BRAIN AND LANGUAGE lo, 281-286 (1980)

Dichotic Listening Pattern in Conduction Aphasia

HANNA DAMASIO AND ANTONIO R. DAMASIO

University of Iowa College of Medicine

Judging from classic postmortem reports (cf., Goldstein, 1948; Kleist, 1952; Benson et al., 1973) and from recent studies using CT scanning (Damasio & Damasio, 1979a), conduction aphasia is often associated with damage to the left cortical auditory complex (the primary areas, Brod- mann’s fields 41 and 42, and the association area, field 22). It follows that arrival of auditory input in the left temporal lobe via the geniculo-cortical pathway must be precluded or greatly diminished in patients with that syndrome and that conduction aphasics should show a marked extinction to verbal stimuli presented to the right ear in a dichotic listening (DL) task. We report two instances of patients with conduction aphasia in whom there was complete right channel extinction in a verbal DL task, and discuss the functional significance of the phenomenon.

MATERIAL AND METHOD

Subjects Two patients with conduction aphasia were studied with the Multilingual Aphasia Exam-

ination (Benton, 1969, 1977). The diagnosis of conduction aphasia was made in accordance with the Boston Diagnostic Classification of the Aphasias (Goodglass & Kaplan, 1972). Pertinent clinical details were as follows:

Case 1. A 45-year-old woman, right handed, with 10 years of education suffered a thrombotic stroke which produced aphasia, right central facial paralysis, and right brachial paralysis. Speech was fluent and meaningful but repetition of words and sentences was greatly disturbed with frequent blocking and phonemic paraphasias. Word finding ability was mildly impaired and so was aural comprehension for syntactically complex items but not for colloquial conversation. Reading comprehension was normal but reading aloud elicited literal paraphasias. Memory was excellent in all spheres. Constructional praxis and calcula- tion were normal.

Case 2. A 43-year-old man, right handed, college graduate suffered a thrombotic stroke which produced aphasia, right central facial paralysis, and right brachial paralysis. The facial

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282 DAMASIO AND DAMASIO

and brachial weakness improved but aphasia persisted. Speech was fluent and meaningful but with phonemic paraphasias. Repetition of verbal material was severely impaired with phonemic paraphasias and omission of phonetic and lexical components. Word finding ability was defective, though less so than repetition. Aural comprehension was normal for simple conversational items but mildly impaired for syntactically organized material. Read- ing comprehension was mostly normal but reading aloud was compromised by phonemic paraphasias. Recall of recent and remote events was excellent as were calculation and visuospatial abilities.

Anatomical Study The anatomical study of both patients was carried out with CT scan, according to a

method described by Damasio and Damasio (1979a, 1978b) where the lesions seen in each cut are plotted in standard templates of the corresponding levels of the human brain. The results of the record are read as a three dimensional reconstruction.

Dichotic Listening Study A dichotic listening task consisting of 110 pairs of different words of common usage, with

one, two, or three syllables (e.g., nice/tall; sister/listen; popular/comical) was used. Each pair appears twice in reversed ear order. The tape is played on a high-fidelity, steady-speed Sony TC-880-2 stereo tape player, and the patient listens to it through cushioned Nakamichi HP-100 earphones. Sound level of the stimuli varies according to the threshold of the subject but is constantly given at approximately 70 dB above threshold. Patients are tested previ- ously for pure tone thresholds with an Eckstein EB-400 audiometer and DL is not given to patients with an ear asymmetry greater than 10 dB. The task has been standardized in normal right-handed adults and used in previous studies (Damasio & Damasio, 1979b).

RESULTS

Anatomical Study

In both patients plotting of the area of decreased density of the CT scan indicates that the lesion involves the insula in most of its extent com- promising its cortex and underlying white matter. The lesion reaches into the temporal lobe (fields 41, 42, and 22 and subjacent white matter) and also involves parietal cortex (area 40 and subjacent white matter). A composite of the two lesions is shown in Figs. 1 and 2.

Dichotic Listening Study Case 1. Complete extinction of right channel (0 responses). Normal left

channel (110 responses). Also reported hearing the sound of words but being unable to understand any of them (“words sound far away”). The volume of the right channel was turned up to a point at which the patient reported that words were “close now.” But she was still unable to distinguish them. She reported difficulty with telephone listening with the right ear requiring her to switch the receiver to the left in order to comprehend speech. If stimuli were given one at a time to each ear, the score was normal for both right and left.

Case 2. Complete extinction of right channel (0 responses). Normal left channel (110 responses). Patient reported “hearing words” in the right

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channel but being unable to “distinguish the sounds.” Manipulation of intensity of the right channel produced no alteration in performance. He recalled that this was similar to the diaculty he had with telephone conversations and that, for that reason, he now always held the receiver close to the left ear, with which he felt speech comprehension was normal. Under monotic listening conditions the patient had a perfect score for both ears.

COMMENT

Other aphasic syndromes, particularly if the underlying lesions are predominantly located in the infrasylvian region, produce extinction of the right channel in DL so that the pattern seen in the two patients is in keeping with previous descriptions (Sparks et al., 1970; Rubens et al., 1978). But cases of conduction aphasia, by virtue of their preserved speech fluency, near normal aural comprehension, and intact nonverbal cognition, allow a technically appropriate administration of the procedure and hence a more reliable measure of the defect. Among aphasics only those with conduction aphasia are able to comment on the phenomenon with insight.

Both cases provide good examples of the “lesion effect” as described by Kimura, which consists of extinction of the channel opposite the side of the lesion, under competition, contrasted with full patency of both channels under monotic conditions (Kimura, 1961). This extinction is the laboratory counterpart of the symptom reported by both patients in rela- tion to the telephone, in itself an abnormal enhancement of the “cocktail party” effect (Ainsworth, 1976). Speech, as transmitted by a telephone receiver, is filtered to such an extent that in patients with left central auditory defects ambient noise provides adequate competing stimuli. Given the amount of damage to the primary auditory areas in the left hemisphere it appears that the competition from which results suppres- sion of the right channel, takes place in the right auditory cortex. By the same token it is the right auditory cortex which adequately “listens” to the left or right channels when they operate separately.

The results obtained with DL in conduction aphasia are pertinent to the debate of whether or not the remarkably preserved linguistic abilities of these patients are the result of a functional takeover by the right hemi- sphere of covertly ambidominant subjects, a view championed, among others, by Kleist (1962) and Kinsboume (1971a, 1971b). At first glance our results would appear to favor that opinion since there is little question that if the left primary auditory areas are damaged, auditory information can only arrive in the cortex via right auditory structures. However, nothing precludes it from immediately travelling to the left auditory association areas by a patent interhemispheric auditory pathway. Work in the Rhesus monkey demonstrates the presence of both anterior commissure and

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CASE 1

DAMASIO AND DAMASIO

FIG. 1. Composite representation of the low density areas of the CT scans of Case 1 plotted in corresponding templates. The lesions involve the auditory complex on the left side, extending forward into the insular cortex and backward and upward into the supra- margynal gyros.

cahosal connections joining the two auditory complexes, the former con- necting the rostral portion of the association area to its homologous contralateral region, the latter linking the caudal portion also to its homol- ogous contralateral region (Pandya et al., 1969). Furthermore the medial portions of the primary areas themselves are interconnected, (a structural detail which may be irrelevant for conduction aphasia). The caudal por- tion of the association field in turn projects to parietal lobe, frontal lobe and cingulate region (Pandya et al., 1969; Pandya & Sanides, 1973). In man the size of the caudal portion of the association cortex seems particu- larly large on the left, as noted in cytoarchitectonic studies (Galaburda,

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CASE 2

FIG. 2. Composite representation of the low density areas of the CT scans of Case 2 plotted in corresponding templates. The lesions involve the auditory complex on the left side, extending forward into the insular cortex and backward and upward into the supra- margynal gyrus.

Sanides & Geschwind, 1978), clearly spilling into the lateral surface of the hemisphere, in the parietal lobe, in agreement with electrophysiological finding of Celesia (1976) who has mapped human auditory cortex in the superior lip of the fronto-parietal operculum. It is to this wide and par- tially intact area in conduction aphasics that the main interhemispheric cortico-cortical auditory pathway projects. Its fibers arrive by a route different from that adopted by the geniculo-cortical pathway (cf. Damasio & Damasio, 1979b), descend in the depth of the parietal lobe, lateral to the trigone, and thence travel to the supratemporal and lateral temporal planes, as well as to the lateral parietal cortex, in which zones

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the auditory association cortex is located. The combination of consider- able sparing of the left auditory cortex, with preserved interhemispheric auditory connections, opens the possibility for further processing of au- ditory information in the left hemisphere. Theoretically, then it is con- ceivable that the linguistic performance seen in conduction aphasics results from predominantly left hemisphere processing rather than com- pensatory right hemisphere takeover.

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