Perception of Auditory-Visual Temporal Synchrony in ... - CiteSeerX

Journal of Experimental Psychology: Human Perception and Performance 1996, Vol. 22, No. 5, 1094-1106

Copyright 1996 by the American Psychological Association, Inc. 0096-1523/96V$3.00

Perception of Auditory-Visual Temporal Synchrony in Human Infants David J. Lewkowicz New York State Institute for Basic Research in Developmental Disabilities

Using a habituation/test procedure, the author investigated adults' and infants' perception of auditory-visual temporal synchrony. Participants were familiarized with a bouncing green disk and a sound that occurred each time the disk bounced. Then, they were given a series of asynchrony test trials where the sound occurred either before or after the disk bounced. The magnitude of the auditory-visual temporal asynchrony threshold differed markedly in adults and infants. The threshold for the detection of asynchrony created by a sound preceding a visible event was 65 ms in adults and 350 ms in infants and for the detection of asynchrony created by a sound following a visible event was 112 ms in adults and 450 ms in infants. Also, infants did not respond to asynchronies that exceeded intervals that yielded reliable discrimination. Infants' perception of auditory-visual temporal unity is guided by a synchrony and an asynchrony window, both of which become narrower in development.

Many everyday objects and events are specified by information that is concurrently available to different sensory modalities. In general, multimodal information provides for greater perceptual and response accuracy than does unimodal information (Gibson, 1979). To achieve greater accuracy, however, the observer must be able to detect the various types of relations that often unite concurrently available multimodal inputs. One ubiquitous perceptual attribute that provides an important basis for unifying multimodal sources of information is temporal synchrony. In fact, the ubiquity of intersensory temporal synchrony, and the fact that its detection is likely to require relatively simple mechanisms, makes temporal synchrony ideal as a basis for intersensory integration during the earliest stages of development (Lewkowicz, 1992a, 1994a). Indeed, empirical findings from a number of studies indicate that beginning as early as the second month of life, human infants can integrate concurrent auditory and visual inputs on the basis of temporal synchrony (Bahrick, 1987, 1992; Lewkowicz, 1986, 1992b, 1992c; Spelke, 1979; Spelke, Born, & Chu, 1983). For example, when infants are shown two side-by-side visual stimuli bouncing out of phase with respect to one another, they prefer to look at the visual stimulus whose bounce corresponds to a sound that occurs at the same time (Lewkowicz, 1992c). In addition, infants who are habituated to a synchronous auditory-visual

This work was supported in part by funds from the New York State Office of Mental Retardation and Developmental Disabilities. I thank Marci Dabbene for her assistance in data collection and Robert Freedland for helpful suggestions. I also thank Albina Claps and her staff at Saint Vincent's Medical Center of Richmond County for their cooperation and assistance in participant recruitment. Correspondence concerning this article should be addressed to David J. Lewkowicz, New York State Institute for Basic Research in Developmental Disabilities, 1050 Forest Hill Road, Staten Island, New York 10314. Electronic mail may be sent via Internet to [email protected].

event exhibit recovery of the habituated response when the synchrony between the auditory and visual events is disrupted, indicating that they can discriminate the difference between synchronous and asynchronous multimodal events (Lewkowicz, 1992c, 1992b). In contrast, infants first exhibit detection of intersensory equivalence based on duration at 6 months of age (Lewkowicz, 1986) and do not exhibit intersensory detection of rate equivalence even as late as 8 months of age (Lewkowicz, 1985, 1992c, 1994b). These kinds of findings have led Lewkowicz (1992a, 1994a) to propose that intersensory temporal synchrony provides the earliest developmental basis for the integration of temporally based multimodal events and that the emergence of responsiveness to other intersensory temporal attributes such as duration, rate, and rhythm is dependent on the prior differentiation and emergence of perceptual mechanisms that are responsive to temporal synchrony. The perception of intersensory temporal synchrony does not require that the heteromodal components of multimodal events occur at the same instant in time. The perceptual system can tolerate some amount of temporal discrepancy and still provide the observer with a unified perceptual experience. The magnitude of this temporal discrepancy has been shown to be dependent on the order in which the heteromodal components of an auditory-visual compound event occur. For example, McGrath and Summerfield (1985) showed that when adult participants were presented with liplike Lissajou figures that mimicked the opening of lips and a tone that was sounded at various intervals either before or after the opening of the lips, participants first reported asynchrony only when the auditory event preceded the visual event by 79 ms (auditory-visual [A-V] asynchrony threshold) and only when the visual event preceded the auditory event by 138 ms (visual-auditory [V-A] asynchrony threshold). Dixon and Spitz (1980) reported similar findings in a study where participants were presented with a film of a hammer hitting a peg and were allowed to either advance or delay the sound until they judged it to be out of synchrony with the hammer hitting the peg. They found that

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INTERSENSORY TEMPORAL SYNCHRONY IN INFANTS participants needed a minimum of 74.8 ms to detect A-V asynchrony and 187.5 ms to detect V-A asynchrony. The point at which participants first begin to perceive the asynchrony between the auditory and visual components of a multimodal event defines the limit of what might be termed the intersensory temporal synchrony window. Heteromodal components of multimodal events that fall inside this window are perceived as belonging together, whereas those that fall outside of it are perceived as temporally separate events. The size of the intersensory temporal synchrony window is likely to be larger in infants for at least two reasons. One is that, relative to adults, infants are inexperienced with temporal phenomena, and if developmental experience contributes to the general improvement in temporal discriminative abilities, then infants should be relatively poor in this regard. The second reason is that the rate of neural transmission in the immature nervous system is slower. For example, the latencies of both the auditory and visual cortical evoked potentials are substantially longer in infants and become progressively shorter with maturation (Regan, 1989). Behavioral studies in human and animal infants also provide evidence of slower processing. For example, conditioning studies with human infants have shown that conditioned stimulus—unconditioned stimulus (CS-UCS) intervals that are two or more times the length of the optimal interval for adults are most effective in producing classical conditioning (Ingram, 1978). Likewise, conditioning studies with rats have shown that young rats require longer CSUCS intervals for optimal conditioning (Caldwell & Werboff, 1962). The size of the intersensory temporal synchrony window can be viewed as yet another measure of the speed of processing in early development because it is a way of indexing the temporal resolution of the nervous system. In addition, as noted earlier, Lewkowicz (1992a, 1994a) has argued that intersensory temporal synchrony may provide the foundation for the perceptual differentiation of other intersensory temporal attributes in early development and that it is likely to narrow in size during development. Currently, no estimates of the size of the intersensory temporal window are available for infants. Thus, the purpose of the current set of experiments was to provide that estimate. The general experimental method used in the experiments with infants consisted of first habituating each infant to an auditory-visual compound composed of a small green disk that bounced up and down on a video screen and a short sound that occurred every time the disk bounced. As soon as the infant reached a predetermined habituation criterion, three asynchrony test trials and one familiar test trial were administered. During the asynchrony test trials, the visible bounce of the disk and the presentation of the sound were made asynchronous by making them occur at different times. To determine whether the infant detected the asynchrony in any of these asynchrony test trials, the magnitude of response in a given asynchrony test trial was compared to the magnitude of response obtained in the familiar test trial. Given infants' propensity to exhibit recovery of a habituated response following the presentation of a novel event, the response in the asynchrony test trial was expected to sig-

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nificantly exceed the response magnitude in the familiar test trial if the infant discriminated the degree of asynchrony specified by that test trial.

Experiment 1: Detection of A-V and V-A Asynchrony in Adults As noted previously, evidence suggests that temporal processing and responsiveness improves with development. One of the questions that motivated this research was whether temporal processing of intersensory temporal synchrony also changes during development. To answer this question, first it was important to establish whether the methods used in the experiments with infants would yield adult asynchrony thresholds comparable to those found in the two previous studies conducted with adults (Dixon & Spitz; 1980: McGrath & Summerfield, 1985). If they did, then this would indicate that the methods are appropriate for estimating the size of the intersensory temporal asynchrony threshold in infants and for making direct infant-adult comparisons. Thus, the purpose of Experiment 1 was to estimate the adult A-V and V-A asynchrony thresholds with the same methods that were used in the subsequent experiments to estimate infant thresholds.

Method Participants. Ten adults (6 women and 4 men) were tested. The participants' mean age was 28.8 years, ranging between 17 and 47 years. None of the participants reported any visual or hearing problems. Apparatus and stimuli. During testing each participant sat in front of a 25-in. (63.5 cm, measured diagonally) video monitor. The visual stimulus was a computer-generated sprite graphic resembling a green disk and was produced by a Supersprite video display board operating inside an Apple lie microcomputer. The disk was two dimensional and subtended 3° and 48 min of arc of visual angle and was displayed in a window created by covering the front of the video monitor with a poster board containing a rectangular cutout. The left edge of the window was located 10 cm from the left edge of the monitor and measured 7.5 cm in width and 34 cm in height. The participant was seated directly in front of the center of the monitor. To view the disk, the participant was required to look 45° to his left toward the monitor window. The distance from the participant to the disk was 50.5 cm. During each trial the participant could see a single disk moving up and down in the monitor window. The distance traversed by the disk from top to bottom was 31.5 cm. The sound, which was generated by a sound-generating chip on the Supersprite board, was a complex tone whose envelope descended in time. The duration of the sound was 271 ms, and it measured 63 dB (re. 0002 dynes/cm2, A scale) at the participant's ear. Spectrum analysis indicated that the sound had a fundamental frequency of 62.5 Hz and several harmonic peaks. The sound was presented through two built-in speakers located on each side of the monitor. A baffle, oriented at a 45° angle with respect to the side of the monitor and located behind each speaker, projected the sound forward toward the participant. Procedure. Participants were tested individually in a dimly illuminated, quiet room. The ambient sound pressure level in the room, as measured at the participant's ear, was 56 dB (re. 0002 dynes/cm2, A scale). Each trial began with the disk appearing at

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the top of the screen and then moving in the downward direction. As soon as it reached the end of its downward trajectory, it reversed its direction without any delay and began to move up. The velocity of the disk, which was calculated on the basis of the off-axis distance of 50.5 cm between the participant and the disk, was 30" and 48 min of arc of visual angle per s. In cyclical terms (a full cycle was defined as the time from the beginning of disk motion at the top of the monitor to the time when it returned to that position) the visual stimulus moved at a rate of 0.42 Hz. The sound was presented at various intervals either before (A-V experiment) or after (V-A experiment) the disk reversed its trajectory at the bottom of the screen. The experimental session began with the participant seeing and hearing a synchronous event (i.e., the disk bounced up and down on the screen and the sound occurred each time it bounced). Participants were told explicitly before the start of the test session that the sound will occur in synchrony with the bounce, that they will view this type of event across several trials, and that the duration of these trials will decrease progressively. Participants were asked to simply watch and listen during this phase of the session. There were seven such trials; their durations were 39.8, 17.9, 14.8, 3.9, 3.9, 3.9, and 3.9 s. The number and durations of these trials were based on the average number of trials needed to reach habituation in the infant experiments described later and on then- average durations. This was done to approximate as closely as possible the testing procedures used with infant participants so as to permit direct comparisons of thresholds. Following this initial familiarization phase, two blocks of five test trials each were administered to each participant. For half of the participants, response to A-V asynchrony was tested in the first block of trials and response to V-A asynchrony was tested in the second block of trials. The order of the two test blocks was reversed for the other half of the participants. During each block of test trials, a participant was given a familiar (F) test trial and four asynchrony test trials. The F test trial involved the presentation of the synchronous A-V event. During the A-V block of test trials, participants were presented with the following four asynchrony intervals: 50, 80, 110, and 140 ms. During the V-A block of test trials, participants were presented with the following four asynchrony intervals: 110, 140, 170, and 200 ms. The order of the test trials within each block was counterbalanced across participants such that each test trial was presented equally often in each ordinal position. The participant's task was to decide, during each trial, whether the sound and disk were synchronous. They were instructed to respond "yes" if they thought that they were and "no" if they thought that they were not. Participants were permitted to hear and view the A-V event during each test trial for as long as they wished, and when they felt confident about their decision, they were asked to give their response.

Table 1 Adult Auditory-Visual (A-V) Asynchrony Thresholds Participant

A-V threshold (ms)

V-A threshold (ms)

1 2 3 4 5 6 7 8 9 10

95 25 25 65 25 65 95 95 65 95

155 155 55 125 55 55 155 125 55 185

M

112

with a "yes" to the 50-ms A-V interval, then with a "no" to the 80-ms interval, and a "yes" to the 110-ms interval. Participant 10 responded with a "yes" to the 140-ms V-A interval, then with a "no" to the 170-ms interval, and a "yes" to the 200-ms interval. As Table 1 shows, the average A-V asynchrony threshold was 65 ms and the average V-A asynchrony threshold was 112 ms. These values are slightly lower than those obtained in the other two studies of asynchrony detection in adults and might reflect the simpler nature of the stimuli used in the current experiment. McGrath and Summerfield (1985) found the A-V asynchrony threshold to be 79 ms and the V-A asynchrony threshold to be 138 ms, and Dixon and Spitz (1980) found the A-V threshold to be 74.8 ms and the V-A threshold to be 187.5 ms. Despite the slightly lower thresholds, however, it was also found that the V-A threshold was nearly twice as large as the A-V threshold. The results from this experiment demonstrate that the method used to study infants' asynchrony thresholds in the subsequent infant experiments yields values in adults that are similar to those obtained with considerably different methods and stimulus materials, and thus, validates its use in infants. The specific thresholds obtained in adults in this experiment provide a direct baseline measure of the mature form of the A-V temporal asynchrony threshold and make it possible to compare infants and adults directly.

Experiment 2: Detection of A-V Asynchrony in Infants

Results and Discussion Table 1 shows the individual thresholds for each participant. The thresholds were computed by calculating the midpoint value between the highest asynchrony interval that the participant judged to be synchronous and the lowest asynchrony interval that the participant judged to be asynchronous. For 8 of the 10 participants, these two intervals were adjacent to each other. Participants 7 and 10 gave what appeared to be a false positive response by saying "yes" to an asynchrony interval but then responding "no" to a higher interval. In these two cases, it was the higher interval that was taken as the threshold. Thus, Participant 7 responded

As noted earlier, the size of the A-V temporal synchrony window is likely to be substantially greater in infants as compared to adults. Because no previous estimates of the size of the temporal synchrony window in infants were available, it was necessary to adopt a convergent-operations approach to zero in on the threshold. As a result, several different groups of infants were tested with different ranges of asynchrony intervals that were intended to span a range from slightly above the adult threshold of 65 ms to substantially above that value. In all, three separate groups of infants were tested. The first group was tested with asynchrony intervals of 100, 200, and 300 ms; the second group

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INTERSENSORY TEMPORAL SYNCHRONY IN INFANTS

Table 2 Infant Age Characteristics in Experiment 2 Infant age (months)

Age (weeks)

SD

Age (weeks)

SD

Age (weeks)

SD

2 4 6 8

11.2 20.0 28.6 37.5

0.6 0.5 0.8 0.4

11.1 20.3 29.0 37.4

0.5 0.4 1.1 0.6

10.9 19.5 29.0 37.0

0.6 1.3 0.4 0.7

Group 2

Group 1

was tested with intervals of 150, 250, and 350 ms; and the third group was tested with intervals of 300, 350, and 400 ms. Because many sensory and perceptual functions undergo marked changes during infancy (Banks & Salapatek, 1983), infants ranging in age from 2 to 8 months of age were included in each group to determine whether changes in the size of the intersensory temporal synchrony window occur during the first year of life.

Method Participants. Three separate groups, each consisting of 32 infants, were tested. Each of these groups was composed of four separate age groups of eight infants each. Groups 1 and 2 were composed of equal numbers of boys and girls. Group 3 also was composed of an equal number of boys and girls at each age except at 2 months where there were 5 boys and 3 girls. Table 2 shows the age characteristics for all three groups of infants. For Group 1, the data from three additional 2-month-old infants were not used because one infant refused to look and two cried during the test session, and the data from three 6-month-old infants were not used because one fussed and two were inattentive. For Group 2, the data from one additional 2-month-old infant were not used because this infant refused to look at the stimulus. For Group 3, the data from one additional 2-month-old and one additional 4-month-old infant were not used because these infants fussed during testing. All the infants in this experiment, as well as in all the subsequent experiments, were healthy at the time of testing and were full term at the time of birth. Each infant's birth weight was greater than 2,500 g, gestational age was greater than 37 weeks, and Apgar scores were greater than 7. Apparatus and stimuli. During testing, each infant sat directly in front of the center of the monitor, at a distance of 50.5 cm. The monitor was enclosed on both sides with a curtain that extended on each side past where the infant was seated. To attract the infant's visual attention toward the location where the disk was presented, a schematic face was displayed in the center of the monitor window before the start of each trial. The infant's visual behavior was viewed and recorded throughout the test session with a video camera located on top of the video monitor. The lens of the camera was pointed at the infant's face, allowing the observer to see clearly the direction of the infant's visual gaze. All scoring of visual gaze was done on-line, although the entire test session also was videotaped. Procedure. Most of the infants sat in an infant seat that was reclined at a 45° angle with respect to the monitor. Some infants refused to sit in the infant seat and instead were tested in their parent's lap.1 The parents who held their infants during the test were not aware of the specific hypothesis being tested and were requested to sit as still as possible and to refrain from any interactions with their baby. The testing session began as soon as the

Group 3

infant was seated and looked at the schematic face on the screen. At this time, the face disappeared and the green disk appeared at the top of the screen and began to move. The duration of each trial depended on the length of the infant's look toward the monitor; as long as the infant continued to look at the window, the infant could see the disk and hear the sound. As soon as the infant looked away from the monitor window for more than 1 s, the disk disappeared, the sound ceased, the schematic face reappeared, and the trial ended. The next trial began when the infant looked back at the schematic face. The total number of habituation trials depended on how quickly an infant met a predetermined habituation criterion. The habituation criterion required that the total duration of looking during the last three trials decline to less than 50% of the total duration of looking during the first three habituation trials. As a result, the minimum number of habituation trials administered to an infant was six, whereas the maximum depended on the infant meeting the habituation criterion and, thus, was open ended. As soon as the last trial satisfying the habituation criterion was completed, the habituation phase was terminated and the next look at the schematic face initiated the test phase. Four types of test trials, consisting of an F and three asynchrony test trials, were administered to each infant. The original A-V compound was presented during the F test trial and the magnitude of response in this trial was used as a baseline against which the response magnitude in each of the asynchrony test trials was compared. If the magnitude of response in a given asynchrony test trial significantly exceeded the magnitude of response in the F test trial, then that asynchrony interval was deemed discriminable. The three asynchrony intervals were produced by making the sound occur at some interval before the point at which the disk reversed direction at the bottom of the screen (i.e., before the bounce). The order of the four test trials was counterbalanced across infants within each age group such that each test trial was presented equally often in each ordinal position. As in the habituation phase, the length of each test trial was controlled by the infant. The experimenter, who could not be seen, observed the infant on a second video monitor. While observing the infant's eyes, she controlled the presentation of the stimuli by initiating trials whenever the infant looked at the schematic face in the window. Because the number of habituation trials could vary in an unpredictable manner across infants, there was no way for the experimenter to know when the habituation phase was over and when the test phase began. Also, the experimenter could not see the disk because she could not see the monitor on which it was displayed

1 Group 1: one 2-month-old, two 4-month-old, one 6-month-old, and one 8-month-old infants were tested on the lap. Group 2: three 2-month-old, one 6-month-old, and two 8-month-old infants were tested on the lap. Group 3: two 6-month-old and one 8-month-old infants were tested on the lap.

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and could not hear the sound because she listened to music on a set of headphones.

Results and Discussion Group 1: Asynchrony intervals of 100, 200, and 300 ms. A preliminary analysis was carried out to determine whether such factors as age and gender affected response recovery in this group of infants in the three asynchrony test trials. To control for differences in absolute amount of looking across age, a response recovery score was computed for each infant by taking the difference between an infant's duration of looking in each asynchrony test trial, respectively, and the infant's duration of looking in the F test trial. The three resulting recovery scores were then entered into a 4 (age) X 2 (gender) X 3 (trials), repeated measures multivariate analysis of variance (MANOVA), with age and gender as betweensubjects factors and test trials (the three recovery scores) as a within-subjects factor. No significant effects were found. Because age and gender did not play a significant role in responsiveness, the data from the four test trials were collapsed across these two variables and a new analysis was performed. This analysis used the raw scores from each of the four test trials and was a repeated-measures MANOVA with Trials as the repeated measures factor. To determine whether the infants discriminated any of the asynchronies presented during the asynchrony test trials, the MANOVA was followed by a set of planned comparisons in which the magnitude of looking in each of the asynchrony test trials was compared to the magnitude of looking in the F test trial (see Figure 1). The planned comparisons indicated that there was no significant response recovery to the 100-ms and the 200-ms asynchrony intervals and that there was marginal recovery of response to the 300-ms asynchrony interval, F(l, 31) = 3.86, p < .06. These results suggest that infants, unlike adults, perceive the auditory and visual attributes of an event as synchronous even when they are separated by as

much as 300 ms. Also, the absence of an age effect suggests that the shortening of the intersensory synchrony window, which occurs sometime before adulthood, does not occur during the period between 2 and 8 months of age. Group 2: Asynchrony intervals of 150, 250, and 350 ms. The preliminary MANOVA of the recovery scores, with age and gender as me between-subjects factors and trials as the within-subjects factor yielded a significant trials effect, F(2, 48) = 3.87, p < .05, indicating that there was an overall difference in response across the different test trials. No gender and no age effects were found, replicating this finding in Group 1. As for Group 1, the data for the four test trials were then collapsed across age and gender and a new MANOVA of the raw scores, with trials as the repeated factor, was carried out. The planned comparison tests that were based on this analysis indicated that response recovery to the 150-ms and the 250-ms asynchrony intervals was not significant, but that response recovery to the 350-ms asynchrony interval was statistically significant, F(l, 31) = 11.49, p < .01 (see Figure 2). Given the marginal response recovery to the 300-ms asynchrony interval in Group 1, the most conservative interpretation of the significant response recovery to the 350-ms asynchrony interval is that me threshold for the detection of A-V asynchrony lies somewhere between 300 and 350 ms. In other words, regardless of age, infants between 2 and 8 months of age require a temporal separation between a leading auditory event and a subsequent visual event of more than 300 ms to perceive the two events as temporally separate. Group 3: Asynchrony intervals of 300, 350, and 400 ms. The purpose of testing responsiveness to this range of asynchronies was twofold. The first was to replicate the finding of a successful discrimination of the 350-ms interval. The second was to determine whether the effect found in Group 2 was dependent on the specific range of asynchrony intervals used, or whether the results reflect a stable

20-i

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5-

c a o>

Familiar

100ms 200ms Test Trials

300ms

Figure 1. Mean duration of looking in the familiar and the asynchrony test trials in Group 1 hi Experiment 2. Error bars represent standard errors of the mean.

Familiar

150ms 250ms Test Trials

350ms

Figure 2. Mean duration of looking in the familiar and the asynchrony test trials in Group 2 in Experiment 2. Error bars represent standard errors of the mean.

INTERSENSORY TEMPORAL SYNCHRONY IN INFANTS threshold that is not dependent on the specific intervals used to test for it. If the specific range did not play a role, then the infants should exhibit significant response recovery to the 350-ms and the 400-ms asynchrony intervals but not to the 300-ms interval. The results of the preliminary analysis indicated that there were no significant gender, age, or trials effects. The planned-comparison analysis of the four test trials indicated that, as predicted, there was a marginally significant response recovery to the 300-ms asynchrony interval, F(l, 31) = 3.46, p < .08, and that there was a significant response recovery to the 350-ms interval, F(l, 31) = 5.71, p < .05, and to the 400-ms interval, F(l, 31) = 4.66, p < .05 (see Figure 3). The significant recovery to the 400-ms interval indicates that infants detected asynchronies greater than 350 ms and provides further support for the conclusion that the A-V asynchrony threshold lies beyond 300 ms.

Experiment 3: Detection of V-A Asynchrony As noted earlier, the V-A synchrony window is considerably larger than the A-V synchrony window in adults. Consequently, it is reasonable to expect that the V-A synchrony window might be larger in infants as well. To test this possibility, the order of the auditory and visual events was reversed in this experiment such that the sound occurred after the visual stimulus bounced. As in Experiment 2, it was necessary to adopt a convergent operations approach to zero in on the threshold. Consequently, three different groups of infants were tested with different asynchrony interval ranges. Given that the V-A asynchrony threshold is likely to be longer and that the A-V asynchrony threshold was found to lie between 300-350 ms in Experiment 2, infants were tested with a range of asynchrony intervals that were both below and above the 300-350 ms region.

20-1

'15-

| o 10Q 5-

Familiar

300 ms 350 ms Test Trials

400 ms


1099

Method Participants. Three separate groups of 32 infants each were tested. Each group was composed of four separate age groups of 8 infants each, with equal numbers of boys and girls. The one exception was Group 3 where 3 girls and 5 boys were tested at 4 months of age and 5 girls and 3 boys were tested at 8 months of age (see Table 3 for the age characteristics of Groups 1-3), For Group 1, the data from 6 additional infants were not used because three 2-month-old infants fussed and two 4-month-old infants and one 8-month-old infant refused to look at the stimulus display. For Group 2, the data from one additional 4-month-old infant were not used because of fussing. For Group 3, the data from one additional 2-month-old infant were not used because of fussing. Procedure. The procedure used in the current experiment was the same as the procedure used in Experiment 2 except that in this experiment the sound occurred after the bounce of disk. Some infants refused to sit in the infant seat and instead were tested in the parent's lap.2

Results and Discussion Group 1: Asynchronies of 250, 350, and 450 ms. The preliminary analysis indicated that there were no significant gender, age, or trials effects. The planned comparison analysis showed that response recovery to the 250-ms interval was not significant and that response recovery was marginal to both the 350-ms interval, F(l, 31) = 3.61, p < .08, and the 450-ms interval, F(l, 31) = 3.50,p < .08 (see Figure 4). The marginal nature of these findings suggests that the V-A asynchrony threshold may be located near the 450-ms region. Group 2: Asynchronies of 400, 450, and 500 ms. The results from Group 1 suggested that the V-A asynchrony threshold may lie somewhere around the 450-ms region. To investigate this possibility, the infants in Group 2 were tested with the 450-ms interval as well as intervals that were 50 ms less or more than that interval. The preliminary analysis yielded a significant age effect, F(3,28) = 4.40, p < .05, which was due to a decrease in the overall amount of looking as a function of age, with the greatest amount of looking found in the 2-month-old infants. Because age did not interact with gender and trials, the raw data from the test trials were collapsed across these two factors and the planned contrast analyses were performed as before. They yielded no significant recovery of response to any of the asynchrony intervals. These findings were surprising, given the findings from Group 1. One reason for this outcome may be that the discrimination of an asynchrony when the sound follows the visual event is more difficult than is the discrimination of an asynchrony when the sound precedes the visual event. The joint results from Groups 1 and 2 are consistent with such an interpretation. 2

Group 1: one 4-month-old, three 6-month-old, and two 8-month-old infants were tested on the lap. Group 2: one 2-monthold, two 4-month-old, two 6-month-old, and three 8-moath-old infants were tested on the parent's lap. Group 3: two 2-month-old, two 4-month-old, three 6-month-old, and three 8-month-old infants were tested on the parent's lap.

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Table 3 Infant Age Characteristics in Experiment 3 Infant age (months)

Age (weeks)

SD

Age (weeks)

SD

Age (weeks)

SD

2 4 6 8

11.2 20.0 28.7 37.5

0.7 0.6 0.9 0.9

11.4 20.4 28.5 37.2

0.4 0.8 0.8 0.7

11.3 20.2 29.0 37.6

0.7 1.4 0.6 0.5

Group 2

Group 1

An additional possibility is that the 450-ms asynchrony interval may be the threshold for the detection of V-A asynchrony but that the infants' performance is affected by the specific interval range that is used in testing. In other words, it may be that the greater difficulty of the V-A discrimination task makes the infants more sensitive to the contextual effects introduced by the other intervals. Group 3: Asynchronies of 450, 500, and 550 ms. One way to find out whether contextual factors influence responsiveness is to test infants with an interval that is most likely to yield positive discrimination. Given that the 450-ms interval yielded marginally significant discrimination in Group 1, and given that the adult data show that the V-A asynchrony threshold is 47 ms longer than the A-V threshold, it would be reasonable to suppose that the V-A asynchrony threshold for infants should be at least 100 ms longer than the A-V asynchrony threshold. Thus, this group of infants was tested with the 450-ms interval, as well as a 500-ms and a 550-ms interval. The two longer intervals were chosen because they are different from the intervals that accompanied the 450-ms interval for Groups 1 and 2, and, as a result, provided the opportunity to test the possibility that contextual factors influenced responsiveness. In addition, these two higher intervals were designed to examine responsiveness to greater degrees of V-A asynchrony. The preliminary analyses indicated that neither gender nor age affected responsiveness. The subsequent planned 20-1

Group 3

comparison analyses showed that the infants discriminated the 450-ms asynchrony interval, F(l, 31) = 4.73, p < .05, that they exhibited marginal discrimination of the 500-ms interval, F(l, 31) = 3.14, p < .10, and that they did not discriminate the 550-ms interval, F(l, 31) = 2.29, p >. 10 (see Figure 5). The fact that the infants in this group discriminated the 450-ms interval suggests that this is the minimum interval necessary for infants to detect V-A asynchrony. It is interesting to note that the minimum interval needed by the infants to discriminate V-A asynchrony is longer than the minimum asynchrony interval needed to discriminate A-V asynchrony. This is consistent with findings from studies with adult participants. The infants in all three V-A groups were tested with the 450-ms asynchrony interval, but only infants in Group 3 exhibited reliable discrimination of this interval. The difference among Groups 1, 2, and 3 is that the 450-ms asynchrony interval was embedded in a different series of asynchrony intervals in each group. The fact that infants in Group 3 discriminated the 450-ms interval suggests that contextual factors (i.e., the specific asynchrony intervals comprising the test trials) may affect responsiveness. That is, responsiveness to the 450-ms test trial may have been contaminated by carryover effects from other test trials in those cases where other test trials preceded presentation of the 450-ms test trial. The carryover effects might be due either to lingering arousal effects from a preceding test trial

20 -,

Familiar


450ms


Familiar


550 ms


INTERSENSORY TEMPORAL SYNCHRONY IN INFANTS

if it resulted in a recovery of response, or they might be due to lingering depression effects if the preceding trial did not produce response recovery. One way to examine the context effect is to ask whether infants discriminated the 450-ms asynchrony interval when its presentation was not preceded by the presentation of any other test trials. Thus, responsiveness to the 450-ms asynchrony interval was examined in just those infants in Experiment 3 who received the 450-ms test trial right after they reached the habituation criterion. Eight infants (two at each age) in each group received the 450-ms test trial first in the series of test trials and, thus, contributed data to this analysis. A one-way MANOVA, comparing the duration of looking in the 450-ms test trial versus the duration of looking in the F test trial, indicated that these infants discriminated the 450-ms asynchrony interval, F(l, 23) = 4.35, p < .05. What is notable about this effect is that it is statistically reliable despite the fact that the total number of participants contributing data to this analysis is 24 rather than the 32 that contributed data to each of the groups. Although the preceding analysis yielded significant discrimination, the F test trial was embedded within the sequence of the three test trials in each of the three experiments and, thus, also was subject to the possible contaminating effects. To control for this, a second analysis was conducted where the measure of baseline responsiveness was the last habituation trial. This trial is least contaminated because it always occurred before the presentation of any of the test trials. Thus, the data from the last habituation trial from the same 24 infants who contributed to the previous post hoc analysis were compared with the data from the 450-ms asynchrony test trial when it was the first test trial in the sequence of test trials. As in the previous post hoc analysis, a one-way MANOVA yielded a significant discrimination effect, F(l, 23) = 7.25, p < .025. It is interesting to note, however, that this analysis yielded a more robust statistical effect. On the one hand, this might be due to the fact mat the F test trial was to some extent contaminated. On the other hand, it might be because responsiveness to the last habituation trial was, to some extent, artificially depressed because of the operation of the habituation criterion. Either way, the finding of a significant discrimination in this post hoc analysis indicates once again that presentation of other test stimuli did play a role in responsiveness to the 450-ms asynchrony test interval. It was possible to carry out a post hoc analysis on the response to the 500-ms asynchrony as well because data from Groups 2 and 3 could be combined for this asynchrony interval yielding a sufficient sample of infants (n = 16) who received this test trial first. A comparison of responsiveness in this test trial versus responsiveness in the F test trials yielded no significant recovery but a comparison of responsiveness in this test trial with responsiveness in the last habituation trial did yield a significant recovery of response, F(l, 15) = 5.48, p < .05. The post hoc analyses support the context hypothesis in showing that clear evidence of discrimination of the 450-ms asynchrony interval and some evidence of the discrimination of the 500-ms interval emerges when the contaminating

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influence of the other test trials is removed. It should be noted, however, that no evidence of discrimination of what would be expected to be more easily discriminable V-A asynchrony intervals was found when the kinds of a priori, planned comparisons that yielded significant results in Experiment 2 were conducted on the data from the V-A experiments. That is, the findings from Group 2 indicated that infants did not respond to the 500-ms asynchrony interval, and the findings from Group 3 indicated that infants did not respond to the 500-ms and the 550-ms asynchrony intervals. These findings suggest that there may be a region of sensitivity to violations of V-A synchrony that not only has lower temporal limits but higher temporal limits as well. The lower temporal limit may allow infants to distinguish between those auditory and visual events that are perceptually synchronous and those that are asynchronous. This lower temporal limit may provide the infant with a psychological present snapshot of the tnultimodal world. The principal function of this psychological present is to enable the infant to process and respond to auditory and visual events in terms of their temporal relationship. The upper limit may be the temporal boundary beyond which the synchrony between an auditory and visual event is no longer relevant and, as a result, the two events are treated as belonging to different psychological presents. Of course, it is possible that the failure to discriminate V-A asynchronies as large as 500 and 550 ms was due to the fact that the sound in those two test trials was processed as being temporally synchronous with the direction reversal of the disk when it reached the top of the monitor. This is not likely, however, because the sound occurred too far in advance of the disk's direction reversal at the top of the screen. It took the disk 1.2 s to traverse the distance from the point where it reversed direction at the bottom of the screen, to the point where it reversed direction at the top of the screen. Even at the longest V-A asynchrony interval of 550 ms, the sound was 650 ms away from direction reversal at the top. Assuming that the infant was now actually performing an A-V asynchrony discrimination task (sound preceding the direction reversal at the top), significant discrimination would be expected because the interval separating the sound and the bounce was 650 ms, which exceeds considerably the 300-350-ms A-V asynchrony threshold found in Experiment 2. Given that significant discrimination was not found in the 550-ms asynchrony test trial, it is unlikely that the sound was being perceived in relation to the bounce at the top of the motion trajectory. Alternatively, if there is an upper limit to the detection of A-V asynchrony (see below), as there appears to be for the detection of V-A asynchrony, then the sound and the visible bounce at the top would simply be treated as two perceptually distinct events.

Experiment 4: Test of the Upper Limit of A-V Asynchrony Detection The postulation of an upper limit for the detection of temporal intersensory synchrony in the case of V-A asynchrony requires that the same kind of upper limit operate in

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the A-V case. The highest asynchrony interval presented in Experiment 2, examining the detection of A-V asynchrony, was 400 ms, and infants exhibited significant discrimination of this interval. Thus, support for the operation of an upper limit on the perception of A-V asynchrony would require evidence that infants fail to discriminate A-V asynchrony intervals beyond 400 ms. To determine if that is the case, in the current experiment infants were tested with A-V asynchronies of 400, 450, and 500 ms.

Method Participants. A new sample of 32 infants, consisting of four separate age groups of 8 infants each, was tested. Half of the infants at each age were boys and half were girls. The mean ages of the infants were as follows: 11.3 weeks (SD = 0.7), 20.2 weeks (SD = 1.4), 29 weeks (SD = 0.6), and 37.6 weeks (SD = 0.5). Apparatus and stimuli. The apparatus and stimuli were the same as those in Experiment 2. Procedure. The procedure was the same as the procedure in Experiment 2. Thus, following habituation to the synchronous A-V compound stimulus, each infant was given a familiar test trial and three test trials during which the audible bounce occurred either 400, 450, or 500 ms before (he visible bounce.

Results and Discussion The preliminary analysis indicated that there were no significant gender, age, or trials effects. As predicted on the basis of the postulated existence of an upper limit on the detection of A-V asynchrony, no significant recovery of response was found for any of the asynchrony intervals. As in the case of the detection of V-A asynchrony, it was found that when an A-V asynchrony interval exceeded an asynchrony interval that was reliably discriminable, infants no longer exhibited evidence of discrimination. It is also interesting to note that a similar context effect found in the experiment examining detection of V-A asynchrony was found in the current experiment. Whereas the 400-ms A-V asynchrony interval was reliably discriminated by Group 3 in Experiment 2, it was not reliably discriminated when it was embedded in a sequence of asynchrony intervals that were different in value.

General Discussion The purpose of the current set of experiments was to investigate developmental differences in the perception of intersensory temporal synchrony. To accomplish this goal, the same stimulus materials and similar testing methods were used to estimate A-V and V-A asynchrony thresholds in adults and in infants. Although results showed that a certain degree of temporal slippage is permitted at all ages in the perception of intersensory temporal synchrony, the amount of that slippage differs markedly across development. Thus, adults were found to require a minimum temporal separation of 65 ms to detect A-V asynchrony and 112

ms to detect V-A asynchrony. In contrast, infants were found to require a minimum temporal separation of as much as 350 ms to detect A-V asynchrony and 450 ms to detect V-A asynchrony. Rather than viewing these results in terms of thresholds, they also can be viewed from a perceptual unity point of view. In terms of perceptual unity, the results from the current experiments suggest that the perception of intersensory temporal unity is governed by an intersensory temporal synchrony window—a period of time during which the heteromodal components of a compound multimodal event can occur sequentially and still be perceived as unified. There are at least two possible reasons why the intersensory temporal synchrony window is greater in infants. One reason may be the relatively unmeyelinated and undifferentiated state of the infant's central nervous system (Yakovlev & Lecours, 1967). Myelin speeds conduction velocity and, as a result, the present results may reflect a generally slower neural transmission rate in the infant. A slower rate of neural transmission might result in a longer time constant in the infant's nervous system leading to poorer temporal resolution. One interesting finding was that there were no developmental changes in the size of the intersensory temporal synchrony window between 2 and 8 months of age. This finding most likely reflects the fact that myelination is a very slow process; it is not completed until 11 years of age. The second possible reason for the greater size of the synchrony window in infants is their relative lack of experience with temporally based, multimodal events. There is evidence that auditory and visual sensory thresholds, as well as many other higher level functions in the auditory and visual modalities, improve as a result of developmental experience with specific kinds of inputs (Banks & Salapatek, 1983; Rubel, 1985). There is also evidence that specific intersensory experiences affect the subsequent organization of intersensory responsiveness (Knudsen, 1985; Lickliter & Banker, 1994; Tees, 1994). Consequently, long-term experience with temporally based, multimodal events is likely to improve performance. The route by which narrowing of the intersensory temporal synchrony window occurs is most likely through a highly complex process of reciprocal interaction between neuromaturational and experiential factors (Gottlieb, 1991; Lewkowicz & Lickliter, 1994). The fact that the V-A synchrony window is larger in both age groups suggests that mere is some inherent characteristic of either the external input or of the nervous system that might account for this difference. The differential propagation time of auditory and visual signals and their consequent differential arrival time at the sensory receptor level cannot account for the difference because the difference in arrival time is negligible at the short distance between the participant and the stimulus used in the current experiments. The much more likely reason is the way in which the auditory and visual modalities process input. Posner, Nissen, and Klein (1976) noted that there is an inequality in the way that visual and auditory systems process incoming information. They argued that the inequality is due to the fact that unlike visual signals, auditory signals are automatically alerting and that they have immediate access to cen-

INTERSENSORY TEMPORAL SYNCHRONY IN INFANTS tral processing mechanisms. One possible reason for the difference in the alerting properties of auditory and visual signals might be the difference in the rate of neural transmission in these two modalities. Evoked potential data from adults indicate that, on average, the latency of the M component of the auditory evoked potential is 30-40 ms shorter than the latency of the visual evoked potential (Regan, 1989). The neural transduction difference also may explain the difference in the magnitude of the A-V and V-A synchrony window. Specifically, the results from the adult experiment yielded a value of 65 ms for the A-V temporal synchrony window and 112 ms for the V-A temporal synchrony window. The additional 30—40 ms that is required for the neural transduction of visual stimulation suggests that the actual amount of time that must pass between the perceptual registration of the auditory and visual stimuli is the same in each case. The period of time that passes between the perceptual registration of the stimuli is referred to as the psychological interval. Figure 6 illustrates how the transduction time difference figures in the calculation of the psychological interval. The upper panel illustrates the concept of the psychological interval in the case of A-V asynchrony, and the lower panel illustrates this concept in the case of V-A asynchrony. As can be seen in the upper panel of Figure 6, the psychological interval for the processing of A-V asynchrony consists of the estimated size of the physical interval (65 ms) and the additional time of 30—40 ms required for the transduction of the visual information specifying the visible bounce. As can be seen in the lower panel of Figure 6, the psychological interval for the processing of the V-A asynchrony consists of the estimated size of the physical interval (112 ms) less the neural transduction time of 30—40 ms. If the calculations designed to take neural transduction time into account are performed, then the psychological interval is between 72 and 105 ms if a neural transduction time difference of 40 ms is used and is between 82 and 95 ms if the neural transduction time difference of 30 ms is used.

Physical Interval

Psychological Interval

Physical Interval

Figure 6. Schematic representation of the role of the psychological interval and the physical interval in the processing of intersensory temporal asynchrony in the case of auditory-visual (A-V) asynchrony (top) and in the case of V-A asynchrony (bottom).

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Although the precise magnitude of the A-V transduction time difference is difficult to specify in infants, on the basis of the fact that the infant's nervous system is less myelinated and that response latencies at various levels of the neuraxis decrease with age (Sanes, 1992), it is reasonable to assume that there is a larger intersensory transduction time difference in infants. Assuming that the transduction time difference is 50 ms, a psychological interval of 400 ms is obtained for infants (350 ms + 50 ms for the A-V synchrony window and 450 ms - 50 ms for the V-A synchrony window). Viewing detection of A-V and V-A asynchrony in this way, shows that the psychological interval required for the processing of these two types of intersensory temporal asynchronies is the same and that the apparent difference in the size of the synchrony windows can be accounted for by the inherent difference in the two sensory modalities that process the information. If the psychological interval that underlies the processing of A-V asynchrony is the same regardless of the sequential order of the auditory and visual events, then this suggests that a common mechanism may mediate responsiveness in each case (Meek, 1984). In addition to finding that the intersensory temporal synchrony window is markedly larger in infants as compared to adults, the findings from the infant experiments suggest that sensitivity to violations of intersensory temporal synchrony in infants is governed not only by a lower temporal limit but by an upper one as well. The data show that there is a relatively narrow temporal region of sensitivity to violations of synchrony and that when violations of synchrony become too large, infants no longer respond to the synchrony of the auditory and visual events. Figure 7 summarizes the findings from the current set of experiments and shows that perception of closely occurring heteromodal events can take one of three forms. If the heteromodal components of a multimodal event are temporally noncontiguous but their offsets are separated by a psychological interval of no more than 400 ms, then they fall into the synchronous region and are perceived as temporally synchronous. If the heteromodal events are temporally noncontiguous by more than the psychological interval of 400 ms but not more than approximately 500 ms, then they fall into the asynchronous region and are perceived as asynchronous. It should be noted that the leftmost boundary of the asynchronous region is the point where the traditional threshold for the detection of intersensory temporal asynchrony lies. Finally, if the heteromodal events are temporally separated by more than the maximum allowed for the perception of asynchrony, the effect is the perception of two distinct, temporally unrelated events. The absence of a terminal boundary for the distinct region in Figure 7 reflects the fact that the temporal limits of this region are open-ended or, at the very least, unknown at this point (thus, the question mark denoting the temporal extent of the distinct region). This region is, of course, speculative because of the negative nature of the evidence in support of it. There are several reasons why postulation of a region of sensitivity to violations of intersensory temporal synchrony makes more sense than simply viewing the problem of the perception of intersensory temporal synchrony as a question

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Event 1

Synchronous

Asynchronous

Distinct

400ms

100 m

Figure 7. Schematic representation of the perceptual effects of different degrees of asynchrony between the offset of an event in one modality (Event 1) and the offset of another event in a second modality (Event 2). The question mark denotes the absence of a terminal boundary for the distinct region and reflects the fact that the temporal limits of this region are open-ended or unknown.

of identifying a single, well-defined threshold. First, from a developmental perspective, a perceptual system that is dichotomous in its sensitivity to intersensory temporal asynchrony is not as useful to the infant as is one that can permit the infant to make one of two decisions in the face of temporally contiguous heteromodal events. Such a system is consistent with the often-reported finding that infants respond to different degrees of discrepancy in a nonmonotonic, curvilinear way. Many studies (Hopkins, Zelazo, Jacobson, & Kagan, 1976; Lewkowicz & Turkewitz, 1980; McCall, Kennedy, & Appelbaum, 1977; Weiss, Zelazo, & Swain, 1988; Zelazo, Hopkins, Jacobson, & Kagan, 1974) of infants of various ages and using various kinds of stimulus materials and techniques have shown that infants respond most often to a moderate amount of novelty and less often, or not at all, to departures from that moderate level. In other words, infants not only fail to respond when the departure from novelty is in the direction of familiarity, but as found here, also fail to respond to a departure in the direction of greater novelty. Thus, the finding of a failure to discriminate A-V and V-A asynchronies that exceed discriminable levels of asynchrony is consistent with the curvilinear response profile in infants' response to novelty. Figure 7 shows that if auditory and visual events are sufficiently close in time, then infants attend to the temporal relationship between them and, as a result, determine whether the heteromodal events are part of a single, perceptually unified event. To accomplish this task, infants use those mechanisms that make it possible to distinguish between synchronous and asynchronous events. If, however,

the temporal separation of the two components is greater than the region of asynchrony, then they can be treated as separate events, and infants do not unify them perceptually. From a developmental standpoint, a mechanism that makes it possible to determine whether two heteromodal events are close enough in time to be perceptually unified, and that also makes it possible to ignore their temporal relationship when a certain threshold is exceeded, makes it possible for infants to parse the world more effectively into those multimodal objects and events that belong together as one unit and those that do not. Such a process is fundamental to the acquisition of perceptual and cognitive categorization. A mechanism that enables infants to determine whether two heteromodal events belong together or not leads to some interesting predictions. For example, if a pair of heteromodal components of a multimodal event fell into the distinct region, then infants would be expected to be able to attend to and encode modality specific dimensions associated with each heteromodal event, but would not be able to attend to and encode amodal dimensions that are common to them because that requires the processing of the relationship between the two components. In contrast, if two heteromodal components of a multimodal event were so temporally contiguous that they fell into the synchronous region, then infants would be able to attend to the amodal attributes of the components (e.g., their common duration), and this selective attention to amodal attributes might override the encoding of modality specific attributes. It has been already noted that the difference in the size of the intersensory temporal synchrony window between in-

INTERSENSORY TEMPORAL SYNCHRONY IN INFANTS fants and adults indicates that the window narrows during development. An interesting question is whether the size of the intersensory asynchrony window also narrows in development. Is it possible that cognitive factors play a role in the size of the two windows? For example, if experience contributes in a general way to perceptual differentiation and perceptual sharpening (Gibson, 1969), then the size of the windows either may be constrained or become constrained more quickly during development for highly relevant and often experienced multimodal events (e.g., audible and visible language) than for unfamiliar and infrequently experienced multimodal events. In conclusion, the results from the present set of experiments provide the first estimate of the size of the intersensory temporal synchrony and asynchrony windows in infants. In addition the results suggest that marked changes in the size of the two types of windows occur during development. The postulation of three perceptually different regions associated with the detection of intersensory temporal relations provides a useful conceptual tool for the future examination of the long-term developmental changes in the perception of intersensory temporal unity.

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Received May 23, 1994 Revision received May 30, 1995 Accepted June 26, 1995

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