The functional relation of visual evoked response and reaction time to stimulus intensity.

Latency of the average visual evoked response (VER) and motor reaction time (RT) were studied as a function of stimulus intensity for brief photic stimuli subtending 4° and 1.5 of visual angle in two subjects. Both VER latency and RT showed an accelerating increase for each tenfold diminution in intensity down to the region of foveal threshold. Below foveal threshold no responses were obtained for the 1.5° stimuli; there was an inflexion in the VER latency and RT curve of responses to the 4° stimuli. Over the photopic range of intensities, VER latency and RT were closely described by power functions varying in exponent from −0.29 to −0.44. The values for VER were −0.36 for the 4° stimuli and −0.40 for the 1.5° stimuli, which were significantly different ( p < 0.01). Although latency of VER was the same for both subjects for each stimulus condition, RT showed a consistent difference between subjects of about 25 msec. RT is considered to be determined by at least two independent mechanisms. The first, retinal in location, follows a power function of intensity; the second is related to variability in efferent processes.

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