[The P 300 potential in schizophrenia]

The P3 component of the event-related potential (E.R.P.) is an endogenous positive wave with a latency of approximately 300 msec, which is typically elicited by rare target stimuli in a detection task. The P3 has been applied to the study of cognitive impairments. One of the most consistently replicated biologic observation in schizophrenia is the P3 amplitude reduction. More recently, a prolonged P 3 latency has been observed by some authors. Nevertheless, when the heterogeneity of the schizophrenic syndrome is taken into consideration, results reported in the literature do not show much agreement. In the present study, we attempted to explore the P 3 component in a group of schizophrenic patients with a precisely defined clinical symptomatology.

Subjects. The study included 19 inpatients and 19 healthy control subjects age and sex-matched. Patients were medication-free for at least 15 days prior recording. According to the DSM III-R diagnosis criteria, 10 patients were classified as "paranoid subtype" and 9 as "disorganized subtype". Furthermore, on the day of the neurophysiological examination, the patients were interviewed using the Scale of the Assessment of Positive Symptoms (S.A. P.S.) and the Scale of the Assessment of Negative Symptoms (S.A.N.S.). The median age (SD) of the patients was 35.3 +/- 8 years, with a median duration of illness 12.5 +/- 7.2 years, a median duration of hospitalization 4.2 +/- 4.3 years and a median IQ 82.8 +/- 13.7. E.R.P. Recording. Subjects performed a two-tone auditory discrimination task: they were instructed to count silently infrequent high-pitched tones of 2000 Hz randomly presented within a serie of low-pitched tones of 1000 Hz. E.R.P. were recorded from 16 scalp electrodes (international 10-20 system) referred to nose. The P 3 was defined as the most positive wave between 250 and 500 msec after the stimulus onset. The amplitude was measured referred to the prestimulus baseline; the latency was measured at the point of maximum amplitude. Statistical analysis. The data were submited to a statistical analysis (ANOVA, Student's t-test, Fisher PLSD and Scheffe F-test, Spearman's correlation and linear regression analysis).

1) The schizophrenic group (N = 19) showed both a significantly prolonged latency and a reduced amplitude, compared to the control group (p < 0.01). 2) In the paranoid subgroup (N = 10), P 3 response was later and smaller than in the control group (p < 0.05). The P 3 latency and amplitude were not significantly different between the two groups of disorganized schizophrenics and controls. 3) A significant correlation between the global score of the S.A.P.S. and the P 3 amplitude was found (r = -0.67). There was no significant correlation between the P 3 parameters and a) the global score of the S.A.N.S. and b) the different items of each evaluation scale. 4) The P 3 amplitude showed a trend toward a negative correlation with the duration of hospitalization. 5) The duration of hospitalization was more important and the global score of the S.A.P.S. was higher in the paranoid than in the disorganized subgroup. There were no significant differences concerning the global score of the S.A. N.S., age, duration of illness, duration of hospitalization, IQ and performances in the detection task between the two subgroups of patients.

We confirmed the P 3 parameters modifications in schizophrenic patients. The P 3 reduced amplitude and prolonged latency only in the paranoid subgroup, while the disorganized subgroup had both values comparable to those of the controls, is the main finding in our study. Kütcher or Louza studies tend to the same results. Furthermore, we showed that task performances or clinical measures such as age, duration of illness, duration of hospitalization or IQ did not account as biases of P 3 parameters. These results could be a further indication that different subtypes of schizophrenia may have different biological substrates.