Different laminar distribution of flash evoked potentials in cortical areas 17 and 18 b of freely moving rats.

The laminar distribution of flash evoked field potential (VEP) was investigated in the visual cortical areas 17 and 18 b in freely moving rats. Averaged VEPs were recorded during stable relaxed wakefulness characterized behaviorally and polygraphically. The depth profiles of the main VEP components displayed polarity reversals in lamina V in every electrode track through area 17 while no polarity reversal was observed in any track through area 18 b. The primary negative component (N30) had an amplitude maximum in lamina IV in both areas. Current source-density analysis (CSD) in connection with amplitude depth profiles indicated that the surface-negative component N30 is generated mainly by the synaptic excitatory inputs in lamina IV. The negative peak of the flash-evoked afterdischarges in area 17 are generated the same way. The data imply a more concentrated field generator in area 17 with densely packed cellular elements responding synchronously to the flash. In area 18 b, either the anatomical distribution or the temporal relationship of the responding elements seems to be different.