Locus of the Stimulus to Visual Accommodation: Where in the World, or Where in the Eye?

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Abstract
Eight observers judged the apparent size of a "moon" simulated by projecting a collimated disk of light just above the horizon in real scenes and virtual images. In each of three viewing conditions, Back-lighted Screen (collimated), Back-projected Photograph (collimated), and Natural Campus Scene, masks inserted in the viewing aperture of the "moon machine" obscured various horizontal bands of the lower half of the visual field. With each experimental manipulation, both the perceived size of the moon and the observer's visual accommodation distance (measured with a laser optometer) covaried systematically. From the combined results of two experiments, it appears that (1) viewing a collimated image of a natural vista does not have the same effects as viewing the actual scene, (2) the retinal locus of visible texture is the primary determinant of perceived size of objects of constant visual angle embedded in natural scenes, and it has a strong effect on accommodation to virtual images, and (3) the overall correlation between apparent, or perceived, size and accommodation shift from the individual's dark focus, averaged across observers, approaches unity (r = 0.97).

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