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A.C. Love, G.L. McCormack; Bihaptic Measurement of Perceived Distance in Far and Near Vision . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4817.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose We previously reported that pointing the unseen forearms at a target (bihaptic localization) can indicate perceived distance in near vision. In this study we evaluated the validity and reliability of bihaptic localization in far and near vision. Methods Bihaptic distance estimates were made by twelve normally binocular subjects while viewing targets at 25, 50, 100, 200 and 400cm. Each target consisted of three luminous 10mm letters viewed in darkness during normal binocular vision or monocularly through pinhole apertures. Informed consent was obtained prior to conducting the experiment. The subjects started each trial with their forearms pointed beyond parallel, and with onset of the target had six seconds to rotate their arms inwards to point at the target. Target distances and protocol selections were block randomized. Results Responses were quantified by the slope of indicated distance on target distance and by the residual variance of the regressions. Slopes were identical in binocular (0.668) and monocular (0.667) viewing (f=0.001, p=0.97). Residual error was lower during binocular viewing (0.55), compared to monocularly pinhole viewing (0.65). In both conditions near targets were generally overestimated and far targets were underestimated, with correspondence of indicated distance and target distance at roughly 0.5 m. Conclusions The lack of difference between mean monocular and mean binocular indicated distance can be explained by a combination of three arguments: (1) binocular and nonbinocular cues to depth were in agreement in this protocol, so the loss of binocular cues should not cause a change of mean value, (2) convergence is a short-lived absolute depth cue*, unlikely to affect depth percepts under our protocol, and (3) the inherent slowness of bihaptic localization makes it relatively insensitive to convergence as a depth cue*. The increased variance during monocularity probably relates to the loss of stereopsis. The shape of the function of indicated distance on target distance compares with previous studies of perceived distance based on verbal reporting. This supports our contention that the bihaptic method is similar in behavior to verbal distance indices*. We conclude that bihaptic localization is valid for recording static distance perception. We intend to use it to evaluate distance perception in divergence excess intermittent exotropia. * McCormack & Love, ARVO ‘03
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