EFFECT OF AN INTERMITTENT LIGHT STIMULATION ON THE CRITICAL FUSION FREQUENCY

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Abstract
The effect of intermittent light stimulation (ILS) on the critical fusion frequency (CFF) has been studied. Some stimulation frequencies (effective frequencies) induced a drop in the CFF. These frequencies are lower than the initial CFF and higher than 2 cps. The maximum drop was obtained for a frequency equal to about half of the CFF of the subject. The relation between the drop of the CFF and the frequency of stimulation can be expressed as a U-shaped curve. It can be mathematically formulated for each subject with three constants which varied remarkably little from subject to subject. The time course of the drop for a given frequency as well as the kinetics of recovery were exponential. The time constant was independent of the frequency of the ILS. If the stimulus consisted in printed letters read by the subject, a drop of the CFF followed by a recovery was observed, both with an exponential time course. The influence of these effects on the techniques used to measure the critical fusion frequency has been discussed. The value of a CFF decrease as a criterion of mental fatigue has been questioned.