A comparison of hemispheric asymmetries in speech-related brain potentials of autistic and dysphasic children

BRAIN

AND

LANGUAGE

37, 26-41 (1989)

A Comparison of Hemispheric Asymmetries in SpeechRelated Brain Potentials of Autistic and Dysphasic Children GERALDINE DAWSON University of Washington CHARLES FINLEY Research Triangle Institute, North Carolina SHEILA PHILLIPS University of North Carolina at Chapel Hill AND ART LEWY University of Washington In a previous study (G. Dawson, C. Finley, S. Phillips, & L. Galpert, 1986, Child Development, 57, 1440-1453) it was found that measures of hemispheric asymmetry during speech processing were predictive of level of language ability in autistic children. The purpose of the present study was to determine whether a similar relationship between pattern of hemispheric asymmetry and language ability exists for language-impaired children without autism. Ten autistic children (8-13 years), 10 dysphasic children (6-1.5 years), and 10 normal children (8-13 The present study was supported by National Institute of Mental Health Grant MH36612 awarded to Geraldine Dawson. The authors express their appreciation to the children who participated in the study and their families, and to the staff at Division TEACCH, for their many hours of cooperation. Drs. Harold Pillsbury and Grady Thomas, Department of Surgery, Division of Otolaryngology, School of Medicine, UNC-CH, generously made available the equipment for evoked response testing. Susan Brinn, Karen Cotten, Mary Evers, Teresa Frei, Larry Galpert, and Mary Hyde assisted in data collection and coding. Faulder Colby and Chris Gullion assisted in data analyses. Address correspondence and reprint requests to Geraldine Dawson, Department of Psychology, University of Washington, Seattle, WA 98195. 26 0093-934x/89 $3.00 Copyright All rights

0 1989 by Academic Press, Inc. of reproduction in any form reserved.

HEMISPHERIC

ASYMMETRIES

27

years) were compared in terms of their patterns of hemispheric asymmetry in the averaged cortical evoked response to a simple speech stimulus, and the relationship between pattern of hemispheric asymmetry and language ability was assessed for each clinical group. It was found that, for both the autistic and dysphasic groups, the majority of subjects showed a reversed direction of hemispheric asymmetry from that characteristic of the normal group. A strong relationship between pattern of asymmetry and level of language ability was found for autistic subjects; autistic subjects with more severe language impairments were more likely to show reversed asymmetry than subjects with less severe language impairments. In contrast, no relationship between language ability and direction of hemispheric asymmetry in the evoked response was found for dysphasic subjects. Separate analyses of right and left hemisphere evoked responses indicated that language ability was related to right hemisphere activity for autistic subjects, and to left hemisphere activity for dysphasic subjects. 4~ 1%~ Academic PWS\. Inc

Given the variability in neurological findings in individuals with autism and the evidence indicating that autism has multiple etiologies (Gillberg & Gillberg, 1983; Lobascher, Kingerlee, & Gubbay, 1970; Knoblock & Pasamanick, 1975; Folstein & Rutter, 1977; Chess, 1971; Lotter, 1974), it is unlikely that the disorder can be explained in terms of dysfunction of one specific brain locus. Rather, the disorder may arise from dysfunction of a brain system or systems involving a number of brain structures. One part of this complex puzzle has been provided by studies of hemispheric functioning in autistic individuals. These studies have indicated that autism is associated with abnormal patterns of hemispheric activation during language and other left-hemispheric-mediated tasks. In four out of five studies using ear preference for auditory stimuli to assess hemispheric asymmetries in speech processing (Arnold & Schwartz, 1983; Blackstock, 1978; Hoffman & Prior, 1982; Prior & Bradshaw, 1979; Wetherby, Koegel, & Mendel, 1981), the majority of autistic subjects failed to show normal hemispheric asymmetries. The abnormality was either a lack of asymmetry or right hemisphere dominance for speech. Similarly, several studies using electrophysiological measures to assesshemispheric activity (Dawson, Warrenburg, & Fuller, 1982, 1983; Dawson, Finley, Phillips, & Galpert, 1986; Ogawa et al., 1982; Small, 1975; Tanguay, 1976) found that autism was associated with reversed or absent hemispheric asymmetry. In one study (Dawson, et al., 1982), it was reported that atypical patterns of hemispheric activation were found for the autistic group only during tasks traditionally mediated by the left hemisphere (i.e., language and motor imitation tasks), and that normal hemispheric activation was exhibited during right-hemisphere-related tasks (i.e., visuospatial tasks). Although abnormal patterns of hemisphere activation are commonly found in autism, most investigators report substantial intersubject variability. In response to speech stimuli, patterns may be reversed (right hemisphere), absent, or normal (left hemisphere). In a recent study (Dawson et al., 1986), the pattern of hemispheric asymmetry in cortical evoked

28

DAWSON

ET AL.

potentials to a speech stimulus was found to be highly predictive of level of language acquisition in autistic children. Autistic children with the most severely impaired language were more likely to show greater rightthan left-hemisphere activity during speech processing, as well as a greater-than-normal degree of asymmetry. Autistic children with more highly developed language tended to show normal hemispheric asymmetries (greater left hemisphere activity). No such relationships were found for normal children. Furthermore, when right and left hemisphere evoked potentials were examined separately for autistic children, it was found that language ability was strongly related to right, rather than left, hemisphere activity (Dawson et al., 1988). Specifically, greater right hemisphere activity, as reflected in the speech-evoked potential, was strongly correlated with poorer language ability. Dawson and her co-workers (Dawson, 1987, 1988; Dawson & Lewy, 1989) and others (Kinsbourne, 1987) have hypothesized that right hemisphere overactivation may be interfering with language processing in autism. In the present study, we sought to examine whether similar relationships between patterns of hemispheric activation and language ability exist for children with developmental language disorder without autism (dysphasia). It is possible that the relationship between hemispheric asymmetries and language ability found by Dawson et al. (1986) may be characteristic of all developmental language disorders, and not specific to autism. However, we predicted that, since it is likely that dysphasic children do have inherent dysfunction of the language areas of the left hemisphere, language ability for these children would be related to left, rather than right hemisphere activity. Patterns of hemispheric activation for speech in children with developmental receptive language disorder (dysphasia) have been examined in a few studies (Hughes & Sussman, 1983; Rosenblum & Dorman, 1978; Springer & Eisenson, 1977). The results of these studies suggest that many dysphasic children do show abnormal patterns of cortical activation for speech, that is, reduced or reversed patterns of asymmetry. However, direct comparisons of autistic and dysphasic children on the same measures of brain activity are generally lacking. Moreover, there exist few, if any, investigations of the relationship between language ability and hemispheric asymmetries in dysphasia. Thus, the present study may shed light on the role of hemispheric processing in dysphasia. Although autistic and dysphasic children share the common symptom of impaired receptive and expressive language, the two disorders differ in many ways. In contrast to autistic children, dysphasic children are capable of other modes of communication besides speech, such as the use of gesture. The autistic child has deficits in other forms of nonverbal communication such as eye contact, and social relatedness, whereas the dysphasic individual’s communicative deficits primarily involve verbal

HEMISPHERIC

ASYMMETRIES

29

language (Bartak, Rutter, & Cox, 1975). These language deficits have been found to be strongly related to specific impairments in processing rapidly presented sequential information (Tallal, Stark, & Mellits, 1985a, 1985b). Furthermore, although both disorders involve impaired language ability, the specific nature of the language impairments differs. Autistic persons generally have more severe comprehension deficits and are more likely to have difficulty in the social use of language than persons with dysphasia. Dysphasic individuals tend to have more difficulty in articulation (Tager-Flusberg, 1981). In light of these differences, it seems likely that the brain mechanisms underlying the two disorders, as well as their developmental courses, are different. In the present study, autistic and dysphasic children were directly compared on measures of hemispheric asymmetry of their averaged evoked potentials to a speech stimulus. Autistic and normal subjects were selected from larger samples of subjects who had participated in a previous study which has already been reported (Dawson et al., 1986). These subjects were chosen to match dysphasic subjects as closely as possible on age and language ability. Because of the rarity of both disorders, and the difficulty of finding subjects who could participate in such a study without sedation, it was not possible to match on handedness. Instead, we chose to directly examine the relationship between hemispheric asymmetries and handedness in order to determine the importance of this variable in our results. Language abilities were comprehensively assessed in the clinical groups using a battery of standardized tests that measured syntax, semantics, articulation, and vocabulary. METHOD

Subjects Dysphasic subjects. Ten children with dysphasia participated. having met the following selection criteria: Based on a recent administration of the Wechsler Intelligence Scale for Children-Revised, a significant difference (215 points) between Verbal and Performance IQ scores with the Performance IQ score in the normal range (270) and the Verbal IQ score below 70, and a Peabody Picture Vocabulary Test (PPVT) IQ score below 70. If these test scores were not available in the child’s school records, the tests were administered. In addition, children with sensory or motor handicaps, or emotional disturbance were excluded. Dysphasic subjects ranged from 6 to 15 years of age (X = IO years, SD = 35 months). For all subjects, handedness was assessed by demonstrated hand usage on I I unimanual tasks. Six dysphasic subjects were right-handed and 4 were left-handed. Autistic subjects. Data from 10 autistic subjects who participated in a previous study, which used an identical experimental protocol as was used for dysphasic subjects, were used for comparison. These autistic subjects were chosen to match dysphasic subjects as closely as possible on language ability and age, and without knowledge of their evoked potential data. The autistic and dysphasic groups had similar age ranges, and the groups did not differ significantly on any of the language measures. These data are reported in Table 1. All autistic subjects had some verbal language ability (at minimum, one-word speech) and were diagnosed as autistic in early childhood based on an interdisciplinary

30

DAWSON ET AL. TABLE 1

LANGUAGE

ABILITIES

OF AUTISTIC

AND DYSPHASIC SUBJECTS: GROUP MEANS

AND RANGES

Language measures

Autistic (N = IO)

Dysphasic (N = 10)

Peabody Vocabulary (raw scores) Range NSST-Receptive (raw scores) Range NSST-Expressive (raw scores) Range Wechsler Vocabulary (scaled scores) Range Wechsler Comprehension (scaled scores) Range Articulation (percentage correct) Range

66.0 25-101 27.3 12-40 28.4 12-40 4.5 1-13 3.4 l-13 95.2 79.5-100

64.2 43-75 30.3 20-38 21.7 o-34 2.8 1-7 6.0 o-12 89.0 57-100

diagnostic evaluation and administration of the Childhood Autism Rating Scale. In addition, according to medical records, all subjects met the DSM-III diagnostic criteria for infantile autism at the time of their evaluation. These 10 subjects ranged from 8 to 13 years (X = 10 years, 4 months, SD = 17 months). Their IQ scores based on standardized intelligence testing ranged from 54 to 84 (X = 71, SD = 9). Six of the autistic subjects’ IQ scores were in the mentally retarded range (<70). Based on demonstrated hand usage on 11 unimanual tasks, 2 autistic subjects were determined to be ambidextrous. The remaining 8 subjects were right-handed. Normal subjects. Data from 10 normal subjects who also participated in the previous study were used for comparison. These subjects were originally recruited through the local public school system. Each normal subject was selected to match an autistic subject on the basis of chronological age (? 12 months) and gender. The mean age of the normal group was 10 years 4 months (SD = 23 months). By parent report via questionnaire, normal subjects were free of learning disabilities, sensory, motor, or neurological impairments, histories of head trauma, and current medications. All normal subjects were right-handed. The Peabody Picture Vocabulary Test (PPVT) was administered to all normal subjects. PPVT IQ scores ranged from 79 to 109 (X = 96, SD = 10). All subjects were screened for possible hearing loss by pure tone audiometry. One autistic subject showed evidence of hearing loss. This subject’s loss was evident in frequencies much higher than that of normal speech and thus should not have affected the results of cortical testing.

Auditory

Cortical

Evoked Responses

The reader is referred to Dawson et al. (1986) for a more detailed description of the stimuli and method. Stimuli. A series of auditory stimuli consisting of 80% clicks, 10% speech stimuli (“Da”), and 10% musical chord stimuli (piano chord) was presented binaurally over earphones at 70 dB above normal hearing threshold. Stimuli were presented in random order with interstimulus intervals ranging from 3 to 5 (X = 4.0). Click stimuli consisted of computergenerated square pulses with durations of 300 psec. EIecfrophysiological recording. Scalp recordings were made using silver disk electrodes positioned at Cz (vertex), midway between C3 and T5 (left hemisphere, LH), and midway

HEMISPHERIC

ASYMMETRIES

31

between C4 and T6 (right hemisphere, RH); International 10-20 system. Linked ear electrodes served as a reference and Fpz was ground. Eye movements were monitored on a separate channel via electrodes placed at the upper outer canthus of each eye. Interelectrode impedance was maintained below 10 k0. Signal bandwidth was filtered to 0.5 to 70 Hz. using a two-pole filter. Averaging was executed by a Nicolet Pathfinder II Evoked Response System that had been modified for selective synchronization with taped stimuli. Artifact-contaminated records (levels greater than 225 PV peak-to-peak) were rejected by the averager according to preset criteria. Averages were computed for a SOO-msecperiod following onset of the stimulus. Fifty responses to the speech stimulus and 50 responses to the musical stimulus were collected and averaged for each subject. Responses to the click stimuli were not recorded. Procedure. Subjects were seated in a comfortable recliner located in a sound-attenuated, electrically shielded chamber. No sedation was used. During testing, subjects were told to sit quietly with their eyes open and to listen to sounds heard over their earphones. Subjects were told to slowly raise their right hand upon hearing the sound, “Da.” This procedure was a replication of that previously used by Grabow, Aronson, Offord, Rose, and Greene (1980) which kept hand used for responses constant across subjects and allowed the experimenter to monitor the alertness and attention of the subject. From pilot studies, it was determined that counterbalancing hand used for responses would have been confusing to autistic subjects. All subjects raised their hands to virtually all speech stimuli (at most one stimulus was missed). Thus, there were no significant differences in attention to the speech stimulus for the three groups. During breaks, the experimenter talked to the subject in order to help maintain alertness. If a subject began to get drowsy, he sometimes took a break outside the chamber (to walk around. get a drink of water. etc.). The experimenter stayed with the subject throughout testing. The testing session required about I hr: and 15: min and was usually held in the morning. Data processing and analyses. Grand averages based on 50 responses were computed for the speech and musical stimulus, for each of the electrode sites, LH, RH, and Cz. Responses were scored in terms of the amplitudes and latencies of the major wave components: Pl, N 1, P2, N2, and P3. Amplitudes were measured relative to an average of the prestimulus baseline period. All peaks were judged independently by three raters according to the confidence with which a peak could be identified using a S-point scale (I, not identifiable; 2, guess; 3, good confidence; 4, almost certain; and 5, absolutely certain). Only data on which two of the three raters identified the peak with good confidence (rating of 3 or higher) were included in the present analyses. Ninety-three percent of the peaks were identified with good confidence or better. Only data from the LH and RH recording sites will be reported here.

Language

Assessment

The tests used to assess children’s language were Peabody Picture Vocabulary Test (PPVT), Northwest Syntax Screening Test-Receptive and Expressive (NSST-R, NSSTE), Vocabulary and Comprehension subtests from the Wechsler Intelligence Scales, and Arizona Articulation Test. The language assessment was carried out on the same day as the neurophysiological testing after an hour break for lunch.

RESULTS Language

Measures

Table 1 displays the group means and ranges for the autistic and dysphasic subjects for each of the language measures. The groups did not differ significantly on any of the measures.

32

DAWSON ET AL.

Patterns of Hemispheric

Asymmetry for Speech

Within-group, between-hemisphere comparisons. The musical stimulus failed to elicit consistent hemispheric asymmetries in the normal group and, thus, group comparisons on this measure were not meaningful. However, as will be reported below, the speech stimulus did elicit consistent asymmetries for the normal group. The following analyses will pertain only to this stimulus. Figure 1 displays the right and left hemisphere averaged evoked potentials to the speech stimulus for the normal, autistic, and dysphasic groups.

RH NORMAL SUBJECTS

I---

RH AUTISTIC SUBJECTS

R Ii DYSPHASIC SUBJECTS

ONSET MILLISECONDS FIG. 1. Right and left hemisphere speech-related potentials for normal, autistic, and dysphasic groups (IV = 10, each group).

HEMISPHERIC

33

ASYMMETRIES

The apparent lack of asymmetry in the group-averaged evoked responses of autistic and dysphasic subjects is the result of averaging across subjects with substantial intersubject variability. Indeed, it was not reasonable to display superimposed group data from the dysphasic subjects because the group-averaged evoked potentials overlapped almost precisely. Representative subject data from each group are shown in Fig. 2. For the normal group, the difference between the LH and RH evoked potentials was found to be significant only for Nl amplitude (t(9) = 2.67, p < .02, two-tailed), and approached significance for Nl latency (t(9) = 2.19, p = .07, two-tailed). Thus, only this early component was used to evaluate direction of hemispheric asymmetries in the autistic and dysphasic groups. This pattern of results is similar to those reported for normal subjects by Grabow et al. (1980) and by Wood, Goff, and Day (1971). For the autistic group, significant hemispheric differences were found for Nl latency; the direction of this difference was opposite of that found for normal subjects (t(9) = -2.52, p < .03). No significant between-hemisphere differences were found for the dysphasic group.

NORMAL SUBJECT

AUTISTIC SUBJECT

DYSPHASIC SUBJECT

MILLISECONDS FIG. 2. Right and left hemisphere speech-related and dysphasic subjects.

potentials for individual

normal, autistic.

34

DAWSON ET AL.

Group Comparisons

of Patterns of Hemispheric

Asymmetry for Speech

Since the purpose of the present study was to examine patterns of hemispheric asymmetry for individual subjects, a more meaningful method of analyzing data was to examine right minus left hemisphere difference scores, which were calculated for each subject. Table 2 and Fig. 3 show the means for the RH minus LH difference scores for Nl amplitude and latency for each group. One-way analyses of variance of the right minus left hemisphere difference scores showed significant group effects for N 1 latency (F(2, 27) = 3.61, p < .05) and a trend toward statistical significance for Nl amplitude (F(2, 27) = 2.73, p = .08). Multivariate analyses of variance of the separate RH and LH Nl amplitudes and latencies were also carried out. For both measures, no significant main effects of group or hemisphere were found. However, significant group x hemisphere interactions were found for Nl latency (F(2, 27) = 3.61, p < .02) and approached significance for Nl amplitude (F(2, 27) = 2.73, p < .08). Post hoc between-group t-test comparisons for RH minus LH difference scores yielded significant differences between dysphasic and normal groups for both evoked potential measures (Nl amplitude and latency), and between autistic and normal groups for both measures. However, autistic and dysphasic subjects did not significantly differ on either measure. These results are shown in Table 3. Thus, it was found that the autistic and dysphasic groups showed similar patterns of hemispheric asymmetry for the speech stimulus, in a direction opposite that found for the normal group. Individual subjects. For each group, individual subjects were categorized as showing right, left, or absent asymmetry according to the following conservative criterion: Right minus left hemisphere latency and amplitude scores were calculated for normal subjects. The scores that defined the third of normal subjects falling nearest to zero were used to define the limits of the range of scores considered to reflect absent asymmetry on TABLE 2 MEAN

RIGHT MINUS

LEFT HEMISPHERE

DIFFERENCES IN EVOKED POTENTIAL

MEASURES

Evoked potential measures Group Autistic Dysphasic Normal

Nl amplitude (WV) SD

x

3.11 7.88

X

2.36

SD

7.09 -3.08 3.64

X SD

Nl latency (msec) -7.41 9.30 - 7.28 17.77 8.32 16.67

HEMISPHERIC

35

ASYMMETRIES

8 7

Autistic

.--

Autistic

Normal Group

Dysphasic

Normal Group

Dysphasic

FIG. 3. Mean right minus left hemisphere difference scores for Nl amplitudes and latencies of the speech-related potentials for normal, autistic, and dysphasic groups (N = 10, each group).

these measures. Scores falling outside this range were considered to reflect either normal (positive scores) or reversed (negative scores) asymmetry. Table 4 shows the percentages of subjects showing right, left, or absent asymmetry for each group for each measure, Nl amplitude and Nl latency. Based on x2 analyses, it was found that both the autistic and dysphasic groups’ distributions of right, left, and absent asymmetries differed sigTABLE 3 BETWEEN-GROUP I-TEST COMPARISONSOF RIGHT MINUS LEFT HEMISPHEREDIFFERENCES

Evoked

Autistic vs. normal df=

18

Dysphasic vs. normal df=

18 18

t P

Dysphasic vs. autistic df=

I P

________

t P

potential

measures

Nl amplitude @V)

Nl latency (msec)

2.26 <.02

-2.61 <.02

2.16 c.05 .22 N.S.

~ 2.02 c.05 ~ .02 N.S.

DAWSON ET AL.

36

TABLE 4 PERCENTAGEOF SUBJECTSEXHIBITING RIGHT, LEFT, OR ABSENT HEMISPHERIC ASYMMETRY FOR SPEECH“

Pattern of asymmetry

Autistic (N = 10) Nl amplitude*** R handed/Non-R handed Nl latency***

Right

Left

50% (10) 40/10

(60)

70%

(20) R handed/Non-R handed Dysphasic (N = 10) NI amplitude** R handed/Non-R handed Nl latency*

50/20 80% (10) 50130 50%

(20) R handed/Non R-handed

30/20

10%

Absent

IO/O

40% (30) 30/10

0% (50) 0

30% (30) 30/o

20% lO/lO

0% (30) 0

20% (50) lO/lO

30% (30) 20/10

(60)

” Numbers in parentheses are percentages for normal group ,$ analyses. * p < .05. ** p < .Ol. *** p < .Oool.

nificantly from that of the normal group for each of the evoked potential measures. However, the autistic and dysphasic groups did not differ significantly from each other in their distributions. Age and Handedness

No significant relationships were found between chronological age and pattern of hemispheric asymmetry, as measured by RH minus LH Nl latency difference or by RH minus LH Nl amplitude difference for any of the groups. Handedness of clinical subjects was not systematically related to pattern of hemispheric asymmetry. The percentages of right-handed and nonright-handed subjects displaying each pattern of hemispheric asymmetry (right, left, or absent asymmetry) are shown in Table 4. Only one autistic subject showed greater left than right hemisphere activity, and this subject was right-handed. The remaining autistic subjects (two non-right-handed subjects and seven right-handed subjects) showed either absent or greater right hemisphere activity. Two dysphasic subjects exhibited greater left hemisphere activity; one of these subjects was right-handed and one was non-right-handed. The remaining dysphasic subjects (three non-right-handed

HEMISPHERIC

37

ASYMMETRIES

subjects and five right-handed subjects) showed either absent asymmetry or greater right hemisphere activity. Chi-square analyses were conducted to determine if right-handed clinical subjects exhibited a different distribution of pattern of hemispheric asymmetry from that of non-righthanded clinical subjects. None of these analyses was found to be statistically significant. Relationship between Pattern of Hemispheric Asymmetry and Language Abilities In order to assess the relationship between pattern of hemispheric asymmetry and level of language ability, correlations between right minus left hemisphere difference scores (for Nl amplitude and Nl latency) and each of the language measures were carried out. These correlations are shown in Table 5. For the autistic group significant correlations between pattern of cerebral asymmetry for speech and language abilities were found (11 of the 12 correlations were significant, and the remaining 1 approached significance). Greater right relative to left hemisphere activity was associated with poorer language abilities for virtually all of the language measures taken, whereas greater left hemisphere activity for speech was associated with superior language abilities. However, these relationships were not found for dysphasic subjects. Of the 12 possible correlations between cerebral asymmetry and language ability, only I reached statistical significance, and thus could have resulted from chance. Correlations between the only language measure available for the normal group, PPVT, and RH minus LH difference scores were not significant. On three of the six language measures, the dysphasic subjects had more restricted ranges than those of autistic subjects which could have TABLE 5 CORRELATIONS

BETWEEN LANGUAGE ABILITY AND PATTERN OF HEMISPHERIC AUTISTIC AND DYSPHASIC SUBJECTS (Ns = 10)

ASYMMETRY

FOR

Language measures

Autistic Nl amplitude

Vocabulary

Comprehension

Articulation

NSST-R

NSST-E

PPVT

- .73 .004 .53 .03

- .63 .02 .50 .04

- .70 ,005 .33 .09

- .48 .04 .56 .02

- .61 .02 .59 .02

- .60 .02 .72 ,005

N.S.

r

.09 N.S. .20

- .04

-.03

P

N.S.

N.S.

N.S.

r P

Nl latency

r P

Dysphasic Nl amplitude

r P

Nl latency

.06

.36

- .06

.08

N.S.

N.S.

N.S.

- .47 .04

- .I5 N.S.

- .22 N.S.

.04

.24

38

DAWSON ET AL.

accounted for the failure to find significant correlations between language and the evoked response measures for the dysphasic group. However, for the remaining three language measures, dysphasic subjects’ ranges were similar to or greater than those of the autistic subjects; for these language measures (NSST-expressive, comprehension, and articulation), autistic subjects still showed strong correlations between hemispheric asymmetry and language ability (T’S = - .61, - .63, and - .70, respectively), whereas dysphasic subjects did not (T’S = 44, .06, .36, respectively). Correlations between language abilities and evoked potential measures also were calculated separately for each hemisphere. For the autistic group, significant correlations were found between level of language ability and measures of right hemisphere activity (see Table 6). The direction of these correlations indicated that greater right hemisphere activation was associated with poorer language ability. Autistic subjects’ language ability was not correlated with left hemisphere measures. For dysphasic subjects, significant correlations between language abilities and evoked potential measures primarily were found for left hemisphere Nl latency; longer left hemisphere latency was associated with poorer language ability for four out of six of the language measures. No significant correlations were found for the normal group. TABLE 6 AND SEPARATE RIGHT AND LEFT HEMISPHEREEVOKED POTENTIAL MEASURES FOR AUTISTIC AND DYSPHASICGROUPS(Ns = 10)

CORRELATIONS

BETWEEN LANGUAGE

Language measures Vocabulary

Comprehension

Articulation

PVVT

NSST-R

NSST-E

- .57*

- .70*

- .14*

Autistic group RH Nl amplitude

r

- .81*

- .79*

- .59*

RH Nl latency

r

.56*

.57*

.59*

.20

.34

.29

.26

- .05

- .03

- .09

.42

-.09

.03

- .09

.04

.07

-.25

LH Nl amplitude LH Nl latency Dysphasic group RH Nl amplitude RH Nl latency LH Nl amplitude LH Nl latency * p < .05.

r

- .Ol

r

.28

.30

r

.12

.23

r

-.04

r

- .03

r

.31

-.12

-.15

- .45

- .40

- .35

- .43

- .67*

.12

.45

- .07

.14

.13

- .47

-.71*

- .58*

- .89*

- .60*

HEMISPHERIC

ASYMMETRIES

39

Thus, it appears that although dysphasic subjects were similar to autistic subjects in their direction of hemispheric asymmetry for the speech stimulus, these measures of hemispheric activity do not bear the same relationship with language abilities for the two language-impaired groups. DISCUSSION

In the present study, it was found that the majority of both autistic and dysphasic individuals showed greater right than left hemispheric activity in their speech-related cortical potentials, a pattern that was reversed from that characteristic of normal subjects. Interestingly, although dysphasic subjects were similar to autistic subjects in their patterns of hemispheric asymmetry, these patterns did not bear the same relationships with language abilities for the two clinical groups. Whereas strong relationships between direction of hemispheric asymmetry, as measured by right minus left hemisphere difference scores for Nl amplitude and latency, and language abilities were found for autistic subjects, no such relationships were found for dysphasic subjects. When the correlations between language abilities and separate right and left hemisphere speech-related cortical potentials were examined for autistic subjects, it was found that poorer language abilities were associated with increased right hemisphere Nl amplitude and shorter right hemisphere Nl latency (or alternatively, superior language abilities were associated with decreased right hemisphere activity). Language ability was not associated with left hemisphere evoked response measures for autistic subjects. Similar analyses for dysphasic subjects revealed that superior language ability generally was associated with increased left hemisphere activity, as reflected in shorter left hemisphere Nl latency, although the relationships between language ability and hemisphere activity were not as consistent across language measures as those found for autistic subjects. Taken together, these results suggest that the atypical patterns of hemispheric asymmetry found during speech processing in autistic and dysphasic individuals may reflect somewhat different neuropathological processes. The results are consistent with the notion that overactivation of the right hemisphere may be interfering with normal, left hemisphere language processing in autism, and that this abnormality in cortical activation may subside as language improves (Dawson, 1987, 1988; Dawson & Lewy, 1989; Kinsbourne, 1987). In contrast, the pattern of hemispheric activity found in dysphasic subjects may reflect inherent dysfunction of left hemisphere language areas. The significant correlation between left hemisphere Nl latency and language ability for the dysphasic subjects is particularly interesting in light of recent studies (Tallal et al., 1985a, 1985b) that suggest that dysphasic subjects have specific perceptual deficits in processing sequential information, for which the left hemisphere is specialized.

DAWSON ET AL.

40

There are several issues that remain to be addressed if we are to better understand the relationship between hemispheric functioning and language ability in handicapped children. First, since both the autistic and dysphasic subjects had lower-than-average overall IQ, it will be important to include mentally retarded children as a comparison group in future studies in order to assess the role of IQ in hemispheric functioning. Second, other measures, such as the cr-blocking technique, which allow for the assessment of ongoing brain activity during task-related conditions, would allow for more meaningful measures of language processing than can be provided by the simple, repetitive stimuli used in most evoked potential studies. Third, in order to determine whether atypical patterns of hemispheric activity are specific to certain functions, such as language, comparisons with a wide variety of task stimuli and behavioral measures are needed. Finally, in order to address the question of how patterns of hemispheric activation are related to language acquisition, longitudinal studies that employ concurrent measures of brain activity and behavioral development will be essential. REFERENCES Arnold, G., & Schwartz, S. 1983. Hemispheric lateralization of language in autistic and aphasic children. Journal of Autism and Developmental Disorders, 13, 129-139. Bartak, L., Rutter, M., & Cox, A. 1975. A comparative study of infantile autism and specific developmental language disorders: The children. British Journal of Psychiatry, 126, 127-145. Blackstock, E. G. 1978. Cerebral asymmetry and the development of early infantile autism. Journal

of Autism and Childhood

Schizophrenia,

8, 339-353.

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