Visual evoked potentials and positron emission tomographic mapping of regional cerebral blood flow and cerebral metabolism: can the neuronal potential generators be visualized?

Visual evoked potentials (VEPs) and visual evoked spectrum array (VESA) to flashes and pattern-reversal were correlated with regional cerebral blood flow (rCBF) or local cerebral glucose metabolism in 4 hemianopsic patients and one subject with cortical blindness. Normal VEPs, topographical distribution and occipital rCBF were noted in hemianopsic patients with macular sparing. A dissociation of topographical distribution of VEPs to flashes and pattern-reversal was demonstrated in one patient with hemianopsia and macular splitting. In this case, rCBF showed unilateral activation of visual areas 17, 18 and 19 of the cortex. The distribution of surface-recorded potentials reflected the complex interaction of electrical field potentials within at least 3 cortical areas rather than volume transmission of striate dipoles alone. VEPs were preserved in a cortically blind patient. rCBF and local cerebral glucose metabolism revealed a functioning island of occipital cortex that most likely represented the generator of the VEP. The combination of VEP and PET permits the correlation of electrophysiological events with the visualization of cortical areas presumably activated by the same visual stimulus.