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S. Kelly, D. Baker, M. Henrickson, E. Ittner, M. Saeed, Y. Pang; The Implicit Surface in the Fellow Eyes of Amblyopes. Invest. Ophthalmol. Vis. Sci. 2009;50(13):3821.
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© ARVO (1962-2015); The Authors (2016-present)
In full cue conditions, humans can veridically locate a target at intermediate and far distances if they can accurately perceive the target’s angular declination and the slant of the near ground surface. However, in the dark, observers usually underestimate the distance of self-illuminated targets and overestimate the height. Since subjects can correctly perceive angular declination in the dark, these perceptual errors arise from the subject’s forced dependence on an internal reference plane, or implicit surface, as the ground surface is not visible. The intersection of the target’s angle of declination with this implicit surface is used to determine the target’s perceived location in darkness. The presence of an amblyogenic factor during development may adversely affect the formation of the implicit surface; specifically its slant. We compared distance and height perception, angle of declination and implicit surface slant obtained from visual normals to those obtained from the fellow eyes of amblyopic observers.
The perceived distance of a self-illuminated target was measured at 6 distances ranging from 4m-7.5m using the blind-walking paradigm. Hand gestures were used to indicate perceived height. All subjects were given practice trials prior to testing. Twenty visually normal observers served as controls. Distance and height perception were measured in the fellow eyes of four strabismic amblyopes, three anisometropic amblyopes and one deprivation-induced amblyope.
Our experiments revealed that: 1) like visual normals, the amblyopes underestimate target distance and overestimate height; 2) visual normals, anisometropic and strabismic amblyopes correctly perceive the angular declination of the self-illuminated targets; 3) the slant of the intrinsic surface for the visually normal observers is 16.9 deg; for anisometropic amblyopes it is 10.33 deg; and for the strabismic amblyopes it is 7.98 deg.
Like visual normals the amblyopes misjudge the egocentric height and distance of targets in the dark. However their perceptual errors are not the same. The source of errors for all groups appears to reside, at least in part, in the slant of the implicit surface.
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