Investigated the finding of Busch and Wachholder that flicker fusion thresholds increase under conditions of mental activity and decrease during inactivity. Flicker fusion thresholds for each eye of 21 undergraduates were measured after 2-min. rests, then after the performance of a mathematical counting exercise, and then after another 2-min. rest. The average thresholds for both the right and left eyes were significantly higher following the counting exercise than after the rests. Features of the experiment included a training/screening session for the subjects to detect and correct “pseudothreshold” problems, and an automatic adjustment of the light frequency to minimize possible experimenter effects.