There is abundant evidence that the cerebral white matter and in particular the corpus callosum show several structural abnormalities in both schizophrenia (SCZ) and bipolar disease (BD). However, which cognitive functions are impaired as a result of these anomalies is still unclear. Previous behavioural tests of interhemispheric crosstalk have shown a differential impairment in SCZ with interhemispheric transmission time, as tested with the Poffenberger paradigm, essentially normal but with an abnormally enhanced interhemispheric summation effect, as tested with the redundant signal effect. The present study was inspired by this discrepancy and by the more general question of a possible overlap between the pathophysiology of SCZ and BD. We tested both SCZ and BD patients in the Poffenberger paradigm and redundant signal effect and found a similar dissociation, namely a normal interhemispheric transfer time and an abnormal redundant signal effect. The only difference between the two groups was a selective slowing of speed of response of the dominant right hand in the SCZ group suggesting an impairment of left hemisphere functions. These results cast further light on the question of common and differential impairments of basic psychological functions in the two diseases.