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Li Li, Joseph Cheng; Looking where you are going does not help path perception. Journal of Vision 2010;10(7):1043. doi: 10.1167/10.7.1043.
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It has been mathematically shown that when travelling on a circular path and fixating a target on the future path, flow lines for environmental points on the path would be vertical. Thus, by integrating all the vertical lines in the flow field, observers could recover the path trajectory directly from retinal flow without recovering heading (e.g., see Wann & Swapp, 2000). Here we test whether fixating a target on the future path helps path perception. Observers viewed displays (110°Hx94°V) simulating their traveling on a circular path over a textured ground (T=3 m/s, R=±3°/s or ±6°/s) for 1 s. Three display conditions were tested. In the path-fixation condition, the simulated gaze direction in the display pointed to a target along the path at 20° away from the starting position; in the non-path-fixation condition, the simulated gaze direction was on a target 10° inside or outside the path at the same distance; and in the heading-fixation condition, the simulated gaze pointed to the instantaneous heading (i.e., the tangent to the path). At the end of the trial, a probe appeared at 10 m. Observers used a mouse to place the probe on their perceived future path. For five observers (3 naïve), path errors (defined as the deviation angle between the perceived and the actual path at 10 m) were accurate only for the heading-fixation condition (mean error: 2.72° & 0.52° for R=3°/s & 6°/s, respectively). For the path- and non-path-fixation conditions, path errors displayed a positive slope (0.6 & 0.98, respectively), consistent with the fact that observers estimated the path curvature based on the total amount of rotation in the flow field. The findings suggest that fixating a target on the future path does not necessarily help the perception of the path trajectory. Path perception largely depends on solving the translation and rotation problem in retinal flow.
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