Background: We compared performance on a dichotic listening (DL) task between schizophrenic, depressed, and healthy control subjects. A variant of the traditional DL paradigm was used in which the subjects were required to focus attention either on the left (forced-left condition) or right (forced-right condition) ear stimulus.
Methods: The subjects were 51 patients with a DSM-IV diagnosis of schizophrenia, 49 patients with recurrent unipolar major depression, and 49 healthy control subjects. They were tested with a consonant-vowel syllables dichotic listening task under three attentional instructions.
Results: There was a significant overall right ear advantage during the nonforced condition, which increased dramatically during the forced-right condition and was eliminated during the forced-left condition. The depressed patients showed no signs of impairments compared with the healthy control group. Thus, they showed a right ear advantage during the nonforced and forced-right conditions, which was shifted to a left ear advantage during the forced left condition. The schizophrenic patients, however, were impaired on the forced-left condition compared with the healthy control and depressed subjects.
Conclusions: The results are discussed in terms of separating attentional and inhibitory executive impairments in schizophrenia and depression, taking into consideration illness duration and information-processing demands.