Flicker masking depends on the method of measurement

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.

Peripheral and central delay in processing high spatial frequencies: reaction time and VEP latency studies

Visually evoked potentials (VEP) and reaction time (RT) were recorded under stimulation with sinusoidal gratings. Grating spatial frequency (SF) was 0.5, 5 or 12 cd and grating contrast was varied. Consistent with previous findings, both VEP latency and RT increased with the increase of grating SF and with the decrease of grating contrast. It was found, in addition, that RT and VEP latency increased by approximately the same amount when SF increased from 0.5 to 5 cd, thus suggesting that the main source of the RT delay at 5 cd in comparison with RT at 0.5 cd is of peripheral origin. However, in comparison with the data at 0.5 and 5 cd, RT at 12 cd increased much more than VEP latency. We conclude that the RT delay at high SF involves a substantial central component in addition to the peripheral delay.