NEURONAL GENERATORS OF VISUAL-EVOKED POTENTIALS IN HUMANS - VISUAL PROCESSING IN THE HUMAN CORTEX

Purpose: We wished to define the localization of cortical generators o f visual (pattern) evoked potentials (VEP) and the temporal sequence o f activation in the occipital region. Methods: In 4 candidates for epi lepsy surgery, a large array of subdural electrodes was placed over oc cipital areas. Checkerboard pattern reversal stimuli were generated an d the epileptogenic focus was localized and functionally mapped. Magne tic resonance imaging did not show any occipital lesions in any of the 4 patients. Results: The area first activated was the lingual gyrus i n the mesial occipital lobe (negative potential peaks at similar to 70 ms), followed by an area superior to the calcarine fissure (negative peaks at similar to 80 ms). Later (starting at similar to 90 ms), ther e were positive potentials over the occipital pole and lingual gyrus, followed by potentials at the lateral occipital lobe.Conclusions: Thes e data support the idea that VEP are generated in the mesial and later al occipital cortex by different circumscribed neuronal generators wit h different latencies of activation. The scalp-recorded N1 and P1 pote ntial peaks most likely derive from the progressive activation of neur onal masses in different regions of the occipital lobe.