Exposure-time and the apparent length of an illuminated rotating arc-line

Eighteen male Ss made estimates of the apparent length of an illuminated arc-line rotating at 75 rpm. Two arc-lengths (6 and 12 cm) were exposed for 200, 100 and 40 msec. Apparent shrinkage occurred for both arc-lengths at 200 msec, but not at the other exposures. This result was interpreted as indicating that stimulation of a certain proportion of retinal units during a specific time interval is a necessary condition for the contraction effect to occur.