The early wave of the visual evoked potential to sinusoidal gratings: responses to quadrant stimulation as a function of spatial frequency.

VEPs were elicited by sinusoidal grating patterns of differing spatial frequencies presented in each of the 4 quadrants of the visual field. Analysis of the early wave of the VEP indicated a pattern of polarity inversions when wave forms were compared across the horizontal meridian of the visual field. With the exception of low spatial frequencies, clear polarity inversions also occurred across the vertical meridian of the visual field in the case of laterally placed electrodes. Comparison of the clearly identifiable early peak with records for the same subject using central field stimulation showed the complexity of the latter records and the inherent difficulty of isolating and interpreting specific component waves of the VEP without adequate topographical data from quadrant stimulation.