Juliet Horenziak¹, Keithan Ducre¹, Peter Xu¹, Kalanit Grill-Spector¹; ¹Stanford University

Recent research from our lab discovered a new psychological phenomenon – perceptual magnification – wherein observers perceive small and distant objects as larger. Surprisingly, we found that perceptual magnification is substantially reduced when viewing the same scene monocularly. However, it is unknown whether this phenomenon differs for observers with impaired depth perception. Here, we tested perceptual magnification in participants with varying degrees of stereovision. 31 participants completed size estimation tasks (20 female, 11 male). Stereoacuity was assessed using the Randot Stereo Test. 17 participants were stereotypical (≤70 arcseconds), 6 had reduced stereovision (71-500 arcseconds), and 8 were stereoblind (>500 arcseconds). Participants made size estimates of objects while seated in Stanford’s Main Quad using reference probes placed on a table in front of them. They first completed a perceptual matching task in which they binocularly viewed 8 target objects at different distances [15.20–151.89 m], visual angles [0.38–7.15°], and elevations [-3.183º– 9.43º]. For each object, they were asked to select the probe that best matched the size of the target in their visual field. Then, they repeated the experiment, now holding the probes up to each object and adjusting the chosen probe until a size match was found. The second experiment was done monocularly. We find that all participants, stereoblind, stereoreduced, and stereotypical, magnify the size of distant objects. Perceptual magnification is greater for smaller, more distant, and less elevated objects in the binocular case. In the monocular case, perceptual magnification is greater for smaller, more distant, and more elevated objects, but the effects of distance and elevation are overall smaller in magnitude. These results suggest that stereoacuity impacts the size perception of distant objects, even at distances of up to 150 m, typically not considered to use stereovision.

Acknowledgements: Stanford Vice Provost for Undergraduate Education Small Grants