This study will explore the validity of the now-popular notion that information processing of the left hemisphere is more dysfunctional than the right in patients with schizophrenia, while the right is more dysfunctional than the left in patients with depression. However, in light of recent critical reviews of this dichotomy, we plan to use carefully-selected patients, controlled drug effects together with three validated and independent measures of laterality. These will be a newly-developed test battery (the Cognitive Laterality Battery, CLB) made up of tests that assess processing normally attributed to the right and left hemispheres, along with a standard neuropsychological test battery (Luria-Nebraska) that assesses lateralized dysfunction and tests of dichotic listening that assess processing asymmetries and/or attentional biases. Secondly, we propose to determine whether improvement in clinical course following drug treatment is related to a corresponding reduction or change in cognitive asymmetry as assessed by each of the three measures. If the "right/left" dichotomy maintains for schizophrenia or depression for any or all of the three measures, and if a consistent change in asymmetry is concordant with a change in symptomatology following drug therapy, the measures of laterality will be useful in differential diagnosis and in monitoring clinical course.