Differentiation of retinal and nonretinal contributions to averaged evoked responses obtained with electrodes placed near the eyes

Recent visual evoked potential (VEP) studies suggest that during sustained voluntary attention, irrelevant or unattended visual stimuli may be filtered precortically. Testing of this hypothesis in humans requires knowledge of the origin of specific components of the VEP that are affected by selective attention. This study describes techniques and procedures for distinguishing between retinal and nonretinal components of VEPs recorded from electrodes placed strategically around the eyes and on the forehead. By visually stimulating one eye at a time while recording simultaneously from both, and by stimulating a given retinal area with the eyes fixated on the target at different angular deviations from the normal line of regard, components of the VEPs around the eyes can be distinguished as of retinal or nonretinal origin. The results are discussed in terms of volume conduction theory.