VISUALLY PERCEIVED EYE LEVEL IS INFLUENCED IDENTICALLY BY LINES FROM ERECT AND PITCHED PLANES

The physical elevation that appears to correspond to eye level (VPEL), as measured with a small visual target, changes systematically with t he orientation in depth ('visual pitch') of a visual field consisting of one or two pitched-from-vertical lines in darkness. The influence i s large and, with a one-line stimulus, is only 15% smaller than the in fluence exerted by a complexly structured, well-illuminated, pitched v isual field. A line from a frontoparallel plane can be presented to th e same retinal locus as a pitched-from-vertical line; the three experi ments in the present report were aimed at determining the influence on VPEL from such lines. In the first two experiments the subject viewed a visual field consisting of a one-line or two-line pitched-from-vert ical stimulus from a pitched-only plane or an oblique one-line or two- line stimulus from an erect plane. Each of the pitched-from-vertical s timuli was presented at seven different orientations separated by 10 d egrees over a +/-30 degrees range. Each of the oblique-line stimuli wa s presented at an orientation that resulted in stimulation to the same retinal locus as one of the conditions with pitched-from-vertical lin es, and thus a range of 'equivalent pitches' was examined that corresp onded to the range of pitches for the pitched-from-vertical lines. The variation in orientation of the oblique-line stimulus and the pitched -from-vertical stimulus each produced systematic changes in VPEL; the two were indistinguishable. A third experiment specifically designed t o examine the possibility that either stimulus sequencing or lack of n aivity of the subjects might have been involved turned up no such effe cts. It is concluded that the aspect of a line stimulus that controls the influence on VPEL is the orientation of the image of the line on a projection sphere centered on the nodal point of the eye or, as in th e present experiments with viewing in primary position, the retinal lo cus stimulated; the orientation-in-depth of the stimulating line provi des no additional influence on VPEL for the stationary, erect, monocul arly viewing observer. The results are interpreted within the framewor k of the great-circle model.