The hemispheric distribution of the transient rat VEP: a comparison of flash and pattern stimulation.

We investigated the hemispheric distribution of the rat visual evoked potential (VEP) to pattern reversal and flash stimuli, presented monocularly at a rate of 1 Hz. Pattern VEP components could be recorded only over an area bounded by the anatomical coordinates of area 17, while some flash VEP components were recordable outside the primary visual area. Monocular pattern stimulation, as expected, evoked dominant contralateral VEPs. Surprisingly, ipsilateral responses could be also recorded with the reference electrode near the nasal bone. These VEPs showed partial polarity inversion compared to the contralateral EPs. To assess the origin of the ipsilateral EPs, we also recorded EPs following surgical deafferentation of the ipsilateral cortex. Our data reveal that ipsilateral VEPs represent volume conducted potentials.