Hemispheric asymmetry in schizophrenia: a “dual deficits” model

Background: The aim was 1) to investigate left hemisphere functional integrity for auditory language processing in schizophrenic patients; and 2) to investigate the interaction between brain laterality and attentional processing by having subjects shift attention to the left or right ear. Methods: The subjects were 33 schizophrenic inpatients, and 33 healthy comparison subjects with the same age, handedness, and gender distribution as the patient subjects. All subjects were tested with dichotic listening (DL) to consonant–vowel syllables, which is a measure of lateralized temporal lobe language processing. The subjects were tested under three different attentional conditions: a non–forced attention condition, attention focused to the right ear stimulus, and attention focused to the left ear stimulus. Results: The main findings were 1) an absence of the expected right ear advantage in the schizophrenic group during the non–forced attention condition; and 2) a failure to modify DL performance through shifting of attention to either the right or left ear. The comparison group showed a right ear advantage during the non-forced and forced-right attention conditions (increased right ear advantage during the forced-right condition), and a left ear advantage during the forced-left attention condition. There were no significant effects of handedness. Conclusions: This pattern of results may indicate a “dual deficit” involving both automatic and controlled processing deficits in schizophrenia.