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S. Kelly, K. Goodman, T. Makoni, C. Misener, A. Yeh, Y. Pang; Geometric Variables May Cause Errors in Distance Perception. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1825.
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We have previously reported that strabismic amblyopic subjects over-estimate the distance of objects when monocularly viewed at intermediate distances in a full cue situation with their non-amblyopic (i.e. fellow) eye (Tran et al, 08). According to recent models of space perception egocentric distance can be geometrically determined if the visual system can accurately determine 3 variables; the eye height, the angle of declination subtended by the object and the perceived slant of the surface the object is located on. If the visual system employs the trigonometric relationship between these variables to compute egocentric distance then errors in such perceptions should be explainable with these same variables. Thus the purpose of the current study was to investigate which of the above variables might underlie the over-estimation of perceived distance in the fellow eyes of strabismic observers.
We monocularly tested visually normal observers (n=22) and the fellow eye of strabismic amblyopes (n=3) on the following tests five tests: Perceived distance at near (50, 150 and 250 cm) with a blind-walking paradigm, verbal estimates of near distance (30, 60 and 90 cm), perceived slant of a near surface with both a palm-board and verbal estimate- tasks (at 0,20,40 and 60 deg) and perceived eye height.
The results indicate two variables may contribute to the over-estimation of target distance made by strabismic observers when using their fellow eye: the first is slant estimation. Like visual normals, strabismic observers over-estimate the slant of a near surface when measured with a verbal-report task, and are more veridical when estimates are obtained with a palm-board. However, the slope obtained from these subjects is flatter (0.56) than that observed by visual normals (0.81). A second finding is that strabismic subjects over-estimated eye height in comparison with the visual normals (z=-2.754, p=0.006).
When surface slant is under-estimated and eye height over-estimated then the perceived distance will be over-estimated. Although additional strabismic subjects need to be tested, the results obtained thus far indicate two potential visual cues that might underlie perceptual errors such as mistakes in distance estimates.
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