The locus of perceived equidistance in the eye-level plane was determined at distances of 1.2, 2.2, 3.2, and 4.2 m from the observer. The stimuli were small, point-like light sources viewed in complete darkness. The observer's head was held fixed; his eyes were allowed to move freely. There were five lights, one in the median plane which remained fixed on every trial, and two variable lights on each side of this at angles of 12° and 24° with respect to the median plane. The locus of perceived equidistance was found to be concave toward the observer at all distances, usually slightly asymmetric with respect to the median plane, and with a variable curvature generally intermediate between that of the physically equidistant circle and that of the corresponding Vieth—Müller circle. The results are inconsistent with an assumption made by Luneburg in his theory of space perception. The pattern of disparities provided by the locus of perceived equidistance was found to vary with viewing distance. This indicates that the perception does not depend on the spatial distribution of retinal stimulation alone and poses a problem as to the nature of the cues that determine perceived equidistance in this situation.
JOHN M. FOLEY, "Locus of Perceived Equidistance as a Function of Viewing Distance," J. Opt. Soc. Am. 56, 822-823 (1966)