The purpose is to determine which aspects of speech production, speech perception and language are independently controlled by the central nervous system and the brain regions associated with each. To determine at what level in the central nervous system speech motor control is independent of language and speech perception, patients with neurological diseases affecting only one aspect of speech and language are studied, and the location of their neuropathology determined. In one study, patients with normal speech production and language processing following penetrating head injury were examined on experimental tests of speech discrimination and identification. Two groups of patients were identified; one with speech discrimination deficits and the other without such deficits. CT scan data comparisons between the two groups suggested that the upper levels of white matter tracts in either hemisphere, and not the cortical regions, were associated with speech discrimination deficits. These data suggest that speech sound discrimination skills can be affected independently from speech sound identification, language and speech production skills, and are not associated with left sided damage. A case of chronic cortical deafness with normal speech secondary to herpes encephalitis was studied. Normal brain stem potentials with severely reduced cortical responses indicated that auditory signals were not reaching the cortex. The patient's phonological information processing, speech perception and speech repetition were only mildly impaired in contrast with a central deafness and severe auditory agnosia. This case demonstrates the independence of speech perception and phonological processing from auditory perception. Data analyses are ongoing for studies of penetrating head injuries to examine the independence of various speech production deficits and the associated lesion locations.
