Structural and functional abnormalities of the left brain hemisphere, more often involving the temporal lobe, were frequently observed in patients with schizophrenia. However, negative and discrepant findings were also reported. Our study aimed to investigate the presence of lateralized brain dysfunction in patients with schizophrenia by using event-related potentials recorded during a monaural and dichotic presentation of tones to patients with clinically stable schizophrenia and to healthy control subjects.
Dichotic listening consists in presenting the subject with two different simultaneous auditory stimuli to either ear. By using this technique, the left and right auditory cortices can be separately tested. In the present study ERPs were recorded from 19 patients with schizophrenia and 26 healthy subjects, subjects were comparable for age, handedness and gender distribution. The Positive and Negative Syndrome Scale (PANSS) for Schizophrenia was used for symptom ratings (Table I). Each subject was given a randomized series of complex tones which were delivered in monaural and dichotic conditions with increasing intensities. A passive dichotic listening task was used to exclude the effect of attention on possible ERP abnormalities. We analyzed the N100 ERP component, related to sensory processing of stimuli and generated in the temporal lobe cortex.
In both patients and controls, dichotic listening inhibited the augmenting pattern of N100 amplitude with increasing tone intensity. Both groups exhibited a positive relationship between stimulation intensity and N100 amplitude over the right central sites in the monaural condition. On the other hand, patients failed to show the augmenting pattern shown by healthy controls over the left central lead (Fig. 1). This abnormality did not correlate with the severity of psychopathology.
Our results suggest a state of functional inhibition of the left auditory cortex, akin to that induced by dichotic listening, in people with schizophrenia. This inhibition is independent from psychopathology or drug therapy.