On the delay in processing high spatial frequency visual information: reaction time and VEP latency study of the effect of local intensity of stimulation

Saleh and Bonnet [Fechner Day 98, p. 344] have shown that, upon parafoveal stimulation and up to 6.5 c/deg, reaction time (RT) is a function of grating contrast multiplied by grating period. The present experiments extend these findings to foveal stimulation within a wider spatial-frequency (SF) range and to stimuli of different duration. Both RT and latency of visually evoked potentials (VEP) were measured. The findings might be explained by the following assumption: Most RT and VEP latency variations across the SF range are a result of local intensity factors (retinal contrast and width of grating bars). Residual RT variations were found that might be due to processing of high SFs by slower mechanisms than those processing low and medium SFs.