The effect of blur and contrast on VEP latency: comparison between check and sinusoidal and grating patterns.

Pattern defocusing was used to evaluate the contribution of different spatial frequency components in checks to VEP latency. Latency shifts with increasing blur (-2.5 to +2.5 diopters) were determined for sinusoidal grating and check patterns equated in fundamental spatial frequency. With both check and grating patterns, the effect of blur was greater the higher the spatial frequency. Given an equal fundamental spatial frequency, however, the latency of checks was more effected. This latency difference between check and sine patterns was pronounced at low fundamental spatial frequencies (large pattern) and decreased with higher spatial frequencies (small pattern). Latencies were then compared for patterns which were defocused vs. simply reduced in contrast. Results show that the increase in latency with defocused large checks is due to both fundamental and higher harmonic spatial components but with small checks, to the fundamental spatial frequency alone.