Relation of Entoptic Stray Light to Flicker and the Perception of Movement. ∗

Abstract

Under certain conditions, flicker is seen in the visual field surrounding the test-object, presented against a dark background, when this object subtends only a small visual angle. Unlike flicker in the test-object itself, field flicker has received little or no study. It appears when the test-object is very bright, or large, or when the flicker of the test-object is much below its critical fusion frequency. The question arises whether the phenomenon is due either to some sort of neural interaction, or to entoptic stray light. Recently considerable evidence has been adduced to make it entirely probable that it arises from the latter source. To determine this conclusively, 2 tests were made. Using monocular vision and locating the image of the test-object on the “blind spot” where no neural interaction can take place, field flicker was not abolished. On the other hand, when 2 test-objects equal in all respects are alternately presented on 2 different parts of the visual field, the first disappearing as the second appears, thus keeping the stray light constant, field flicker is abolished, demonstrating its dependence upon actual intermittent illumination rather than upon neural interaction between parts stimulated and those not.

The relation between the critical flicker frequencies of the test-object and the surrounding field was measured under a variety of conditions. With areal increase of the test-object, the c.f.f.'s of both test-object and the field were raised in such a manner as to indicate some interaction between the 2 areas. With increase in intensity, the c.f.f.'s of both were at first raised till a point was reached at which the c.f.f. of the test-object began to fall, so that with further increase in intensity the c.f.f.'s of the 2 areas became identical. Finally the c.f.f. of the test-object became even lower than that of the field. This was true for both foveal and peripheral vision.