Neuronal generators of visual evoked potentials in humans: visual processing in the human cortex.

Purpose: We wished to define the localization of cortical generators of visual (pattern) evoked potentials (VEP) and the temporal sequence of activation in the occipital region. Methods: In 4 candidates for epilepsy surgery, a large array of subdural electrodes was placed over occipital areas. Checkerboard pattern reversal stimuli were generated and the epileptogenic focus was localized and functionally mapped. Magnetic resonance imaging did not show any occipital lesions in any of the 4 patients. Results: The area first activated was the lingual gyrus in 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), there were positive potentials over the occipital pole and lingual gyrus, followed by potentials at the lateral occipital lobe. Conclusions: These data support the idea that VEP are generated in the mesial and lateral occipital cortex by different circumscribed neuronal generators with different latencies of activation. The scalp-recorded N1 and P1 potential peaks most likely derive from the progressive activation of neuronal masses in different regions of the occipital lobe.