title: Manipulating stereopsis and vergence in an outdoor setting: moon, sky and horizon.
abstract: A simple stimulus generator has been constructed that permits a small illuminated target to be seen with variable inter-ocular disparity, when superimposed upon the binocular view of an outdoor landscape. This device was applied to several questions involving perception of size, distance and orientation, with the following results: (1) when the apparent distance to an "artificial moon", as perceived through stereopsis, is decreased by about 50-fold (from near horizon to about 60 m), its apparent size is reduced by only a miniscule amount (8% on average); hence, the moon illusion is probably not due to compensation--conscious or subconscious--for its apparent distance; (2) those changes in apparent size known as convergence micropsia vary as a function of the visual surround; for a vergence change of 1 deg, greater perceived change in size of a small target arises when a landscape is seen nearby than with empty sky as surround; (3) when a target is shown somewhat above the horizon against an empty sky, it must be viewed with divergence of the visual axes (image positions for "hyper-infinite" distance), in order to be perceived as vertically above objects on the skyline; this effect implies a strong backward tilt to the apparent vertical and probably reflects an attempt to "null out" the perceptual consequences of the convergence that typically occurs during downward saccades.
authors.affiliation: Neurobiology Unit, Scripps Institution of Oceanography, La Jolla, CA 92093.
authors.forename: J T
authors.initials: JT
authors.lastname: Enright
mesh_terms: D003867:Depth Perception / Q000502:physiology; D004215:Distance Perception / Q000502:physiology; D004867:Equipment Design; D005133:Eye Movements; D006801:Humans; D009903:Optical Illusions; D012858:Size Perception / Q000502:physiology; D014787:Vision Tests / Q000295:instrumentation; D015348:Vision, Binocular / Q000502:physiology*