IMPLICATIONS OF NEURAL PROCESSING ON SENSORY/PHONETIC APHASIA

Abstract

The present research attempts to investigate, from a theoretical point of view, what neural processes might underlie phonetic aphasia. Phonetic aphasia is a defect in the processing of the sound structure of words, specifically in the ability to perceive sound stimuli.;Neuropsychological and psychophysical research has considered the ability of a listener to detect a stimulus. The central nervous system processes stimulus inputs at differential rates which subsequently either suppress or facilitate the acoustic stimulation presented at the periphery. Consequently, the internal perception of input may substantially differ from objective presentation of acoustic stimulus events which include intensity, duration, frequency, and interstimulus interval. Neuropsychological research suggests that brain injury results in a neural processing disorder: a damaged nervous system processes the same information more slowly than does an intact one. As a result of the inhibition-facilitation effects of the aphasic nervous system, different sound stimuli reach awareness at different rates. Only some and only those stimuli which reach awareness are effective for the aphasic.;Two separate defects, then, are thought to occur within the aphasic nervous system which bear relation to the recognition of speech among aphasics. Firstly, afferent neural processing suppresses parts of acoustic stimulation, which renders parts of afferent neural stimulation effective and parts ineffective. Since speech is a sequence of sound stimuli, parts of the word are perceived and parts of the word are blotted out. Secondly, the longer latency period for recovery in the aphasic nervous system renders the nervous system "less ready" to accept new afferent stimulation. Because speech proceeds at a rate of 80 msec. per phoneme, the rate of speech may be too fast for the resolving power of the aphasic nervous system. Operative concurrently and/or singularly, these defects may underlie difficulties in the perception of speech and may result in different degrees of the phonetic aphasic process. Recent studies with aphasic individuals lend support to this point of view.;Phonetic aphasia in this vein can be viewed as an interference of ongoing process rather than being due to a loss of certain learned items. A proposed model for empirical investigation is offered.