Developmental parameters of the ear asymmetry: A multivariate approach

BRAIN AND LANGUAGE 2, 171-185 (1975)

Developmental Parameters of the Ear Asymmetry: A Multivariate Approach 1 PAUL SATZ

University of Florida D I R K J. BAKKER AND JETTY TEUNISSEN

Free University of Amsterdarn RON GOEBEL

University of Florida AND HARRY V A N DER V L U G T 2

University of Leiden "[he present paper has two objectives: (1) a critical review of dichotic listening studies in children, and (2) an investigation of developmental parameters of the ear asymmetry, based on factors revealed in the review. A dichotic listening task was administered to approximately 20 boys and 20 girls at each of five ages (5,6,7,9 and 11). The results were as follows: (1) significant ear asymmetry was not found in children younger than nine years of age; (2) the magnitude of the differences between ears, while not significant until age nine, continued to increase with age until eleven at which time the slope functions for each ear plateaued; and (3) ear asymmetry was independent of sex. An attempt is made to explain the present results, and those of previous research, within a developmental framework.

The left cerebral hemisphere has long been known to subserve speech and language functions in right-handed adults and in some smaller proportion of left-handed adults (Penfield & Roberts, 1959; Gloning & Quatember, 1966; Satz, 1970, 1972). The distribution of speech lateralization, while less clear in left-handers, is virtually unknown in children, regardless of hand preference. Clinical reports have suggested that the process of speech lateralization may represent a gradual transition from bilateral to increasing unilateral hemispheric participation as the brain reaches full maturation around puberty (Lenneberg, 1967; Basser, 1962; 1 This collaborative research was supported in part by funds from the National Institutes of Health (NS 08208), the National Institutes of Mental Health (MH 19415) and the Paedologish Institute, Free University of Amsterdam. 2 The authors express their appreciation to the Head and Staff of the Johannesschool and the kindergarten "De Gelaavsde Kat," as well as to Messr. Piet Reitsma and Ad de Haas for their kind cooperation. 171 Copyright© 1975by AcademicPress, Inc. All rightsof reproductionin any formreserved.

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Zangwill, 1960). The evidence for this interpretation is as follows: (1) that recovery of speech and language functions, after unilateral brain injury, is inversely related to age, i.e., with less recovery and more permanent sequelae as the child reaches puberty and (2) that the occurrence of aphasic symptoms in children is commonly associated with either leftor right-sided lesions. The preceding observations are clearly compatible with an age difference, however grossly defined (e.g., children vs adults), in the cerebral lateralization of speech. The data, however, do not permit a more rigorous specification of the rate or age at which this process of lateralization is established. Recent advances in the application of binaural separation procedures (i.e., dichotic listening) provide a unique opportunity to examine developmental parameters of this hemispheric phenomenon in normal children. Unfortunately, a review of the literature reveals a number of contradictory findings. Some of the results are due to procedural artifacts in the studies, and some are clearly incompatible with developmental theories of language acquisition. For this reason a brief evaluation of six studies is presented. The studies can be distinguished largely by the method of stimulation (multiple digit lists vs single CV syllable or word pairs) and the type of response employed (recall vs recognition). The first study, which has been cited extensively, is by Kimura (1963). She presented three different groups of digits (one, two and three pairs) to 120 right-handed children of both sexes between the ages of four and nine. Using simple comparison t-statistics she demonstrated that digits presented to the right ear were recalled better than those presented to the left for most of the within age/sex comparisons-even as early as age four. Similar findings were reported in a later study by Knox and Kimura (1970) for children between the ages of five and nine. No sex differences were found in either study. The curious finding, in each study, was the fact that the magnitude of the ear asymmetry decreased with age with the greatest asymmetry evident in the youngest ages (four and five). One might conclude from these data that the lateralization of speech is established early in childhood and that hemispheric polarization decreases with age. This interpretation is at variance with the evidence on the incidence and recovery of speech disturbances in brain-injured children (Lenneberg, 1967; Basser, 1962). Moreover, the interpretation is incompatible with a developmental process of increasing lateralization' with age. One is therefore left with the tenuous conclusion, based on these two studies, that the cerebral lateralization of speech may be established very early in childhood (e.g., age four or earlier) and that it undergoes no development thereafter. This conclusion has been implicitly accepted for the past decade.

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However, the data that support it can be explained, in part, as artifacts of the stimulus procedures used by Kimura (1963) and Knox and Kimura (1970). The fact that the stimulus lists were composed of one-, two- and three-pair trials suggests that the task became increasingly easier for the older children, particularly on the one- and two-pair trials. This bias would tend to increase overall recall for both ears in the older age groups which, in turn, would decrease the magnitude of the difference in recall between ears. Inspection of their data confirms this ceiling effect. However, this effect cannot account for the significant ear asymmetry in the younger age groups. Although the risk of Type I errors increases when multiple t-statistic comparisons are made, particularly on large samples (Hays, 1963), the t values in both studies were both high and replicable. Geffner and Hochberg (1971) recently presented similar groups of digit lists (two pairs/trial) to 208 children from low and middle socioeconomic levels between the ages of four and seven, with an equal number of boys and girls at each age. They also found a significant ear asymmetry at each age level in the middle socioeconomic groups, including the four-year-olds. By contrast, the lower socioeconomic children did not reveal an ear asymmetry until seven years of age? Curiously, the results again revealed a decreasing magnitude of the ear asymmetry with age in the middle socioeconomic group. While these results are compatible with the hypothesis of delayed development in the lower socioeconomic children, the hypothesis is weakened by the study's lack of a developmental increase in the magnitude of the ear asymmetry with age. That is, no evidence of an ear by age interaction was found. Nor was there a main effect for sex. The demonstration of a significant ear asymmetry in the younger middle socioeconomic children is compatible with the Kimura (1963) and Knox and Kimura (l 970) studies. The fact that the magnitude of the ear asymmetry decreased with age, however, could once again be explained as a possible artifact of the procedures used. For example, the authors employed a simple two pairs of digits/trial task which would be too easy for the older subjects and might thereby produce a ceiling effect similar to that of Kimura and Knox (1963, 1970). Inspection of the ,tables, in fact, confirms this artifact in the older children. The authors also commented: "A point was reached when they were able to report 'all four digits presented dichotically. Consequently, there was a decrease in error scores, although the right ear superiority was maintained (p. 199)." The authors also presented their digits at a slower rate for This finding, incidentally, is incompatible with the Knox and Kimura study (1970) w h i c h comprised children from lower socioeconomic backgrounds. They reported an ear asymmetry even in their youngest age groups (i.e., five).

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dichotic than for monotic tasks (2 pairs/1.5 sec vs 2 pairs/sec) which would further decrease the task complexity for older subjects (Satz, 1968). These two procedural artifacts would therefore prevent an adequate developmental test of an ear by age interaction. Without this interaction it would be hazardous to postulate a lag in the process of speech lateralization in lower socioeconomic children. Yet Geffner and Hochberg (1971) disregard the problem and conclude: " . . . dominance is achieved by the gradual concentration in one hemisphere of an originally bilateral function. Conceivably, for low socioeconomic children the process of cerebral lateralization is slower and less complete, and consequently, auditory asymmetry delayed (p. 200)." These conclusions, particularly with reference to process and rate, are unsubstantiated by their data. If the younger lower socioeconomic children are lagging, then so are the older middle socioeconomic children. Similar problems exist in studies employing other dichotic paradigms (e.g., single pairs of CV nonsense syllables or words presented for recognition). For example, Nagafuchi (1970) presented two and three syllable words, one pair/trial, to 80 children of both sexes between the ages of three and six. The author concluded that ear asymmetry exists as early as three years of age, although " . . . the dominance effect was not always observed as clearly in young children (p. 411)." This study is also subject to criticism for the following reasons: (1) No analysis of variance was computed to test for the effects of ear, age or sex; nor was any attempt made to test for an ear by age interaction. Once again, multiple t-statistic comparisons were computed between ears (N = 16) for each type of syllable for boys and girls; (2) inspection of the tables for each sex reveals that total recognition (R ÷ L) decreased rather than increased with age, and this is clearly incompatible with developmental studies of language acquisition (Brunet, 1968); (3) boys showed twice the number of significant differences between ears as girls, suggesting that boys may be more precocious than girls with respect to attainment of the ear asymmetry (contrary to the author's conclusion). A recent study by Berlin, Lowe-Bell, Hughes and Berlin (1973) found similar results of an ear asymmetry in children as young as five. The authors presented single pairs of CV nonsense syllables (e.g.,/pa/,/ta/, /ka/) in a recognition paradigm in which phonetic and acoustic parameters were precisely controlled and synchronized. The study was based on a total of 150 children between the ages of five and thirteen with an equal number of boys and girls at each of five ages (5,7,9,11 and 13). The results again disclosed a right ear advantage across all age and sex groups including the five-year-old children. Also, there was no ear by age interaction: the magnitude of the ear asymmetry was constant at all age levels. The authors were puzzled by this latter finding which

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suggests that, once hemispheric lateralization is established, no further development in ear asymmetry occurs. The only developmental trend in their data was an increase in double-correct responses with age, which they refer to as an increase in overall channel capacity or total accuracy. However, closer inspection of their data indicates that total accuracy (R + L) remained constant over ages and that only the number of double-correct response pairs increased with age (Age 5, ) ( = 11; Age 13, X = 17). In fact, the mean numbers of fight a n d left responses were almost identical across age groups which contradicts their interpretation of an increase in total accuracy or channel capacity with age. What the authors apparently mean is that older subjects tended to report correctly the syllables from both ears more often than did the younger subjects. Yet inspection of their tables reveals that this tendency occured on less than 25% of the trials in the older age groups. Hence, the authors are misleading or confused with respect to the term "total accuracy." Berlin and associates (1973) did, however, recognize that their results were discrepant with reports on the incidence and recovery of speech disturbances in brain-injured children (Lenneberg, 1967). They also recognized the discrepancy between their results and the developmental fact that mnemonic, perceptual and linguistic competencies (including reading and writing) increase dramatically with age (Bruner, 1968). They concluded, therefore, " . . . that the fight ear advantage as measured by dichotic listening to n o n s e n s e CV's is not an index of total overall language performance (p. 9)." This caution may be valid, but it fails to explain why a recent study, employing similar pairs of CV syllables, found completely different results (Bryden, 1973). This again suggests that artifacts in the task (i.e., stimulus and procedural parameters), or in the scoring and analysis of responses, may have accounted for the identical right and left ear responses across age groups in the Berlin et al. study. This possibility is strengthened particularly in view of the scoring and 'analysis procedures used by Bryden (1973). The study employed similar pairs of CV syllables differing only in the initial stop consonant. The dichotic pairs were administered to 120 children between the ages six and fourteen with an equal number of boys and girls at each of five ages (6,7,10,12 and 14). In contrast to the study by Berlin and associates (1973), Bryden (1973) analyzed his data on first responses only because ,of the dramatic depression of second response accuracy at all age levels. 4 In an analysis of variance, the author found a main effect for age and ears and a highly significant ear by age interaction. Based on this interaction, separate between ear within age comparisons were made 4 Berlin et al. (1973) analyzed their data on both single correct and double correct items (pooled) which may have confounded some of the age by ear relationships.

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which revealed a trend in the developmental magnitude of the ear asymmetry by age 10, which was not significant until ages 12 and 14. A pronounced sex difference in the ear asymmetry was also found in favor of the girls. Further analyses on the phonetic properties of the stimuli indicated that the magnitude of the ear asymmetry varied as a function of the voicing contrast between consonants. This study represents the first demonstration of a much later a n d more gradual development of the ear asymmetry. The results are also more compatible with clinical reports on the recovery of speech functions in brain-injured children (Basser, 1962; Lenneberg, 1967). Bryden (1973) also reported preliminary (i.e., unpublished) results on the presentation of multiple digit lists (two to four pairs/trial), using a free recall procedure similar to Kimura and Knox (1963, 1970) and Geffner and Hochberg (1971). By increasing the trial length complexity of the task, Bryden probably circumvented the ceiling effect and thus again found a (unspecified) developmental increase in the ear asymmetry with age. However, he apparently overlooked the importance of the ceiling effect in accounting for the discrepancy between his study and preceding studies. Intuitively, the presence of a ceiling effect in dichotic tasks is bound to obscure any investigation of developmental differences, particularly the important ear by age interaction. Furthermore, the investigation of possible lags in hemispheric speech lateralization between younger and older children, or between low and middle socioeconomic children, must rest on the demonstration of a developmental increase in efficiency of channel separation with age (Geffner & Hochberg, 1971). The ceiling effect, however, does not seem to account for the marked discrepancy between the Berlin et al. (1973) and Bryden (1973) studies, both of which employed single pairs of CV nonsense syllables in a recognition paradigm. Although it was suggested that differences in scoring procedure may have been involved, this explanation fails to explain why Berlin and associates found almost identical ratio differences between ears at all age levels. That is, 5-year-old children produced the same number of correct responses for the right ear as did 13-year-old children. The same relationship between ages was observed for left channel correct responses. This latter finding is almost as confusing as Nagafuchi's (1970) demonstration of a decrease in total channel capacity over age. Both studies, in fact, conflict with developmental theory which is based on an increase in performance competency with age. Thus errors or idiosyncracies in scoring and analysis procedures must be entertained as well as concern for signal-to-noise ratios in recording (Berlin et al., 1973).

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THE PROBLEM The preceding review, in snmmary, highlights a number of problems concerning developmental parameters of the ear asymmetry. Three basic questions seem to emerge: (1) at what age is the ear asymmetry established? (2) does the magnitude of the ear asymmetry increase developmentally with age? and (3) is the ear asymmetry independent of sex? The answer to the first question, based on five of the six studies, seems to be that the ear asymmetry may be established early in childhood, probably as early as four years of age. Despite possible procedural and analysis artifacts in these s t u d i e s - a n d the discrepancy between these results and recovery phenomena in brain-injured children-the fact still remains that most of the studies demonstrated early lateralization on dichotic listening tasks. The answer to the second question is less certain. We have pointed out that most of the studies used tasks too easy for valid assessment of developmental changes. The presentation of simple digit or word trials, particularly at slower rates, is likely to wash out possible differences between younger and older children due to a ceiling effect. The only study with sufficient task complexity (Bryden, 1973) did report a developmental increase in the ear asymmetry with age. Bryden also employed a scoring procedure (for CV syllables) which adequately assessed changes in increased performance with age. Berlin et al. (1973) apparently did not. The answer to the third question, also based on five of the six studies, seems to be that ear asymmetry at both younger and older ages is independent of sex. The only study to report a sex difference in the ear asymmetry (in favor of girls) is by Bryden (1973). The fact that Bryden's study was based on large sample N and employed analysis of variance procedures is sufficient warrant for further investigation of this problem. Thus, the need exists to re-examine more rigorously each of the preceding questions. The fact that answers to these questions involve multiple measurements on large numbers of subjects for several variables and their effects (ears, age and sex) dictates that multivariate analysis of regression models be employed, in order to supplement statistical significance with additional information concerning the importance of an effect (i.e., proportion of total variance it accounts for). METHOD

Subjects One hundred and ninety-eightchildrenfrom a kindergartenand a normalprimary school participated in the experiment.The childrenlivedin a suburban middleclass communityof Amsterdam.There were 19 boys and 19 girls fiveyears of age, and 20 boys and 20 girls at

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each of the ages six, seven, nine and eleven. Subjects were randomly sampled from their classes. Subjects 7-years-old and older were all right-handed; of the five and six year olds, 28 (74%) and 35 (88%) subjects respectively were right-handed. There were no reported neurological difficulties or auditory defects in the children.

Test and Procedure. Subjects were presented with a Dichotic Listening Test prepared by the University of Florida Neuropsychology Laboratory. Thirty series of four Dutch-spoken digit-pairs were relayed to the ears by a Ferrograph Model 722 H/P tape recorder and a set of Beyer DT 480 stereophonic earphones at the rate of 2 pairs/sec and at an intensity-level of approximately 70 dB (SPL). Left and right channel series were randomly chosen from a pool of 60 series. The first four series of pairs were considered practice trials and were not included in the final analysis. Each subject was instructed to recall as many digits as possible in any order. Headphones were reversed halfway, that is, after the 13th trial. Manual preference was defined by performance on subtest two of the Harris Tests of Lateral Dominance (Harris, 1957). A subject was considered right-handed when the right hand was used for at least nine of the ten tasks. This cut-off point was chosen because one task (question 2: wind a watch) is impracticable for Dutch school children: Few wear watches, and those who do, wear the watch on their right arm as much as on their left arm. Of the 5- and 6-year-olds, only five subjects (6%) more often used their left hand than their right hand for the tasks of the manual preference test. Testing was done in a quiet room of the school. The manual preference test was presented first, followed by the Dichotic Listening Test.

Type o f Analysis The type of statistical analysis to which the data were subjected is known as Analysis of Regression. Herein, the experimenter formulates hypotheses about his data in terms of linear mathematical models,a analyzes these models as regression equations, and compares them for their relative efficiency in predicting the data. 6 The basic procedure in the present study was as follows. The dichotic listening scores were first predicted by a "Full" model, using all independent variable sources of variance as predictors, and imposing no restrictions on these predictors' interrelations. The R 2 for this model represented the total amount of accounted-for dichotic performance variance, and was therefore a logical base against which subsequent models could be compared. An analysis of trend was then performed by comparing the Full model with models which "restricted" the performance-over-age relations to rectilinear or curvilinear functions. The model which was found not significantly different from the Full model revealed the nature of the trend. Finally, models were constructed which, one by one, eliminated different sources of variance from the predictor set. Since the comparison model contained all sources of variance and each restricted model contained all but one, the F ratio comparing the two indicated the significance of the eliminated source of variance. Moreover, the difference in R2's for the two models indicated the relative size of the source of variance in terms of the amount of dichotic performance variance it uniquely predicted. Throughout the analyses, whenever a restricted model was found not significantly different from the comparison model, it was substituted for the comparison model and became the standard against which subsequent models were compared. The move effected 5 The term "linear model" refers only to the fact that the weighted components of the model are summed; it says nothing about the shape of the model's function when plotted, which is dependent upon the properties of the model's components. 6 The computer program used in this study was R E G R A N (Veldman, 1967), a Fortran IV program, specifically designed for this analytic technique.

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a reduction in the " n o i s i n e s s " of the data and increased power, and similarly to the standard pooling techniques of m o r e familiar statistical methods. Additionally, the restricted model (if true) provided better estimates of the parameters, since it had fewer predictors (Ward & Jennings, 1973, p. 98). T h e dichotic listening data were analyzed in two stages. T h e purpose of the first stage was to identify the nature of the effects of sex and age on the performance functions for each ear separately. T h e purpose of the second stage was to select a m e a s u r e of ear asymmetry (using the right- and left-ear data) and conduct tests upon it aimed at uncovering the developmental course of this p h e n o m e n o n .

RES U LTS

Stage 1 Table 1 summarizes the results of the separate analyses of the rightand left-ear data. For both ears, performance over age was found to be curvilinear, with positive slope and negative acceleration. The lack of significant F ratios for the curvilinear models indicated that they were not significantly different from their respective full models. No significant sex effect or sex by age interaction was found for either ear. The small sizes of these effects attest to their relative inability uniquely to predict dichotic listening performance. Essentially all the accounted-for variance was due to the subject's ages. The fact that the age effect for the right ear was greater than for the left indicates that performance increased with age to a greater degree for the right ear. TABLE 1 SUMMARY OF THE SEX BY AGE ANALYSIS OF REGRESSION ON THE RIGHT- AND L E F T - E A R PERFORMANCE DATA

Data

Source of variance

Size (% of total variance)

F

Right ear

Full model Linear trend Curvilinear trend Sex by age B e t w e e n sexes within ages B e t w e e n ages

47.60 41.02 46.30 0.06 0.69 45.55

-3.95** 1.14 0.11 2.53 82.39***

Left ear

Full model Linear trend Curvilinear trend Sex by age B e t w e e n s e x e s within ages B e t w e e n ages

28.19 23.34 27.13 0.22 1.30 25.60

-2.13" 0.70 0.29 3.48 33.90***

* p = .0503. ** p ~ .005. *** p ~ .0001.

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Stage 2

At the outset, the measure thought to represent the ear asymmetry best was the difference between right- and left-ear scores. This measure was therefore derived, and a full model using all sources of variance involving sex and age was constructed to predict it. However, the total power of the model to predict this measure was extremely limited (7%). While the ear difference measure preserves the relations between ears, it also eliminates information concerning the overall level of accuracy. Since Stage 1 indicated that performance increased in accuracy with age, it is apparent that substitution of the ear difference score for the actual performance score threw out a lot of previously accounted-for variance. Even if age and]or sex were found significant with this measure, they would appear small by comparison, since each would account for something less than 7% of the total variance. More importantly, the use of the ear difference score does not permit a direct analysis of the main effect of ear as an independent variable nor of the interaction of ear by age. For these reasons, Stage 2 proceeded with the original measure, i.e., total correct recall per ear, and the data were then arranged for an ear by age analysis. The sex variable was not included in this analysis since it was shown during Stage 1 to be too weak and insignificant to warrant inclusion. Table 2 summarizes the results of the Stage 2 analysis. Not surprisingly the data were best fit by curvilinear functions with positive slopes and negative accelerations. However, additional tests indicated that the ear functions differed significantly in slope but not in acceleration. Enough difference in slope occurred for a highly reliable, though relatively weak, ear by age interaction.

TABLE 2 SUMMARY OF THE EAR BY AGE ANALYSIS OF REGRESSION OF THE DICHOTIC LISTENING DATA

Source of variance

Size (% of total variance)

Full model L i n e a r trend Curvilinear trend Curvilinear with c o m m o n acceleration E a r b y age B e t w e e n ears within ages Between ages within ears

41.02 36.16 40.59 40.38 1.71 6.64 35.45

* p _< .005. ** p --< .0001.

F -5.35** 0.70 1.42 11.35" 21.99"* 78.28* *

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i i Right O--R--O Left 55 50 O0

~if)

45

35 30 25

U

20

AGE FIG. 1. The best fitting curves, predicted by analysis of regression, for ear scores (percentage correct), as a function of age in years.

Post hoc tests were performed to determine the loci of the significant between-ear differences. Five t tests for correlated means were computed, one for each between-ear comparison at each of the five age levels. Correcting for the shifting alpha level caused by performing independent comparisons within a set of means, significant differences between ears were found only at ages 9 and 11 (p ~< .05). These age by ear differences can be visualized in Fig. 1.

DISCUSSION An attempt was made in the present paper to evaluate studies of the development of the ear asymmetry in children, to identify certain procedural and conceptual factors that accounted for some of the discrepant findings and to discuss the problems which still exist. Based on this review, the present study was conducted. A major attempt was made to control procedural artifacts (particularly the ceiling effect) and to use methods of analysis that would provide better assessment of the ear asymmetry and its developmental parameters. The results showed that a curvilinear model represented the best developmental estimate of ear asymmetry. The developmental functions for each ear were curvilinear, with positive slopes and negative accelerations. The positive slopes indicate that subjects recalled increasingly more information from both ears up to nine years of age, at which time the slopes leveled off and produced the curvilinear function. However, the two slopes (i.e., ears) were not parallel; in fact, the difference between slopes indicates that right ear functions (on this dichotic task) develop significantly faster than left. Although a trend for right ear superiority was apparent as early as 5- to 6-years-old, the differences in recall between ears were not significant until age nine. The functions for each ear also shared the same negative acceleration, which indicates that the

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rates at which gains in performance from year to year grow smaller, were essentially the same for both ears. These findings, in summary, clearly indicate the importance of age in the development of the ear asymmetry. Moreover, the significant ear by age interaction indicates that the magnitude of the ear asymmetry increased with age but was not significant until nine years of age. Finally, the separate slope functions for each ear were shown to be largely independent of sex. That is, a single curve model, based on age, accounted for the lion's share of the variance for both the right- and left-ear scores. On the basis of the present results one could conservatively conclude that there are no sex differences associated with developmental changes in ear asymmetry. This interpretation is supported by the fact that six of the seven studies failed to demonstrate an ear by sex effect. Two unpublished studies, recently completed in our laboratories (Borowy, 1973, Bakker & Teunissen, personal communication) also found no association between sex and developmental parameters of the ear asymmetry, bringing the tally to eight negatives. These findings, however, should not rule out the possibility of more basic underlying sex differences in maturation of the cerebral hemispheres (Taylor, 1969). A more difficult problem concerns the age of onset of the ear asymmetry. The box score, based on five of the previous studies, favors an early age of onset (i.e., ages 5-7), despite some of the methodological problems previously discussed. Yet the present results, and those of Bryden (1973), demonstrate a relatively late onset of the ear asymmetry (i.e., ages 9-12), despite a trend as early as 5 years of age. This problem is further complicated by two recent unpublished studies in our laborat o r y - b o t h of which found different results. A study by Borowy (1973), based on 120 children from ages five to eleven, demonstrated a significant ear effect at all ages, including five, which contrasts with present results. On the other hand, a study by Darby and Satz (1973), sampling normal and dyslexic children at ages five, seven, and twelve (longitudinal-predictive project), found a significant ear effect in only the twelve-year-old normal children. 7 Both studies employed a similar three pair digit recall task. If one includes the present results and the unpublished results of Borowy (1973), Darby and Satz (1973), and Bakker and T e u n i s s e n (personal communication) the box score is increased to seven out of ten studies in favor of an early age of onset of the ear asymmetry.8 It is unlikely that this discrepancy is due to list length (stimulus complexity), 7 This finding was interpreted as additional support for a delay in left h e m i s p h e r e maturation, postulated to occur in specific reading disability (Satz & V a n N o s t r a n d , 1973). 8 T h e recent study by Bakker & T e u n i s s e n (personal communication) reported an ear a s y m m e t r y as early as age s e v e n in normal children.

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to type of materials presented (digits vs CV syllables) or to variations in response (recognition vs recall). For example, Bryden (1973) found late onset of the ear asymmetry using both CV syllables (recognition) and digits (recall), whereas Borowy (1973) and Darby and Satz (1973) found differences in onset using similar stimuli (digits) and response (recall) procedures. Therefore, the weight of the evidence favors an early onset of the ear asymmetry in normal children. The fact that the ear asymmetry appears early in development is compatible with the view that the left cerebral hemisphere is prepotent (genetically or otherwise) for language acquisition and that this predisposition is reflected in some degree of increased hemispheric asymmetry (and decreased bilateral symmetry) by five years of age. This formulation is certainly compatible with clinical reports on the incidence and recovery of aphasic symptoms in brain-injured children (Basser, 1962; Lenneberg, 1967; Semmes, 1968). The problem, however, concerns translating what is meant by degree of hemispheric asymmetry into statistically meaningful terms. If one postulates a bilateral though unequal hemispheric representation of speech by five years of age, then more than a simple dichotomy in brain lateralization is implied. Translated into statistical terms one should expect to find at least a reliable trend, if not significance, in the ear asymmetry at this age. The data, in fact, from all the published and unpublished studies (N = 10) are consistent when viewed in this light? However, it should be clear that a significant ear effect at age five does not imply complete unilateral hemispheric dominance. This question can only be examined within a developmental context: Does the magnitude of the ear asymmetry increase as a function of age and at what age, if any, does the asymmetry asymptote? This question represents a more substantive problem than age of onset of the ear asymmetry. If, for example, no further changes in the magnitude of the ear asymmetry occur after onset, then one would have strongly to question the validity of a brain maturation model (i.e., age changes in hemispheric participation of speech) and clinical reports on recovery of speech in brain-injured children (Basser, 1962; Lenneberg, 1967; Zangwill, 1960). The present results and those of Bryden (1973) were shown to be most compatible with the concept of developmental changes in the degree of hemispheric speech lateralization. Similar results were also found in the two unpublished studies from :~bur laboratory (Borowy, 1973; Darby & Satz, 1973). However, if the two latter studies are tallied, the box score is still discrepant with a developmental brain matu9 If this hypothesis is true it reduces the possibility of sampling bias in those studies which have reported a later onset of the ear asymmetry (Bryden, 1973; Darby & Satz, 1973; and the present study).

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ration model. Five of the studies (Kimura, 1963; Knox & Kimura, 1970; Nagafuchi, 1970; Geffner & Hochberg, 1971; Berlin et al., 1973) have shown either a decrease or plateau in the ear asymmetry after three to five years of age. These results, however, were shown on a priori grounds to be artifacts of stimulus procedure (e.g., ceiling effects) and grossly at variance with theories of language development (Bruner, 1968) and brain maturation (Lenneberg, 1967). 1° Thus, one might conclude, on the basis of the present results, that the ear asymmetry, regardless of its age of onset, does undergo major changes after five years of age. Further, this increase in the magnitude of the ear asymmetry is compatible with the process of speech-brain lateralization which has been postulated to occur during childhood. Additional support for these normative findings would certainly provide a more substantive context for investigating possible delay in hemispheric lateralization in children who are dyslexic or subject to marked cultural deprivation (Bloom, 1962; Geffner et al., 1971; Satz & Van Nostrand, 1973). REFERENCES Basser, L. S. 1962. Hemiplegia of early onset and the faculty of speech with special reference to the effects of hemispherectomy. Brain, 85, 427-460. Berlin, C., Hughes, L., Lowe-Bell, S., & Berlin, H. 1973. Dichotic right ear advantages in children 5 to 13. Paper presented at the International Neuropsychology Society, New Orleans, LA, February, 1973. Bloom, S. 1964. Stability and Change in Human Characteristics. New York: John Wiley. Borowy, T. D. 1973. Developmental parameters of the ear asymmetry phenomenon: The effects of sex, race, and socioeconomic class. Unpublished doctoral dissertation, University of Florida. Bruner, J. S. 1968. The course of cognitive growth. In N.S. Endler, L. R. Boulter, & H. Osser (Eds.), Contemporary Issues in Developmental Psychology. New York: Holt, Rinehart and Winston. Pp. 476-494. Bryden, M. 1973. Dichotic listening and the development of linguistic processes. Paper presented at the International Neuropsychology Society, New Orleans, LA, February. Darby, R., & Satz, P. 1973. Developmental dyslexia: A possible lag mechanism. Unpublished Master's thesis, University of Florida.

10 Whereas the ceiling effect may have obscured developmental changes in the ear asymmetry in other studies (i.e., with digits), a floor effect may have occurred in the present study, making the task more difficult at the earlier ages, and influencing both the age of onset and developmental changes of the phenomenon. Although Bryden's (1973) use of CV syllables mitigates against this possibility, it suggests that future studies should control for this bias by equalizing difficulty level between ages. This could be accomplished by varying list length (or stimulus complexity) such that 50 percent of the items are correctly recalled at each age. In an effort to control for complexity in the present study the R/R + L and L/R + L scores were plotted against ages. The right and left ear slopes were similar to the slopes in Fig. 1 which suggests that the ear by age interaction was not an artifact of task complexity at the younger ages. Nevertheless, this variable should be controlled in future studies.

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Geffner, D. S., & Hochberg, I. 1971. Ear laterality performance of children from low and middle socioeconomic levels on a verbal dichotic listening task, Cortex, 7, 193-203~ Gloning, K., & Quatember, R. 1966, Statistical evidence of neuropsychological syndromes in left-handed and ambidextrous patients. Cortex, 2, 484-488. Harris, A. J. 1957. Lateral dominance, directional confusion and reading disability. Journal of Psychology 44, 283-294. Hays, W. L. 1963. Statistics for Psychologists. New York: Holt, Rinehart and Winston. Kimura, D. 1963. Speech lateralization in young children as determined by an auditory test. Journal of Comparative and Physiological Psychology, 56, 899-902. Knox, C. and Kimura, D. 1970. Cerebral processing of nonverbal sounds in boys and girls. Neuropsychologia, 8, 227-237. Lenneberg, E. H. 1970. Biological Foundations of Language. New York: John Wiley. Nagafuchi, M. 1970. Development of dichotic and monaural hearing abilities in young children. Acta Otolaryngologica, 69, 409-415. Penfield, W., & Roberts, L. M. 1959. Speech and Brain Mechanisms. Princeton, N J: Princeton University Press. Satz, P. 1968. Laterality effects in dichotic listening: A reply. Nature (London), 218, 277-278. Satz, P. 1970. Cerebral mechanisms in motor asymmetry. Paper presented at the Academy of Aphasia, Tulane University Medical School, New Orleans, LA. Satz, P. 1972. Pathological left-handedness: An explanatory model. Cortex, 8, 121-135. Satz, P., & Van Nostrand, G. K. 1973. Developmental dyslexia: An evaluation of a theory. In P. Satz and J. J. Ross (Eds.) The Disabled Learner: Early Detection and Intervention. Rotterdam, The Netherlands: Rotterdam University Press. Pp. 121-148. Semmes, J. 1968. Hemispheric specialization: A possible clue to mechanisms. Neuropsychologia 6, 11-26. Taylor, D. C. 1969. Differential rates of cerebral maturation between sexes and between hemispheres: Evidence from epilepsy. The Lancet, July 19th. Veldman, D. J. 1967. Fortran Programming for the Behavioral Sciences. New York: Holt, Rinehart, and Winston. Ward, J., & Jennings, E. 1973. Introduction to Linear Models. Englewood Cliffs, NJ: Prentice Hall. Zangwill, O. L. 1960. Cerebral Dominance and Its Relation to Psychological Function. Edinburgh: Oliver and Boyd.