Fall in Critical Flicker Frequency associated with Adaptation to Intermittent Light

Abstract

IN a series of experiments in which the effects of centrally acting drugs on the adaptation of the critical flicker frequency (CFF) to intermittent light of varying frequency was investigated, a consistent placebo effect was noted, namely a fall in mean CFF^1–3. This had not been described by other workers investigating the effects of drugs and placebo on mean CFF alone, and it was therefore possibly due to the method used. The method used involved exposing subjects for periods of 1 min to an intermittent light of 25 or 50 c/s, before measuring ascending and descending thresholds of CFF³. To investigate this further, the CFF was measured in six subjects at 0, 1.5 and 3 h on 3 consecutive days, beginning at 9 a.m., about 1.5 h after a light breakfast. The subjects abstained from tea, coffee and smoking during the experimental period. On one day, ascending and descending thresholds were measured without previous adaptation, while on the others, subjects were exposed for 1 min to intermittent light at 25 or 50 c/s before the thresholds were measured. The method of measurement was the same as that previously described³ except that the light source was viewed through an artificial pupil 2 mm in diameter and was placed at the focal point of a lens which delivered parallel light to the eye. This procedure eliminated any effect of changes in pupil size of the subject. An interval of 1 min elapsed between each determination.