Interhemispheric asymmetries in the visual evoked response: effects of stimulus lateralisation and task.

VER's were elicited by briefly presented lateralised letters in two experiments. In experiment 1 the subjects engaged in verbal processing of the letters and in experiment 2 they engaged in visuospatial processing. In both experiments the latencies of the first two components of the VER's were consistent with previous findings and with the anatomy of the retino-cortical visual pathways. The pattern of hemisphere asymmetries of the amplitudes of the middle and late components differed in the two experiments in that (i) in the second experiment the amplitude of the middle component from the left hemisphere did not vary with field of stimulus presentation although this was the case in the first experiment and (ii) a hemisphere asymmetry (left greater than right) was observed in the late component only in the second experiment. The results are interpreted as evidence of difference in the cerebral processing of the stimuli in respect of the task demands of the two experiments.