Neural mechanism for speech sound discrimination :
Findings from the study of aphasic patients

Abstract

    We extracted consonants from syllables by computer, prepared well-controlled but easily perceivable syllable and consonant discrimination tests, and applied them to examine 21 aphasic patients and 18 normal geriatric subjects. The aphasic patients showed disturbances in the syllable discrimination test, but disturbance in the consonant discrimination test was milder. There was no significant difference between the scores of the aphasics and those of the normal controls when the consonants were presented to the healthy (left) ear. In the aphasics, there was a significant correlation between the results of the syllable discrimination test and auditory word-comprehension ability (auditory retention span). At the same time, there was a significant correlation between the consonant discrimination test and auditory resolution performance (click fusion threshold). Factor analysis of the responses of the aphasic patients in the consonant discrimination test showed that acoustic feature analysis of speech sounds in the human brain is performed based on the coordinate axes of the participation of the vocal code (voiced vs. unvoiced) , the participation of the velum palati (oral vs. nasal) , the point of articulation, and the manner of articulation. Thus the theory which classifies phonemes based on these phonetical axes is neuropsychologically valid. The findings suggest that in cases where comparison between the results of acoustic feature analysis and memory of phonemes stored in the Wernicke's area is disturbed, identification of speech sounds is impossible even though phonetical feature analysis may be valid.