at the short SOA\ but that the saccadic responses themselves were also slightly faster in this condition "M249 ms# than in the contraversive condition "M259 ms#[ Recall that this pattern was also apparent in the saccade latency data from the initial overall analysis "i[e[ without the saccade time!window exclusions^ see Table 0#[ It has previously been reported that saccade latencies can be enhanced when an irrelevant sound is presented in the direction of the required eye! movement ð27Ł[ Could auditory targets presented on the side of the upcoming saccade inadvertently have pro! duced this e}ect in our experiment\ speeding up the sub! sequent saccade in their direction< If so\ then one might perhaps argue that manual responses to the auditory targets at the short SOA were faster in the ipsiversive condition only because saccades were also faster in this case[ Having completed the eye! movement response sooner\ participants might then pro! ceed sooner to their manual response[ Note that our study involved a dual!task situation\ with both saccades and manual responses having to be made in a speeded fashion\ and with the imperative stimuli for these two tasks occurring close together in time at the short SOA[ It is well known that in many such cases\ delays in the _rst response can be propagated into the second response "re~ecting the so!called {{psychological refractory period||\ or PRP\ see Refs[ ð3\ 23\ 24\ 26\ 48Ł#[ One popular account for such PRP phenomena is that response selec! tion in the second task "here\ auditory discrimination# may have to wait in a {{queue|| until response selection for the _rst task "here\ the saccade# has been completed[ Could this alone explain the di}erence between auditory RTs in the ipsiversive and contraversive conditions\ at the short SOA< The auditory responses might have to queue for longer at the short SOA condition with con! traversive saccades\ because these saccades took on aver! age 09 ms longer[ Pashler et al[ ð25Ł have previously shown that lateral saccades in response to symbolic central cues "as here# can delay subsequent manual choice responses to sounds in a PRP manner\ provided the sounds are presented very shortly after the saccade cue "as in the present study at the 199 ms SOA#[ To examine this possibility further\ we conducted a further two!way ANOVA "SOA×saccade direction#\ this time on the saccade latencies rather than on the manual RTs from the auditory task[ No e}ect of SOA was found "p³9[38#\ but there was a trend for an e}ect of saccade

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direction\ F"0\ 4#5[1\ p³9[95^ with slightly faster ipsi! versive than contraversive saccades overall "249 vs 243 ms#[ Moreover\ there was an interaction between SOA and saccade direction ðF"0\ 4#00[7\ p³9[91Ł[ Inspection of Table 1 indicates that this was due to slower saccades for contraversive versus ipsiversive trials only at the short SOA "a similar pattern was also found in the overall data\ before the time!window exclusions^ see Table 0#[ This pattern is unfortunate\ as it does leave open the possibility that in the critical short SOA conditions "where the target sound both began and terminated before the eye movement#\ the e}ect of saccade direction on manual RTs to the auditory targets might simply be a consequence of PRP factors\ with faster saccades in the direction of the sound leading to faster manual responses to that sound[ A _nal two!way ANOVA examined the percentage of eye movement errors "see Table 1# for trials with saccade latencies in the critical 294Ð394 ms window "these errors were mainly saccades in the wrong direction\ though some blinks would also be included#[ Note that these saccade!error trials had been excluded from all of the above analyses[ The ANOVA revealed a main e}ect of SOA\ F"0\ 4#07[8\ p³9[996\ with more errors at the short SOA "02[0)# than the long SOA "5[4)#\ pre! sumably due to the temporal overlap of saccades with auditory targets in the former case only[ There was also a main e}ect of saccade direction\ F"0\ 4#81[1\ p³9[990\ with less errors when ipsiversive "5)# rather than con! traversive "02[5)# saccades were required[ Finally\ an interaction was also apparent\ F"0\ 4#09[5\ p³9[91\ because saccade direction had a larger e}ect on saccade errors at the short SOA[ These saccade accuracy results strengthen the possibility that the auditory RT data in the elevation task\ at the short SOA\ may have been contaminated by changes in performance on the saccade task\ since contraversive saccades were clearly harder and more error!prone than ipsiversive saccades at this short SOA[ Thus\ our e}ect of saccade direction on auditory RTs at the short SOA might conceivably re~ect just a PRP e}ect\ driven by the relative ease of making eye!move! ments in the ipsiversive versus contraversive conditions at this short SOA\ with saccades being faster when a target sound was played on the same side as their des! tination just before the saccade commenced0[ In fact\ the e}ect of saccade direction at the short SOA was numerically larger for auditory elevation RTs than for saccade latencies "see Table 1#\ suggesting that a PRP e}ect may not provide the whole story[ On standard accounts of the PRP phenomenon ð26Ł\ the maximum {{queuing|| e}ect one could expect would be for all of the slowing in response!selection for task 0 "here\ a mean 0 This possibility may also apply to some previous studies of pre! saccadic shifts of attention using visual targets[

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09 ms slowing for contraversive versus ipsiversive sac! cades# to propagate into task 1^ yet the e}ect of saccade direction on auditory discrimination was twice as large as this "mean of 10 ms#[ This lends some encouragement to the view that there may be an in~uence from a pre! saccadic shift of attention at the short SOA\ in addition to any PRP e}ect[ However\ the issue is unresolved for now[ Experiments 2 and 3 return to this issue\ seeking to eliminate PRP factors[ To summarize\ the present experiment found clear evi! dence for the predicted e}ect of saccade direction on auditory judgements[ Auditory elevation discriminations were better for target sounds presented on the side of the saccade destination[ This was true at the long SOA\ where saccades were usually completed before the sound began "this always applied for those trials that passed our sac! cade!latency criterion#[ The e}ect of saccade direction on auditory performance was also found at the short SOA\ where on the analyzed subset of trials the sound had always terminated before the eye began to move[ However\ the e}ect on auditory performance at this short SOA might conceivably be due to a PRP phenomenon\ since sounds presented on the side of the required saccade speeded up the eye!movement and this alone could in principle explain the faster manual RTs to sounds in the ipsiversive condition[ Fortunately\ this potential PRP problem cannot apply to the e}ect on auditory performance at the long SOA "see Table 1#\ since ipsiversive and contraversive saccades showed equivalent latencies here "as would be expected\ since the eye!movement was made before the sound was presented#[ Hence there was no saccade latency e}ect to propagate into the auditory RTs in a PRP manner and so the ipsiversive versus contraversive di}erence in auditory performance at the long SOA requires a di}erent expla! nation[ It must be attributed either to the predicted in~u! ence from a saccade upon hearing or instead simply to an e}ect from where the eye was pointing once the sac! cade had been completed\ prior to the onset of the target sound[ Note that neither of these possibilities is trivial\ since an actual shift in eye!position cannot change the input to auditory receptors any more than internal sac! cadic programming and so even an eye!position e}ect would have to operate via internal processes[ Experiment 1 tested whether eye!position alone\ in the absence of saccades\ can in~uence performance in the auditory elev! ation task[

5[ Experiment 1 If the long SOA e}ect on hearing in experiment 0 had been due to the direction in which the eye pointed at the end of the saccade\ rather than to the saccade itself\ then a similar in~uence should be found when visual _xation is maintained towards one side or the other\ without any

on line saccades taking place[ Several previous studies have suggested that the direction of _xation can some! times in~uence hearing "e[g[ Refs[ ð00\ 06\ 07\ 10\ 16\ 17\ 20\ 32Ł#\ although none have used a task like the elevation discrimination RT employed here[ We brie~y review some of these previous _ndings below[ Gopher ð00Ł monitored gaze while people shadowed one ear during dichotic presentation of spoken messages over headphones\ and observed a tendency for listeners to look spontaneously towards the shadowed ear[ Further studies reported better selective shadowing when listeners looked towards an external loudspeaker presenting rel! evant sounds\ rather than to the loudspeaker presenting irrelevant sounds ð16\ 20Ł[ Reisberg et al[ ð32Ł had listeners look toward the relevant loudspeaker\ the distracting loudspeaker or directly in between and found a small cost from _xating distractor sounds[ However\ this result has proved controversial and di.cult to replicate "see Refs[ ð02\ 59Ł#[ Thus\ there are a few precedents for suspecting that hearing can be better when gazing towards the source of a sound\ even in situations where this cannot add any useful visual information and even though it will not change the initial input to auditory receptors[ However\ the previous evidence for such _xation e}ects in audition is rather mixed and comes from very di}erent paradigms to our own[ Accordingly\ our next study tested whether main! taining constant _xation towards one side or another would a}ect performance in our auditory elevation task[ If the long SOA e}ect in experiment 0 had been caused by the deviated direction of gaze which resulted after a lateral saccade\ it should be found once again when devi! ated gaze is simply maintained towards one side\ with no on!line saccade[ On the other hand\ if maintaining gaze towards one side were found to have no e}ect\ this would imply that the long SOA e}ect in experiment 0 had been due to the immediately preceding saccade[ Finally\ we note that if maintaining _xation towards or away from the target sound were found to have no e}ect in our elevation task\ this would mean that when returning to study any e}ect on hearing from saccades "as we do in experiments 2 and 3#\ we could be less concerned about whether the target sound had com! pletely terminated before the eye moved[ As noted earlier\ tests for presaccadic shifts in covert attention are very constrained\ because often sensory performance can only be considered in a very narrow time!window\ ideally when the saccade has been fully programmed but has not quite been executed yet[ Restricting analysis to this very narrow time!window is clearly essential in studies of vis! ual performance\ since once the eye has shifted the input to visual receptors will change for trivial reasons[ However\ it is not so obvious that the same temporal restriction must apply to studies of hearing\ as moving the eye does not shift auditory receptors[ If experiment 1 were to _nd no e}ect of eye position upon hearing in our

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elevation task\ then in subsequent studies on the possible in~uences of saccades\ we could relax the constraint that all sounds must terminate before the saccade begins in our critical conditions\ as eye!position would have been shown not to matter[ 5[0[ Method The present study was designed to investigate whether eye!position has any in~uence on auditory performance\ when the ears and head are held in a constant straight ahead position by our apparatus "as in the previous experiment#\ but the eyes now maintain _xation on a location which di}ers between blocks "left\ right or straight ahead#[ The method was similar to experiment 0 except that instead of saccading to the left or right on each trial following a central cue\ participants held constant _xation at one side or the other "or at the center# for an entire block of trials\ refraining from any saccades[ Our question was whether auditory elevation judgements would be more e.cient on the left than the right when subjects _xated left and vice!versa[ This would indicate that the direction of the fovea can indeed modulate audi! tory performance in our task[ 5[0[0[ Participants Ten new volunteers completed this experiment and were paid -6[19 for their participation "between 0[4 and 1 h\ depending on how much adjustment of the eye!move! ment monitor was required#[ Two additional volunteers were rejected for failure to complete the auditory up:down task at above 69) correct[ The participants ranged in age from 06Ð20\ with a median of 14[4[ All had normal hearing and normal or corrected!to!normal vision by self report[ Four were male and the remaining six were female and all were right!handed except for one woman who was left!handed by self!report[ Again\ participants were not informed as to the purpose of the experiment until after they had completed all trials[ 5[0[1[ Apparatus and materials The set!up was similar to that used in experiment 0\ except that the location of the _xation LED now varied between blocks[ The _xated LED was either directly in front of the participant\ or 04> to the left or right of the sagittal midline\ aligned with the column of two loud! speakers on one side[ An LED was always visible as a marker on each side\ but which position had to be _xated "left\ center or right# changed between blocks[ Eye pos! ition was monitored with an ASL EyeTrac 109\ which measured the position of the left eye as before[ Eye pos! ition was sampled every 4 ms and trials where the eye! position signal deviated more than 1[4> from the required direction of _xation were discarded from analysis "3[6) of trials\ though many of these apparent deviations were due to blinks#[

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5[0[2[ Procedure Participants _xated the appropriate light in each block\ and this was monitored as described above[ They made a speeded manual response to each auditory target to specify its perceived elevation regardless of its laterality\ as in experiment 0[ This response terminated the trial "or a maximum of 1999 ms elapsed if no response was made#[ There was then a 569 ms interval prior to the next target sound[ In the case of a sound localization error\ a 1999 Hz feedback tone was emitted for 199 ms from a central loudspeaker\ as before[ Each block contained 019 trials\ comprising 29 trials at each of the four possible target locations "top left\ bottom left\ top right\ bottom right# in a random order[ Every participant completed seven blocks of trials[ The _xation point was always directly ahead of the participant for the _rst block\ which was excluded from analysis as practice[ During the next six blocks _xation towards the center\ left or right was changed between each block\ in an order that was counterbalanced across participants\ with the constraint that during the whole experiment there were two blocks of left\ right and center _xation "excluding the practice block# for every participant[ The eye monitor was adjusted and recalibrated between each block\ to allow maximum accuracy of eye movement monitoring around the required _xation position[ 5[1[ Results The _rst block was excluded as practice[ Outlier trials\ where auditory RTs were less than 099 ms or more than 0499 ms\ were again excluded from analysis "less than 0) of trials#[ We compared trials where the sound target was ipsilateral to the point that participants looked towards "i[e[ at 9> eccentricity from deviated _xation\ with the sound source above or below the lateral _xation point#\ versus contralateral to the _xated point "with sounds now at 29> eccentricity from _xation\ but still at 04> from the sagittal midline#[ We also compared both these conditions against trials with a {{neutral|| eye position\ where the participant looked straight ahead "with sound targets now appearing at 04> eccentricity to the left or right of _xation#[ Note that both the ipsilateral and contralateral conditions were obtained within a block of deviated gaze\ while the data for the neutral condition come from separate blocks where the eyes _xated straight ahead[ Each participant completed 139 trials in each of the three _xation conditions[ The mean results per con! dition are shown in Table 2[ It appears that auditory responses were faster when the subject looked towards rather than away from the side of the sound target[ A two!tailed t!test was conducted on participants| mean RTs to determine whether they were signi_cantly faster when looking towards the target sound rather than away[ This comparison was signi_cant^ t"8#1[4\ p³9[92[ A similar comparison for the auditory error

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Table 2

RT ) Fix)

Ipsilateral gaze

Neutral gaze

Contralateral gaze

321 7[6 3[6

316 6[7 3[6

334 09[2 3[7

Experiment 1] mean auditory reaction times "RT#\ auditory errors ")# and _xation failures "Fix)#[ Gaze direction was ipsilateral or con! tralateral with the side of the auditory target\ or was directed centrally between the possible auditory target locations "neutral gaze#[

rates did not reach signi_cance ðt"8#0\ p³9[23Ł\ but note that any error trend was in the same direction as the RT e}ect[ The contralateral condition showed sig! ni_cantly more errors in the auditory task than the neu! tral!gaze condition ðt"8#1[5\ p³9[92Ł[ The comparison of ipsilateral versus neutral gaze did not approach sig! ni_cance for RTs or error rates[ This pattern of results suggests that a cost of looking away from the target sounds was responsible for the ipsilateral versus con! tralateral e}ect\ rather than a bene_t of looking towards the sounds[ Finally\ there were no di}erences between conditions in terms of the low percentage of trials excluded due to inadequate _xation[ 5[2[ Discussion The results indicate that the current direction of gaze "rather than the on!line programming of an upcoming saccade# can in~uence auditory discrimination in our elevation task\ independent of head position "which was kept _xed throughout by the head restraint# and of audi! tory inputs "which remained constant#[ This supports several previous suggestions ð00\ 06\ 07\ 10\ 16\ 17\ 20\ 32Ł that eye position may a}ect hearing and provides a particularly clear demonstration of this in a choice RT task[ Our method has the advantage that the result cannot be attributed to any response bias "since the required up! down discrimination was entirely orthogonal to which! ever lateral position participants looked towards#[ More! over\ the RT e}ect was supported by an error trend\ thus ruling out speed:error tradeo}s[ The results extend the long SOA _ndings of experiment 0\ where auditory elevation judgements were better on the side participants looked towards after making the saccade\ than on the side they looked away from[ The present study shows that this in~uence from the direction of _xation upon hearing can be found even without a saccade[ This has two implications for our primary ques! tion of whether auditory attention shifts in a presaccadic manner[ First\ although the long SOA e}ect found in experiment 0 cannot be dismissed as a PRP artifact "as discussed earlier#\ it might nevertheless re~ect an in~u! ence from deviated eye position\ just as in the present

experiment\ rather than a true in~uence from the saccadic programming that led to this deviated eye position[ Second\ when testing for any truly saccadic in~uences\ it is clearly vital to probe auditory performance with sounds that terminate immediately prior to the saccade com! mencing "as in our restricted analysis of the short SOA data in experiment 0\ with a narrow time!window of saccadic latencies#^ otherwise any e}ect observed might be due to eye position\ rather than to the saccadic pro! gramming itself[ There is apparently no way of escaping the fact that in order to test for presaccadic attentional shifts\ even within audition\ the sensory probe must be presented in a very narrow critical time!window\ which overlaps with saccade programming\ yet terminates before saccade execution[ Our next study aimed to achieve just this\ while also seeking to eliminate the potential PRP problem that we discussed extensively for the short SOA data in experi! ment 0[ Thus\ experiment 2 returned to the central ques! tion of our paper\ whether saccade programming in~uences auditory performance\ but now did so armed with the knowledge that the mere direction of sustained _xation can a}ect hearing\ as found in experiment 1[ 6[ Experiment 2 Experiment 0 had sought to determine if auditory attention precedes eye movements to their destination by using central cues to direct saccades either to the left or to the right[ In the crucial subset of short SOA trials in that study\ an auditory target terminated immediately before the saccade was executed[ Auditory judgements were signi_cantly faster when the upcoming saccade was ipsiversive rather than contraversive to the target sound even in this situation[ While this might appear to support a presaccadic attention shift in audition\ in fact the sac! cades themselves were also faster in the ipsiversive con! dition at the short SOA[ It was therefore possible to argue that the faster auditory responses in this condition might simply follow on from the faster saccade\ in a PRP manner\ as discussed above[ Pashler et al[ ð25Ł have previously shown that lateral saccades made in accordance with a symbolic central cue can indeed produce PRP e}ects on manual responses to closely following sounds "in their study\ the sounds were non!lateralized tones that had to be discriminated in pitch#[ That _nding seems consistent with a PRP account for the short SOA data in our experiment 0[ Importantly\ however\ Pashler et al[ also observed that more {{re~ex! ive|| saccades\ made towards the onset of a peripheral light\ clearly did not produce any PRP e}ect[ "This makes good functional sense\ as we typically make so many saccades to peripheral visual targets in everyday life that our behavior would be very disrupted if every single eye! movement of this kind held up other ongoing cognitive tasks[#

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Thus\ we might avoid the PRP problem altogether if the saccades in our experiment were made toward a peripheral light\ rather than in accordance with a sym! bolic central cue that must _rst be decoded in order to select the appropriate saccade response[ The only poten! tial problem with this approach is that one might argue that the onset of a peripheral light\ serving as the saccade cue\ might itself draw auditory attention to its side[ How! ever\ Spence and Driver ð42Ł have shown that peripheral visual events do not attract auditory attention when unin! formative as regards the location of auditory targets[ They consistently found a null e}ect from uninformative peripheral visual cues upon auditory elevation judge! ments\ provided that central _xation was required and monitored throughout[ The very same visual cues did produce enhanced elevation judgements for closely fol! lowing visual targets on the same side[ Spence and Driver therefore concluded that while uninformative visual events attract covert visual attention "as demonstrated by Posner ð30Ł and by many others since#\ they do not attract covert auditory attention when central _xation is required[ Of even more importance for present purposes\ in a further experiment Spence and Driver ð42Ł found that when instructions to withhold saccades were no longer given "and eye!position was no longer monitored#\ audi! tory performance now did for the _rst time become sig! ni_cantly a}ected by an uninformative peripheral visual event\ with faster judgements for target sounds appearing on the side of the visual cue\ shortly after it[ Spence and Driver "Ref[ ð42Ł\ pp[ 04Ð05# speculated that this in~uence from irrelevant visual events\ found only when central _xation was not insisted upon\ may have arisen because participants then saccaded towards the visual cue on a substantial proportion of trials[ In other words\ Spence and Driver suggested that saccades may have shifted auditory attention in their experiment\ when eye!move! ments were not monitored[ However\ while highly encouraging for our hypothesis of a presaccadic shift in auditory attention\ Spence and Driver|s results ð42Ł are insu.cient to prove such an e}ect\ as they only manipulated whether _xation instruc! tions were given "and eyes monitored# or not[ They never explicitly required saccades to the visual events\ nor mea! sured whether any such saccades were executed when the requirement for central _xation was relaxed[ Further! more\ since they had no measure of saccades\ they could not determine whether the auditory e}ects found in the absence of central _xation instructions should be attri! buted to truly presaccadic attention shifts\ or merely to the change in eye!position following any saccades "in accord with the eye!position e}ect found in our experi! ment 1 and at the long SOA of experiment 0#[ In our next study\ just like Spence and Driver ð42Ł\ we presented a peripheral visual event on one side shortly prior to an auditory target on either side and required a

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manual response discriminating the elevation of the tar! get sound as usual[ However\ unlike Spence and Driver\ we explicitly required "and measured# a rapid saccade to the peripheral visual event[ Since Pashler et al[ ð25Ł have previously shown that such saccades to peripheral lights do not produce any PRP e}ects on manual responses to sounds\ our previous PRP problem "which arose for the short SOA data in experiment 0# should now be cir! cumvented[ Moreover\ Spence and Driver ð42Ł have repeatedly con_rmed that lights themselves do not attract auditory attention when no saccade is allowed "our own experiment 3 tests this issue once again\ as a control#[ Thus\ any e}ect upon audition should be due to the saccade itself rather than to the peripheral saccade cue[ If presaccadic shifts of auditory attention do exist\ we should now _nd better auditory elevation judgements on the side of the light\ due to the upcoming saccade towards it^ this should be apparent even for sounds which ter! minate just before the eye actually moves[ A further advantage of this method is that pro!saccades towards peripheral lights are highly compatible and very fast ð7Ł[ This might eliminate the e}ect that the auditory target had exerted upon saccade latency for centrally cued eye!movements in experiment 0\ at the short SOA[ If so\ the potential PRP problem we have been grappling with would certainly be avoided[ That is\ if saccade lat! encies became equivalent in the ipsiversive and con! traversive conditions\ as we now expected\ there would no longer be any di}erence in saccade RT to propagate into the auditory RTs for these conditions in a PRP manner[ Finally\ saccade latencies to peripheral visual events should presumably not only be faster\ but also less vari! able than the centrally cued saccades of experiment 0[ This should hopefully result in a greater proportion of trials _tting our ideal time!window\ whereby the saccade is fully programmed yet not quite executed when the auditory target is presented[ 6[0[ Method 6[0[0[ Participants Eight new volunteers took part in this experiment\ and were paid -6[19 for their participation "which required just over 0[4 h#[ One was rejected for failing to reach the 69) accuracy criterion on the auditory discrimination task[ The remaining participants were all right handed[ They ranged in age from 19Ð13 years\ with a median of 10^ two were male and the remaining _ve were female[ The participants were not informed as to the purpose of the experiment until after they had completed all trials[ 6[0[1[ Apparatus and materials The set!up was as for experiment 0\ but with the addition of two red cue LEDs mounted immediately above the yellow peripheral marker LEDs\ to serve as

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saccade cues[ One of these was 04> to the left and one 04> to the right of the participant|s sagittal midline[ Saccade recording and criteria were the same as in experiment 0[ 6[0[2[ Procedure At the start of each trial\ the central LED which par! ticipants initially had to _xate turned green\ and remained so throughout the trial[ A red peripheral LED was illuminated 399 ms later on either the left or right[ The participant was instructed to saccade directly to this peripheral light as rapidly as possible[ The red saccade cue was in no way predictive of the location of the audi! tory target\ which occurred above or below on either side\ at equiprobable SOAs of 099 or 329 ms after the peripheral saccade cue "note that the critical short SOA was now shorter than the 199 ms SOA in experiment 0\ as the saccade latencies were expected to be faster\ given the use of peripheral saccade cues rather than central symbolic cues#[ The participant made a speeded manual response indicating whether the sound target came from an upper or lower speaker\ just as before[ Upon this response\ or a maximum of 1999 ms after the onset of the sound if no manual response was recorded\ the central light switched back from green to red and the saccade cue!light was extinguished\ ending the trial[ In the case of a manual response error\ a feedback tone came from the center as in the previous studies[ During the 599 msec inter!trial interval the _xation light remained red[ Yellow peripheral marker lights remained on at the possible sac! cade destinations "one on each side# throughout the experiment^ these were immediately below the possible red saccade cues[ Each block contained 001 trials\ comprising seven trials where the auditory target came from the same side as the saccade cue "ipsiversive trials# and seven trials where it came on the opposite side "contraversive trials#\ for each of the four target locations\ and at each of the two SOAs\ all in a random order[ The participants were initially trained on 45 trials where they were only required to make manual responses to the auditory targets[ They then completed 39 trials where they were only required to saccade to the cue light "auditory targets were still presented\ but participants did not have to respond to these targets#[ Every participant then completed _ve blocks of 001 trials each\ where they were required both to saccade towards the peripheral light cue\ and also to judge the elevation of the target sounds[ The _rst such block was excluded as practice[ 6[1[ Results Given the _ndings of experiments 0 and 1\ the critical data concerned auditory performance on trials where the auditory target terminated before the eye moved\ at the short SOA[ Accordingly\ trials were now only accepted for analysis if the saccade latency fell in the range of 194Ð

Table 3 099 ms SOA

RT ) EM

329 ms SOA

ipsiversive

contraversive

ipsiversive

contraversive

421 01[2 135

444 01[8 135

405 02[6 135

431 01[5 116

Experiment 2] mean auditory reaction times "RT#\ auditory errors ")#\ and eye!movement latencies "EM# for trials where a saccade was executed in the time!window from 194Ð294 ms following the onset of the central cue[ The saccades were either ipsiversive or contraversive to the side of the auditory target[

294 ms "20) of trials were excluded due to saccades occurring too early\ while 3[8) of trials were excluded for saccades occurring too late or not occurring at all#[ This 194Ð294 ms saccade!latency range is the critical time!window for which the auditory target "094 ms in duration# has terminated at the short "099 ms# SOA prior to the saccade commencing\ yet with the saccade being executed within the next 099 ms\ indicating that it was likely to have been programmed while the auditory target was present[ For completeness\ we applied the same time! window criterion to the long SOA data as well\ although these can only assess post!saccadic e}ects of gaze devi! ation\ rather than the presaccadic shifts tested by the short SOA data[ Applying the same time!window cri! terion to all of the data ensures comparable data!sets for the two SOAs\ and also serves to eliminate any trials with atypically slow or anticipatory saccades[ The mean results per condition for all trials meeting the stringent time!window criteria are shown in Table 3[ It can be seen that\ as in experiment 0\ auditory dis! crimination RTs were faster for trials with ipsiversive saccades than contraversive saccades\ at both the long and the short SOAs[ The long SOA result replicates the long SOA _nding from experiment 0 and is also con! sistent with the e}ect of eye position on hearing that was found in experiment 1[ The short SOA result for auditory responses is similar to that seen in experiment 0 at the short SOA[ However\ unlike that study\ there was now no di}erence in saccade latencies for the ipsiversive versus contraversive conditions at this short SOA\ just as we had hoped "see Table 3#[ This was presumably because the peripheral visual event now used as the saccade cue was su.cient to induce an equally rapid saccade regard! less of where the sound appeared[ This aspect of the results means that possible PRP e}ects can no longer account for the e}ect of saccade direction on auditory RTs at the critical short SOA\ as there was no di}erence between ipsiversive and contraversive saccade latencies that could propagate to auditory RTs[ The pattern described above was con_rmed by stat! istical analysis of the data from those trials which fell

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within our time!window criteria for saccadic latency[ We _rst conducted a two!way repeated ANOVA "saccade direction×SOA# on the auditory RTs[ There was no main e}ect of SOA ðF"0\ 5#0\ p³9[24Ł\ but a reliable e}ect of saccade direction ðF"0\ 5#04[0\ p³9[997Ł\ with sig! ni_cantly faster auditory RTs on trials with an ipsiversive saccade "M413 ms# rather than a contraversive saccade "M438 ms#[ There was no interaction between SOA and saccade direction ðF"0\ 5#9[0\ p³9[79Ł\ showing that the e}ects of saccade duration were similar at both SOAs[ The critical test of presaccadic shifts in auditory atten! tion comes from the accepted short SOA trials\ where the saccade was launched immediately after the auditory target terminated[ In these trials\ the eye has not yet shifted from central _xation while the sound is still present\ yet the saccade is likely to have been pro! grammed during the sound "since it occurred within 099 ms of the sound terminating on the accepted trials#[ A t!test found that auditory RTs were indeed signi_cantly faster when the upcoming saccade was ipsiversive rather than contraversive within just this critical subset of short SOA trials ðt"5#2[5\ p³9[90Ł[ A similar two!way ANOVA "saccade direction×SOA# on the auditory error data found no signi_cant terms "no p³9[5#[ Note\ however\ that the numerical trend at the short SOA was in support of the critical ipsiversive versus contraversive di}erence in RTs\ arguing against any speed!error trade!o} in the auditory task for this e}ect[ Finally\ a similar two!way ANOVA on the saccade latency data also found no signi_cant terms ðfor SOA\ F"0\ 5#0[2\ p9[18^ for saccade direction\ F"0\ 5#1[6\ p9[04^ for the interaction\ F"0\ 5#9[2\ p9[47Ł[ Note that there was now absolutely no trend for faster saccades on ipsiversive than contraversive trials\ thus discounting any PRP account for the saccade direction e}ect on audi! tory RTs[ There was no systematic pattern in the very small num! ber of trials "³0)# on which a clear saccade in the appropriate direction did not occur[ 6[2[ Discussion Experiment 2 found a reliable advantage in the audi! tory discrimination task on the side of an upcoming saccade[ Saccade latency to the peripheral visual cue was not in~uenced by the location of the auditory target[ Hence\ the advantage found for auditory targets pre! sented on the same side as the upcoming saccade can no longer be attributed to any PRP!like propagation of saccade latency e}ects into auditory RTs[ In any case\ Pashler et al[ ð25Ł have previously found that prosaccades to peripheral visual events\ as here\ do not produce any PRP e}ects on manual choice RTs to sounds[ The long SOA results replicate the _ndings of experi! ment 0 and are consistent with the e}ect of eye!position

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in experiment 1\ since the saccade had always been made "and thus gaze was already deviated# before the sound target appeared at this long SOA[ These long SOA results thus suggest an e}ect of gaze deviation on hearing[ By contrast\ the short SOA results appear to provide the _rst clear evidence for truly presaccadic shifts of auditory attention in the direction of the upcoming saccade\ in a situation where auditory RTs could not be contaminated by any PRP in~uence from saccadic latency[ Only one possible objection to this short SOA e}ect remains[ Perhaps the visual saccade cue itself attracted auditory attention\ rather than the requirement to sac! cade towards it[ This would be inconsistent with Spence and Driver|s ð42Ł repeated _nding\ using the very same task in the same laboratory\ that peripheral visual cues do not attract auditory attention\ and thus have no in~u! ence on auditory elevation judgements\ when eye move! ments are prevented[ However\ rather than simply relying on extrapolation from their studies\ we thought it wise to test this possibility once again\ by repeating our experi! ment 2\ but now requiring central _xation throughout[ If the results of experiment 2 were indeed due to making the saccade\ they should not be replicated in our _nal study[ However\ if they were merely due to the onset of the peripheral visual cue\ then they should be found again even when no saccade is made towards that cue[ On the basis of Spence and Driver|s ð42Ł results\ we predicted that the visual cue would have no such e}ect when sac! cades were prohibited[

7[ Experiment 3 The only modi_cation from experiment 2 was that the participants were now instructed to maintain _xation on the central light for the duration of each trial\ rather than saccading to the peripheral visual cue[ If the peripheral visual cue attracts auditory attention automatically\ elev! ation judgements should be faster for a sound target presented on its side rather than on the opposite side[ If this were found\ our interpretation of experiment 2 in terms of a presaccadic shift of attention would be thrown into serious doubt[ However\ based on Spence and Driver ð42Ł\ we did not expect to _nd any e}ect of the visual cue when saccades were prevented[ 7[0[ Method 7[0[0[ Participants Fourteen new volunteers took part in this experiment and were paid -5 for their participation "which required slightly less than 0[4 h#[ Four were rejected because they failed to reach the accuracy criterion of 69) in the audi! tory elevation task[ Of the remaining ten subjects\ eight were female and two were male[ All were right handed\ their ages ranging from 05Ð21 with a median age of 08[

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Table 4 099 ms SOA

RT ) Fix)

329 ms SOA

ipsilateral

contralateral

ipsilateral

contralateral

316 6[8 4[8

329 7[6 3[4

328 6[5 5[8

321 7[7 5[5

Experiment 3] mean auditory reaction times "RT#\ auditory errors ")# and _xation failures "Fix)#[ The peripheral visual cues were either ipsilateral or contralateral to the side of the auditory target[

They were not informed as to the purpose of the experi! ment until they had completed all trials[ 7[0[1[ Apparatus and materials The setup was identical to that used in experiment 2[ However\ unlike experiment 2\ the participants were asked to maintain _xation on the central LED through! out each trial[ Eye position was monitored as before[ Trials where the eye deviated more than 1[4> to the left or right of the central LED between the onset of the visual cue and the manual response to the sound were excluded from analysis[ 7[0[2[ Procedure As before\ each experimental block contained 001 trials\ identical to the previous study[ Obviously\ there was now no saccade training\ as central _xation had to be maintained throughout each trial[ Each participant was trained with 45 trials in the auditory elevation task[ They then completed _ve blocks of this task\ the _rst being excluded as practice[ 7[1[ Results Outlying trials with a manual RT of less than 099 or more than 0499 ms were excluded from analysis as before "less than 0) of trials#[ The mean results for each con! dition are shown in Table 4[ It can be seen that auditory discriminations were no longer faster on the side of the visual cue\ unlike experiment 2[ A two!way repeated ANOVA "SOA×direction of the visual cue "i[e[ con! tralateral versus ipsilateral to the subsequent auditory target## was conducted on auditory RTs[ There were no signi_cant main e}ects of SOA ðF"0\ 8#1[2\ p³9[06Ł or cue direction\ ðF"0\ 8#9[2\ p³9[51Ł and no interaction ðF"0\ 8#1[9#\ p³9[08Ł[ No signi_cant terms were found in a similar analysis of auditory error rates "no p³9[2#[ Finally\ a similar ANOVA on the number of excluded trials where the eye deviated from _xation found no sig! ni_cant terms "no p³9[1# except for a trend "probably artifactual# for more _xation errors in the long SOA versus the short SOA ð4[1 vs 5[64)\ F"0\ 8#2[2\ p³9[0Ł[

This is probably due merely to a higher probability of blinking over longer periods[ A _nal analysis directly compared the size of the ipsi! lateral:contralateral e}ect on auditory RT between the current experiment versus experiment 2[ The only di}er! ence in procedure between the two studies was that a saccade was required towards the visual cue in experiment 2\ while in this experiment such saccades were prohibited\ with central _xation being required on every trial[ For each participant in each experiment\ the advantage for ipsilateral over contralateral trials in auditory RT at the short SOA was computed[ These data were then subjected to a one!tailed between!subject t!test "with the prediction of a reduced ipsilateral:contralateral di}erence in the cur! rent no!saccade study#[ This analysis showed a signi_cant di}erence between the positive result of experiment 2\ versus the null result of the present study ðt"04#1[3\ p³9[903Ł\ with a mean validity e}ect of 12 ms in experi! ment 2\ versus a nonsigni_cant trend of only 2 ms in this experiment[ This con_rms that the critical di}erence in outcome between the two experiments was indeed reliable[ 7[2[ Discussion The results of this experiment clearly replicate the _n! dings of Spence and Driver ð42Ł^ uninformative peripheral visual events do not exogenously attract auditory atten! tion when eye!movements towards them are prevented[ Of more importance for our present purpose\ this study used an identical set!up and procedure to the previous experiment\ with the single exception that participants no longer executed a saccade to the peripheral visual cue[ The signi_cant disappearance of the previously obtained advantage for ipsilateral trials implies that the e}ect in experiment 2 must have been due to the requirement to make a saccade\ rather than to the mere onset of a visual transient in the periphery[ 8[ General discussion The primary goal of our study was to examine the possible link between saccades and auditory attention\ following on from recent studies which have documented a tendency for visual attention to shift in the direction of an upcoming saccade just before that saccade is actually executed "e[g[ Refs[ ð2\ 5\ 03\ 15Ł#[ In the course of testing for any presaccadic shifts of auditory attention\ we also had to consider the possible e}ect that deviated eye! position might exert on hearing and in addition the poss! ible in~uence of a sudden sound on the speed of executing a particular saccade[ Taken together\ our results provide the _rst ever evidence that auditory performance is a}ec! ted by the direction of an upcoming saccade\ with better auditory localization in the vicinity of the saccade des!

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tination[ We also documented an in~uence of eye position per se "i[e[ the direction of deviated gaze# upon hearing\ but were able to exclude this in~uence from the pre! saccadic e}ect in our experiment\ by testing with sounds that terminated before the eye actually moved[ Our series of experiments also illustrates many of the intricate methodological issues that must be considered when assessing presaccadic shifts of attention[ For instance\ one must assess the possibility that the sensory probe used to measure any presaccadic shift might itself a}ect the speed of the saccade and thus lead to an appar! ent e}ect on sensory performance which is in fact due merely to a PRP e}ect[ Fortunately\ we were able to exclude such factors with our _nal experiments[ All our studies tested auditory performance with a speeded up:down discrimination task\ as introduced by Spence and Driver ð41Ð43Ł[ The main advantage of this task for our purposes is that it is known to provide a sensitive measure for the distribution of auditory atten! tion[ In addition\ criterion!shift accounts for any e}ects can be assessed and discounted "because accuracy as well as RT is measured\ unlike simple RT tasks# and response priming accounts are ruled out "as the manipulation of saccade direction\ towards the left versus right\ is entirely orthogonal to the up:down dimension that must be judged#[ Using this auditory task\ experiment 0 found that elev! ation discrimination was faster "and tended to be more accurate# for sounds on the side that a centrally cued saccade was made towards\ consistent with a shift of auditory attention towards the saccade destination[ Our analysis focused primarily on a subset of short SOA trials\ where the sound target terminated shortly "no more than 099 ms# before the eye began to move[ Better auditory performance was found in the direction of the upcoming saccade even for these trials\ apparently suggesting a truly presaccadic attentional shift[ However\ the saccades themselves were faster when ipsiversive to the sound tar! get at the short SOA "consistent with some previous _ndings that sounds can in~uence visually!cued saccades\ ð27Ł#[ The faster auditory RTs in the ipsiversive condition at the short SOA might then simply follow on from the faster saccades\ in a PRP fashion[ This potential PRP problem did not apply to the long SOA data\ which again showed better auditory per! formance on trials with ipsiversive saccades[ At this SOA\ the saccades were of equivalent latency regardless of whether they were ipsiversive or contraversive to the sound "as would be expected\ given that the sound was now presented only after the saccade#[ Hence there was no saccade latency di}erence to propagate to the auditory RTs in any PRP manner and the e}ect on auditory per! formance thus appears genuine at this SOA[ However\ since at the long SOA the saccade had already been made before the sound target was presented\ its e}ect on auditory performance might be due to the deviated eye!

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position which it produced\ rather than to the underlying saccadic programming[ Experiment 1 examined whether deviated eye position is su.cient to a}ect hearing when no on!line saccade is made[ Earlier studies on the possible e}ects of gaze direction on hearing have a rather mixed history ð32\ 59Ł and none have ever used a straightforward auditory discrimination like the one used here "typically\ more complex tasks such as shadowing of continuous speech have been employed#[ Experiment 1 found that auditory elevation judgements were slower when gaze was main! tained away from the side where the sound target appeared[ This implies that eye!position alone\ in the absence of an on!line saccade\ can in~uence auditory performance[ It also provides one plausible explanation for the saccade direction e}ect found at the long SOA e}ect in experiment 0 "and replicated in experiment 2#[ At this SOA the eye was already deviated when the sound target appeared\ and so eye!position may have produced the e}ect\ as shown in experiment 1[ These behavioral _ndings of an in~uence from current gaze direction upon auditory performance accord well with some electrophysiological _ndings from single!cell recordings in animals\ within brain structures responsive to sounds[ Such studies have revealed that the neuronal response to auditory stimuli can be in~uenced in various brain regions "e[g[ in auditory cortex\ superior colliculus\ and parietal lobe# by where the animal is looking relative to the location of the sound source ð05\ 44\ 45Ł\ even though this of course does not change the auditory input to the cochlea[ Moreover\ one human study measuring event!related responses to sounds\ by means of voltage ~uctuations at the scalp\ has similarly reported a stronger neural response to sounds when gazing towards their source rather than away ð22Ł[ Such neural modulation might underly the in~uence of eye!position on auditory discriminations that we observed here[ Experiment 2 returned to the issue of possible saccadic in~uences on hearing\ rather than e}ects from eye!pos! ition per se[ We now used peripheral visual lights\ rather than central colours\ to cue the saccade[ This change was made for several reasons[ First\ Pashler et al[ ð25Ł have shown that saccades to peripheral lights do not produce PRP e}ects on manual RTs to sounds\ so the pro!saccade task should circumvent the PRP problem which may have a}ected the short SOA data of experiment 0 with its centrally cued saccades[ Second\ we anticipated that in any case there might now be no di}erence in saccade latency for eye!movements that were ipsiversive versus contraversive to the target sound at the short SOA "since pro!saccades to peripheral lights are very fast and may be di.cult to disrupt by other events such as sounds#[ This expectation was borne out\ which rules out any possible PRP account\ as there was now absolutely no di}erent in saccade latencies to propagate to the auditory RTs[ Third\ Spence and Driver ð42Ł have repeatedly

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shown that visual events do not attract auditory attention when saccades are prevented\ so we could be con_dent that any advantage for auditory judgements on the side of the peripheral saccade cue would be attributable to the saccade itself\ rather than merely to the visual cue[ Experiment 3 provided a further control for this[ Experiment 2 found reliably faster auditory elevation discriminations on the side the saccade was made towards[ This was observed at the long SOA\ where the eye shifted prior to the sound occurring "so this particular e}ect might be due to deviated eye!position\ as in experi! ment 1#[ A similar pattern was also found at the short SOA\ even within just those trials where the eye did not begin to shift until after the sound target terminated[ This provides strong evidence for a truly presaccadic shift of auditory attention in the direction of the upcoming saccade[ Experiment 3 con_rmed that\ as Spence and Driver ð42Ł had previously found\ the peripheral visual cues did not in themselves attract auditory attention when no saccade was made towards them[ Hence the e}ect on auditory performance in experiment 2 can be unam! biguously attributed to the upcoming saccade[ The methodological approach taken in this study when testing for presaccadic attentional shifts was to analyze those trials that passed a critical saccade!latency criterion\ extending 099 ms after sound o}set[ This was to ensure that the upcoming saccade was likely to have been programmed when the sound appeared\ but had yet to be executed when the sound terminated[ We think this cautious method of analysis\ focusing on just a critical subset of trials\ is the most appropriate[ However\ a ref! eree suggested that a fuller "albeit less conservative# pic! ture of the timecourse of saccadic in~uences on hearing might be obtained by plotting the advantage in auditory performance for ipsiversive versus contraversive trials\ for various di}erent time!windows of saccade latency relative to the target sound[ Such a plot is presented for experiment 0 "which had centrally cued saccades# in Fig[ 1"A# and for experiment 2 "which had peripherally cued saccades# in Fig[ 1"B#[ Both Fig[ 1"A# and "B# suggest that the e}ect of saccade direction on auditory performance builds up in the 199 ms prior to saccade execution "being larger for trials where the saccade will be executed within the next 099 ms ðgrey bar in histogramsŁ than for trials in the preceding bin\ where more than 099 ms will elapse before the saccade occurs ðcompare grey bar with bars to its immediate left in the histogramsŁ#[ The most critical point for present purposes is that the e}ect on hearing is clearly present before the saccade actually occurs "grey bar in histo! grams#\ as was shown by our more conservative analyses earlier[ The e}ect then appears to peak in size\ for sac! cades whose execution overlaps with the presence of the sound "indicated by the labelled white horizontal bar in the histograms#[ This may suggest that separate e}ects from saccadic programming and from gaze deviation can

summate within this time!window "or alternatively\ that a higher proportion of trials within this bin had fully programmed saccades than in other bins#[ The e}ect on hearing is then largely maintained after the saccade\ when gaze is deviated[ While this pattern is suggestive\ note that several caveats must be placed on the apparent timecourse of the saccadic e}ect as conveyed by Fig[ 1[ First\ the number of trials which contribute to the scores shown varies for the di}erent bins\ meaning that some values are more reliable than others\ being based on a larger sample[ When designing the experiment\ our intention had not been to derive a complete picture of the full timecourse for any saccadic in~uences\ but rather to collect as much data as possible at just two critical time!windows^ for sounds presented just before the saccade and for sounds presented after the saccade[ As shown by the numbers printed above each bar in Fig[ 1\ many of the trials did indeed fall into these two regions of time[ A second caveat is that the data shown in Fig[ 1 do not take into account possible PRP e}ects\ in the manner allowed for by the more restricted analyses we presented earlier[ Finally\ there were insu.cient data for some of the bins shown to permit a full statistical analyses of the timecourse pattern\ beyond that provided by the more conservative analyses we presented earlier[ Future studies could adapt our methods to collect more data across the full range of time!bins[ For now\ our present data su.ce to show that both the direction of an upcoming saccade "experiment 0 and experiment 2# and also the direction of deviated gaze "experiment 1# can a}ect hearing[ In many respects\ our _nding that auditory attention shifts in the direction of an upcoming eye!movement is even more remarkable than previous reports "e[g[ Refs[ ð2\ 5\ 03\ 15Ł# of analogous shifts in covert visual attention[ After all\ an eye!movement will dramatically alter the input to the visual system\ whereas by contrast eye!move! ments cannot directly change the sensory input to the ears[ However\ as we noted in our Introduction\ even presaccadic shifts of visual attention must be due to chan! ges in internal representation\ rather than changes in the input to receptors and when viewed from this perspective it may be less surprising to discover that saccades can a}ect hearing as well[ Indeed\ there is already some pre! liminary evidence from electrophysiology demonstrating that an upcoming saccade can in~uence the internal neu! ral representation of sounds ð01Ł[ Many neurophysiological studies have documented {{presaccadic visual enhancement|| for single cells in vari! ous species\ whereby the neuronal response to a visual event at a particular location is enhanced if that location becomes the target for a saccade\ with the enhancement arising shortly before the saccade is actually executed ð51Ł[ This phenomenon has now been observed in several brain regions\ including the superior colliculus\ parietal cortex\ substantia nigra and frontal!eye _elds ð1\ 8\ 09\

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Fig[ 1[ Histograms of the relative advantage for auditory reaction times in the ipsiversive! versus contraversive!saccade conditions\ as a function of saccade latency relative to the target sound] "A# for experiment 0 and "B# for experiment 2[ The e}ect on hearing was computed by including all trials from all subjects that fell within a particular saccade!latency bin "each bin was 099 ms in width#^ computing the grand medians for ipsiversive trials and for contraversive trials and then plotting the resulting di}erence[ Since saccade latency on each trial was not under direct experimental control\ di}erent bins contain di}erent number of trials^ the grand total of trials contributing to each bin is printed above each bar in the histogram[ The period of time in which the sound was presented is marked by the labeled horizontal white bar at the centre of each histogram[ Bars on the left half of each histogram "negative values along the abscissa# show trials where the sound terminated before the saccade was executed\ so that any in~uence of saccade direction could be due only to saccadic programming[ For instance\ the grey bar represents trials where the saccade was made within 099 ms of the sound terminating[ The bars on the right half of each histogram "positive values along the abscissa# show trials where the sound occurred after the saccade was completed\ so that any apparent in~uence of saccade direction might now be due instead to the deviated gaze which the saccade produced[ Thus\ time runs from right!to!left for the white horizontal bar indicating the sound\ in order that the development of saccadic e}ects through time "i[e[ from initial programming through to execution and gaze deviation# runs from left!to!right in the histogram[

18\ 21\ 36\ 37\ 40\ 50\ 52Ð55Ł[ Almost without exception\ such studies have only tested for presaccadic enhance! ment of the neural response to visual events near the saccade destination[ However\ Hikosaka and Wurtz ð01Ł reported preliminary physiological evidence that certain cells in the monkey substantia nigra exhibit enhanced responses to a sound when a saccade is made soon after! wards towards the sound source[ Their study thus pro! vides both a precedent for own _nding that an upcoming saccade can lend an advantage to auditory events near the saccade destination\ and also a possible neural sub! strate for our e}ect[ Future neuroscience studies could usefully adapt the various methods that have been

developed for studying presaccadic visual enhancement in single cells\ to study the neural mechanisms underlying the in~uence that saccades can evidently exert upon hear! ing[ Our own particular methods could be exploited in future behavioral studies[ For instance\ it would be useful to determine whether the presaccadic shifts of auditory attention that we have documented are strictly obligatory[ The sound target was always equally likely on either side in our experiments[ It would be interesting to test performance when auditory targets are made much more likely on the contraversive side to the upcoming saccade\ by analogy with previous studies of whether visual atten!

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tion shifts obligatorily towards a saccade destination\ even when targets are strongly expected elsewhere ð15\ 49Ł[ It would also be interesting to test whether saccades can in~uence performance in touch\ as well as in vision and hearing[ A few previous studies have suggested that tactile performance can be a}ected by the direction of deviated gaze ð04\ 28Ł\ even in the dark\ suggesting an in~uence of eye!position on touch[ However\ we know of no study concerning whether saccades will in~uence tac! tile performance[ Tentatively\ this might be expected given our own _ndings\ plus neuroscience evidence that structures which have been implicated in presaccadic enhancement\ such as the colliculus and parietal lobe\ code tactile events in close spatial register with visual and auditory events ð44Ł[ If saccades were found to a}ect touch in addition to hearing and vision\ this would imply a supramodal shift of attention in the direction of an upcoming saccade[ Such questions await future research[ For now\ our studies provide the _rst demonstration that saccades can a}ect human auditory performance and thus open up a whole new line of inquiry[ Acknowledgement This work was supported by a program grant from the Medical Research Council "U[K[#[

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