Abstract Divided visual field tasks were given to normal subjects, and patients with schizophrenia and affective disorder, to investigate hemisphere differences in the visual processing of standardised pictorial stimuli. There were two conditions: in the first, subjects were asked to decide whether a common entity represented by a picture was living or non-living, a task involving a categorical judgement based on semantic information; a left hemisphere task. In the second condition, subjects judged whether these depictions represented entities which were bigger or smaller than a cat; a right hemisphere task requiring visual imagery to compare spatial dimensions. It was found that the patient groups, while showing slower reaction time (RT) overall, both displayed a right hemisphere (RH) advantage on the imagery task. Furthermore, the schizophrenics' RHs showed the normal relationship between closeness of size comparison and RT, additional evidence that the visual imagery mechanism is intact. However, these patients failed to show the expected left hemisphere advantage on the visual-semantic task. There was a suggestion that performance on the semantic task was related to the experience of vivid imagery in normals and visual hallucinations in the schizophrenics. The possible contribution of hemispheric imbalance in the production of visual hallucinations from a disordered semantic system is discussed.