The hypothesis was proposed, that the perceived depth, which results from the relative height cue, depends on "optical adjacency." A 3 X 3 factorial experiment was conducted to examine this hypothesis. The 2 factors were vertical separation (3.5, 5.5, 7.5 in.) and background conditions (0 background, outline background without surface texture, textured background). Verbal estimates of the depth between pairs of frontal parallel points were obtained under the 9 conditions. In one experiment, the backgrounds simulated a floor surface, and in another, the backgrounds simulated a ceiling surface. The results in both experiments were comparable. Both main effects, separation and background, and the interaction effect were significant. All the effects were in the direction predicted by the optical adjacency hypothesis. Relative 'height is often included in general discussions of the cues for perceived depth. The higher of two objects will generally appear more distant. A search of the literature has failed to uncover any systematic experimentation dealing with this cue. Apparently the assertion that there exists a lawful relationship between relative height and relative perceived distance is based on informal observations and demonstrations. Two interesting demonstrations are provided by Gibson (1950, Fig. 72) and Kilpatrick (1952, p. 5). Although these demonstrations, and others like them, may be impressive, an experimental demonstration of this cue under controlled conditions is still desirable. However, let us assume for the moment that the vertical position cue is valid. What accounts for the effect of this variable? On this question, the experimental literature is also silent. Nevertheless, there are several possible answers, and one of these has been 1 This work was supported by Grant MH 4153 from the National Institute of Mental Health of the United States Public Health Service. examined in the present experiment. The explanation is derived from Gibson's (1950 Gibson's ( ,1959 analysis of the stimulus conditions for perceived space. In Gibson's view, the first step in accounting for the perceived distance of objects is to determine the optical stimulation responsible for the perception of an extended continuous ground surface. Gibson proposes that the optical gradient of textural density is one of the important stimulus correlates of perceived surface. The perceived distance of an object will be determined by its optical relationship to the ground surface, and can be specified by noting at what point, in the textured optic array, the contour of the object interrupts the texture of the ground. The foregoing analysis provides an explanation of the height-in-the-field cue. Objects which are higher up are normally optically adjacent with portions of the optical gradient of texture correlated with more distant (physically and perceptually) portions of the ground surface, than is the case for relatively lower objects. Therefore, the higher of two objects will appear farther away. Experiments have been 335 336 WILLIAM EPSTEIN designed to examine this interpretation. EXPERIMENT I General plan.-Reports of the perceived depth between a pair of frontal parallel points were obtained. The points were vertically aligned and were separated by one of three vertical interspaces. The prediction was that the depth perceived between the points would increase as the vertical separation increased. The points were observed under three background conditions: zero background, outline background without surface texture, textured background. The expectation was that perceived depth would increase as the specificity of the background (from zero to textured) increased Furthermore, a significant Separation X Background interaction was expected. The effects of varying separation were expected to be greater for the highly specific background. Subjects.-The 5s were 30 undergraduates. Ten 5"s were assigned to each of the three background conditions. Materials.-The standards were fluorescent points, § in. in diameter, located at a disance of 57 in. from .S's eye. In the completely dark laboratory room they were visible by the light of an ultraviolet tube. The S viewed the points monocularly through a red filter which eliminated any perceptible illumination of the surroundings. Under the condition of zero background, the points were viewed against a black background. For the outline background, the points were fastened onto a frontoparallel black cardboard sheet, on which the trapezoid illustrated in Fig. la was drawn in fluorescent paint. The separation between the two parallel sides was 8 in.; the longer of the parallel sides was 26 in.; the shorter, 8J in. The textured background is shown in Fig. Ib . Its outer dimensions were the same as the outline background. Both backgrounds yielded the impression of a receding ground. Figure Ib had the appearance of a floor. In addition to these stimulus materials a fluorescent footrule was prepared. Procedure.--Each S 1 made two verbal estimates of the depth between the points for each of three degrees of vertical separation: 3.5, 5.5, 7.5 in. The six trials were run in random order. Each 6" made judgments under one background condition only. For all background conditions, the absolute heights of the points were the same. The points were located approximately at eyelevel. The instructions called for a verbal estimate of the depth between each pair of points based on an immediate impression of their depth separation (phenomenal instructions). To facilitate performance, 6" was shown the fluorescent frontoparallel footrule immediately prior to the depth estimation task. The 6" was asked to make his judgments in multiples or fractions of a foot. The ruler was not present during the depth estimation task. The instructions also informed 6" that a report of zero depth was entirely acceptable.
For instance, Epstein shows that perceived distances do not solely depend on the vertical position in the visual field, but also on the background REF .
Perceived depth as a function of relative height under three background conditions
Journal of Experimental Psychology
Psychology Medicine