Perceptual asymmetry on the dichotic fused words test and cerebral speech lateralization determined by the carotid sodium amytal test

002X-3932.89 5300+0.00 c 1989PergamonPress plc

PERCEPTUAL ASYMMETRY ON THE DICHOTIC FUSED WORDS TEST AND CEREBRAL SPEECH LATERALIZATION DETERMINED BY THE CAROTID SODIUM AMYTAL TEST ROBERT Montreal

Neurological

Institute

(Recriued

J. ZATORRE*

and Hospital,

I1 February

McGill

1988: accepfed

University,

Quebec,

Canada.

12 Mur 1988)

Abstract-The rhymed fused dichotic words test was administered to 61 epileptic patients whose lesions were atrophic and predominantly unilateral. Subjects were categorized according to the side of speech representation, as determined by intracarotid sodium Amytal injection (left-hemisphere representation, 35 subjects; right-hemisphere representation, 4 subjects; bilateral representation, 22 subjects). Results indicated that 33 of the patients with left-hemisphere speech representation obtained right-ear advantages, and all four of the patients with right-hemisphere speech representation obtained left-ear advantages. The distribution of scores for patients with bilateral speech straddled the zero ear-advantage point, but overlapped both of the other two groups to some extent. Neither handedness, familial handedness, sex, nor side of epileptogenic focus had any significant effect on the observed asymmetries. It was also found that stimulus dominance effects have an important influence on the results, and must be taken into account in the interpretation ofdichotic listening asymmetries. It is concluded that this test yields valid estimates of speech lateralization: it is also apparent that the magnitude of ear advantage may be an important variable insofar as the more extreme asymmetries appear to be exclusively associated with speech dominance of the contralateral hemisphere, whereas small-magnitude asymmetries are often associated with bilateral speech representation.

IN 196 I KIMURA [ 133 published the first report applying BROADBENT’S [2] dichotic listening technique to investigate hemispheric lateralization of language. The validity of the technique rests on Kimura’s demonstration that a right-ear advantage for verbal materials was obtained from among subjects whose speech was represented in the left hemisphere, and conversely, that a left-ear superiority was found among those with right-hemisphere speech representation. In that study patients were given intracarotid sodium Amytal injection prior to neurosurgical intervention for the relief of epilepsy [29]; side of speech representation was then inferred according to the pattern of disruption observed following injection into each hemisphere [3, 17, IS]. In the intervening quarter century, a large body of experimental work has been carried out using the dichotic listening paradigm (cf. [4, 261 for reviews), with results that have often tended to confirm KIMURA’S [13] initial observations. Nevertheless, there is now considerable controversy regarding what variables may influence ear asymmetries, and whether dichotic listening data may or may not be used as a true index of hemispheric speech lateralization, particularly for individual subjects. The most common difficulty in this regard

*Address H3A 2B4.

for correspondence:

Montreal

Neurological

Institute,

1207

3801 University

St., Montreal,

Quebec,

Canada

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ROBERT J. ZATORR~

has been the relatively large number of normal right-handed subjects who do not show the right-ear advantage predicted from clinical estimates of the incidence of left-hemisphere speech representation. The many conflicting findings, poor reliability, and high variability that has often been reported for dichotic listening have led some investigators (e.g. [21, 281) to question its usefulness altogether. Kimura’s pioneering results were crucial in establishing the paradigm. However, many questions still remain unanswered. First, the initial study [ 131 presented group means of ear scores, which does not permit an evaluation of the validity of a dichotic score for an individual subject. The way the data were presented also precluded an analysis of how the magnitude of an ear asymmetry may be related to underlying speech dominance, an issue which has now become more urgent. A related problem is that KIMURA [13] retained in her sample subjects whose atypical speech dominance was related to extensive damage of the left hemisphere. She did show that left-hemisphere damage per se was not sufficient to account for the left-ear advantage obtained in right-hemisphere speech dominant subjects, but the data did not allow an unambiguous evaluation of the effect of this factor on the degree of asymmetry. This issue may be important because it is well-established that surgical or vascular lesions affecting the temporal lobe can lead to a suppression of the contralateral ear in dichotic listening [l, 12, 221. Of the 107 subjects Kimura classified as left-hemisphere speech dominant, only a certain unspecified number actually received the sodium Amytal test. Furthermore, in this study no data were reported for subjects with bilateral speech representation, an issue which is also significant. Finally, Broadbent’s original dichotic test involved free recall of pairs of digits; this technique leaves many sources of variation uncontrolled, including potentially important mnemonic and attentional factors. Given these questions, it is remarkable that very few studies have examined the validity of dichotic listening more closely. A recent report by STRAUSS et ul. [27] has attempted to investigate further the relation between dichotic perceptual asymmetries and speech representation as determined by sodium Amytal injection. They administered a free-recall dichotic test similar to Kimura’s, but using words rather than digits. Their results showed that 86% of subjects with left-hemisphere speech representation obtained right-ear advantages, but no more than halfof their sample with right-hemisphere speech obtained the expected left-ear advantage. Most subjects with bilateral speech representation also obtained right-ear advantages, but their scores did not appear to differ significantly from the group whose speech was entirely represented in the left hemisphere. As in KIMURA’S [13] study, Strauss et ul. included in their sample subjects whose lesion may have caused or otherwise influenced the ear asymmetry obtained, thus precluding a direct analysis of individual asymmetries as related to side of speech dominance, rather than to possible lesion effects. Also, the dichotic test used could be sensitive to attentional and mnemonic factors that are difficult to control. One other study, by GEFFEN and CAUDREY [Xl, has reported somewhat more promising findings linking performance on a dichotic monitoring task to side of speech dominance as measured by dysphasia following unilateral ECT treatments in 30 subjects, or by sodium Amytal study in 6 subjects. The findings were consistent with the hypothesis that the ear contralateral to the speech-dominant hemisphere has an advantage in detection of verbal materials presented dichotically. Using a discriminant function analysis, they were able to classify correctly all 28 of their subjects with left-hemisphere speech, but two of their seven subjects with right-hemisphere speech were misclassified by their procedure. The purpose of their study was to establish the validity of the technique in terms of correct classification of

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patients, therefore, these authors did not examine individual differences in the obtained perceptual asymmetries. They also did not describe the nature of the lesions, if any, in their sample. The present investigation was undertaken to examine more directly the relation between individual asymmetries in dichotic listening and side of hemispheric speech dominance as determined by sodium Amytal testing. Unlike previous studies, a principal aim was to examine the magnitude of ear advantage on an individual basis according to the side of speech representation. Furthermore, all subjects known to have a lesion which might have interacted with the ear asymmetry were excluded from study. The fused rhymed dichotic words test used in the present study was developed by WEXLER and HALWES [30, 311 as a modification of a test originally reported by JOHNSON et al. [l 11. Briefly, it consists of pairs of monosyllabic rhymed CVC words differing only on the initial consonant. The stimuli are constructed and aligned in such a way that partial interaural fusion occurs: subjects generally experience and report only one stimulus per trial. This procedure has the distinct methodological advantage of minimizing attentional factors, since the percept is unitary and localized to the midline. A further important improvement in this test as compared to free-recall dichotic presentation is that stimulus dominance effects [20, 241 may be explicitly calculated, and their influence on ear asymmetries assessed and eliminated. Wexler and Halwes have shown that a large proportion of normal right-handed subjects obtain right-ear advantages on this test, and that the test-retest reliability is suitably high (r=0.85). However, until now this test has not been validated with an independent method ofdetermining speech dominance. The present report represents an attempt at such a validation.

METHODS Subjects Testing was carried out on 67 patients at the Montreal Neurological Hospital who had undergone intracarotid sodium Amytal injection of each cerebral hemisphere as part of a preoperative investigation for seizure surgery. All patients suffered from a seizure disorder related to a static, atrophic cerebral lesion, usually lateralized predominantly to one side. All but seven patients had a primary epileptogenic focus in one temporal lobe; the others had either frontal (five cases) or parietal (two cases) foci, either in isolation or in addition to a temporal focus. Patients with evidence of objective or structural lesions, such as tumors, cysts, calcifications, hamartomas, etc. were excluded from study. Patients with arteriovenous malformations were included, but those with other vascular lesions were not. No patient had undergone previous neurosurgical intervention. Most patients were native speakers of English, but a few spoke French as a mother tongue. Among this latter group, the test was administered if a sufficient number ofcorrect responses were obtained on the binaural screening task (see below). Subjects who were known or suspected of having a hearing loss were given a pure-tone audiometric examination, and were excluded if an asymmetry greater than 10 dB existed between the ears at any frequency below 6 kHz. Speech lateralization was determined following the Amytal study by the usual criteria in use at the Montreal Neurological Hospital 13, 171. Sodium Amytal injection (15Gl75 mg of sodium Amytal dissolved in 3 cc) was carried out following catheterization of the internal carotid artery and angiography. Two injections were performed. one on each side, on two separate days. Language testing in the sodium Amytal procedure includes naming of objects, serial speech tasks (counting, days ofthe week), and reading, as well as receptive language tasks (pointing to named objects, simplified Token test). Baseline testing is carried out prior to injection in order to compare performance to that under the influence of the drug. Subjects whose language IS clearly represented in one cerebral hemisphere typically demonstrate speech arrest following injection into the dominant hemisphere, followed by varying degrees of both receptive and productive dysphasia; injection into the non-language dominant hemisphere produces no speech disturbance. Subjects whose speech representation is considered to be bilateral are more heterogeneous; they include those with substantial disturbance in speech production and comprehension from both injections, as well as those with a major disturbance from one side but only mild interference from the other side. Demographic details regarding the patient groups, subdivided according to stde of speech representation are shown in Table 1. Six subjects who produced too few storable responses for meaningful analysis (see below) were excluded from the table and from further analysis, leaving a total of 61 patients. Handedness was determined by a

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ROBERT J. ZA~ORRE

modified version of the scale developed by CROVITZ and ZEN~R [6]. Familial sinistrality was defined as having at least one left-hander among the subject’s parents or siblings, or in the case of families with more than ten members, having at least two left-handers among them. In addition to the patient groups, 30 neurologically normal right-handed subjects (13 male, 17 female) were tested. These subjects were included only to ensure that the fused words test in fact yielded data similar to those reported by WEXLER and HALWES 1301, and to ascertain if the range of responses obtained by patients was comparable to that obtained by control subjects. The control subjects, most of whom were university students or psychologists. wcrc not matched in any systematic fashion, therefore, to the patients. Procedure

The rhymed fused words test has been described in detail elsewhere 1301. It consists of 15 pairs of words which differ only on the first consonant (e.g. coat/goat, pigidig, etc.). Before starting the dichotic test, a binaural screening test was administered in which each of the 30 words was presented once binaurally. The subjects’ task was to choose the one correct answer out of four possibilities (the correct choice, the other member of the dichotic pair, and tw’o foils differing only in the first consonant). The choices were printed on a page, and subjects selected one by circling it. For the dichotic test, each pair was presented twice per set of trials (once in each of the two possible channel assignments), and four randomizations ofthese 30 trials were presented. for a total of 120 trials (8 repetitions ofeach pair). Subjects responded in the same way as for the binaural test. Stimuli were recorded on audio tape and played back on a Uher tape recorder through matched Tclephonics TDH-39P earphones equipped with supra-aural rubber cushions. Earphones were calibrated to a continuous vowel sound at 75 dB(A) using a GenRad 1565-B sound-pressure meter and a type 9A ~-CCacoustic coupler. The effect of any remaining channel asymmetries was minimized by switching the earphones between the ears. once after the first 30 trials, and again after 90 trials. Subjects were not informed that a dichotic test was to be given. They were simply told to circle the word they heard on each trial. Before beginning the dichotic test they were instructed that the words might not always seem clear, but that they were still to make the best choice possible, even if unsure. Testing was carried out individually. in the context of an extensive neuropsychological battery.

Scoring was carried out as follows: first, the number of non-stimulus-dominated responses was calculated for each ear. This was accomplished by an approach originally suggested by HALWES [IO]. For each pair. the two words appear four times in each ear. Stimulus-dominated responses (as opposed to ear-dominated responses) are considered to occur when a subject responds with the same word regardless of ear of presentation. For example. if “coat” is presented to the left ear and “goat” to the right, the subject may respond that he heard “coat”: if”goat” is then presented to the left ear and “coat” to the right ear, and the subject still reports “coat,” then a stimulusdominated response has occurred. The aim of the scoring method was to eliminate such responses. Thus, out of the set of eight responses per pair, all responses that were attributable to stimulus dominance were subtracted from the total. Errors (i.e. when a subject chose a response that did not correspond to either member of the dichotic pair) were noted, but otherwise ignored in the scoring procedure (an approach justified by the negligibly small number oferrors typically obtained). The total number of non-stimulus-dominated responses was then calculated for each ear across all 15 stimulus pairs. These values were used in all subsequent analyses. Subjects were excluded from study if fewjer than ten such responses from both ears were obtained, since the ear advantage was deemed not reliable under such circumstances. Six patients who met the other criteria for study had to be excluded on this basis. These corrected scores were then treated in two ways for analysis. First, a simple difference score between the right and left ears was calculated. Second, a laterality index (lambda) first suggested by BRYDENand SPK~TT [S] was used. This index is the natural logarithm of the ratio between the right and the left ear scores, and possesses several statistical properties which make it a useful metric to assess laterality. In the case of a score of zero for one ear, the lambda index cannot be calculated; however, this only occurred in four cases among the patients and once among the controls, so that one was arbitrarily substituted for the zero in these cases to permit calculation of the index. Results were also transformed according to the r* index, (R - L),/(R + L) 120. 311. hut the results were essentially identical to those using the lambda index, and so will not be reported.

RESULTS The results are shown in Figs 1 and 2, which plot each subject’s ear advantage according to the side ofspeech representation as demonstrated by the sodium Amytal test. Figure 1 shows the distribution of the difference score, and Fig. 2 shows the distribution of scores using the lambda index. The most immediately notable finding is that the distributions are clearly displaced from one another: the scores for the left-hemisphere speech group clearly tend to

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TEST

TABLE I. Characteristics of patient population studied. Number, mean age, mean full-scale Wechsler IQ score, and mean number of years of education, together with distributions of handedness, familial handedness, sex, and side of maximal electrographic abnormality.

Left N Age Full-scale IQ Education Handedness Right Left or mixed Familial sinistrality Present Absent Unknown Sex Male Female Epileptogenic focus Right Left

Side of speech representation Bilateral

Right

35 28.7 9X.0 13.1

22 26.0 97.5 I I.0

4 24.3 90.3 10.5

24 11

12 10

2 2

12 18 5

11 9 2

0 4 0

20 15

13 9

2 2

17 18

12 10

1 3

Side of Speech sentatKm Left mara, Right INC) -40

-30

-20

-10

D=i-L

m!

NC

.

FIG. I. Distribution of difference scores (R-L) on fused words test according to side of speech representation as determined by intracarotid sodium Amytal testing. Each symbol represents one subject.

right-ear advantages, the right-hemisphere speech group showed exclusively left-ear advantages, and the bilateral speech group’s scores fall in between. It is also notable that there is considerable overlap, particularly between the left-hemisphere and bilateral speech groups. This overlap, however, seems to be largely limited to a relatively narrow range approx between 0 and 10 on the difference score continuum, and between 0 and + 1.OOon the lambda continuum. The scores of the normal control group basically mirror those of the lefthemisphere speech group both in terms of range of asymmetries and central tendency of the distribution.

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ROBERT J. ZATORRE

Side of Speech sentation Left m3terai Right WC1

I

NC

.

FIG. 2. Distribution of lambda scores [In (R/L)] on fused words test according to side of speech representation as determined by intracarotid sodium Amytal testing. Each symbol represents one subject.

Analysis of variance on asymmetry

scores

Statistical analysis was undertaken to evaluate the contribution of several factors to the magnitude of the ear asymmetry among subjects whose speech representation was known from Amytal testing. Analysis of variance was performed on the sample of subjects with lefthemisphere and bilateral speech only; those with right-hemisphere speech were excluded because of their small number. A four-way analysis of variance was carried out, including the following factors: side of speech representation, handedness, sex, and side of epileptogenic focus. Because of the lack of sufficient observations in each cell, only main effects, two-way interactions, and three-way interactions including the speech factor were computed, the other sources of variation being pooled within the error term. The result for the difference score indicated a highly significant effect of side of speech [F (1,42) = 26.01; P < O.OOOl], but no other significant effects or interactions. That is, the distributions of ear asymmetry scores differ significantly between the two groups, but are not affected by the other factors. A similar finding was obtained for the analysis using lambda: only the side of speech factor was significant [F(l, 42)=30.19; P
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PERCEPTUALASYMMETRY ON THE DICHOTIC FUSEDWORDS TEST TABLE 2. Mean values for total score, difference speech representation Side of speech representation Left Left Bilateral Bilateral Right Right Control

Handedness Right Left or mixed Right Left or mixed Right Left or mixed Right

score, and lambda and handedness

Total score 29.00 26.18 25.58 28.70 29.50 37.00 29.90

index according

Difference score 17.78 IO.36 - 1.42 -6.30 -21.50 -31.00 14.70

to side of

Lambda

index

1.60 0.96 -0.10 - 0.49 -1.81 -2.72 1.14

[F(l, 34)=26.42; P
of subjects

per group

The distribution of handedness, familial sinistrality, sex, or side of epileptogenic focus among the three patient groups (Table 1) was also analyzed, independently of the scores on the dichotic test. No significant differences among the groups could be uncovered for any of the variables (handedness, x2 = 1.22; familial sinistrality, x2 =4.2; sex, x2 =0.12; side of lesion, x2 = 1.18; P> 0.10 in all cases). Thus, the number of subjects in each category (rightvs left-handed, male vs female, etc.) did not differ according to their speech representation. Subsidiary

analyses

Within the bilateral speech group some distinctions could be drawn according to the pattern of speech interference observed after each injection. In seven cases there was more interference with speech following injection into the left than into the right carotid artery, in four cases the reverse was true, and in the remaining eleven cases the degree of speech interference was not sufficiently different after each injection to be able to make any distinctions. A KruskallLWallis statistic did not give evidence for any differences in degree of ear asymmetry within these groups (H=2.32, df=2). It is nevertheless interesting to note that all four patients in the subgroup with greater right than left speech representation did obtain left-ear advantages, whereas four out of seven subjects classified as having greater left than right hemispheric speech representation obtained right-ear advantages. Analyses were also computed to determine if the scores of the normal control subjects differed in any way from those of the patient group with left-hemisphere speech representation. No significant difference was found comparing these two groups on either the difference score or the lambda index (F< 1 in both cases). A comparison was also run of the total number of non-stimulus-dominated responses (see Table 2) between the controls and the two groups of patients with left-hemisphere or bilateral speech representation. No differences were found on this variable across any of these groups [F (2, 83) = 0.54, P> 0.581.

ROBERTJ. ZATORRE

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Examination

of’ homogeneity

with lambda scores.

The lambda index permits a statistical evaluation of the homogeneity of the distribution of scores [S]. When this analysis was applied to the present data, it was found that none of the groups except those with right-hemisphere speech representation could be considered homogeneous. For the left-hemisphere speech group the analysis yielded x2 (34)=95.88, P 0.10; however, it is uncertain if the observed homogeneity in this group could be expected to hold with a larger sample. Correlational

analysis

A correlational analysis was also carried out to examine the relationship between the two dependent variables (difference and lambda scores). Pearson product-moment correlation coefficients were calculated separately for each group, and the results were as follows: for all groups, the two indices of ear asymmetry were highly correlated (left-hemisphere speech group, r=0.85, PO.25; for the lambda index, r = -0.11, P> 0.60). The trends for the normal control group were in the same direction as for the left-hemisphere speech group (difference score, r = 0.73, P
DISCUSSION The results clearly indicate that are related to side of hemispheric the test can now be established for the more extreme asymmetries are

perceptual asymmetries on the fused words dichotic test speech representation. More importantly, the validity of individual subjects. It would appear from this sample that nearly exclusively associated with unilateral hemispheric

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speech representation. However, the distribution of scores is such that there is considerable overlap between the bilateral speech group and the others, and this is confined largely to a region straddling the zero (no ear difference) point (Figs 1 and 2). Moreover, there is a wide range of possible ear advantages, even within the group that is known to have exclusively lefthemisphere language representation, as underscored by the analysis of homogeneity. Although the results presented in this study are based on a population with definite cerebral pathology, it seems unlikely that the epileptogenic lesions played any important role in the ear asymmetries observed. First, all cases with lesions that would be expected to affect the ear advantage were excluded. Second, there was no relation at all between side of maximal EEG abnormality and ear preference. Third, the distribution of scores in the righthanded normal control subjects did not differ from that of the patients with left-hemisphere speech. Finally, the total number of non-stimulus-dominated items on which the ear advantages were based did not differ between the patient groups and controls. Thus, the behavior of the patients on the fused words test appears in every way comparable to that of controls, and the results obtained can probably be safely evaluated in this light. An important point to keep in mind regarding these data is that the sample studied was most definitely not a random one. Rather, the sodium Amytal test is only carried out (1) in patients whose memory function is at risk from a planned temporal lobectomy [16], and (2) in patients who are suspected of having atypical speech representation based on handedness, clinical seizure pattern (e.g. postictal dysphasia), or pattern of results on neuropsychological tests. In fact, in a few cases a sodium Amytal procedure was carried out because of the results of the dichotic fused words test; conversely, in other cases the Amytal test was not carried out because a large right-ear advantage was observed on the fused words test. Because the present study was carried out over the course of several years, it became apparent after some time that the fused words test predicted speech representation to some extent. It would therefore have been unethical to withhold potentially important data in a patient’s neuropsychological assessment. These facts necessarily bias the results, so that atypical speech cases are vastly overrepresented in this sample as compared to a general population, for whom more appropriate estimates of the incidence of atypical speech have been reported by several investigators (e.g. [3, 91). The bias also may distort the results somewhat, in that large right-ear advantages may be somewhat underrepresented in the left-hemisphere group (since subjects obtaining such scores were less likely to receive Amytal testing and thus be included in the study). Based on these considerations, it is not possible to determine the independent contribution of such factors as handedness, familial handedness, sex, and ear asymmetries, in order to arrive at a prediction regarding a subject’s hemispheric speech representation. Thus, the approach suggested by GEFFEN and CAUDREY [S],in which a discriminant function analysis uses both handedness and ear advantages to determine a side of speech representation, would not be applicable here. Indeed, such an analysis on the present sample would lead to the absurd conclusion that handedness bore little relation to side of speech representation. Despite the foregoing, it is possible to determine the effect of various factors on the degree of ear asymmetry observed. Thus, in the analyses presented above it is clear that the only variable that affects the distribution of scores is whether or not speech is lateralized to the left hemisphere. Neither sex nor side of epileptogenic focus was shown to affect the scores. Handedness did not affect the scores either; although there was an apparent tendency (Table 2) for the left-handers to show a bias favoring the left ear, this effect did not reach conventional levels of statistical significance. There was, however, a handedness by familial

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sinistrality interaction on both dependent variables. Curiously, it was the left-handers with no family history of left-handedness that differed from the others in that they exhibited an average left-ear dominance, regardless of side of speech representation. The importance of this finding remains to be established. The two measures ofear advantage used in this study were highly intercorrelated across all groups, and similar findings have been reported before [25,3 11. It is unlikely that any other transformations would yield substantially different results. There is therefore no overwhelming empirical reason to prefer one index to another. The lambda index [S] may nonetheless be preferable because of its statistical properties, in particular the fact that it allows for the calculation of the significance level for a given score, and gives both an estimate of variability for each subject as well as the homogeneity of the sample. Also, it is somewhat less correlated with the total number of responses than is the difference score. The distribution of scores for the left-hemisphere speech group and for the normal control group in the present study is similar to that reported by WEXLER and HALWES [30] for normal right-handers, who noted that 12% of their sample obtained left-ear advantages. Because this would represent too high an estimate or right-hemisphere speech representation, these authors [32] suggested establishing a statistical significance criterion, whereby only subjects exhibiting a certain degree of asymmetry would be reliably classified. This approach has the merit ofarriving at proportions ofleft-hemisphere speech that are in accord with its incidence in the normal population, as estimated from the neurological literature. It has the distinct disadvantage, however, that a large proportion of subjects do not yield sufficiently large asymmetries, and therefore remain unclassified. The results presented in this study would support the interpretation that “non-significant” ear asymmetries may indeed be meaningful insofar as many subjects who yield small asymmetries were found to have speech bilaterally represented. Thus, the degree of ear advantage seems to be one clue to speech representation, and may be a useful way of distinguishing between bilateral vs unilatera! hemispheric speech dominance. It is still necessary to account for the overlapping distributions between the lefthemisphere and bilateral speech groups, however. Many subjects in both groups yielded small-magnitude ear advantages. It is reasonable to hypothesize that subjects may yield a small degree of ear asymmetry for any of several different reasons. The most theoretically interesting reason is that there may be competition from both hemispheres for processing the words, which would yield, on average, a roughly equal number of responses from each ear. Presumably, this would explain the generally small asymmetries observed in the bilateral speech group, since they have some speech representation in both hemispheres. A second factor which may lead to small asymmetries may have no relation to the underlying speech representation, but may be a simple matter of sampling: if relatively few non-stimulus-bound words are available for calculating the asymmetry, then large asymmetries will be less likely to emerge. Presumably, many of the subjects with lefthemisphere speech who nevertheless obtained small asymmetries did so for this reason. This hypothesis is supported by the finding that there is a strong correlation between number of non-stimulus-dominated words and degree of asymmetry in the left-hemisphere speech group (and in the normal control group), but not in the bilateral speech group. If this line of reasoning is correct, then it should be a relatively simple matter to improve the results by increasing the number of available items (e.g., by repeating the test several times, until a suflicient number of non-stimulus-dominated items is obtained). It would then be predicted that subjects with bilateral speech would continue to show small-magnitude asymmetries,

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despite an increase in the number of items, whereas subjects with left-hemisphere speech would tend to increase their right-ear advantage. Several authors [20,25] have emphasized the importance of a large number of trials in order to obtain reliable data; it would appear from the present data that it is important to obtain a large number of non-stimulusdominated items. On the other hand, WEXLER and HALWES [31] reported only a weak relationship in their sample between asymmetry and the number of non-stimulus-dominated scores. It is clear that more research will have to be carried out to examine this issue fully. However, if we reconsider the data from this study by establishing an arbitrary criterion of 30 non-stimulus dominated items in order to consider a given result reliable (yielding 15 such subjects in the left-hemisphere group), the degree of overlap between left-hemisphere and bilateral speech groups becomes very small. Only three out of these 15 subjects with lefthemisphere speech obtained difference scores lower than 15 words, and only one subject with bilateral speech obtained a larger difference score than this. The group with right-hemisphere speech does not overlap at all with the left-hemisphere group, and only slightly with the bilateral speech group. Thus, it appears that it may be entirely possible to establish criteria that will permit a three-fold classification of hemispheric speech representation according to degree of ear advantage. In this respect, control of stimulus dominance effects would appear to be crucial: the clearest results emerge only when stimulus dominance can be overcome by the ear advantage. If stimulus dominance is ignored, then many subjects will not yield sufficiently asymmetric scores to permit classification. Note that this fact would have been obsured had raw scores (i.e. without correction for stimulus dominance) been used to calculate asymmetries. In that case stimulus dominance would still exert an influence on the degree of ear advantage, but there would be no way of evaluating it. There are many other factors that have been posited as possible explanations for the lack of perfect agreement between dichotic listening results and hemispheric speech representation. One class of such factors, relating to attentional processes (e.g. [14]), would not appear to be relevant with the fused words test insofar as a single partially fused percept is phenomenologically experienced, centred at the midline. Thus, deployment of attention to one ear or another would not bias the result [19]. The analysis of lambda scores indicates that the distributions cannot be assumed to be homogeneous. Thus, a number of important sources of variability must exist, in addition to those discussed above. A complete discussion ofthese issues is beyond the scope of this paper, but one hypothesis that should be mentioned is that anatomical asymmetries in the brainstem auditory pathways and/or in the central auditory system may vary across individuals, resulting in different degrees of perceptual asymmetry 17, 15, 231. These considerations should be taken into account in interpreting degree of measured laterality effects and their relation to underlying hemispheric functional asymmetry. Such explanations may eventually account for the wide range of ear asymmetry scores that can be observed even within a group whose members share the same underlying speech representation. A related factor of importance in interpreting these results is that the category of bilateral speech represents a very heterogeneous group. Included among this group are subjects who showed marked disruption in speech from both right- and left-sided injections, as well as those who showed only a slight, transient verbal disruption from injection into one hemisphere, and a clear dysphasia from the contralateral injection. Fifteen patients could be unambiguously classified as bilateral speech but with greater representation in one

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hemisphere. The fused words results did not distinguish among these subgroups reliably, although there was a trend for more consistent left-ear advantages among those with more right- than left-hemisphere speech. A resolution of the relation between bilateral speech and ear asymmetries will most likely have to await a better understanding of the phenomenon of bilateral speech itself. In conclusion, this study has demonstrated a definite relationship between perceptual asymmetry for individual subjects and cerebral functional asymmetry as determined independently via intracarotid sodium Amytal testing. It would therefore appear somewhat premature to accept TENG’S [28] conclusion that dichotic ear differences as an index of hemispheric specialization are “not well founded on either logical or empirical grounds” (p. 235). Caution must be exercised, nevertheless, in the interpretation of differences in magnitude ofear advantage across subjects. On the one hand, the present results suggest that small asymmetries may be related to bilateral speech organization in some subjects, particularly if stimulus dominance is excluded. On the other hand, a large range of scores was observed within the group of persons with clear left-hemisphere speech representation. There is therefore no simple relationship between an individual score on a dichotic test and the underlying cerebral representation. Ackno~v/edgementsl wish to thank Drs B. Wexler and T. Halwes for making available the fused rhymed dichotic words test, Drs A. Olivier, W. Feindel, J. -G. Villemure, and R. Leblanc for permission to study their patients, and the patients themselves for their cooperation. This research was supported by funds from the National Institute for Neurological and Communicative Disorders and Stroke of the United States (National Research Service Award, F32-NSO6784), the Fonds de la Recherche en Santt du Qutbec (Establishment Grant No. 840155), and the Medical Research Council of Canada (Operating Grant MA 9598 to the author, and MT 2624 to B. Milner). I also thank A. Bruemmer for assistance in data compilation.

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