Stimulus size and visual angle in a cue-reduced experimental setting

Abstract

The effect of stimulus size and visual angle on the perception of luminescent objects was studied. Forty subjects were asked to estimate the size of one of four stimulus objects viewed through a pinhole eyepiece. Their estimates corresponded to the retinal angle, rather than the metric size of the stimuli. The probable roles of the specific distance tendency and aerial perspective as perceptual anchors that govern these estimates were discussed.In independently conducted research, Baird (1964), Gogel (1969), and Gogel and Sturm (1971 reported that, in the absence of uncontrolled depth cues, retinal image, not metric size, is discriminated. A more recent investigation reported by Fitzpatrick, Pasnak, and Tyer (1982), however, indicated that retinal image was not discriminated in a cue-reduced experimental situation. Fitzpatrick et al. (1982) used a pinhole eyepiece to control for accommodation and monocular accommodative convergence. This method of cue control was also employed by Gogel and Sturm (1971), but it produced conflicting results in their situation. The present investigation was designed to test the effect of stimulus size and retinal angle on size perception using the pinhole eyepiece method of cue control. Comparisons that facilitate a statistical assessment of the success of cue reduction were incorporated in the experimental design. METHOD SubjectsSubjects were 40 volunteers who were naive regarding the nature of the experiment and were not familiar with the experimental suite. Only individuals with 20/30 vision or better in one eye as measured by a Snellen Eye Chart were used as subjects, and none wore corrective lenses. ApparatusThe lighttight laboratory suite was internal to other laboratory rooms and separated from them by an exterior hallway so that subjects could gain no idea of the shape or size of the experimental chamber . The experimental chamber measured 18.24 x 2.74 x 3.04 m , Accommodation cues were controlled by mounting a 1.0-mm pinhole eyepiece in the door of the experimental chamber 1.17 m from the floor . The effect of mounting the eyepiece in the closed door was to deny subjects an opportunity to estimate the size of the chamber .Four stimuli were constructed from luminescent strips (John Meshna, Inc., Lynn, Massachusetts) that emitted a uniform faint green glow when alternating current was applied to them. The stimuli's dimensions were (A) 2.54 x .64 em, (B) 5.08 x 1.27 em, (C) 20.53 x 2.54 em, and (D) 41.06 x 5.08 em.All stimuli were mounted on wooden stands 1.17 m from the floor. A and B were placed 2.44 and 4.88 m, respectively, from 29 the subject's eye, subtending a visual angle of 1 deg 12 min. C and D were situated 4 .88 and 9.75 m, respectively, from the subject's eye, subtending a visual angle of 4 deg 48 min.Prior to the experiment, brightness matches between all stimuli were determined by adjusting supply voltages to the strips using ac variacs. Luminance at the eyepiece was -.03 log f'L, The low luminance, unfamiliar dimensions of the experimental chamber, and monocu...