DIFFERENTIAL ACTIVATION OF MULTIPLE CURRENT SOURCES OF FOVEAL VEPS ASA FUNCTION OF SPATIAL-FREQUENCY

The purpose of the present study was to map sensory-evoked potentials to foveally presented square-wave gratings of different spatial freque ncies with a high density electrode montage. Scalp isocontour voltage and scalp current density (SCD) maps were computed to assess differenc es in scalp topography of the sensory-evoked responses as a function o f spatial frequency. Topographic analysis showed a segregation of evok ed responses to stimuli of different spatial frequencies. While low fr equency patterns elicited a bilateral positive potential localized at lateral occipital sites from 60-120 msec, high frequency patterns elic ited a prominent midline occipital negative potential. SCD revealed th at, for any spatial frequency, two current density foci were evident: an earlier negative focus (current sink), centered at mesial-occipital areas, and a later positive focus (current source), centered at later al-occipital regions of the scalp. The current source was much more pr ominent than the sink for lower spatial frequency, and vice versa. Mor eover, the source was larger over the right side of the scalp, whereas the current sink shifted from the right to the left side as spatial f requency increased. The present electrophysiological findings suggest the view that: (1) visual sensory-evoked potentials elicited by low ve rsus high spatial frequencies have different polarity and topographic localization, (2) these potentials might reflect the activation of fun ctionally distinct, topographically segregated, neural generators diff erentially activated as a function of spatial frequency, and (3) these generators seem asymmetrically distributed over the left (LH) and rig ht (RH) hemispheres.