Visual Evoked Potentials: Analysis of the Fovea and Perifovea

Visual evoked potentials (VEP) were used to measure how stimulus properties (pattern VEP check sizes/spatial frequency) and retinal eccentricity (fovea versus perifovea) interact to give rise to the final VEP response. The purposes of this study were to investigate how stimulus check size (spatial frequencies) and retinal eccentricity affect the VEP response, re-examine whether the cortical magnification factor is applicable to VEP measures, and to determine optimal sized VEP checks for foveal and perifoveal stimuli. In this study, we used a foveal target that was a two degree circle with a diameter of 3.6 cm; a perifoveal target that was a circular ten degree annulus with an inner diameter of 3.6 cm and an outer diameter of 17.5 cm; and a full field target that was a ten degree circle with a diameter of 17.5 cm. These stimuli were chosen because they stimulate approximately the same amount of cortical area. VEPs were performed on ten healthy adult subjects monocularly through the dominant eye. Measurements of the implicit time (N1 and P1) and amplitude (N1-P1) were taken using four different VEP check sizes, 0.23, 0.52, 0.83, and 1.78 degrees). The findings of this study indicate that differences exist in sensitivity to specific check sizes (spatial frequencies) depending on the type of VEP measure used (implicit time or amplitude) and area of retina stimulated. These results are not consistent with a single stimulus being optimal for all measures and that there is a complex interaction between visual targets and responses