Light reflexes of the eye pupil are measured with an infrared scanning pupillometer in response to a 74-deg field. At threshold, the pupillary system responds to stimulus energy for short flashes and stimulus magnitude for long flashes. On a plot of log-flash magnitude vs log-flash duration, the intersection of the constant-energy asymptote and the constant-magnitude asymptote is defined as the critical duration. The critical duration was measured at threshold for the pupillary light reflex and visual perception, using seven subjects and three background levels. The critical durations were about 70 msec for most tests, but were shorter for very high backgrounds and longer with no background. Since other investigators have shown that the critical duration cannot be determined within the receptor, this suggests that the pupillary light reflex and visual perception share common temporal processing involving the nervous system beyond the receptor level.
JOHN G. WEBSTER, "Critical Duration for the Pupillary Light Reflex," J. Opt. Soc. Am. 59, 1473-1478 (1969)