Visual evoked potentials following abrupt contrast changes

The timing of visual evoked potential (VEP) amplitude and phase changes following abrupt increases or decreases in contrast was examined. Gratings (1 c/deg) were presented at a low contrast for 8 sec, increased to a higher contrast for 8 sec, and then decreased to the initial lower contrast for another 8 sec. Second harmonic VEP amplitude and phase were recorded continuously and averaged in 1 sec epochs. Both amplitude and phase exhibited delays in reaching a stable level following the contrast change. For amplitude, the length of the delay was dependent on the magnitude and direction of the contrast step and on the spatial frequency of the stimulus. Time constants for the change in amplitude following step increases in contrast ranged from 0.2 sec for a 12% contrast step to 1.34 sec for a 37% contrast step. The timing of phase changes, however, was independent of the size of the contrast increases (tau = 0.7 sec). For step decreases in contrast, both amplitude and phase were relatively independent of the size of the change (tau = approx. 0.9 sec for amplitude and tau = 0.15 sec for phase). Amplitude time constants also increased with increasing spatial frequency (tau = 1.2 sec for 1 c/deg, tau = 1.6 sec for 4 c/deg and tau = 2.3 sec for 8 c/deg); phase time constants, however, did not change as a function of spatial frequency (tau = 0.7 for all spatial frequencies). These findings demonstrate that a unitary process may not always be tapped by signal averaging techniques. Additionally, swept stimulus VEP techniques may produce considerable errors in threshold estimation depending on the stimulus spatial frequency and on the slope and direction of the contrast change.