Spatial summation in human vision: simple reaction time measurements.

The reaction-time technique was applied to examine spatial summation or area-intensity reciprocity at suprathreshold levels in the fovea. A family of reaction time vs luminance curves was measured in two experiments, and the luminance required to produce a criterion reaction time was computed from these curves to estimate the extent of summation. The first experiment showed that the upper level of spatial summation, the Ricco area, defined for constant reaction time increases with decreasing luminance level and that the upper limit of temporal summation is independent of the change in target size. The second indicated that the Ricco area decreases with increasing pulse duration up to 20-30 ms and then remains constant.