Differential activation of multiple current sources of foveal VEPs as a 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 frequencies with a high density electrode montage. Scalp isocontour voltage and scalp current density (SCD) maps were computed to assess differences in scalp topography of the sensory-evoked responses as a function of spatial frequency. Topographic analysis showed a segregation of evoked responses to stimuli of different spatial frequencies. While low frequency patterns elicited a bilateral positive potential localized at lateral occipital sites from 60-120 msec, high frequency patterns elicited a prominent midline occipital negative potential. SCD revealed that, 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 lateral-occipital regions of the scalp. The current source was much more prominent than the sink for lower spatial frequency, and vice versa. Moreover, 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 frequency increased. The present electrophysiological findings suggest the view that: (1) visual sensory-evoked potentials elicited by low versus high spatial frequencies have different polarity and topographic localization, (2) these potentials might reflect the activation of functionally distinct, topographically segregated, neural generators differentially activated as a function of spatial frequency, and (3) these generators seem asymmetrically distributed over the left (LH) and right (RH) hemispheres