If both eyes are subjected to flickering stimuli of equal luminance and frequency, the critical fusion frequency is usually somewhat higher when the stimuli are in the same phase than when they are in the opposite phase. The difference in fusion frequency for the two cases is herein termed the Sherrington effect after its discoverer. Apparatus for measuring this effect is described in the present paper. It was found that, for a 2° visual field and dark surround, the Sherrington effect was proportional to the mean critical frequency for the two eyes as the field luminance was varied, being about 8 percent of it for the present observer. At constant field luminance, the effect approached zero as the field angle approached zero but was approximately constant for angles greater than 2°. The mean monocular fusion frequency for seven observers was below the simultaneous (in-phase) binocular frequency, and for a 0.25° field it was even below the alternate (out-of-phase) frequency; for three observers, it was below the alternate frequency for a 2° field. The present data appear to be inadequate to serve as a basis for deciding between alternative explanations of the phenomena.
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