Pattern flash visual evoked potentials in patients with homonymous hemianopia

Visual evoked potentials from seven horizontally spaced electrodes were recorded from normal subjects and subjects with homonymous hemianopia in response to hemifield pattern flash stimulation. Stimulation produced a large early peak that was positive on the scalp contralateral to the hemifield and negative on the ipsilateral scalp. From computer fitting of the amplitudes versus electrode position, the position of the equivalent source was found to be in the contralateral hemisphere. The horizontal orientation of the dipole source was approximately tangential (parallel) to the occipital scalp surface with negative polarity toward the medial fissure. In normal subjects, visual evoked potential amplitudes at the first peak were positive on the left and negative on the right for right hemifield stimulation. Left hemifield stimulation showed the opposite results. Three patients with homonymous hemianopia showed normal visual evoked potential results from their functional hemifields and nearly flat results from their hemianopic hemifields. The normal visual evoked potentials originated in their intact cortical hemispheres.