Simultaneous Brightness Contrast and the Pulfrich Phenomenon*

Abstract

If an object, moving from left to right, is viewed binocularly by an observer, and if a brightness reducing filter is placed in front of the observer’s left eye, the moving object will appear to be further away than it actually is. This demonstrates the Pulfrich phenomenon. The brightness of an object can be reduced in another way, through an intense inducing field, or glare source, placed nearby the object. This illustrates simultaneous brightness contrast. The present investigation concerns whether the Pulfrich phenomenon would occur if the brightness of the moving object were reduced (in one eye) not by a filter, but by an inducing field. Eight observers were used. When a filter was placed in front of the observer’s left eye, this produced a displacement of the moving object away from the observer. This was the Pulfrich effect. When an inducing field was presented slightly peripherally to the observer’s left eye, this produced a displacement of the moving object toward the observer, or in the opposite direction of the Pulfrich phenomenon. Displacement was directly proportional to inducing luminance. A “reverse” Pulfrich phenomenon occurred, therefore, when the brightness (to one eye) of a moving object was reduced by an inducing field rather than by a filter. From these results we can assume that the physiological mechanism for brightness reduction by an inducing field is different in whole or in part than that mechanism for brightness reduction by a filter.