Abstract
The visual system infers the three-dimensional shape and location of objects from two flat retinal images. Binocular disparities between these retinal images, resulting from the horizontal offset of the eyes, provide a shape cue that the visual system is highly sensitive to. Indeed, even minor changes of these disparities can have dramatic effects on perceived shape. For example, corrective spectacles can produce distortions in perceived shape when there are differences in the minification and magnification of the two eyes' images. It is well established that monocular horizontal and vertical magnification make fronto-parallel surfaces appear slanted away or towards the magnified eye, respectively. Previous literature found that monocular uniform magnification produced a small perceived slant in the same direction as horizontal magnification. It was hypothesized that this was a result of averaging the effects of the horizontal and vertical magnification, but this hypothesis has not been tested. In this study, participants judged the slant and shape of objects while experiencing a range of interocular differences in horizontal, vertical, or uniform magnification. Participants adjusted the slant or shape of a stimulus until it appeared fronto-parallel or square, respectively. Consistent with previous literature, monocular horizontal and vertical magnification produced perceived slants in opposite directions and greater magnification produced greater perceived slants. Monocular uniform magnification also produced a perceived slant. This effect was smaller than the effect seen with meridional magnification, but larger than expected if the visual system were averaging the opposing meridional effects. The shape distortion produced by meridional magnification roughly followed the pattern of the perceived slant, indicating that these distortions likely share a common cause. These results suggest that perceptual distortions associated with spectacles are more common than previously thought, and raise new questions about how the visual system processes binocular disparities to infer the shape and slant of objects.