Impaired categorical perception of synthetic speech sounds in schizophrenia

Background: Simple speech sounds such as /ba/ and /da/ differ in the freque ncy composition of their underlying formants. Normal volunteers asked to id entify intermediate phonemes along the /ba/ to /da/ continuum abruptly swit ch 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 schizo phrenic and 14 control subjects, using a forced-choice phonemic discriminat ion paradigm. Results: Patients and controls were equally able to recognize endpoint form s of both phonemes, but differed significantly in their perception of inter mediate forms near the center of the continuum. Patients also showed a sign ificantly shallower response curve, suggesting an impairment in boundary de finition. Despite their impairment in categorical perception, schizophrenic subjects showed normal adaptation of response when test stimuli were prece ded by a series of /ba/ or /da/ stimuli from the endpoints of the continuum . Conclusions: The present results suggest that precision of phonemic process ing is impaired in schizophrenia. This categorical perception deficit may r epresent upward generalization of impaired memory-dependent acoustic proces sing. Deficits in the precision of cortical processing may contribute signi ficantly to cognitive dysfunction in schizophrenia. Biol Psychiatry 1999;45 :82-88 (C) 1999 Society of Biological Psychiatry.