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Frank Durgin, Zhi Li, Brennan Klein; Angular expansion theory turned on its side. Journal of Vision 2014;14(10):154. doi: 10.1167/14.10.154.
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When standing, egocentric distance can be specified angularly by direction of gaze to the point of ground contact (Wallach & O'Leary, 1982). Estimates of egocentric distance show underestimation by 0.7, consistent with an observed overestimation of gaze declination by 1.5 (Durgin & Li, 2011). Moreover, perceptual matching of ground distances to pole heights can be perfectly modeled by a 1.5 expansion of perceived angular declination relative to the horizontal (Li et al., 2011). In azimuth, extent matching corresponds to an angular expansion of about 1.2 (Li et al., 2013). Are these angular biases associated with the coding of gaze position in the head or with the reference frame of the horizontal ground plane? We tested this question in an open field using people as targets by comparing perceptual matching by upright observers and by observers suspended on their sides at eye level. Participants instructed one experimenter to move left or right so as to create a frontal distance from a second experimenter equal to the participant's egocentric distance to the second experimenter. Implicitly, the task is to create a 45째 azimuthal angle. Would matches made by observers on their side show an angular gain of 1.5, consistent with their bodily orientation, or would they show the more typical azimuthal gain of 1.2? A total of 35 participants (18 sideways) matched egocentric distances of 7 to 16 m and made verbal estimates of a 35 m egocentric extent and a 25 m frontal extent 35 m away. In fact, participants on their side showed twice the angular bias as upright participants -- both in their extent matches and in their verbal estimates of distances. The sideways verbal estimates implied an angular expansion by 1.4. These angular distortions do not seem to affect shape perception, but only the estimation of extents between objects.
Meeting abstract presented at VSS 2014
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