Perceptual discrimination of vowels in aphasia

In the present study 3 hypotheses were investigated: first, the notion that an aphasic impairment of vowel perception is not associated with particular aphasic syndromes or lesion sites, second, that it is a disorder comparable to a general impairment of perception in a normal speaker caused by some form of interference, and third, that perceptual phonemic discrimination is a separate process from the phonemic discriminative function necessary for speech production. The hypotheses were tested by means of a vowel discrimination test administered to 50 German-speaking aphasic patients (roughly equally divided between Broca's, mixed non-fluent, Wernicke's and mixed fluent groups); the same test, masked by white noise at -10 dB was also administered to 20 normal native speakers of German. Results were in support of all 3 hypotheses. First, aphasic patients' error patterns were similar across fluent and nonfluent groups and for all lesion sites. Second, the error distributions of aphasics with slight auditory impairment resembled those of normal subjects in the -10 dB white noise condition, while distributions of aphasics with severe auditory impairment were indicative of an added component of guessing behaviour. And third, the patients' performance on the discrimination task differed from that shown on a comparable repetition test. (It was argued that repetition involves a patient's expressive capacity in addition to his perceptual capacity). The differentiation of perceptual and expressive phonemic discrimination was further supported by an analysis of the speech errors occurring in the spontaneous (purely expressive) speech and in the repetition (expressive plus perceptual) tasks of 16 French Canadian and 5 English Canadian aphasics.