Light location influences the perceived locations of internal sounds

Transcription

1 Perception, 2010, volume 39, pages 91 ^ 102 doi: /p6149 Light location influences the perceived locations of internal sounds Scott Dellorso, James Schirilloô Department of Psychology, Wake Forest University, Winston-Salem, NC 27109, USA; schirija@wfu.edu Received 7 August 2008, in revised form 18 February 2009; published online 7 January 2010 Abstract. Numerous studies have shown that visual stimuli can bias the perceived location of auditory stimuli. Here, we sought to determine if a visual stimulus can also bias the perceived location of multiple internal auditory stimuli. Fifty subjects were presented with a train of eight binaural click-pairs over headphones while a single flash of light was presented either to the left or to the right side of a central fixation point on an otherwise black CRT screen. A no-flash baseline was also implemented, as was a no-fixation control. The subjects used a rating scale to indicate the perceived location of each of the eight click-pairs within their heads. The results showed that the flash of light significantly influenced the perceived location of the click-pairs, biasing them in the same direction as the flash of light. This suggests that, even without perceptual correspondence, cross-modal interactions can occur. 1 Introduction A visual stimulus can bias the perceived location of a concomitant sound. Known as the `ventriloquism effect' (see Howard and Templeton 1966), this illusion is quite robust, and is apparent even when pairing neutral, seemingly unrelated, stimuli such as spots of light and noise bursts (Bermant and Welch 1976; Bertelson and Aschersleben 1998; Bertelson and Radeau 1981; Lewald et al 2001; Radeau 1985; Radeau and Bertelson 1987; Slutsky and Recanzone 2001; Welch and Warren 1986). The ventriloquism effect has been studied most often by imposing varying degrees of spatial disparity between visual and auditory stimuli and assessing the extent the former biases the perceived location of the latter (Bertelson and Aschersleben 1998; Choe et al 1975; Hairston et al 2003; Jack and Thurlow 1973; Lewald et al 2001; Radeau and Bertelson 1987; Slutsky and Recanzone 2001; Thurlow and Jack 1973; Thurlow and Rosenthal 1976). Maximum localization bias has been found to be directly correlated with the perception that the visual and auditory stimuli appear `unified', or originating from the same location in space (Bertelson and Radeau 1981; Hairston et al 2003; Wallace et al 2004). Only rarely considered has been how an external stimulus (eg a flash of light) might affect the localization of a sound source heard within the head (Lewald and Ehrenstein 1996; Weerts and Thurlow 1971). However, in these previous studies the light preceded the sound, serving only to redirect the subject's gaze in order to determine how uncoupled eye-centered coordinates from head-centered coordinates might map onto each other. In another strand of research it has been shown how multiple presentations of tactile (Geldrard and Sherrick 1972) or auditory (Hari 1995) stimuli can create mislocalizations, called saltation, depending upon their interstimulus interval (ISI). The current study focuses on a unique aspect of these spatial judgments; those involving how the perceived location of multiple sounds within the head of the listener can be influenced by a single flash of light. ô Author to whom all correspondence should be addressed.

2 92 S Dellorso, J Schirillo When a short binaural click-pair is played through stereo headphones, the resulting sound is localized closer to the ear that is first presented with one of the clicks from the click-pair. If a series of eight 1 ms binaural click-pairs is played in succession with ISIs of less than 120 ms, with four left-ear leading (by 0.8 ms) followed by four rightear leading (by 0.8 ms), the listener clearly perceives the eight click-pairs as moving progressively from left to right through the head (Hari 1995). This phenomenon is known as illusory directional hearing. Mays and Schirillo (2005) extended Hari's (1995) finding by testing how external stimuli, ie flashes of light, affect illusory directional hearing. They used the same procedure of a train of eight binaural click-pairs, reversing which ear led after the fourth click-pair, to produce illusory directional hearing. Each click-pair was presented in conjunction with a small circular flash of light on a computer screen, after which subjects rated the perceived click localizations within the head. The lights flashed either in a progression of eight locations across the screen, or four times on one side of the screen, then four times on the other side of the screen. The findings of this study were remarkable in that the location or direction of the flashing lights produced a strong effect on the subject's perception of where the click-pairs were located within his or her head. In all cases the sounds were mislocated towards the location or direction of the flashing lights. In contrast, related research involving how lights affect internal auditory localization has yielded mixed results. Weerts and Thurlow (1971) used an experimental paradigm that implemented gaze direction, body turning, and expectations to study sound localization. They found that, when gaze was diverted, sound localization appeared to originate from that direction. This effect was augmented by body turn and expectations that the sound would come from the gaze point; however, these two variables alone had no effect on auditory localization. Lewald and Ehrenstein (1996) controlled subject's gaze by using LED fixation points at various peripheral angles, while playing a constant tone with an interaural level difference (ILD). Subjects adjusted the ILD so that the tone was perceived as equal in both ears, making the sound appear to come from the midline of the head. However, the results of this study showed subjects overadjusted the ILD in the direction opposite to where their gaze was fixed. The purpose of the current study was to extend the research of Mays and Schirillo (2005) by investigating how a single (external) flash of light can affect illusory directional (internal) hearing. Likewise, the bias of the sounds (eight click-pairs) was predicted to follow the position of the light flash being either on the left side or on the right side of a computer screen. In this simple experiment we use the minimal conditions possible to bias internal sounds. 2 Experiment Method Subjects. All procedures were approved by the Institutional Review Board of Wake Forest University and were performed in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki. Fifty naive Wake Forest University undergraduate introductory psychology students (twenty-one males) aged 18 ^ 22 years, with normal or corrected-to-normal vision and normal auditory thresholds (20 Hz ^ 8 khz) participated in the experiment in exchange for course credit Apparatus and stimuli. The subject sat in a quiet (ie ambient sound fluctuated between 5 db and 12 db), dimly illuminated room and wore circumaural headphones (Koss; Pro/4AA; 50 db A at peak measurement). His/her chin was on a chin-rest 24 inches away from a dark (2.5 cd m 2 ) CRT screen (except for a central fixation point and an occasional flash of light). The CRT was a 19 inch CTX, model EX95950F,

3 Light influences sound localization 93 with a 75 Hz refresh rate. Subjects were instructed to focus on the fixation point and press any key on a computer keyboard to begin each trial, which occurred after a random interval of between 1 to 2 s. After each trial subjects filled out a response questionnaire, which consisted of eight columns on which to rate each binaural click-pair Procedure. E-Prime software generated the experimental program. On each selfinitiated trial eight binaural click-pairs were presented over headphones (45 db) and a single light (96 cd m 2 ; 1.3 deg radius of visual angle) flashed for 8 ms. The click-pairs temporal order presented a 1 ms rectangular pulse click to one ear 0.8 ms prior to an identical click being presented to the other ear. Thus, the click-pairs, which lasted just over 1 ms, occurred during the middle of the flash which was presented on either the left or right side of the screen ( 14 deg= 14 deg from fixation, respectively). Clickpairs that presented the right-ear sound before the left-ear sound were termed `right ear leading', and conversely click-pairs that presented the left-ear sound before the right-ear sound were termed `left ear leading'. Each trial consisted of eight click-pairs, with the first four presented as right ear leading and the second four presented as left ear leading, or vice versa. The ISI between click-pairs was 64 ms. One sequence of click-pairs with left ear leading is illustrated in figure ms 1.0 ms Left ear Right ear 0.8 ms ISI Left ear Right ear Click order Figure 1. Temporal sequence (not to scale) of left-to-right auditory click-pairs. Light bulbs represent the eight possible flash occurrences, but only one flash was presented per trial. Subfigure shows the relative timing of first flash and clicks.

4 94 S Dellorso, J Schirillo After completing each trial, subjects rated the perceived location of each of the eight click-pairs within the head, using a continuous scale ranging from 0 to 20. A rating of 0 corresponded to a click being perceived in the left ear, and a rating of 20 corresponded to a click being perceived in the right ear. Any number between 0 and 20 indicated the sound was perceived somewhere within the head along the axis from left ear to right ear, with a rating of 10 indicating a sound being perceived in the middle of the head. For each trial, the subjects completed eight ratings, one for each click-pair that was presented in the trial. Subjects completed the experiment at their own pace and were observed by the experimenter for the first few trials to ensure they were following the procedure, after which time they were left alone in the room. Once subjects completed ratings for all the trials of the experiment, they were debriefed. An experimental session took approximately 50 min. The trials were divided into 32 different conditions that varied across three dimensions. The first dimension was the randomly determined direction of the click-pairs; with either four left-ear-leading click-pairs followed by four right-ear-leading ones, or four right-ear-leading click-pairs followed by four left-ear-leading ones. The second dimension was whether the light was flashed on the left or the right side of the screen. This was also determined randomly so that subjects would not turn their eyes toward a given side of the screen before the flash occurred. This makes this experimental methodology uniquely different from those of Lewald and Ehrenstein (1996) and Weerts and Thurlow (1971). The latter presented their light before their sound, so that subjects would gaze in a particular direction prior to their auditory simulation. The last dimension was which of the eight click-pairs contained a temporally coincident flash of light. These 32 conditions (2 click-pair directions62 light-flash locations68 temporal flash occurrences) were presented randomly and constituted a block of trials. An experimental session was composed of 6 blocks, resulting in a total of 192 trials per subject. Because the random trials were predetermined, each subject received the trials in the same sequential order. 2.2 Results Each subject's data were transcribed from the paper grid to an Excel (Microsoft) spreadsheet. Data from eight subjects were removed from the analysis owing to incomplete trials. The data were organized into 32 conditions; for which we computed the mean reported localization (N ˆ 42) for each click-pair. The means are plotted as two separate graphs, which reflect left-to-right click-pair direction (figure 2) and right-to-left click-pair direction (figure 3). Each dotted gray line indicates one of the eight possible times a flash occurred on the right and the thick black line indicates the average across all eight flash presentations. Each dashed gray line indicates one of the eight possible times a flash occurred on the left and the thick light gray line indicates the average across all eight flash presentations. The graphs show an overall increase in ratings for all eight click-pairs in the trials when a light was flashed on the right as opposed to being flashed on the left. This indicates subjects perceived the clicks as being closer to the right ear within the head. This effect was found for both directions of click-pairs (figures 2 and 3). The magnitude of the effect can be seen by examining the error bars of the thick lines, which represent the standard error of the means. To test for statistical significance, we ran a 2 (flash left or right)62 (click-pair direction right-to-left or left-to-right)68 (timings of the flash) repeated-measures withinsubjects ANOVA. We collapsed the ratings from all eight click-pairs in each trial into an overall mean for the trial. From the overall means for each trial we calculated each subject's overall means for all 32 conditions. Kolmogorov ^ Smirnov tests were performed and indicated that each of our conditions had a normal distribution. Thus, eight separate ANOVAs were run, differing by which click-pair contained the temporally

5 Light influences sound localization 95 Perceived location Flash right 1 ± 8 Flash right average Flash left 1 ± 8 Flash left average Click Figure 2. Experiment 1: Left-to-right click-pair direction. Click-pair ratings for flash right (black) versus flash left (gray) conditions. Dotted (flash right) and dashed (flash left) gray lines represent click-pairs containing different temporally coincident flashes. Solid lines represent the average of the eight click-pairs. Error bars represent 1 SEM, N ˆ 42. Left ear perceived location ˆ 0; right ear perceived location ˆ 20. Perceived location Flash right 1 ± 8 Flash right average Flash left 1 ± 8 Flash left average Click Figure 3. Experiment 1: Right-to-left click-pair direction. Click-pair ratings for flash right (black) versus flash left (gray) conditions. Dotted (flash right) and dashed (flash left) gray lines represent click-pairs containing different temporally coincident flashes. Solid lines represent the average of the eight click-pairs. Error bars represent 1 SEM, N ˆ 42. Left ear perceived location ˆ 0; right ear perceived location ˆ 20. coincident flash of light, and one additional ANOVA was run on the overall means averaged across all temporal occurrences of the flash. The results showed main effects for both click-pair direction (F ˆ 26:655, p 5 0:001) and flash-of-light location (F ˆ 28:971, p 5 0:001); however, there was no effect of the time of the flash (F ˆ 0:976, p ˆ 0:329), no two-way interaction (F ˆ 0:665, p ˆ 0:420), and no threeway interaction (F ˆ 0:425, p ˆ 0:425). The results also showed a significant difference between the overall means of flashes on the left and flashes on the right for click-pairs 1 ^ 6 and 8 (table 1).

6 96 S Dellorso, J Schirillo Table 1. Results of ANOVA for significance between flash-left and flash-right conditions. * Significant at a ˆ 0:05. ** Significant at a ˆ 0:01. Flash occurrence F p Click ** Click ** Click ** Click * Click ** Click ** Click Click ** Overall ** 2.3 Discussion We found that the perceived spatial locations of internal (ie within the head) auditory stimuli can be biased in the direction of a single external visual stimulus. While comparable findings have been reported for external sound sources, in most of these cases perceptual correspondence of the cross-modal stimuli occurred (Bertelson and Radeau 1981; de Gelder and Bertelson 2003; Hairston et al 2003; Wallace et al 2004). Unique to the current study is the fact that, since the auditory signal was presented over headphones, it could not perceptually group with the flash of light from the CRT screen, yet bias still occurred. In Mays and Schirillo (2005) subjects tracked eight flashes of light that traversed a CRT screen, so it may be that systematic eye movements (from left-to-right or right-to-left) produced the auditory bias. In the current study, subjects had to fixate prior to viewing a single flash of light. This prevented them from making a saccade prior to the flash of light, since they could not predict which side of the screen would be subsequently illuminated. This suggests that eye movements are not necessary to produce bias. In our second experiment half of the trials had no fixation, thus testing whether any presumptive bias of eye movements to either side of the screen could affect the results. 3 Experiment Method Subjects. All procedures were approved by the Institutional Review Board of Wake Forest University and were performed in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki. Five paid naive Wake Forest University undergraduate students (two males) aged 18 ^ 21 years, with normal or correctedto-normal vision and normal auditory thresholds (20 Hz ^ 8 khz) participated in the experiment Apparatus and stimuli. These were the same as in experiment 1, except that after each trial the subject used a computer keyboard to type his/her eight responses directly into E-Prime Procedure. This was the same as in experiment 1, except for three differences. First, only on half of the trials did subjects have to fixate. On the other half of the randomly determined trials there was no fixation cross present. In these cases subjects were instructed to simply gaze at the center of the screen and wait for the eight clicks-pairs to be presented over the headphones before responding. Second, besides a flash of light occurring during one of the eight click-pair presentations, a no-flash condition was included. This condition occurred eight times for every eight flash conditions. However, flash and no-flash conditions were randomly intermixed (ie there

7 Light influences sound localization 97 could be several trials in succession that either contained no flash or contained a flash). Last, after completing each trial, subjects rated the perceived location of each of the eight click-pairs within the head, using a continuous scale ranging from 10 to 10. This is because one aspect of the Weber ^ Fechner law is that humans have an innate logarithmic representation of numbers. The use of an equal positive-tonegative scale ensures that any use of directional ratings was not an issue (Dehaene et al 2008). For example, it circumvents the concern that a rating of 2 is the same distance away from the left ear as 18 is from the right ear. Thus, we used a rating of 10 to correspond to a click being perceived in the left ear, and a rating of 10 to correspond to a click being perceived in the right ear. Any number between 10 and 10 indicated that the sound was perceived somewhere within the head along the axis from left ear to right ear, with a rating of 0 indicating a sound being perceived in the middle of the head. An experimental session took approximately 35 min. The trials were divided into 128 different conditions that varied across four dimensions. As in experiment 1, the first dimension was the randomly determined direction of the click-pairs, and the second dimension was whether the light was flashed on the left side or on the right side of the screen. The third dimension was which of the eight click-pairs contained either a temporally coincident flash of light or the no-flash condition. The last dimension was whether there was a fixation cross or not. These 128 conditions [2 click-pair directions64 light-flash locations (left, right, and 26 none) 68 flash occurrences62 fixation conditions] were presented randomly and constituted a block of trials. An experimental session was composed of one block, resulting in a total of 128 trials per session. Each subject completed 42 sessions overall, averaging 2 ^ 3 sessions per day, which were separated by at least 2 h each. As the random trials were predetermined, each subject received the trials in the same sequential order. 3.2 Results The data were organized into 128 conditions, for which we computed the mean reported localization (N ˆ 42) for each click-pair. As there were no differences between fixation and no-fixation conditions, these conditions were collapsed. The means of three subjects (BA, RP, and PB) who generated a relatively smooth function are plotted as two separate graphs, organized by (i) click-pair direction, and (ii) flash location or no-flash condition (figure 4). The means of the data for the other two subjects (LS and EG) who generated a step function are plotted as two separate graphs, organized by (i) click-pair direction, and (ii) flash location or no-flash condition (figure 5). Each subject showed an average increase in ratings for click-pairs in the trials when a light was flashed on the right (thick black line) as opposed to being flashed on the left (thick dark-gray line). This indicates subjects perceived the clicks as being closer to the right ear within the head. This effect was found for both directions of click-pairs. The no-flash condition fell between the right-flash and left-flash conditions for each subject. To test for statistical significance, we ran a 2 (click-pair direction right to left or left to right)63 (flash left, flash right, or no flash)62 (fixation or no fixation) repeated-measures within-subjects ANOVA. We collapsed the ratings from all eight click-pairs in each trial into an overall mean for the trial. From the overall means for each trial we calculated each subject's overall means for all 128 conditions. Komogorov ^ Smirnov tests were performed and indicated that each of our conditions had a normal distribution. Eight separate ANOVAs were run, differing by which click-pair contained the temporally coincident flash of light, and three additional ANOVAs were run on the overall means averaged across all temporal occurrences of the two-flash and the no-flash conditions.

8 98 S Dellorso, J Schirillo Perceived location (a) Flash right 1 ± 8 Flash right average Flash left 1 ± 8 Flash left average No flash Click (b) Click Figure 4. Experiment 2: The average click-pair ratings for flash right (black) versus flash left (gray) conditions of three subjects (BA, RP, and PB) who produced relatively smooth functions combining conditions of whether a fixation was present or not. (a) Left-to-right click-pair direction; (b) right-to-left click-pair direction. Dotted (flash right) and dashed (flash left) gray lines represent click-pairs containing different temporally coincident flashes. Solid lines represent the average of the eight click-pairs. Black dashed line represents when no flash was present. Error bars represent 1 SEM. Left ear perceived location 10; right ear perceived location 10. Perceived location (a) Flash right 1 ± 8 Flash right average Flash left 1 ± 8 Flash left average No flash Click (b) Click Figure 5. Experiment 2. The average click-pair ratings for flash right (black) versus flash left (gray) conditions of two subjects (LS and EG) who produced step functions combining conditions of whether a fixation was present or not. (a) Left-to-right click-pair direction; (b) right-to-left click-pair direction. Dotted (flash right) and dashed (flash left) gray lines represent click-pairs containing different temporally coincident flashes. Dark-colored lines represent the average of the eight click-pairs. Black dashed line represents when no flash was present. Error bars represent 1 SEM. Left ear perceived location 10; right ear perceived location 10. For each subject, the results showed a statistically significant main effect for both click-pair direction ( p 5 0:001) and flash-of-light location ( p 5 0:001) (see table 2 for a summary of F and p values for each subject). For each subject, there was also a statistically significant main effect between the flash (either right or left) and the no-flash condition (ie either p 5 0:01 or p 5 0:05). However, there was no main effect for fixation versus no-fixation nor was there an interaction for click-pair direction6flash/no-flash condition. There was also no interaction for fixation6click-pair direction; or for fixation6flash/no-flash condition. There was also no three-way interaction for fixation6click-pair direction6flash/no-flash condition (see table 2 for a summary of F and p values for each subject).

9 Light influences sound localization 99 Table 2. Results of ANOVA for significance between conditions. * Significant at a ˆ 0:05. ** Significant at a ˆ 0:01; nsˆ not significant. Condition Subject F p Click-pair direction LS ** BA ** EG ** RP ** PB ** Flash left versus flash right LS ** BA ** EG ** RP ** PB ** Flash left versus no flash LS * BA * EG * RP ** PB * Flash right versus no flash LS * BA * EG * RP ** PB * Click-pair direction6flash/no flash LS ns BA ns EG ns RP ns PB ns Fixation versus no fixation LS ns BA ns EG ns RP ns PB ns Fixation6click-pair direction LS ns BA ns EG ns RP ns PB ns Fixation6flash/no flash LS ns BA ns EG ns RP ns PB ns Fixation6click-pair direction6flash/no flash LS ns BA ns EG ns RP ns PB ns 3.3 Discussion In experiment 2 we showed that each of our five subjects produced results comparable to the group data collected in experiment 1. This is important in that our measurements of click-pair locations were subjective, leaving no way to know whether a given rating is comparable across subjects. In fact, while subjects produced unique patterns of responding (see figures 4 and 5), all retained the main effect of having the flash of light bias the direction of the entire train of click-pairs in the direction of the light.

10 100 S Dellorso, J Schirillo This occurred even on trials that contained no fixation, minimizing the possibility that eye movements could affect the localizations. Moreover, having either a left-side flash or a right-side flash differed from a no-flash baseline condition in the expected directions. In all of our plots, the data show that clicks concurrent with the flash do not pop out. While this is a null result, it reveals that click-pairs seem to influence each other. Even so, a few individuals (eg subjects LS, EG; figure 5) show an abrupt jump between click 4 and click 5. This suggests that the 64 ms ISI may have been too long for some subjects to create a seamless transition between left- and right-leading clickpairs. That is, the ISI was at a temporal delay that was perceptually longer for certain subjects, thus minimizing their saltatory effect. Taking a group average in experiment 1 minimized this finding. Interestingly, the amplitude of the ratings (maximal rating minimal rating) is almost twice as large in the second experiment. Recall that the second experiment used a bifurcated (eg 10= 10) rating scale, while the first experiment used a continuous 0 ^ 20 rating scale. A future investigation will explore how different rating scales affect the rating magnitude of illusory directional hearing. 4 General discussion Our methodology is unlike a number of psychophysical and physiological studies that had subjects (eg humans, monkeys, or cats) use a light to direct their gaze away from straight-ahead prior to hearing a sound. These studies were designed to test if eyecentered and head-centered receptive field coordinates share a single representative map of space (Hartline et al 1995; Jay and Sparks 1984, 1987, 1989; Peck et al 1994; Poppel 1973; Ryan and Schehr 1941; Zambarbieri et al 1981). Thus, our findings cannot resolve the conflicting results found by Weerts and Thurlow (1971) and Lewald and Ehrenstein (1996), since their lights merely established gaze direction prior to the presentation of their sounds. Instead, our findings suggest that visual and auditory sensory (but not necessarily motor) maps overlap while making cross-modal localization judgments, even when one stimulus source is internally located and the other stimulus source is externally located. However, since our ISI was 64 ms it is possible that a saccade could have been to the flash in a specific direction after only the second or third click-pair, in that saccades to peripheral visual targets can take as little as 120 ms (Carpenter 1988). If this were the case, there should have been a different pattern of responses for lights flashed during these later click-pairs compared to lights flashed during the earlier click-pairs. Our data do not support such a possibility. Instead they agree with findings that saccades do not influence auditory localization (Binda et al 2007; Klingenhoefer and Bremmer 2004). If our subject's eyes did move before a train of click-pairs was completed, our findings would be more consistent with those of Weerts and Thurlow (1971) and contrary to those of Lewald and Ehrenstein (1996). This would suggest that in the present study auditory localization is more uncertain than visual localization, resulting in a bias of the auditory perception ratings toward the direction of the light flash. This explanation is consistent with the results of Mays and Schirillo (2005) and the findings that auditory receptive fields in the superior colliculus shift with changes in eye position, allowing the auditory and visual maps to remain in register (Jay and Sparks 1984, 1987, 1989). However, in our methodology we presented eight sounds using interaural time differences (ITDs) to traverse a range of auditory space rather than using interaural level differences (ILDs) to determine midline location. In that auditory saltation can be produced by either ITDs or ILDs (Phillips and Hall 2001; Phillips et al 2002) it may be possible in future studies to make more direct comparisons between these two methodologies.

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