William Marcum¹, Maria Carmichael¹, J. Farley Norman¹; ¹Western Kentucky University

An experiment evaluated the importance of linear perspective for the visual perception of environmental slant. Eighteen observers (ages ranged from 19 to 80 years) judged the slant-in-depth of a rectangular (98 cm wide x 120 cm long) physical surface, where the actual slants varied from 20 to 65 degrees. The rectangular surface possessed six narrow (3.7 cm) wooden slats oriented along the longer axis; a column of 11 LED’s (light emitting diodes) ran along the center of each lengthwise-oriented slat. When viewed by an observer in lighted visual conditions, the physically parallel slats of the slanted surface appeared to converge (in a projected image, such as a retinal image) due to the effects of linear perspective. Likewise, when viewed by an observer in the dark, the six columns of LED’s also converged in the observers’ retinal image due to the effects of linear perspective. The observers’ task was to adjust the orientation of a line on a computer monitor (relative to vertical & horizontal axes) until its orientation matched the perceived slant of the rectangular surface in depth. The observers viewed the experimental stimuli monocularly (to prevent the use of binocular disparity) and used a chin rest to similarly prevent the potential use of motion parallax as a source of information about slant. Multiple judgments were obtained for each physical stimulus slant. Linear regressions obtained for each observer in both full-cue and dark conditions showed that the observers (both younger and older) could perceive environmental slant almost as well in the dark (when only linear perspective information was available) as in fully lighted conditions (average Pearson R correlation coefficients were 0.951 and 0.935 for lighted and dark conditions, respectively). The results clearly demonstrate that linear perspective is a powerful and effective source of information about environmental slant.