Peripheral visual response time and retinal luminance-area relations

Button pressing response time was measured in five trained subjects to the onset of small visual stimuli imaged across the horizontal retinal meridian at 10 degrees arc intervals from 90 degrees arc right to 90 degrees arc left of the line of sight. Four stimulus luminance conditions were presented at each peripheral retinal location, a low luminance condition (approx. 2.6 ft-L), a high luminance condition (arox. 22.2 ft-L), and two conditions in which luminance was varied systematically from low to moderately intense levels from one side of the visual field to the other. The two variable luminance conditions were included to see if it is possible to compensate for the progressive decrease in retinal stimulus image area caused by the apparent pupil. The results indicated that response time to stimuli imaged within approximately 70 degrees arc from the fovea does not differ significantly when all possess about the same luminance. Mean response time is significantly faster to stimuli imaged beyond about 70 degrees arc from the fovea when their luminance is increased by an amount equal to the foveal stimulus luminance multiplied by the cosine of the angle between the peripheral stimuli and the line of sight. These data are discussed in relation to previous psychophysical data and to possible response mechanisms.