Visual-evoked responses elicited by the onset and offset of sinusoidal gratings: latency, waveform, and topographic...

Transient evoked potentials (VERs) elicited by the onset and offset of sinusoidal gratings have been investigated. Results indicated that an increase in amplitude and a decrease in latency characteristics of VER occur as monotonic functions of increasing contrast. The latencies were shorter for low than for medium spatial frequency gratings up to contrasts of 0.51. Previous reports of the latency of the VER increasing as a monotonic function of increasing spatial frequency are confirmed. Topographic studies of the scalp distribution of VERs indicated that the site of maximal response were similar for low and medium spatial frequencies and that potentials elicited by stimulus offset had a similar distribution to the early onset response. These results indicate that the same cortical region may be responsible for generating these potentials despite their different latency and trigger features. Additional data are presented to illustrate the likely effects of differential recording on the waveform of the VER.