Abstract
The present study examined the relation between deficits in speech and frequency transition discrimination in aphasia. Nine aphasic patients with left hemisphere damage and 11 normal subjects participated in the experiments. A speech discrimination task and a non-speech frequency transition discrimination task were employed. The speech discrimination task used 2 stimulus pairs. The first pair consisted of /ba/ and /da/, both of which had rapid second formant (F2) transitions. The second pair consisted of /wa/ and /ra/, both of which had gradual F2 transitions. The non-speech frequency transition discrimination task also used 2 stimulus pairs. The first pair consisted of 2 pure tones with a rapid frequency transition, and the second consisted of 2 pure tones with a gradual frequency transition. The first non-speech pair corresponded to the /ba/-/da/ pair, and the second corresponded to the /wa/-/ra/ pair.
The following results were obtained : (1) Overall discrimination performance with the /ba/-/da/ pairs characterized by rapid F2 transition was not significantly lower than that with the /wa/-/ra/ pairs characterized by gradual F2 transition in the aphasics. (2) However, the error pattern in the /ba/-/da/ discrimination and that in the /wa/-/ra/ discrimination were different. (3) A significant correlation between performances with the /ba/-/da/ discrimination and those with the rapid frequency transition discrimination was observed in the aphasics. (4) Patients who showed poor performances with both the /ba/-/da/ discrimination and the rapid frequency transition discrimination had a lesion in the transverse temporal gyrus of the left hemisphere.
These findings suggest that perceptual disturbance caused by a lesion in the transverse temporal gyrus is not a principal factor behind speech discrimination deficits in aphasic patients, but it does have a negative effect on the discrimination performance of syllables with rapid frequency transition.
