Transient visually evoked potentials to the pattern reversal and onset of sinusoidal gratings

Transient visually evoked potentials (VEPs) recorded in response to the contrast reversal and onset of spatially sinusoidal gratings have been investigated. Previous reports of an increase in the latency of the response at higher spatial frequencies have been confirmed but only for spatial frequencies higher than 2 c/deg. At lower spatial frequencies it is suggested that two positive components interact resulting in a departure from a monotonic relationship between latency and spatial frequency. Below 1 c/deg pattern reversal and pattern onset modes of stimulation produced VEPs of similar amplitude and wave form. Above 1 c/deg the amplitude of the pattern onset response peaked at a higher spatial frequency than the response to pattern reversal, and the response was dominated by a negative (N1) rather than a positive component (P1). This distinction has been corroborated by investigating the effect of field size variation. The peak amplitudes shifted to a lower spatial frequency with increase in field size but at all field sizes the N1 component of the pattern onset response peaked at a higher spatial frequency than the other components measured. It is attempted to relate these findings to previous studies of both grating and checkerboard VEPs and to psychophysical studies of contrast sensitivity.