The Effect of Two Brightness Factors upon the Rate of Fluctuation of Reversible Perspectives

Abstract

In this study an attempt was made to discover the effect of two brightness factors upon the rate of fluctuation of reversible perspectives. The two factors considered were: (1) brightness contrast between contour and background; and (2) illumination. The Necker cube was used as the stimulus-object. It is perhaps questionable whether this figure furnishes the best instance of what Kohler calls contrast between figure and environment, which, together with neighboring areas produce, as he believes, a current in tissue in the visual brain center, the strength of which depends on the brightness difference between them.1 The cube was chosen, however, because it was thought to furnish changes defined more sharply than any other figure. Observers and apparatus. Thirty-six students (women from Sweet Briar College), divided into six groups of six each, served as Os in this study. A 2-in. cube, drawn with dark gray contour lines 1/8 in. wide, was presented under three conditions: (A) on a white ground, under 200-w. illumination; (B) on a medium gray ground, under 200-w. illumination; and (C) on a white ground, under 15-w. illumination. (A) and (B) furnish a difference in degree of contrast, with illumination constant; and (A) and (C) provide a difference in amount of illumination, with contrast constant, as both outline and ground are here reduced by the same fraction. Kohler holds, as will be remembered, that the processes of the visual center tend to block themselves, thus producing a shift in perspective; and, further, that the rate of blocking depends upon the strength of the current which, in turn, depends upon the amount of brightness contrast between figure and environment.2 If this theory is true, then the greater the brightness contrast, the faster should be the rate of fluctuation. It was this point that the present experiment was designed to test under conditions which measured the efficacy of contrast apart from degree of illumination, and of illumination apart from contrast. Procedure. After preliminary instruction, 0, with chin in head-rest and seated 3 ft. from the pattern, closed her eyes for 15 sec., then fixated a neutral point at the center of the cube and reported the shifts in perspective as they appeared until 10 changes occurred. The time of every change was noted. This procedure, repeated 20 times, constituted a series of observations under one of the conditions described above. After a 2-min. rest, a similar series was made under each of the other condi-