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Perception of temporally competing speech stimuli in preschool children
BRAIN AND LANGUAGE
17, 166-179 (1982)
Perception of Temporally Competing Speech Stimuli in Preschool Children GRACE
H. YENI-KOMSHIAN University
AND DIANE PAUL-BROWN of Maryland
The aim of this study was to investigate the perception of competing speech stimuli in 3-, 4-, and 5-year-old normally developing children. A dichotic listening paradigm was used in which the temporal alignment between the two stimuli was varied to represent three levels of competition. Minimal, moderate, and maximal levels of temporal competition were represented by a Separation, Lag, and Simultaneous test condition, respectively. The subjects were behaviorally set to listen for and to report the two stimuli on each trial. The incidence of double correct responses in the test conditions was the measure of interest. The results show a sharp and linear drop in double correct scores from the Separation, to the Lag, and to the Simultaneous condition. There were no age-related differences in the Separation and the Simultaneous conditions. In the Lag condition, the performance of the 3-year-olds was significantly lower than the 4- and 5-yearolds. The findings were interpreted to be indicative of limited auditory processing ability in preschoolers for moderately and maximally competing speech stimuli.
Dichotic listening experiments in which two different auditory stimuli are presented simultaneously one to each ear have been conducted with adults and with children as young as 3 years. The results of such experiments, using speech stimuli, have indicated that even the youngest age group tested shows a right-ear advantage (REA); that is, the accuracy level for the speech stimuli presented to the right ear is greater than that for the left ear (Kimura, 1963; Nagafuchi, 1970; Bever, 1971; Ingram, 1975). The most general interpretation for a REA in verbal dichotic We would like to thank the staff, parents, and children affiliated with the University of Maryland’s Center for Young Children and the University Church Nursery School for their cooperation. Thanks are extended to Marjorie Meltzer, for her help with subject screening procedures. We also thank Sigfried Soli for his help in the initial computer editing of the stimuli and the staff of the Kresge Hearing Research Laboratory of the South, New Orleans, for preparing the test tapes. Address reprint requests to Dr. G. H. Yeni-Komshian, Hearing and Speech Sciences, University of Maryland, College Park, MD 20742.
166 0093-934X/82/050166-14$02.00/O Copyright 0 1982 by Academic Press, Inc. All rights of reproduction in any form reserved.
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experiments is that it reflects left-hemisphere specialization for speech and language function (Kimura, 1967). This interpretation has motivated a number of researchers to utilize dichotic listening procedures to investigate the developmental course of left-hemisphere specialization for speech and language. Although a few studies report a developmental increase in REA (Bryden, Allard, & Scarpino, 1973; Satz, Bakker, Teunissen, Goebel, & Van der Vlugt, 1975), the results of most developmental studies have revealed that the magnitude of the REA does not change, in any linear fashion, with increasing age. (See Witelson (1977) for a comprehensive review of the developmental literature.) A recently published study on dichotic listening with 3- to 12-year-olds reported a REA at all ages, but no developmental change in the magnitude of the REA was observed (Hiscock & Kinsbourne, 1980). The evidence thus far is in favor of the argument that lateralization for speech is established by age 3 and is not altered as a function of age. Another measure that can be obtained from developmental studies of dichotic listening is an index of the child’s ability to successfully process the competing stimuli. This measure is based on the subject’s accuracy in reporting the two competing stimuli on a single trial. Thus, the emphasis here is on the development of the ability to process competing stimuli rather than hemispheric specialization of function. The first study that focused on this issue was conducted with children between the ages of 5 and 12 years (Berlin, Hughes, Lowe-Bell, & Berlin, 1973a). The results of this study showed that the ability to correctly perceive both competing stimuli (double correct) increased with age, whereas the REA did not show a developmental change. Another study with 5- to 9-year-olds also reported an age related increase in the incidence of double correct scores (Satz et al., 1975). The results of studies with children younger than 5 years suggest that they are likely to have low double correct scores. In a study with 3- to 6-year-old children, for example, Nagafuchi (1970) reported that the double correct scores for 3-year-olds occurred on only half of the trials even when the dichotic stimuli were presented at a relatively high sensation level. Older children in the same study had more double correct scores at lower sensation levels. In fact the incidence of double correct scores was extremely low in a study carried out with preschoolers (Yeni-Komshian, 1973). In this study, 62 children between the ages of 3 and 5 were given a verbal dichotic listening test in which a trial consisted of one pair of words. The word stimuli were 12 one-syllable common names. The results showed that the correct responses for the left and right ears combined was at 33.5% for the 3- and 4-year-olds and at 42.0% for the 5-year-olds. However, the double correct scores were at 1.5% for the 3- and 4-yearolds and at 7.5% for the 5-year-olds. In this study, a simple free recall
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procedure was used and even though the subjects were repeatedly reminded to provide two responses for each trial they usually had only a single response. Almost all the 3- and 4-year-olds did not even attempt to guess at a second response. All the 5-year-olds were able to comply with the instructions and attempted to provide two responses for some of the trials, however, in such instances usually only one of their responses was correct. This single-response pattern of performance in preschoolers led us to ask the following questions. Are some children unable to hear both of the competing stimuli? Young children may, in fact, have a limited auditory processing capacity that is subject to maturational growth. Alternatively, do children just not know how to listen for and report two stimuli? That is, children may have the auditory processing ability required for such a task but they may fail to respond correctly due to performance factors such as inattention, memory limitations, lack of understanding of task requirements or apprehension about the strangeness of dichotic stimulation. This experiment was designed to address these questions. That is, does the performance of children who report only one stimulus in a dichotically presented pair reflect an auditory processing limitation or a performance bias? To address the question of processing limitation, we systematically changed the amount of temporal separation between the competing stimuli. We assumed that the greater the degree of temporal competition between the stimuli the greater would be the demand on processing. Thus, this experiment provides an initial estimation of the amount of temporal separation children require in order to perceive two different stimuli correctly. To address the question of performance bias, we enhanced the chances of obtaining two responses from the subjects by producing a behavioral set for listening for and reporting two different stimuli. This plan enabled us to assess whether children can perceive two competing stimuli correctly when they were behaviorally set to do so. In addition, this study provides some information related to the phenomenon of a dichotic lag effect. In such studies, the two stimuli of a dichotic pair are presented with partially overlapping temporal alignments, and within certain lag periods, the lagging stimulus is perceived more accurately than the leading stimulus (Berlin, Lowe-Bell, Cullen, Thompson, & Loovis, 1973b; Studdert-Kennedy, Shankweiler, and Schulman, 1970). In a typical experiment, about five or six different lag periods are investigated. A few studies have examined this lag effect with school-age children (Mirabile, Porter, Hughes, & Berlin, 1978; Tobey, Cullen, Rampp, & Fleischer-Gallagher, 1979). Since, in this study, we varied the amount of temporal separation between the stimuli, one
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of our conditions is similar to a single lag period. This allowed us to examine, in a restricted manner, whether the lag effect could be extended to a younger age group. METHOD
Subjects Right-handed children in three age groups, 3, 4, and 5 years, recruited primarily from two local preschools, served as subjects. All were native speakers of English. Twelve children, half male and half female, were selected in each age group. The age range (and mean age) for each of the three groups was 3;1-3;ll (3;7), 4;1-4;ll (4;6), and 5;0-5;ll (5;6). Handedness was determined by preferred drawing hand and confirmed by teacher or parental report. Subject selection criteria, in addition to chronological age and handedness, included normal hearing acuity, equivalent speech reception thresholds between ears and normal receptive and expressive language abilities. Specifically, all subjects selected were required to demonstrate normal hearing acuity bilaterally on a pure-tone audiometric screening (20 db at 500, 1000, and 2000 Hz and 20-25 db at 4000 and 6000 Hz) and to have equivalent speech reception threshold levels between ears, within 5 db (Monitored Live Voice, pictured spondees). Finally, children included as subjects were required to score at least at age level on the auditory comprehension and verbal ability portions of the Preschool Language Scale (Zimmerman, Steiner, & Evatt, 1969). These screening procedures were completed on the same day as testing or within a 4-month period prior to the experiment.
Stimuli The test stimuli were CVC words which differed in their initial stop consonant and medial vowel. Multiple productions of six familiar words (bat, dot, get, pet, toot, and cat) were recorded by a male speaker. The recording was made on a two-track tape recorder (Nagra, IV-S) with a dynamic microphone (Turner, Model 2302) in a soundproof booth. Broad-band spectrographic analyses and amplitude displays were obtained using a Voice Identification, Inc. (700 series) Voice Print. The six different words that were close in duration and of equivalent amplitude, as determined by the vowel peak (?4 db), were selected for editing. The selected stimuli were then digitized and were equated for total duration (550 + 25 msec) using a PDP-12 computer. The stimuli were digitized at a rate of 12 kHz. These test stimuli were then used to prepare the test tape at the computer facilities of the Kresge Hearing Research Laboratory in New Orleans. The test tape consisted of one practice and three experimental conditions.
Test Conditions The practice trials consisted of 30 pairs of stimulus words that were presented binaurally. Each of the six words was paired with every other word and the order of presentation was random. The interval between each word in a given pair was 350 msec. The experimental conditions consisted of 30 word-pair trials each. One word of each pair was recorded on one channel of the tape. In the first experimental condition (Separation), the two words of each pair were separated by a period of 350 msec of silence. That is, the separation time between the onset of the first and the second stimulus was 900 msec. The stimuli that were presented first in one block of 15 trials were presented last in the second block of 15 trials. This condition is assumed to represent a minimal amount of temporal competition between the two stimuli. In the second experimental condition (Lag), the two stimuli of a given pair overlapped partially. The lag time between the onset of the first and the second stimulus was 150 msec. The onset of the second stimulus coincided with the steady-state portion of the
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vowel of the first stimulus. The 150-msec lag time was determined on the basis of spectrographic analysis and represented the point, measured from the burst, at which the vowel had reached steady state in each of the stimuli. The critical acoustic information for identifying each stimulus, i.e., the formant transitions, was not overlapping. The stimuli that lead in one block of 15 trials were the ones that lagged in the second block of 15 trials. This condition is assumed to represent a moderate amount of temporal competition between the stimuli. The third experimental condition (Simulraneous) is actually the standard dichotic listening test, where the two stimuli overlap completely and their onsets are simultaneous. This condition provided maximal temporal competition between the stimuli. Following the practice trials all the subjects were tested in the order: Separation, Lag, Simultaneous. This order of testing allowed us to place the subjects in a response mode in which they were initially set to expect to hear and to report two different words on each trial. This was accomplished through the Separation condition. The other two experimental conditions, Lag and Simultaneous, represented an orderly increase in temporal competition between the stimulus word pairs.
Procedure Each child was escorted individually to a soundproof room for testing. The experimenter sat across from the child and remained present throughout the session. A Shure (Model 5755) microphone connected to a Teat (Model A-3300SX2T) tape recorder was used to record verbal responses of the subjects. The test tape was played on a Teat (Model A3300SX2T) two-track tape recorder via Telephonics (Model TDH-39P) earphones. Prior to the start of the binaural practice items, the experimenter gave the following instructions to the child: I’m going to put these earphones on you. You will hear a man say two words. I want you to listen and tell me the two words he says. If he says hat-mat, you say hat-mat. You say what he says. Wait until he says both words. Let’s try Now we’ll listen to the man. one, dot-pot-you say-. The separation condition was initiated following five consecutive correct two-word responses on the practice trials. Prior to the Lag and Simultaneous conditions, the child was told that the man would be getting “faster” but would still be saying two words. The child was reminded before each condition to report both words on each trial. The stimuli were presented at 80-db sound pressure level (SPL), as measured with reference to a IOOO-Hz calibration tone equated in peak-to-peak amplitude to the vowel segments of the test stimuli. Earphone placement was counterbalanced and the specific schedule of channel placement was determined on a random basis prior to the initiation of the experimental listening tasks. Four different schedules of earphone placement were prepared and children from each age group were randomly assigned to one of the four schedules. Responses were transcribed by the experimenter on a prepared score sheet and later confirmed with the audio recording. If the child did not give a two-word response to any trial, the experimenter asked if the child heard another word. If the child was not sure, the trial was repeated. The experimenter proceeded with the next trial after a second repetition even if a two-word response was not obtained. Only the response to the last repetition was included in subsequent analyses. Feedback to the child related to general listening and responding behavior, rather than accuracy. Sessions lasted between 30 and 50 min and breaks were given as necessary. Children were generally very cooperative and tolerant of the earphones. Occasionally, some children,
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particularly younger 3-year-olds, needed encouragement to keep the earphones on and to complete the task. Sometimes the accompanying parent was asked to help by talking to his or her child or staying in the room during testing. If a child became upset and could not be consoled very rapidly, the session was terminated. One 3-year-old girl and two 3year-old boys did not complete the experiment. In each case only the separation condition was attempted and partial data obtained was not included in the analyses. In addition, one 3-year-old boy and one 5-year-old girl would not begin the experiment. Otherwise, all other children completed the three experimental conditions in one session. All but one parent contacted agreed to have his or her child participate.
RESULTS Double Correct
The main purpose of this study was to examine the accuracy level of double correct responding in the three test conditions. Double correct scores, where both items are correct on a single trial, define a conservative index of auditory processing efficiency for the test conditions. The results are shown in Fig. 1. An analysis of variance, with two between-subject variables (age and sex) and one within-subject variable (test condition), was used to analyze the double correct scores. There was a significant condition effect, F(2, 33) = 9.64, p < .OOOl. The between-subject variables were not significant; however, there was a significant age x test condition interaction, F(4, 33) = 3.07, p < .023. The condition effect revealed a sharp and linear drop in double correct scores from the Separation to the Lag to the Simultaneous condition. This reduction in performance was significant at each test condition (Newman-Keuls, p < .Ol). Further analysis of the interaction revealed an age effect limited to the Lag condition in which the 3-year-old group DOUBLE CORRECT
loogo-
3 YEAR OLDS * 4 YEAR OLDS 0 5 YEAR OLDS l
80t; 70E 8 60;
50-
I5 402 I.$ 3020IO t SEPARATION I900 rnmc reporotion)
1
I
LAG SIMULTANEOUS I150 mrac (0 swamtim) *sp.m+ion) CONDITION
FIG. I.
Mean double correct scores in the Separation, Lag, and Simultaneous conditions for each age group and mean total correct scores combined across all age groups.
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had significantly (Newman-Keuls, p < .Ol) lower double correct scores (38%) than the 4- and 5-year-old groups (61%). There were no significant age differences in the Separation and Simultaneous conditions. These results show that when the temporal competition between the stimuli was minimal, as in the Separation condition, all subjects had high double correct scores (92%); and in contrast, when the temporal competition was maximal, as in the Simultaneous condition, the scores for the three age groups were low (24%). The drop in performance level from the Separation to the Simultaneous condition was 68% for the three age groups as a whole. The only significant age effect (seen in the Lag condition) suggests that a task with moderate temporal competition affected the auditory processing ability of the 3-year-olds much more adversely than the two older age groups. It appears that the 4- and 5-year-olds were better able than the 3-year-olds to process the nonoverlapping critical acoustic information provided in the Lag condition. Total Correct Total correct scores are based on the sum of the right and the left ear scores. Stated another way, the total correct score is the sum of the double correct and the single correct responses. Thus, the arithmetic difference between the total correct and the double correct scores is a measure of the incidence in which only one of the stimuli was reported correctly. The total correct score represents a less stringent index of auditory processing efficiency than the double correct score. An analysis of variance using total correct scores showed a significant condition effect, F(2, 33) = 9.85, p < .OOOl, but effects due to age or sex were not significant, nor were there significant interactions. The total correct score, combined across the three age groups, was 95% in the Separation condition, 78% in the Lag condition, and 63% in the Simultaneous condition. The reductions in scores at each condition were significant (Newman-Keuls, p < .Ol). The drop in total correct scores from the Separation to the Simultaneous condition was 32%, however; this is less pronounced than the reduction in performance level revealed by the double correct score. The two measures of auditory processing performance can be compared directly by examining the results presented in Fig. 1. It can be seen that there is no difference between the two measures in the Separation condition where the temporal competition between the stimuli was minimal. However, in the Lag condition which represents a moderate level of temporal competition between the stimuli, the total correct score is clearly higher than the double correct score. This distinction is especially marked for the 3-year-old group. The greatest difference between
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total correct and double correct scores, for all subject groups, occurred in the Simultaneous condition. This condition represented the maximum level of temporal competition between the stimuli, and the subjects regardless of age, showed great difficulty in successfully reporting both stimuli per trial. Thus, as the temporal competition between the stimuli was increased, the discrepancy between the two indicies of auditory processing performance was magnified. In other words, as temporal competition increased, the subjects were more likely to report only one stimulus correctly. Differences between Right- and Left-Ear Scores The Simultaneous condition is the same as the standard verbal dichotic listening test and therefore it is of interest to report whether the subjects in the present experiment show a right-ear advantage. The results for the Simultaneous condition are shown in Fig. 2. The accuracy of the right-ear score was significantly higher than the left-ear score at each age level (t tests, p < .Ol). An inspection of Fig. 2 reveals that the difference between the right- and left-ear scores was 23% for the 3-yearold group, 15% for the 4-year-old group, and 19% for the 5-year-old group. These differences in the magnitude of the right-ear advantage at each age level were not statistically significant. Our findings support the studies that have not found an age-related shift in the right-ear advantage. As stated earlier, the Lag condition was used in the present experiment to represent a moderate level of temporal competition between the stimuli. There is, however, a body of information obtained from dichotic lag experiments in which the general finding concerning ear score differences SIMULTANEOUS 100 -
(0 separation
)
go80 6 E
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AGE GROUP
FIG. 2.
condition.
Mean right-ear and left-ear scores for each age group in the Simultaneous
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LEFT EAR
00 L 70 ki a 60 8 50 5 w 40 2 I$ 30 20
4 #GE RIGHT EAR LAG
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AGE EAR LAG
FIG. 3. Mean right-ear and left-ear scores for each age group in the Lag condition. The left panel is for the trials when the right-ear stimulus was lagging and the right panel is for the trials when the left-ear stimulus was lagging.
is that the lagging stimulus is perceived more accurately than the leading stimulus. The ear scores obtained from the Lag condition are displayed in Fig. 3. The left panel of Fig. 3 shows the results for the trials in which the right ear stimulus was lagging; and, the right panel shows the results for the trials in which the left-ear stimulus was lagging. There was a significant right-ear advantage at each age level in the right lag condition (t tests: 3-year-olds, p < .OOl; 4-year-olds, p < .02; and 5-year-olds, p < .OS). As a consequence of the lag effect in the left-ear Lag condition, the right-ear advantage is eliminated and although there appears to be a left-ear advantage for the 3- and 5-year-old groups, these differences were not significant. Thus, the results in the two Lag conditions support, to some extent, the notion that a lag effect is operative in 3-, 4-, and 5year-old children. As reported earlier, performance for all subjects was at a higher level in the Lag than in the Simultaneous condition. We examined if this difference in performance was reflected equally in the right- and left-ear scores as a function of whether it was the lagging or leading channel. Table 1 lists the right- and left-ear scores for the Simultaneous and the two Lag conditions. A comparison of individual ear scores in the Simultaneous condition with scores in the two Lag conditions shows an improvement in performance of over 20% in the right and left lagging ear scores at all age levels. In contrast improvements in the leading ear scores were generally less marked and were only evident for the 4- and
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PERCEPTION OF COMPETING SPEECH SOUNDS TABLE PERCENTAGE
1
CORRECT EAR SCORES FOR THE SIMULTANEOUS
AND LAG CONDITIONS
Condition Ear
Simultaneous
Right lag
Left lag
R L
67 44
90” 45
65 15”
4
R L
72 57
90” 71
78 78”
5
R
66 47
88” 67
76 85”
Age 3
L ’ The lagging ear.
5-year-old groups. That is, 3-year-olds showed improvement only in the lagging channel in comparison with their performance in the Simultaneous condition, whereas 4- and 5-year-olds showed an improvement in both the lagging and the leading ear scores. DISCUSSION Limited Auditory Processing Capacity in Preschoolers The ability of the children tested in this study to accurately report both stimuli correctly on a single dichotic trial was significantly reduced as a function of increasing temporal overlap between the stimuli. Since the level of double correct responding in the Separation condition was over 90% for all age groups, reductions in performance in conditions where the stimuli were partially or completely overlapping were not likely due to factors such as lack of comprehension of task requirements or memory limitation. Furthermore, since subjects were behaviorally set to listen for and report the two stimuli on each trial in the Separation condition, their performance in the ensuing test conditions may be deduced to reflect the upper limit of their auditory processing capacity. One indirect confirmation of the effect of the behavioral set provided in the present experiment is the fact that the level of double correct scores in the Simultaneous condition was at 24% whereas for comparable subjects and test conditions the level of double correct scores was as low as 1.5% and not higher than 7.5% in a study that did not provide such a behavioral set (Yeni-Komshian, 1973). Thus if subjects were in fact induced to attend to both stimuli optimally, within the limits of their auditory processing capacity, then our results support the argument that normally developing preschoolers have great difficulty in processing temporally overlapping stimuli because of auditory processing limitation and not because of performance bias.
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Development of the Ability to Process Competing Speech Stimuli To further evaluate our results, the same test tape was used to obtain comparable data from six adult listeners within the age range of 21 to 30 years. The percentage of double correct responses for these subjects was 100% for the Separation, 98% for the Lag, and 92% for the Simultaneous conditions. Since the dichotic stimuli were highly discrepant, in that both the initial consonant and the vowel were different in each dichotic pair, it is not surprising to note that the adult listeners had very few errors in any of the conditions. The preschoolers performed as well as the adults in the Separation condition, however, there was a major discrepancy between their performance and adult performance in processing stimuli that overlap either partially or completely (Lag and Simultaneous conditions). This discrepancy between child and adult processing capacity points to the possibility that in young children functional maturation of certain auditory pathways is incomplete. In this regard, we present the hypothesis that success in double correct reporting of maximally competing stimuli depends, in part, on interhemispheric transmission of information. This hypothesis is based on a model, proposed to interpret results of verbal dichotic listening performance, that emphasizes the role of the callosal pathway (Sparks & Geschwind, 1968; Sparks, Goodglass, & Nickel, 1970). The impetus for this model was derived from the results obtained from adult patients who have had a complete section of the corpus callosum. These patients show great difficulty in reporting the left-ear stimuli correctly in dichotic listening tasks and virtually all their responses are from the right ear channel (Sparks & Geschwind, 1968; Milner, Taylor, & Sperry, 1968). The model specifies that in dichotic listening conditions, the contralateral pathways are prepotent and the ipsilateral pathways are suppressed. Thus, the right-ear signal is projected to the left hemisphere, and the left-ear signal reaches the left hemisphere through the callosal pathway following initial projection to the right hemisphere. For listeners with an intact and fully functioning corpus callosum, the consequences of at least an additional synaptic connection at the corpus callosum for left-ear signals is thought to produce an advantage for processing right-ear signals. However, for commissurotomized patients the absence of a functioning corpus callosum results in their inability to process the left-ear signals. Even though we are aware that this model specifies callosal transmission for left-ear signals only we would like to present the hypothesis that the degree to which both of the dichotically competing stimuli are perceived correctly depends on the amount of transmission of information through the corpus callosum. This line of reasoning leads us to examine the maturational status of the corpus callosum in children within the age range sampled in this investigation. One gradient of maturation is based on the course of my-
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elination in different structures in the brain. Myelination of the corpus callosum begins at about the fourth month after birth and the process is not complete until about the age of 10 (Yakovlev & Lecours, 1967). With myelination viewed as an index of functional maturation, the lack of complete myelination may indicate that transmission of information between the two hemispheres is not maximally efficient before age 10. A test of the hypothesis presented above would be to find out whether adult levels of performance on our experimental tasks are achieved at about age 10. Amount of Temporal Separation Stimuli
Needed for Identifying
Competing
The Lag and Simultaneous conditions were designed to provide an initial evaluation of the amount of temporal separation required for preschoolers to successfully perceive two competing, but highly discrepant, stimuli. Although the results showed that for all ages there was a significant increase in the double correct scores in the Lag condition in comparison to the Simultaneous condition, performance in the Lag condition was appreciably lower than in the Separation condition. Furthermore, a comparison between the performance of the adult subjects and the preschoolers in the Lag condition, shows that none of the child groups came close to the adult level of performance. Even though the performance of the 4- and 5-year-olds was 37% lower than the adults, they were significantly better than the 3-year-olds. These results indicate that partially overlapping stimuli were identified better that completely overlapping stimuli to varying degrees at each age level. The 4- and 5year-old subjects were better able to make use of the HO-msec separation time between the onsets of the two stimuli to extract linguistically relevant information than the 3-year-old subjects. It appears that the amount of separation time required to consistently identify two competing speech stimuli correctly is greater than 150 msec but clearly less than 900 msec. The results of this study suggest that for 4- and 5-year-olds the required separation time would be shorter than for 3-year-olds. Further analysis of the present data can provide information concerning the relationship between spectral differences and temporal overlap in the subjects’ ability to correctly identify the test stimuli. This information will be presented in a separate paper. Developmental
Aspects of the Lag Effect
The information pertaining to the lag effect provided by this study is that with 150-msec lag time a REA was seen when the right-ear stimulus was lagging; however, when the left-ear stimulus was lagging there was no ear effect, that is, the REA was neutralized. Although there was no age-related ear effect in the Lag condition, the 3-year-old group was
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significantly lower than the 4- and 5-year-old group in double correct scores. The reason for this age effect may be revealed by examining whether there was an enhancement in individual ear scores between the Simultaneous condition, and the right-ear and the left-ear Lag conditions. The results showed that for 3-year-olds there was an enhancement for the lagging stimulus only, whereas for the 4- and 5-year olds there was an enhancement of both leading and lagging stimuli. These results indicate that the reason the 3-year-old group had significantly lower double correct scores than the 4- and 5-year-old groups, is because for the 3-year-olds the leading stimuli were as difficult to identify as the maximally competing (Simultaneous) stimuli. It appears that 3-year-olds require lead times longer than 150 msec to improve their identification of the stimuli. It is apparent that among preschoolers, 3-year-olds require more processing time than 4- and 5-year-olds. It is interesting to note that the results seen for the 3-year-olds in the present study parallel those reported by Tobey et al. (1979) for 9-year-old boys with auditory processing disorders. In their study, they sampled lag times of 30, 60, 90, 120, and 150 msec and found very little increase in identification of the leading stimuli by these children even at 150 msec. Thus, it appears that normally developing 3-year-olds as well as older children with auditory processing disorders require more processing time than 4- and 5-year-olds. Age and Sex Effects in Hemispheric
Specialization
The results from the Simultaneous condition provide additional support to the point of view that left hemisphere specialization for speech is evident in normally developing 3-year-old children. Our results also support previous studies that have reported that the magnitude of the REA does not change over age. In addition, the results of this study did not reveal any sex differences in REA nor in the double correct scores. This finding supports previous research which shows that the pattern of ear asymmetry does not differ as a function of sex in young children, even though differences in overall accuracy are sometimes noted (see Witelson, 1977; Bryden, 1979, for reviews). REFERENCES Berlin, C. I., Hughes, L. F., Lowe-Bell, S. S., & Berlin, H. L. 1973. Dichotic right ear advantage in children 5 to 13. Cortex, 9, 394-401. (a) Berlin, C. I., Lowe-Bell, S. S., Cullen, J. K., Jr., Thompson, C. L., & Loovis, C. F. 1973. Dichotic speech perception: An interpretation of right ear advantage and temporal offset effects. Journal of the Acoustical Society of America, 53, 699-709. (b) Bever, T. G. 1971. The nature of cerebral dominance in speech behavior of the child and adult. In R. Huxley & E. Ingram (Eds.) Language acquisition: Models and methods. New York/London: Academic Press. Bryden, M. P. 1979. Evidence for sex-related differences in cerebral organization. In M. A. Wittig & A. C. Petersen (Eds.) Sex-related differences in cognitive functioning: Developmental issues. New York: Academic Press.
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Bryden, M. P., Allard, F., & Scarpino, F. 1973. Unpublished studies, University of Waterloo. Hiscock, M., & Kinsbourne, M. 1980. Asymmetries of selective listening and attention switching in children. Developmental Psychology, 16, 70-82. Ingram, D. 1975. Cerebral speech lateralization in young children. Neuropsychologia, 13, 103-105. Kimura, D. 1963. Speech lateralization in young children as determined by an auditory test. Journal of Comparative Physiological Psychology, 56, 899-902. Kimura, D. 1967. Functional asymmetry of the brain in dichotic listening. Cortex, 3, 163-178. Milner, B., Taylor, L., & Sperry, R. W. 1968. Lateralized suppression of dichotically presented digits after commissural section in man. Science, 161, 184-186. Mirabile, P. J., Porter, R. J., Jr., Hughes, L. F., & Berlin, C. I. 1978. Dichotic lag effect in children 7 to 15. Developmental Psychology, 14, 277-285. Nagafuchi, M. 1970. Development of dichotic and monaural hearing abilities in young children. Acta Otolaryngologica, 69, 409-415. Satz, P., Bakker, D., Teunissen, J., Goebel, R., & Van der Vlught, H. 1975. Developmental parameters of ear asymmetry: A multivariate approach. Brain and Langunge, 2, 171-185. Sparks, R., & Geschwind, M. 1968. Dichotic listening in man after section of neocortical commissures. Cortex, 4, 3-16. Sparks, R., Goodglass, H., & Nickel, B. 1970. lpsilateral versus contralateral extinction in dichotic listening resulting from hemispheric lesions. Cortex, 6, 249-260. Studdert-Kennedy, M., Shankweiler, D., & Schulman, S. 1970. Opposed effects of a delayed channel on perception of dichotically and monotically presented CV syllables. Journal
of the Acoustical
Society of America,
48, 579-594.
Tobey, E. A., Cullen, J. K., Jr., Rampp, D. R., & Fleischer-Gallagher, A. M. 1979. Effects of stimulus-onset asynchrony on the dichotic performance of children with auditoryprocessing disorders. Journal of Speech and Hearing Research, 22, 197-211. Witelson, S. F. 1977. Early hemisphere specialization and inter-hemispheric plasticity: An empirical and theoretical review. In S. S. Segalowitz & F. A. Gruber (Eds.) Languuge development and neurological theory. New York: Academic Press. Yakovlev, P. I., & Lecours, A. R. 1967. The myelogenetic cycles of regional maturation in the brain. In A. Minkowski (Ed.), Regional development of the brain in early life. Oxford: Blackwell. Yeni-Komshian, G. H. 1973. Dichotic listening studies with 3 to 5 year old children. Unpublished studies, Johns Hopkins University. Zimmerman, I. L., Steiner, V. G., & Evatt, R. F. 1969. Preschool language scale. Columbus. OH: Merrill.
17, 166-179 (1982)
Perception of Temporally Competing Speech Stimuli in Preschool Children GRACE
H. YENI-KOMSHIAN University
AND DIANE PAUL-BROWN of Maryland
The aim of this study was to investigate the perception of competing speech stimuli in 3-, 4-, and 5-year-old normally developing children. A dichotic listening paradigm was used in which the temporal alignment between the two stimuli was varied to represent three levels of competition. Minimal, moderate, and maximal levels of temporal competition were represented by a Separation, Lag, and Simultaneous test condition, respectively. The subjects were behaviorally set to listen for and to report the two stimuli on each trial. The incidence of double correct responses in the test conditions was the measure of interest. The results show a sharp and linear drop in double correct scores from the Separation, to the Lag, and to the Simultaneous condition. There were no age-related differences in the Separation and the Simultaneous conditions. In the Lag condition, the performance of the 3-year-olds was significantly lower than the 4- and 5-yearolds. The findings were interpreted to be indicative of limited auditory processing ability in preschoolers for moderately and maximally competing speech stimuli.
Dichotic listening experiments in which two different auditory stimuli are presented simultaneously one to each ear have been conducted with adults and with children as young as 3 years. The results of such experiments, using speech stimuli, have indicated that even the youngest age group tested shows a right-ear advantage (REA); that is, the accuracy level for the speech stimuli presented to the right ear is greater than that for the left ear (Kimura, 1963; Nagafuchi, 1970; Bever, 1971; Ingram, 1975). The most general interpretation for a REA in verbal dichotic We would like to thank the staff, parents, and children affiliated with the University of Maryland’s Center for Young Children and the University Church Nursery School for their cooperation. Thanks are extended to Marjorie Meltzer, for her help with subject screening procedures. We also thank Sigfried Soli for his help in the initial computer editing of the stimuli and the staff of the Kresge Hearing Research Laboratory of the South, New Orleans, for preparing the test tapes. Address reprint requests to Dr. G. H. Yeni-Komshian, Hearing and Speech Sciences, University of Maryland, College Park, MD 20742.
166 0093-934X/82/050166-14$02.00/O Copyright 0 1982 by Academic Press, Inc. All rights of reproduction in any form reserved.
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experiments is that it reflects left-hemisphere specialization for speech and language function (Kimura, 1967). This interpretation has motivated a number of researchers to utilize dichotic listening procedures to investigate the developmental course of left-hemisphere specialization for speech and language. Although a few studies report a developmental increase in REA (Bryden, Allard, & Scarpino, 1973; Satz, Bakker, Teunissen, Goebel, & Van der Vlugt, 1975), the results of most developmental studies have revealed that the magnitude of the REA does not change, in any linear fashion, with increasing age. (See Witelson (1977) for a comprehensive review of the developmental literature.) A recently published study on dichotic listening with 3- to 12-year-olds reported a REA at all ages, but no developmental change in the magnitude of the REA was observed (Hiscock & Kinsbourne, 1980). The evidence thus far is in favor of the argument that lateralization for speech is established by age 3 and is not altered as a function of age. Another measure that can be obtained from developmental studies of dichotic listening is an index of the child’s ability to successfully process the competing stimuli. This measure is based on the subject’s accuracy in reporting the two competing stimuli on a single trial. Thus, the emphasis here is on the development of the ability to process competing stimuli rather than hemispheric specialization of function. The first study that focused on this issue was conducted with children between the ages of 5 and 12 years (Berlin, Hughes, Lowe-Bell, & Berlin, 1973a). The results of this study showed that the ability to correctly perceive both competing stimuli (double correct) increased with age, whereas the REA did not show a developmental change. Another study with 5- to 9-year-olds also reported an age related increase in the incidence of double correct scores (Satz et al., 1975). The results of studies with children younger than 5 years suggest that they are likely to have low double correct scores. In a study with 3- to 6-year-old children, for example, Nagafuchi (1970) reported that the double correct scores for 3-year-olds occurred on only half of the trials even when the dichotic stimuli were presented at a relatively high sensation level. Older children in the same study had more double correct scores at lower sensation levels. In fact the incidence of double correct scores was extremely low in a study carried out with preschoolers (Yeni-Komshian, 1973). In this study, 62 children between the ages of 3 and 5 were given a verbal dichotic listening test in which a trial consisted of one pair of words. The word stimuli were 12 one-syllable common names. The results showed that the correct responses for the left and right ears combined was at 33.5% for the 3- and 4-year-olds and at 42.0% for the 5-year-olds. However, the double correct scores were at 1.5% for the 3- and 4-yearolds and at 7.5% for the 5-year-olds. In this study, a simple free recall
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procedure was used and even though the subjects were repeatedly reminded to provide two responses for each trial they usually had only a single response. Almost all the 3- and 4-year-olds did not even attempt to guess at a second response. All the 5-year-olds were able to comply with the instructions and attempted to provide two responses for some of the trials, however, in such instances usually only one of their responses was correct. This single-response pattern of performance in preschoolers led us to ask the following questions. Are some children unable to hear both of the competing stimuli? Young children may, in fact, have a limited auditory processing capacity that is subject to maturational growth. Alternatively, do children just not know how to listen for and report two stimuli? That is, children may have the auditory processing ability required for such a task but they may fail to respond correctly due to performance factors such as inattention, memory limitations, lack of understanding of task requirements or apprehension about the strangeness of dichotic stimulation. This experiment was designed to address these questions. That is, does the performance of children who report only one stimulus in a dichotically presented pair reflect an auditory processing limitation or a performance bias? To address the question of processing limitation, we systematically changed the amount of temporal separation between the competing stimuli. We assumed that the greater the degree of temporal competition between the stimuli the greater would be the demand on processing. Thus, this experiment provides an initial estimation of the amount of temporal separation children require in order to perceive two different stimuli correctly. To address the question of performance bias, we enhanced the chances of obtaining two responses from the subjects by producing a behavioral set for listening for and reporting two different stimuli. This plan enabled us to assess whether children can perceive two competing stimuli correctly when they were behaviorally set to do so. In addition, this study provides some information related to the phenomenon of a dichotic lag effect. In such studies, the two stimuli of a dichotic pair are presented with partially overlapping temporal alignments, and within certain lag periods, the lagging stimulus is perceived more accurately than the leading stimulus (Berlin, Lowe-Bell, Cullen, Thompson, & Loovis, 1973b; Studdert-Kennedy, Shankweiler, and Schulman, 1970). In a typical experiment, about five or six different lag periods are investigated. A few studies have examined this lag effect with school-age children (Mirabile, Porter, Hughes, & Berlin, 1978; Tobey, Cullen, Rampp, & Fleischer-Gallagher, 1979). Since, in this study, we varied the amount of temporal separation between the stimuli, one
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of our conditions is similar to a single lag period. This allowed us to examine, in a restricted manner, whether the lag effect could be extended to a younger age group. METHOD
Subjects Right-handed children in three age groups, 3, 4, and 5 years, recruited primarily from two local preschools, served as subjects. All were native speakers of English. Twelve children, half male and half female, were selected in each age group. The age range (and mean age) for each of the three groups was 3;1-3;ll (3;7), 4;1-4;ll (4;6), and 5;0-5;ll (5;6). Handedness was determined by preferred drawing hand and confirmed by teacher or parental report. Subject selection criteria, in addition to chronological age and handedness, included normal hearing acuity, equivalent speech reception thresholds between ears and normal receptive and expressive language abilities. Specifically, all subjects selected were required to demonstrate normal hearing acuity bilaterally on a pure-tone audiometric screening (20 db at 500, 1000, and 2000 Hz and 20-25 db at 4000 and 6000 Hz) and to have equivalent speech reception threshold levels between ears, within 5 db (Monitored Live Voice, pictured spondees). Finally, children included as subjects were required to score at least at age level on the auditory comprehension and verbal ability portions of the Preschool Language Scale (Zimmerman, Steiner, & Evatt, 1969). These screening procedures were completed on the same day as testing or within a 4-month period prior to the experiment.
Stimuli The test stimuli were CVC words which differed in their initial stop consonant and medial vowel. Multiple productions of six familiar words (bat, dot, get, pet, toot, and cat) were recorded by a male speaker. The recording was made on a two-track tape recorder (Nagra, IV-S) with a dynamic microphone (Turner, Model 2302) in a soundproof booth. Broad-band spectrographic analyses and amplitude displays were obtained using a Voice Identification, Inc. (700 series) Voice Print. The six different words that were close in duration and of equivalent amplitude, as determined by the vowel peak (?4 db), were selected for editing. The selected stimuli were then digitized and were equated for total duration (550 + 25 msec) using a PDP-12 computer. The stimuli were digitized at a rate of 12 kHz. These test stimuli were then used to prepare the test tape at the computer facilities of the Kresge Hearing Research Laboratory in New Orleans. The test tape consisted of one practice and three experimental conditions.
Test Conditions The practice trials consisted of 30 pairs of stimulus words that were presented binaurally. Each of the six words was paired with every other word and the order of presentation was random. The interval between each word in a given pair was 350 msec. The experimental conditions consisted of 30 word-pair trials each. One word of each pair was recorded on one channel of the tape. In the first experimental condition (Separation), the two words of each pair were separated by a period of 350 msec of silence. That is, the separation time between the onset of the first and the second stimulus was 900 msec. The stimuli that were presented first in one block of 15 trials were presented last in the second block of 15 trials. This condition is assumed to represent a minimal amount of temporal competition between the two stimuli. In the second experimental condition (Lag), the two stimuli of a given pair overlapped partially. The lag time between the onset of the first and the second stimulus was 150 msec. The onset of the second stimulus coincided with the steady-state portion of the
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vowel of the first stimulus. The 150-msec lag time was determined on the basis of spectrographic analysis and represented the point, measured from the burst, at which the vowel had reached steady state in each of the stimuli. The critical acoustic information for identifying each stimulus, i.e., the formant transitions, was not overlapping. The stimuli that lead in one block of 15 trials were the ones that lagged in the second block of 15 trials. This condition is assumed to represent a moderate amount of temporal competition between the stimuli. The third experimental condition (Simulraneous) is actually the standard dichotic listening test, where the two stimuli overlap completely and their onsets are simultaneous. This condition provided maximal temporal competition between the stimuli. Following the practice trials all the subjects were tested in the order: Separation, Lag, Simultaneous. This order of testing allowed us to place the subjects in a response mode in which they were initially set to expect to hear and to report two different words on each trial. This was accomplished through the Separation condition. The other two experimental conditions, Lag and Simultaneous, represented an orderly increase in temporal competition between the stimulus word pairs.
Procedure Each child was escorted individually to a soundproof room for testing. The experimenter sat across from the child and remained present throughout the session. A Shure (Model 5755) microphone connected to a Teat (Model A-3300SX2T) tape recorder was used to record verbal responses of the subjects. The test tape was played on a Teat (Model A3300SX2T) two-track tape recorder via Telephonics (Model TDH-39P) earphones. Prior to the start of the binaural practice items, the experimenter gave the following instructions to the child: I’m going to put these earphones on you. You will hear a man say two words. I want you to listen and tell me the two words he says. If he says hat-mat, you say hat-mat. You say what he says. Wait until he says both words. Let’s try Now we’ll listen to the man. one, dot-pot-you say-. The separation condition was initiated following five consecutive correct two-word responses on the practice trials. Prior to the Lag and Simultaneous conditions, the child was told that the man would be getting “faster” but would still be saying two words. The child was reminded before each condition to report both words on each trial. The stimuli were presented at 80-db sound pressure level (SPL), as measured with reference to a IOOO-Hz calibration tone equated in peak-to-peak amplitude to the vowel segments of the test stimuli. Earphone placement was counterbalanced and the specific schedule of channel placement was determined on a random basis prior to the initiation of the experimental listening tasks. Four different schedules of earphone placement were prepared and children from each age group were randomly assigned to one of the four schedules. Responses were transcribed by the experimenter on a prepared score sheet and later confirmed with the audio recording. If the child did not give a two-word response to any trial, the experimenter asked if the child heard another word. If the child was not sure, the trial was repeated. The experimenter proceeded with the next trial after a second repetition even if a two-word response was not obtained. Only the response to the last repetition was included in subsequent analyses. Feedback to the child related to general listening and responding behavior, rather than accuracy. Sessions lasted between 30 and 50 min and breaks were given as necessary. Children were generally very cooperative and tolerant of the earphones. Occasionally, some children,
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particularly younger 3-year-olds, needed encouragement to keep the earphones on and to complete the task. Sometimes the accompanying parent was asked to help by talking to his or her child or staying in the room during testing. If a child became upset and could not be consoled very rapidly, the session was terminated. One 3-year-old girl and two 3year-old boys did not complete the experiment. In each case only the separation condition was attempted and partial data obtained was not included in the analyses. In addition, one 3-year-old boy and one 5-year-old girl would not begin the experiment. Otherwise, all other children completed the three experimental conditions in one session. All but one parent contacted agreed to have his or her child participate.
RESULTS Double Correct
The main purpose of this study was to examine the accuracy level of double correct responding in the three test conditions. Double correct scores, where both items are correct on a single trial, define a conservative index of auditory processing efficiency for the test conditions. The results are shown in Fig. 1. An analysis of variance, with two between-subject variables (age and sex) and one within-subject variable (test condition), was used to analyze the double correct scores. There was a significant condition effect, F(2, 33) = 9.64, p < .OOOl. The between-subject variables were not significant; however, there was a significant age x test condition interaction, F(4, 33) = 3.07, p < .023. The condition effect revealed a sharp and linear drop in double correct scores from the Separation to the Lag to the Simultaneous condition. This reduction in performance was significant at each test condition (Newman-Keuls, p < .Ol). Further analysis of the interaction revealed an age effect limited to the Lag condition in which the 3-year-old group DOUBLE CORRECT
loogo-
3 YEAR OLDS * 4 YEAR OLDS 0 5 YEAR OLDS l
80t; 70E 8 60;
50-
I5 402 I.$ 3020IO t SEPARATION I900 rnmc reporotion)
1
I
LAG SIMULTANEOUS I150 mrac (0 swamtim) *sp.m+ion) CONDITION
FIG. I.
Mean double correct scores in the Separation, Lag, and Simultaneous conditions for each age group and mean total correct scores combined across all age groups.
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had significantly (Newman-Keuls, p < .Ol) lower double correct scores (38%) than the 4- and 5-year-old groups (61%). There were no significant age differences in the Separation and Simultaneous conditions. These results show that when the temporal competition between the stimuli was minimal, as in the Separation condition, all subjects had high double correct scores (92%); and in contrast, when the temporal competition was maximal, as in the Simultaneous condition, the scores for the three age groups were low (24%). The drop in performance level from the Separation to the Simultaneous condition was 68% for the three age groups as a whole. The only significant age effect (seen in the Lag condition) suggests that a task with moderate temporal competition affected the auditory processing ability of the 3-year-olds much more adversely than the two older age groups. It appears that the 4- and 5-year-olds were better able than the 3-year-olds to process the nonoverlapping critical acoustic information provided in the Lag condition. Total Correct Total correct scores are based on the sum of the right and the left ear scores. Stated another way, the total correct score is the sum of the double correct and the single correct responses. Thus, the arithmetic difference between the total correct and the double correct scores is a measure of the incidence in which only one of the stimuli was reported correctly. The total correct score represents a less stringent index of auditory processing efficiency than the double correct score. An analysis of variance using total correct scores showed a significant condition effect, F(2, 33) = 9.85, p < .OOOl, but effects due to age or sex were not significant, nor were there significant interactions. The total correct score, combined across the three age groups, was 95% in the Separation condition, 78% in the Lag condition, and 63% in the Simultaneous condition. The reductions in scores at each condition were significant (Newman-Keuls, p < .Ol). The drop in total correct scores from the Separation to the Simultaneous condition was 32%, however; this is less pronounced than the reduction in performance level revealed by the double correct score. The two measures of auditory processing performance can be compared directly by examining the results presented in Fig. 1. It can be seen that there is no difference between the two measures in the Separation condition where the temporal competition between the stimuli was minimal. However, in the Lag condition which represents a moderate level of temporal competition between the stimuli, the total correct score is clearly higher than the double correct score. This distinction is especially marked for the 3-year-old group. The greatest difference between
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total correct and double correct scores, for all subject groups, occurred in the Simultaneous condition. This condition represented the maximum level of temporal competition between the stimuli, and the subjects regardless of age, showed great difficulty in successfully reporting both stimuli per trial. Thus, as the temporal competition between the stimuli was increased, the discrepancy between the two indicies of auditory processing performance was magnified. In other words, as temporal competition increased, the subjects were more likely to report only one stimulus correctly. Differences between Right- and Left-Ear Scores The Simultaneous condition is the same as the standard verbal dichotic listening test and therefore it is of interest to report whether the subjects in the present experiment show a right-ear advantage. The results for the Simultaneous condition are shown in Fig. 2. The accuracy of the right-ear score was significantly higher than the left-ear score at each age level (t tests, p < .Ol). An inspection of Fig. 2 reveals that the difference between the right- and left-ear scores was 23% for the 3-yearold group, 15% for the 4-year-old group, and 19% for the 5-year-old group. These differences in the magnitude of the right-ear advantage at each age level were not statistically significant. Our findings support the studies that have not found an age-related shift in the right-ear advantage. As stated earlier, the Lag condition was used in the present experiment to represent a moderate level of temporal competition between the stimuli. There is, however, a body of information obtained from dichotic lag experiments in which the general finding concerning ear score differences SIMULTANEOUS 100 -
(0 separation
)
go80 6 E
8
70-
60-
v 505 w 408 L 3020IO -
AGE GROUP
FIG. 2.
condition.
Mean right-ear and left-ear scores for each age group in the Simultaneous
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LEFT EAR
00 L 70 ki a 60 8 50 5 w 40 2 I$ 30 20
4 #GE RIGHT EAR LAG
LEFT
AGE EAR LAG
FIG. 3. Mean right-ear and left-ear scores for each age group in the Lag condition. The left panel is for the trials when the right-ear stimulus was lagging and the right panel is for the trials when the left-ear stimulus was lagging.
is that the lagging stimulus is perceived more accurately than the leading stimulus. The ear scores obtained from the Lag condition are displayed in Fig. 3. The left panel of Fig. 3 shows the results for the trials in which the right ear stimulus was lagging; and, the right panel shows the results for the trials in which the left-ear stimulus was lagging. There was a significant right-ear advantage at each age level in the right lag condition (t tests: 3-year-olds, p < .OOl; 4-year-olds, p < .02; and 5-year-olds, p < .OS). As a consequence of the lag effect in the left-ear Lag condition, the right-ear advantage is eliminated and although there appears to be a left-ear advantage for the 3- and 5-year-old groups, these differences were not significant. Thus, the results in the two Lag conditions support, to some extent, the notion that a lag effect is operative in 3-, 4-, and 5year-old children. As reported earlier, performance for all subjects was at a higher level in the Lag than in the Simultaneous condition. We examined if this difference in performance was reflected equally in the right- and left-ear scores as a function of whether it was the lagging or leading channel. Table 1 lists the right- and left-ear scores for the Simultaneous and the two Lag conditions. A comparison of individual ear scores in the Simultaneous condition with scores in the two Lag conditions shows an improvement in performance of over 20% in the right and left lagging ear scores at all age levels. In contrast improvements in the leading ear scores were generally less marked and were only evident for the 4- and
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PERCEPTION OF COMPETING SPEECH SOUNDS TABLE PERCENTAGE
1
CORRECT EAR SCORES FOR THE SIMULTANEOUS
AND LAG CONDITIONS
Condition Ear
Simultaneous
Right lag
Left lag
R L
67 44
90” 45
65 15”
4
R L
72 57
90” 71
78 78”
5
R
66 47
88” 67
76 85”
Age 3
L ’ The lagging ear.
5-year-old groups. That is, 3-year-olds showed improvement only in the lagging channel in comparison with their performance in the Simultaneous condition, whereas 4- and 5-year-olds showed an improvement in both the lagging and the leading ear scores. DISCUSSION Limited Auditory Processing Capacity in Preschoolers The ability of the children tested in this study to accurately report both stimuli correctly on a single dichotic trial was significantly reduced as a function of increasing temporal overlap between the stimuli. Since the level of double correct responding in the Separation condition was over 90% for all age groups, reductions in performance in conditions where the stimuli were partially or completely overlapping were not likely due to factors such as lack of comprehension of task requirements or memory limitation. Furthermore, since subjects were behaviorally set to listen for and report the two stimuli on each trial in the Separation condition, their performance in the ensuing test conditions may be deduced to reflect the upper limit of their auditory processing capacity. One indirect confirmation of the effect of the behavioral set provided in the present experiment is the fact that the level of double correct scores in the Simultaneous condition was at 24% whereas for comparable subjects and test conditions the level of double correct scores was as low as 1.5% and not higher than 7.5% in a study that did not provide such a behavioral set (Yeni-Komshian, 1973). Thus if subjects were in fact induced to attend to both stimuli optimally, within the limits of their auditory processing capacity, then our results support the argument that normally developing preschoolers have great difficulty in processing temporally overlapping stimuli because of auditory processing limitation and not because of performance bias.
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Development of the Ability to Process Competing Speech Stimuli To further evaluate our results, the same test tape was used to obtain comparable data from six adult listeners within the age range of 21 to 30 years. The percentage of double correct responses for these subjects was 100% for the Separation, 98% for the Lag, and 92% for the Simultaneous conditions. Since the dichotic stimuli were highly discrepant, in that both the initial consonant and the vowel were different in each dichotic pair, it is not surprising to note that the adult listeners had very few errors in any of the conditions. The preschoolers performed as well as the adults in the Separation condition, however, there was a major discrepancy between their performance and adult performance in processing stimuli that overlap either partially or completely (Lag and Simultaneous conditions). This discrepancy between child and adult processing capacity points to the possibility that in young children functional maturation of certain auditory pathways is incomplete. In this regard, we present the hypothesis that success in double correct reporting of maximally competing stimuli depends, in part, on interhemispheric transmission of information. This hypothesis is based on a model, proposed to interpret results of verbal dichotic listening performance, that emphasizes the role of the callosal pathway (Sparks & Geschwind, 1968; Sparks, Goodglass, & Nickel, 1970). The impetus for this model was derived from the results obtained from adult patients who have had a complete section of the corpus callosum. These patients show great difficulty in reporting the left-ear stimuli correctly in dichotic listening tasks and virtually all their responses are from the right ear channel (Sparks & Geschwind, 1968; Milner, Taylor, & Sperry, 1968). The model specifies that in dichotic listening conditions, the contralateral pathways are prepotent and the ipsilateral pathways are suppressed. Thus, the right-ear signal is projected to the left hemisphere, and the left-ear signal reaches the left hemisphere through the callosal pathway following initial projection to the right hemisphere. For listeners with an intact and fully functioning corpus callosum, the consequences of at least an additional synaptic connection at the corpus callosum for left-ear signals is thought to produce an advantage for processing right-ear signals. However, for commissurotomized patients the absence of a functioning corpus callosum results in their inability to process the left-ear signals. Even though we are aware that this model specifies callosal transmission for left-ear signals only we would like to present the hypothesis that the degree to which both of the dichotically competing stimuli are perceived correctly depends on the amount of transmission of information through the corpus callosum. This line of reasoning leads us to examine the maturational status of the corpus callosum in children within the age range sampled in this investigation. One gradient of maturation is based on the course of my-
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elination in different structures in the brain. Myelination of the corpus callosum begins at about the fourth month after birth and the process is not complete until about the age of 10 (Yakovlev & Lecours, 1967). With myelination viewed as an index of functional maturation, the lack of complete myelination may indicate that transmission of information between the two hemispheres is not maximally efficient before age 10. A test of the hypothesis presented above would be to find out whether adult levels of performance on our experimental tasks are achieved at about age 10. Amount of Temporal Separation Stimuli
Needed for Identifying
Competing
The Lag and Simultaneous conditions were designed to provide an initial evaluation of the amount of temporal separation required for preschoolers to successfully perceive two competing, but highly discrepant, stimuli. Although the results showed that for all ages there was a significant increase in the double correct scores in the Lag condition in comparison to the Simultaneous condition, performance in the Lag condition was appreciably lower than in the Separation condition. Furthermore, a comparison between the performance of the adult subjects and the preschoolers in the Lag condition, shows that none of the child groups came close to the adult level of performance. Even though the performance of the 4- and 5-year-olds was 37% lower than the adults, they were significantly better than the 3-year-olds. These results indicate that partially overlapping stimuli were identified better that completely overlapping stimuli to varying degrees at each age level. The 4- and 5year-old subjects were better able to make use of the HO-msec separation time between the onsets of the two stimuli to extract linguistically relevant information than the 3-year-old subjects. It appears that the amount of separation time required to consistently identify two competing speech stimuli correctly is greater than 150 msec but clearly less than 900 msec. The results of this study suggest that for 4- and 5-year-olds the required separation time would be shorter than for 3-year-olds. Further analysis of the present data can provide information concerning the relationship between spectral differences and temporal overlap in the subjects’ ability to correctly identify the test stimuli. This information will be presented in a separate paper. Developmental
Aspects of the Lag Effect
The information pertaining to the lag effect provided by this study is that with 150-msec lag time a REA was seen when the right-ear stimulus was lagging; however, when the left-ear stimulus was lagging there was no ear effect, that is, the REA was neutralized. Although there was no age-related ear effect in the Lag condition, the 3-year-old group was
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YENI-KOMSHIAN
AND PAUL-BROWN
significantly lower than the 4- and 5-year-old group in double correct scores. The reason for this age effect may be revealed by examining whether there was an enhancement in individual ear scores between the Simultaneous condition, and the right-ear and the left-ear Lag conditions. The results showed that for 3-year-olds there was an enhancement for the lagging stimulus only, whereas for the 4- and 5-year olds there was an enhancement of both leading and lagging stimuli. These results indicate that the reason the 3-year-old group had significantly lower double correct scores than the 4- and 5-year-old groups, is because for the 3-year-olds the leading stimuli were as difficult to identify as the maximally competing (Simultaneous) stimuli. It appears that 3-year-olds require lead times longer than 150 msec to improve their identification of the stimuli. It is apparent that among preschoolers, 3-year-olds require more processing time than 4- and 5-year-olds. It is interesting to note that the results seen for the 3-year-olds in the present study parallel those reported by Tobey et al. (1979) for 9-year-old boys with auditory processing disorders. In their study, they sampled lag times of 30, 60, 90, 120, and 150 msec and found very little increase in identification of the leading stimuli by these children even at 150 msec. Thus, it appears that normally developing 3-year-olds as well as older children with auditory processing disorders require more processing time than 4- and 5-year-olds. Age and Sex Effects in Hemispheric
Specialization
The results from the Simultaneous condition provide additional support to the point of view that left hemisphere specialization for speech is evident in normally developing 3-year-old children. Our results also support previous studies that have reported that the magnitude of the REA does not change over age. In addition, the results of this study did not reveal any sex differences in REA nor in the double correct scores. This finding supports previous research which shows that the pattern of ear asymmetry does not differ as a function of sex in young children, even though differences in overall accuracy are sometimes noted (see Witelson, 1977; Bryden, 1979, for reviews). REFERENCES Berlin, C. I., Hughes, L. F., Lowe-Bell, S. S., & Berlin, H. L. 1973. Dichotic right ear advantage in children 5 to 13. Cortex, 9, 394-401. (a) Berlin, C. I., Lowe-Bell, S. S., Cullen, J. K., Jr., Thompson, C. L., & Loovis, C. F. 1973. Dichotic speech perception: An interpretation of right ear advantage and temporal offset effects. Journal of the Acoustical Society of America, 53, 699-709. (b) Bever, T. G. 1971. The nature of cerebral dominance in speech behavior of the child and adult. In R. Huxley & E. Ingram (Eds.) Language acquisition: Models and methods. New York/London: Academic Press. Bryden, M. P. 1979. Evidence for sex-related differences in cerebral organization. In M. A. Wittig & A. C. Petersen (Eds.) Sex-related differences in cognitive functioning: Developmental issues. New York: Academic Press.
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