Study of the human visual cortex: direct cortical evoked potentials and stimulation.

The authors studied the visual cortex of 15 patients undergoing studies for medically intractable epilepsy. Although the subdural and strip electrode placement varied in each of these patients, there were enough electrodes over the visual cortex to complete studies involving evoked potentials and direct cortical stimulation. Visual evoked potentials were elicited using two check sizes (50 and 16 min) for pattern reversal studies, 50 min checks for on-off stimulation, 50 min checks for horizontal and vertical hemifields and simple flash for the VEP. These studies demonstrated that the pattern reversal and on-off stimuli caused very complex, multipotential waveforms in striate and vision associational cortex that do not resemble the response obtained at the scalp. Different volumes of visual cortex are activated by stimulation with 16 min checks, 50 min checks and simple flash. Flash activates the largest volume of visual cortex and it is likely that this finding is what makes this test of so little value clinically. Direct cortical stimulation shows that colored responses are generated primarily in the posterior striate cortex and inferior occipital lobe, while movement is primarily generated by the visual association cortex. No complex visual images were obtained by stimulation of either the striate cortex or visual association cortex. The brain mechanisms that lead to formed visual images remain to be identified.