A paradox in the lateralisation of the visual evoked response

Abstract

THE average evoked response in man to stimulation of the visual field with a reversing checkerboard pattern and recorded from the scalp over the occipital region has a waveform which is consistent both within and between subjects. The most characteristic feature for full-field stimulation is a major positive component at about 100 ms which is recorded maximally 5 cm above the inion in the midline and is distributed fairly symmetrically over both sides of the head (Fig. 1). The pathways from the retina to the visual cortex undergo partial decussation in the chiasma, so that information presented to the left half of the visual field passes to the right hemisphere, whereas the left hemisphere receives signals from the right half of the visual field. Consequently, it might be predicted that stimulation of one half field will produce an evoked response which is maximal over the contralateral hemisphere. The data presented here show that, contrary to this prediction, the maximal response occurs at the scalp electrodes situated over the hemisphere ipsilateral to the field stimulated.