Perimetry of contrast detection thresholds of moving spatial sine wave patterns. IV. The influence of the mean retinal illuminance.

Contrast detection thresholds for moving sine wave gratings were obtained at the fovea and at eccentricities of 6 degrees, 21 degrees, and 50 degrees on the nasal horizontal meridian. The targets subtended from 30 X 30 minutes of arc up to 16 degrees X 16 degrees. Mean retinal illuminance was varied between 10 and 0.01 trolands. The transition from the de Vries-Rose to the Weber region occurs in the far peripheral visual field at a 2-3 decades lower illuminance level than at the fovea. The spatio-temporal contrast detection thresholds become comparable over the whole visual field if the mean distance between retinal ganglion cells is taken as a yardstick, and field width, spatial frequency, and quantum density are scaled accordingly. This means that at scotopic illuminance levels coarse or medium gratings are preferentially detected at other than foveal locations. (The fine gratings cannot be resolved at all at such levels.) It is argued that both electrophysiological and psychophysical evidence indicates that Weber behavior starts whenever some small fixed number of quantum absorptions occur within an area of 1 mean interganglion cell distance across. Or, equivalently, if a fixed small number of "neural quanta" enters a 100 X 100 micron2 area of the visual cortex.