Psychophysiological differences in identical twins discordant for schizophrenia: a critical index for the onset of schizophrenia.

It has been hypothesized that not only genetic but also environmental factors contribute to the onset of schizophrenia. We therefore conducted psychophysiological studies on a pair of identical twins discordant for schizophrenia, to differentiate non-genetic from genetic indexes possibly associated with this disease. The affected and unaffected twins were 28 year-old females. The affected twin was diagnosed as having schizophrenia based on the Diagnosis and Statistical Manual of Mental Disorders, third edition revised (DMS-III-R), whereas the unaffected twin had no psychiatric disorders. The brain potentials evoked by visual stimulation (visual event-related potential [visual ERP]) were recorded. The components of the visual ERP, which are believed to reflect pattern cognition, semantic processing and the failure to use preceding word information, were compared with normal subjects. Both twins showed abnormal semantic processing and similar failure to use preceding word information. Abnormality of semantic processing was marked in the affected twin. These results indicate that failure to use preceding word information might reflect only genetic factors, whereas abnormal semantic processing might reflect both genetic and non-genetic factors because the affected twin was considered to show accelerated deterioration after the disease onset. However, only the affected twin showed abnormal pattern cognition, which might be attributable to non-genetic factors such as an acquired vulnerability to schizophrenia. We suggest that the impairment of pattern cognition evaluated by visual ERP may be a critical index for the onset of schizophrenia.