Cerebral laterality and psychopathology: a review of dichotic listening studies

Studies using dichotic listening tasks have reported findings suggestive of alterations of cerebral laterality in schizophrenia and affective disorders. In a review of these findings, an effort was made to take into account four factors: (1) type of dichotic listening task; (2) performance level; (3) clinical state of patients at the time of testing; (4) diagnostic subtype of patients. A convergence of evidence indicates that the last two factors are of major importance. Several studies have found a relationship between clinical state and dichotic ear asymmetry. Greater severity of illness in schizophrenic and depressed patients is associated with reduced laterality, and clinical remission is accompanied by a normalization of laterality. While thie relationship appears to hold for both verbal and nonverbal dichotic tasks in depressed patients, that is not the case for schizophrenic patients. Studies have also reported evidence of differences in dichotic ear asymmetry between diagnostic subtypes of schizophrenia (i.e., paranoid vs. nonparanoid patients) and affective disorders (i.e., bipolar vs. unipolar patients). This evidence suggests the existence of homogeneous subgroups with distinctive laterality patterns and clinical characteristics.