The corpus callosum and cerebral speech lateralization

BRAIN

AND

LANGUAGE

38,

195-206

(1990)

The Corpus Callosum and Cerebral Speech Lateralization MARYSE LASSONDE,* *Groupe

de Recherche

M. P. BRYDEN,? AND PASCALE DEMERS*

en Neuropsychologie,

UniversitP Waterloo

de MontrPal,

and

f University of

In order to specify the callosal involvement in the establishment of cerebral lateralization, a dichotic listening task was administered to six subjects with congenital absence of the corpus callosum, two callosotomized patients, and two hemispherectomized patients. The acallosal subjects were also compared to six subjects matched for age, sex, and hand dominance as well as to six subjects also matched for IQ. Our findings indicate that language functions, as assessed by dichotic listening performance, are more strongly latralized in callosal agenesis subjects than in IQ-matched normal controls. Our results also reveal that hemispherectomized patients typically show a strong ear-advantage favoring the ear contralateral to their remaining hemisphere. Callosotomized patients, on the other hand, show a more variable pattern of results that seems to be related to the postsurgical time interval. Finally, our findings suggest that there might be a relationship between IQ and lateralization in subjects with borderline or mild deficiency. 0 1990 Academic Press, Inc.

It is generally accepted that the left hemisphere is specialized for linguistic functions while the right is involved in the global processing of spatial information. There is, however, less of a consensus as to the origins and development of cerebral specialization (e.g., Bryden, 1986a). It has been suggested that the independence or complementarity of cerebral function results from interhemispheric competition during development (e.g., Denenberg, 1981, 1983), and it has been argued that the corpus callosum is the crucial pathway by which each hemisphere, during This research was supported by grants from the Natural Sciences and Engineering Research Council of Canada to M. Lassonde and M. P. Bryden, and by a team grant from the Minis&e de I’Education du Quebec (FCAR) awarded to M. Lassonde and M. Ptito. The authors thank Dr. Brenda Milner for her helpful comments on the text, Janice Murray for her assistance in preparing the dichotic material and in carrying out the statistical analyses, Mima Vrbancic for her helpful editorial comments, and Katy-Jo Butler for preparation of the figures. Reprint requests should be addressed to Dr. M. Lassonde, Departement de Psychologie, Universite de Montreal, C.P. 6128, Succ, A, Montreal, Quebec H3C 357, Canada. 195 0093-934x/90

$3.00

Copyright 0 1990 by Academic Press. Inc. All rights of reproduction in any form reserved.

196

LASSONDE,

BRYDEN,

AND DEMERS

the course of normal development, exerts an inhibitory action on its counterpart (e.g., Doty, Negrao, & Yamaga, 1973; Moscovitch, 1977). If the corpus callosum plays an inhibitory role, it follows that in the congenital absence of the corpus callosum normally lateralized functions should develop redundantly in both hemispheres (Denenberg, 1981). Acallosal subjects should therefore differ considerably from callosotomized patients operated in adult life at an age when the normal left hemisphere superiority for speech is established (e.g., Milner, Taylor, & Sperry, 1968; Sparks & Geschwind, 1968). There is as yet no compelling evidence of bilateral representation of language in acallosals to support this hypothesis of an inhibitory role of the corpus callosum. One relatively simple way of assessing language lateralization is through dichotic listening (Kimura, 1967). However, the results of verbal dichotic listening tasks in agenesis cases are inconsistent (see Chiarello, 1980, for a review): a few studies find equal left and right ear performance, suggestive of bilateral language representation (e.g., Geffen, 1981), but most have reported clear lateral asymmetries (e.g., Jeeves, 1970), although at times it is the left ear rather than the right that shows superior performance, suggesting a right hemisphere language lateralization (Bryden & Zurif, 1970; Lassonde, Lortie, Ptito, & Geoffroy, 1981). In marked contrast, commissurotomized or callosotomized patients (e.g., Milner et al., 1968; Sparks & Geschwind, 1968) not only show a larger right-ear advantage than normal subjects but also display a left-ear extinction upon dichotic presentation of verbal material. They are thus comparable to right hemispherectomized patients (eg., Nebes & Nashold, 1980). Unfortunately, the data from these patients, and particularly those from agenesis cases, have been collected from a variety of laboratories, using different techniques and frequently only a very few subjects (Chiarello, 1980). The aim of the present study, therefore, was to specify the callosal involvement in the establishment of cerebral lateralization by using a common technique to study a reasonably large sample of agenesis cases, as well as normal, callosotomized, and hemispherectomized subjects. The specific dichotic listening task employed with all subjects in this study was selected because it has been shown to be sensitive to the lateralization of language processes in normal individuals (Bryden & Murray, 1985). METHOD Subjects. The dichotic listening task was administered to six subjects with congenital absence of the corpus cahosum as revealed by pneumoencephalography and/or computer axial tomography. These subjects were compared to groups of subjects with normal, intact brains. One group (Normals) consisted of six subjects with average or superior intelligence matched to the agenesis cases for age, sex, and hand dominance. A second comparison group (IQ-Matched) consisted of six subjects matched for IQ with the agenesis cases, as

CORPUS CALLOSUM

TABLE DESCRIFTIVE

Group

-

Agenesis

IQ-matched

Normals

Hemispherect Call. Sect.

Subject MJ HB AM NL LG MG AL SC NH JB SC DD AR SA MC LL MB DL LE DV SL MP

197

AND SPEECH LATERALIZATION

CHARACTERISTICS

1 OF SUBJECTS

Age at surgery

Age at testing

Sex

Hand

IQ

F F F F F M F F F F F M F M F F F F F M M M

R R L L R L R R L L R L L L R R L R L R L R

63 64 55 74 78 77 65 71 67 88 71 77

12 14 13 21

13 16 16 17 24 17 13 15 17 16 21 16 22 17 12 17 16 22 25 25 17 21

65 6.5 53 78

well as being of the same age, sex, and handedness. Two epileptic patients who had undergone a complete section of the corpus callosum in adult life and two adult patients who had undergone hemispherectomy early in life participated in the study. These are termed Callosotomy and Hemispherectomy groups, respectively. There were equal numbers of right- and left-handers in each of the five groups. Data on age, sex, handedness, IQ, and surgical history are provided in Table I. From this table, it can be seen that the experimental subjects and matched-IQ controls had an IQ that corresponded to the borderline or mild mental deficiency range (mean IQ, Acallosal subjects: 68.5: IQ-matched controls, 73; Hemispherectomized and Callosotomized patients, 65). More detailed case histories of the agenesis, hemispherectomy, and callosotomy cases are reported elsewhere (Lassonde, Sauerwein, Geoffroy, & D&uie, 1986; Lassonde, Sauerwein, McCabe, Laurencelle, & Geoffroy, 1988; Ptito, Lassonde, Lepore, & Ptito, 1987). Briefly, the acallosal group consisted of one male (MG) and five female subjects. The first four patients originate from the Saguenay/Lake St. Jean area, a region of Quebec that has been linked to a large number of hereditary cases of callosal agenesis with concomitant progressive myelopathy. In fact, CAT scan examinations were performed on these four subjects (MJ, HB, AM, NL) because they showed myelopathy; complete callosal agenesis without other brain lesion was demonstrated by these examinations. While the myelopathy may slow the motor responses of these patients, intellectual abilities are often unimpaired and when a low IQ is found, it is usually attributable to familial mental deficiency. The other two acallosal subjects (LG and MG) are siblings who show an IQ close to the low average level (78). Callosal agenesis was revealed in LG after she suffered a

198

LASSONDE,

BRYDEN,

AND DEMERS

mild concussion resulting from a fall. MG was examined because he showed chronic enuresis and motor incoordination at the age of 4 and because his sister had already been diagnosed as acallosal. At our request, LG underwent MRI that showed that the anterior commissure was intact. CAT scans or MRI did not reveal any visible lesion in any of these two subjects. Both siblings live on their own and are gainfully employed. The two hemispherectomized patients, one female (LE) and one male (DV), underwent operations for control of intractable epilepsy. They no longer have any seizures and are now attending sheltered workshops. The two callosotomized patients, both males, presented postoperatively some complications. SL, a left-handed patient, experienced dysphasia and leftsided weakness for which he received physiotherapy; both symptoms cleared in about 2 months, coincident with the resorption of a radiologically diagnosed left frontal hematoma. MP, the right-handed patient, showed postoperatively a large edema in the right hemisphere; clinically, he presented a left-sided hemiplegia, left homonymous hemianopsia, aphasia, and incontinence. After 4 months of intensive rehabilitation, he fully recovered. His cognitive abilities, however, have somewhat decreased especially on nonverbal tasks. MRI examination revealed a partial section of the corpus callosum with the splenium being intact. The callosotomy has considerably reduced the frequency and gravity of seizures in these two patients and MP is now gainfully employed. All subjects underwent initial audiometric testing to ensure they had normal hearing in each ear, with no major differences between the ears. Handedness was assessed by the Halstead-Reitan lateral dominance test. Intellectual capacities were evaluated by means of the French versions of the Wechsler scales (WISC-R or WAIS-R). Stimuli. The dichotic material (Bryden & Murray, 1985) consisted of natural speech tokens of the consonant-vowel stimuli /bo/, /do/, /PO/, and /to/, produced by a female speaker and initially recorded on a Revox A 700 recorder. Stimulus intensity was equalized by monitoring peak VU levels during recording. The stimuli were subsequently digitized on a PDP 11 and the edited versions reconverted to analog signals on magnetic tape, using all nonidentical combinations of the four stimuli two at a time. The syllables in each pair were aligned for simultaneous onset of the initial consonants and recorded on separate audio channels of the tape. The initial four stimuli permitted six different pairings, each of which could be arranged in two ways, depending on which item was assigned to which channel. These 12 pairings were duplicated 40 times each, to provide a total of 480 trials. These 480 trials were distributed in 16 blocks of 30 trials each, with an interval of 3 set between trials and 20 set between blocks. The master tape, prepared at the University of Waterloo, was copied onto a Maxell 35180B tape using a Harman Kardon HK 400XM duplicator and a Teat 3340 recorder. Stimuli were presented on a four-channel Teat A-3340s recorder through Sennheiser 300 earphones which were calibrated prior to each experimental session. Subjects were initially asked to identify each of the CV syllables when they were presented monaurally to the left and right ear, and all were capable of doing so. They were then told that they would hear two different syllables simultaneously, one in each ear, and were asked to nod each time the target syllable /PO/ was presented. Sixty practice trials followed these instructions. The experimentation proper consisted of four blocks of 120 trials each, the headphones being reversed after 240 trials. Within each block of 120 trials, the target appeared in one ear on 30 trials, the other ear on 30 trials, and not at all on 60 trials.

RESULTS

We initially determined the accuracy with which the target was detected as a function of the particular syllable pair that was presented; we took, for example, all trials in which the pair /po-do/ had been

CORPUS CALLOSUM

TABLE DICHOTIC

Group Agenesis

IQ-matched

Normals

Hemispherect. Call. Sect.

199

AND SPEECH LATERALIZATION

LISTENING

2

PERFORMANCE

OF SUBJECTS

Subject

Left ear

Right ear

A

MJ HB AM NL LG MG AL SC NH JB SG DD AR SA MC LL MB DL LE DV SL MP

93 32 90 59 88 96 44 31 61 102 41 38 64 59 67 64 36 52 120 40 66 35

69 97 52 98 102 115 53 40 82 88 54 69 65 109 116 105 95 118 40 118 103 116

-0.935** 2.451** - 1.367** 1.526** 0.723** 1.749** 0.313% 0.357* 0.736** -0.723** 0.455* 1.071** 0.033 2.326** 3.133** 1.812** 2.182** 4.346** - 5.489** 4.771** 1.601** 4.250**

*p < .lO; **p < .05.

presented with the target in the left ear and determined on how many trials the subjects had detected the target. This procedure was repeated for all 24 stimulus combinations. The number of correct responses to presented items was then summed and used to derive Bryden and Sprott’s (1981) laterality index (A), a log-odds ratio comparing the relative accuracy for the two ears. Positive values of h indicate a right-ear advantage (REA) and negative scores a left-ear advantage (LEA). The values of A and their significance levels are shown in Table 2. An analysis of the A values indicated significant group differences (F(2, 15) = 4.06, p = .04), with the normal group having the largest positive values. An analysis of the number of items correct on each ear revealed a highly significant right-ear advantage (F(1, 15) = 12.57, p = .003), a difference between the groups in overall accuracy (F(2, 15) = 3.72, p = .05), and a trend toward an Ear x Group interaction (F(2, 15) = 2.82, p = .09). As can be seen from Table 2, the majority of the control subjects showed REAs, with only one, a left-hander of the IQ-matched group, yielding a LEA. The two hemispherectomized subjects, as expected, gave large values of h favoring the ear contralateral to the remaining

200

LASSONDE,

BRYDEN,

AND

DEMERS

hemisphere. One callosotomized patient, MP, had a A value of the same order of magnitude as the hemispherectomized patients despite having had an incomplete section of the corpus callosum leaving the splenium intact. The large A value may result, however, not from callosal section per se, but rather from the fact that he suffered postoperative right hemisphere complications, resulting in a decrease in performance IQ and a transient left-sided hemiplegia. In fact, the other callosotomized patient, SL, who had 5 years to recover from a postoperative left frontal hematoma, showed a A value comparable to those observed in the normal and acallosal subjects. Among the agenesis cases, four showed REAs and two gave LEAS. With the exception of one of the normal subjects and three of the IQ-matched group, all A values were significantly different from 0 at the .05 level of confidence or better, and the three IQmatched subjects all yielded significance levels better than .IO. Because our primary concern was with the degree of lateralization, and because there were so many left-handers in the study, who are known to be different from right-handers on dichotic tests (Searleman, 1980; Piazza, 1980; Bryden, 1986b), subsequent analyses were done with the absolute, rather than the algebraic, values of A. The average value of absolute A was 2.31 for the normal subjects, 0.61 for the IQ-matched subjects, and 1.46 for the acallosal subjects. These scores are significantly different from one another (F(2, 15) = 5.14, p = .02). Rather paradoxically, the scores of the acallosal group fell between those for the two intact groups, being lower than that of the normal subjects but higher than that of the IQ-matched controls. While this suggests that acallosal subjects have at least as great a lateralization as do people with an intact callosum, it does make the data somewhat more difficult to interpet. Within the IQ-matched group, however, there was a reasonably high correlation between IQ and absolute A value (r = SO), while this was not the case with the acallosals (r = - .16). In Fig. 1, the regression of A on IQ is shown for the IQ-matched group, and the data for the individual agenesis and IQ-matched cases are plotted. In all six cases, the A values for the agenesis subjects are greater than would be predicted from their IQ (p < .05 by binomial test). Thus, the acallosal subjects have larger A values than would be expected in subjects with an intact callosum and the same IQ. Figure 1 also indicates that hemispheric specialization in normal subjects, as assessed by absolute A value, increases with intelligence. While this may be simply a result of the fact that in our sample absolute A values tend to increase with overall accuracy, it may indicate that A values are generally reduced in subjects with low IQs. This is an issue deserving of further research. Further information about the way in which the dichotic task is per-

CORPUS CALLOSUM

2.5 -

201

AND SPEECH LATERALIZATION

0

q Aaenesis . lQMalched 2.0 2 4 If If 3

0 1.5-

0 0

l.O-

n

8

0

2

0.0

*

50

1

60

a

I

I

1

70

80

90

100

IQ FIG. I. Relation between absolute h and IQ for the IQ-matched (filled squares) and acallosal (open squares) subjects. The regression line is computed for the subjects in the IQ-matched group only. Note that all the agenesis cases fall above this line.

formed can be obtained from a detailed examination of performance on the various stimulus pairings. According to Kimura’s (1967) model of dichotic listening, for example, information transmitted by contralateral pathways will block that coming along ipsilateral pathways. By this view, left-ear information reaches the left hemisphere only by first reaching the right hemisphere via the contralateral pathway, and then passing through the corpus callosum to the opposite hemisphere. Therefore, interactions between left- and right-ear stimuli can take place only if the callosum is intact, and should not be expected to occur in the acallosal, callosotomized, or hemispherectomized subjects. One source of information about such interaction lies in an analysis of responses to various stimulus pairs. For instance, with /PO/ as the target, /bo/ shares a place of articulation feature, /to/ a voicing feature, and /do/ neither one. If there is any summation of feature information between ears, one should expect the target /PO/ to be better detected when it is presented in conjunction with /bo/ or /to/ than when it is presented with /do/. In fact, for all groups of subjects, performance is clearly better when one of the target features is redundantly present on the other ear (see Table 3). Accuracy was usually best when the target

202

LASSONDE,

BRYDEN, TABLE

DISTRIBUTION

OF HITS

AND

ERRORS

AND DEMERS 3

AS A FUNCTION

Correct detections: Target features at other ear

OF COMPETING

ITEM

False alarms: Target features present

Group

Place

Voice

None

Both

Place

Voice

Agenesis IQ-matched Normal Hemispherect. Call. Sect.

69.83 73.33 79.00 79.50 66.50

56.50 28.17 42.00 40.50 42.00

38.83 16.50 34.60 39.00 45.50

14.17 7.00 25.80 25.00 23.00

6.67 6.17 5.60 16.00 20.00

1.50 3.50 3.60 0.00 2.50

was paired with /bo/ and worst when it was paired with /do/. Analysis of the hit data indicated that accuracy varied as a function of competing item (F(2, 30) = 4.03, p = .03) and that there was a trend toward an interaction with group (F(2, 30) = 2.89, p = .07). The agenesis and normal groups showed very similar patterns, while the IQ-matched group was relatively poor when either /do/ or /to/ was the competing item. The general pattern of results suggests that information from both ears is reaching a common locus, whether the callosum is present or not, a finding that also applies to hemispherectomized and callosotomized patients. Evidence for interactions between the two ears can also be seen in the false alarms-the “yes” responses made when the target /PO/ is not actually present (Table 3). If the stimulus pair is /bo-to/, both place and voicing information pertinent to /PO/ are present, although not in the proper combination. In contrast, if the stimulus pair is /bo-do/, the target voicing feature is missing and if it is /to-do/ the target place feature is missing. Thus, false alarms should be most common when the stimulus is /bo-to/. Again, this is the case for all groups of subjects including the hemispherectomzed and callostomized patients and regardless of whether the callosum is present or not. Once again, this pattern of results suggests that there must be some common locus at which both left and right ear information is available within one hemisphere and provides evidence against an ipsilateral suppression mechanism. DISCUSSION

Our major finding is that language function, as assessed by dichotic listening, appears to be more strongly lateralized in subjects with callosal agenesis than in normal controls matched for IQ. We also concur with the previous finding (e.g., Nebes & Nashold, 1980) that hemispherectomized patients typically show a strong advantage in favor of the ear

CORPUS

CALLOSUM

AND

SPEECH

LATERALIZATION

203

contralateral to the remaining hemisphere. Callosotomized patients, on the other hand, show more variable results that appear to be influenced by the lapse of time since undergoing surgery; a strong right-ear advantage, in magnitude comparable to that found in the right hemispherectomized patient, was observed only in one individual who had not fully recovered from postoperative edema of the right hemisphere. Finally, our results indicate that there may be a relationship between degree of lateralization and IQ in subjects with mild intellectual impairment. Our results do not, however, agree well with Kimura’s (1967) model of ipsilateral suppression in dichotic listening. Our data indicate that dichotic listening is affected not only by the presence of the designated target item, but also by the characteristics of the stimulus arriving at the other ear; there is thus both feature summation in correct detections and evidence for blending errors in false alarms. This state of affairs can only be explained by assuming that information from both left and right ears is available at the same cortical locus. Furthermore, since these effects were observed in all groups of subjects including callosotomized and acallosal patients, they do not depend on the integrity of the corpus callosum. In that sense, our results confirm Geffen and Quinn’s (1984) observation that there is little evidence from either brain-lesioned patients or normal subjects that dichotic stimulation produces suppression of information on the ipsilateral pathways. The assumption that information from both ipsilateral and contralateral pathways is simultaneously available and is combined in order to discriminate speech sounds is best exemplified by the results obtained with hemispherectomized patients. These individuals have only one hemisphere, yet typically show the same pattern of hits and false alarms as normal subjects. Hemispherectomized patients also show a stronger degree of lateralization than normal subjects, a finding which is easily explained by the fact that contralateral auditory pathways are stronger than ipsilateral ones and thus produce better dichotic performance on the ear contralateral to the remaining hemisphere. This explanation almost certainly accounts for the results obtained in the callosotomized patient MP; his postoperative recovery was complicated by a massive edema of the right hemisphere, and he presented clinical signs of transient left-sided hemiplegia, left homonymous hemianopsia, and diminished nonverbal cognitive abilities. Thus MP had, at the time of testing, a right hemisphere that functioned somewhat abnormally, this accounting for the elevation of his laterality index to a value comparable to that found in the right hemispherectomized patient DV. We suggest that the artifactually enhanced right-ear effect found in MP as well as in other “split-brain” patients (Milner et al., 1968; Sparks & Geschwind, 1968) is simply due to the abnormal functioning of the hemisphere, usually the right, that has been manipulated during surgery

204

LASSONDE,

BRYDEN,

AND

DEMERS

to give access to the corpus callosum. The commissurotimized patients tested by Milner et al. (1968) did not, in fact, show such a strong rightear effect when later retested (Milner, personal communication), and our other callosotomized patient, SL, whom we examined for the first time 5 years after surgery, showed a right-ear advantage comparable to that found in our normal or acallosal subjects. While it is thus possible that the delay since operation and postoperative complications are important variables influencing the magnitude of the laterality index, it can also be argued that callosotomized patients may show anomalies of brain function related to their epilepsy (Meyer, Beck, & Shepherd, 1953, and that an increased degree of lateralization may reflect no more than asymmetrically distributed pathological processes. None of these factors, however, can apply to subjects with callosal agenesis. These individuals had no history of epilepsy, had undergone no neurosurgical procedures, yet were more strongly lateralized than normal subjects matched for IQ. While it could be argued that the brains of the acallosal patients might be abnormal, CAT scan examination showed no detectable cerebral lesions in any of these patients. Most often, callosal agenesis was revealed accidentally through radiological examinations performed for various reasons (myelopathy, mild concussion, or motor incoordination). In fact, some of these acallosal cases are quite asymptomatic: MG and LG are fully autonomous and show no sign of cerebral pathology. In that sense, they resemble their 29-year-old sister in whom callosal agenesis was recently revealed after we had requested all members of their family to be examined through CAT scans for a genetic study. This finding, and the fact that two of our acallosal subjects are otherwise normal (MG and LG), rule out extracallosal pathology as a possible cause of increased lateralization in our sample of callosal agenesis subjects. Our data thus indicate that congenital absence of the corpus callosum does not lead to a bilateral representation of linguistic functions, and the counterintuitive finding of greater lateralization in acallosal subjects calls for further explanation. If language functions are initially lateralized in one hemisphere (cf. Bryden, 1975; Geschwind & Galaburda, 1986), it may be the case that constant interhemispheric communication, during the development of normal individuals with intact corpora callosa, leads to a diminution of the initial asymmetry. This can obviously not take place if the corpus callosum itself does not develop, and hence the initial asymmetry remains high in individuals with callosal agenesis. The present data make it quite clear that lateralization is not reduced in subjects with callosal agenesis; our argument, that it is actually enhanced when compared to appropriate normal controls, is based on our finding of an inverse relation between lateralization and IQ in subjects

CORPUS CALLOSUM

AND SPEECH LATERALIZATION

205

with borderline or mild intellectual deficiency (IQ scores in the range 55-80). However, there does not appear to be any evidence for a relation between IQ and lateralization in subjects with normal or above-normal intellectual capacities, and further work on the relation between lateralization and mild deficiency is obviously called for. REFERENCES Bryden, M. P. 1975. Speech lateralization in families: A preliminary study using dichotic listening. Brain and Language, 2, 201-211. Bryden, M. P. 1986a. The nature of complementary specialization. In F. Lepore, M. Ptito, and H. Jasper (Eds.), Two hemispheres: One brain. New York: R. Liss. Bryden, M. P. 1986b. Dichotic listening performance, cognitive ability, and cerebral organization. Canadian Journal of Psychology, 40, 445-456. Bryden, M. P., & Murray, J. E. 1985. Toward a model of dichotic listening performance. Brain

and

Cognition,

4, 241-257.

Bryden, M. P., & Sprott, D. A. 1981. Statistical determination

of degree of laterality.

19, 571-581.

Neuropsychoiogia,

Bryden, M. P., & Zurif, E. B. 1970. Dichotic listening performance in a case of agenesis of the corpus callosum. Neuropsychologia, 8, 371-377. Chiarello, C. 1980. A house divided? Cognitive functioning with callosal agenesis. Brain and

Language,

11, 128-158.

Denenberg, V. H. 1981. Hemispheric laterality in animals and the effects of early experience. The Behavioral and Brain Sciences, 4, l-49. Denenberg, V. H. 1983. Micro and macro theories of the brain, The Behaviorul and Bruin Sciences, 6, 174-178. Doty, R. W., Negrao, N., & Yamaga, K. 1973. The unilateral engram. Acra Neurohiologiue Experimentale

(Warsaw),

Geffen, G. 1981. Phonological ropsychologia,

18,

33,

711-728.

fusion after partial section of the corpus callosum. Neu-

613-620.

Geffen, G., & Quinn, K. 1984. Hemispheric specialization and ear advantages in processing speech. Psychological Bulletin, %, 273-291. Geschwind, N., & Galaburda, A. S. 1986. Cerebral lateralization. Cambridge, MA: MIT Press. Jeeves, M. A. 1979. Some limits to interhemispheric integration in cases of callosal agenesis and partial commissurotomy. In I. Steele Russell, M. van Hof, & G. Berlucchi (Eds.), Structure and functions of the cerebra/ commissures. New York: Macmillan. Kimura, D. 1967. Functional asymmetry of the brain in dichotic listening. Cortex, 3, 163178. Lassonde, M., Lortie, J., Ptito, M., & Geoffroy, G. 1981. Hemispheric asymmetry in callosal agenesis as revealed by dichotic listening performance. Neuropsychologia, 19, 455-458. Lassonde, M., Sauerwein, H., Geoffroy, G., & Decarie, M. 1986. Effects of early and late transection of the corpus callosum in children: A study of tactile and tactuomotor transfer and integration. Bruin, 109, 953-967. Lassonde, M., Sauerwein, H., McCabe, N., Laurencehe, L., & Geoffroy, G. 1988. Extent and limits of cerebral adjustment to early section or congenital absence of the corpus callosum. Behnvioural Brain Research, 30, 165-181. Meyer, A., Beck, E., & Shepherd, M. 1955. Unusually severe lesions in the brain following status epilepticus. Journul of Neurology, Neurosurgery and Psychiatry, 18, 24-33. Mimer, B., Taylor, L., & Sperry, R. W. 1968. Lateralized suppression of dichotically presented digits after commisural section in man. Science, 161, 184-186.

206

LASSONDE,

BRYDEN,

AND DEMERS

Moscovitch, M. 1977. The development of lateralization of language functions and its relation to cognitive and linguistic development: A review and some theoretical considerations. In S. J. Segalowitz & F. A, Gruber (Eds.), Language development and neurological theory. New York: Academic Press. Nebes, R. D., & Nashold, B. S. 1980. A comparison of dichotic and visuo-acoustic competition in hemispherectomized patients. Brain and Language, 9, 246-254. Piazza, D. M. 1980, The influence of sex and handedness in the hemispheric specialization of verbal and nonverbal tasks. Neuropsychologia, 18, 163-176. Ptito, A., Lassonde, M., Lepore, F., & Ptito, M. 1987. Visual discrimination in hemispherectomized patients. Neuropsychoiogia, 25, 869-880. Searleman, A. 1980. Subject variables and cerebral organization for language, Cortex, 16, 239-254. Sparks, R., & Geschwind, N. 1968. Dichotic listening in man after section of neocortical commissures. Cortex, 4, 3-16.