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H.L. Humble, P. Ghude, J.V. Odom; Effect of Prism Orientation on Heading Detection Bias in Optic Flow Displays . Invest. Ophthalmol. Vis. Sci. 2005;46(13):4612.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Recent studies of ambulatory behavior indicate that when moving toward a target people follow a curvilinear path. These results have been interpreted to indicate that that heading judgments are based on moving toward a target not on optic flow. This interpretation assumes that if heading judgments were based on optic flow information they would not be altered by prism alignment. We tested this assumption. Methods: Three observers made heading judgments using a relatively standard optic flow display. Observers were seated 50 cm from a display that subtended 44.6 arc deg in the horizontal dimension. The observer saw a red line at the center of the screen. Pressing a mouse began a trial. During the 833 mS trial, an optic flow pattern was presented consisting of 100 dots appearing at random locations and moving away from a focus of expansion (FOE). Dots had a lifetime of 467 mS or were replaced as they moved off of the screen. At the end of the trial the central red line reappeared. The observers’ task was to determine if the FOE was to the right or left of the red line. A double staircase was employed to determine a threshold for the FOEs to the right and to the left of the center. Bias was the difference between the right and left thresholds. Precision was defined as ½ the sum of the thresholds. Observers repeated the judgments under 3 binocular conditions: no prism and 2 binocular prism conditions (30 diopters base right and left). A 3x3 Latin square design was used to avoid order effects. Results: As prisms changed from 30 deg base right to 30 deg base left there was a shift to the right in the bias of observers’ judgments of 11.0 to 13.6 deg (mean = 12.2 deg; SEM = 0.46 deg). The precision of judgments of heading was unaffected. The difference between the precision of base right and left judgments ranged from –4.2 deg to 14.0 deg (mean = 1.97; SEM = 3.5 deg). Conclusions: It appears that prism orientation affects the bias of heading judgments in optic flow fields. The change in bias is consistent with the prism orientation induced image shift. This effect is unrelated to the precision (accuracy) of thresholds. If the result proves to be robust across differing optic flow tasks, interpretation of data from ambulatory tests may need to be revised.
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