Impaired categorical perception of synthetic speech sounds in schizophrenia

Background: Simple speech sounds such as /ba/ and /da/ differ in the frequency composition of their underlying formants. Normal volunteers asked to identify intermediate phonemes along the /ba/ to /da/ continuum abruptly switch from perceiving "ba" to perceiving "da". The present study investigates precision of phonemic processing in schizophrenia. Methods: Categorical perception of speech sounds was evaluated in 15 schizophrenic and 14 control subjects, using a forced-choice phonemic discrimination paradigm. Results: Patients and controls were equally able to recognize endpoint forms of both phonemes, but differed significantly in their perception of intermediate forms near the center of the continuum. Patients also showed a significantly shallower response curve, suggesting an impairment in boundary definition. Despite their impairment in categorical perception, schizophrenic subjects showed normal adaptation of response when test stimuli were preceded by a series of /ba/ or /da/ stimuli from the endpoints of the continuum. Conclusions: The present results suggest that precision of phonemic processing is impaired in schizophrenia. This categorical perception deficit may represent upward generalization of impaired memory-dependent acoustic processing. Deficits in the precision of cortical processing may contribute significantly to cognitive dysfunction in schizophrenia. Biol Psychiatry 1999;45:82-88 (C) 1999 Society of Biological Psychiatry.