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Constance S. Royden, Daniel M. Conti; A model using velocity differences to compute heading can explain an illusory transformation of optic flow fields. Journal of Vision 2002;2(7):737. doi: 10.1167/2.7.737.
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© ARVO (1962-2015); The Authors (2016-present)
When an observer moves in a straight line, the image velocities on the retina (the optic flow field) form a radial pattern. The center of this pattern coincides with the observer's direction of motion. When a field of dots moving in an expanding radial pattern is superimposed on a field of dots moving with uniform lateral motion, the perceived position of the center of expansion shifts in the direction of motion of the laterally moving dots (Duffy and Wurtz, 1993). In this study, we show that the vectors computed from the differences between the expanding velocities and the lateral velocities form a radial pattern with its center shifted in the direction of the laterally moving dots, as in the illusion. We further show that a computational model (Royden, 1997) that computes heading using motion opponent operators also shows this shift in the computed center of expansion. To compare the model responses with those of human observers, we created a strong version of the illusion, in which each dot in the expanding field is spatially paired with a dot in the laterally moving field of dots. All dots had limited lifetimes of 240 msec, with a total trial duration of 800 msec. The radial field simulated observer approach toward a plane at a distance of 50 cm with a speed of 42.0 cm/sec. The maximum dot speed was 10.6 deg/sec at the edge of the 25 × 25 deg viewing window. The speed of the lateral dots was varied between trials. The stimulus created a strong illusion of the shifted center of expansion. The average shift in response for 5 observers was −0.09, 2.79, 6.99 and 9.26 deg for lateral dot speeds of 0, 2.0, 6.0, and 10.0 deg/sec respectively. The average shift in the computed center from the model simulations for the same stimuli were 0, 2.02, 6.58 and 11.92 deg, very similar to the human results. These results suggest that a motion opponent process is responsible for this illusory transformation, and may also be important in computing heading.
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