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Lawrence Cormack; Continuous Tracking of Motion Induced Position Shifts. Journal of Vision 2019;19(8):92. doi: https://doi.org/10.1167/19.8.92.
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When a Gabor with a drifting carrier is viewed peripherally, the entire Gabor patch appears to be moving – a motion induced position shift (MIPS). Indeed, an entire global form composed of many such Gabors appears to move despite remaining fixed. Here, subjects attempted to hold such a global form (a large circle comprising 16 Gabor patches) centered on a fixation mark using a trackpad whilst an unseen force attempted to drive it in a random walk. Two conditions were “zebra motion”, in which each Gabor had a fixed phase and the entire circular herd was driven in a 2D random walk, and “cuttlefish motion”, in which the carriers were driven in a random phase walk while each envelope and hence the global circle was driven in a 1D walk in the orthogonal direction. For zebra motion, subjects responded to the motion components in both cardinal directions, but the responses parallel to the carrier stripes were slightly worse than the orthogonal responses (as one might expect). For cuttlefish motion, subjects responded likewise to the motion parallel to the stripes but, crucially, they also responded to the phase walk as if it were motion of the global form (i.e., they manually “nulled” their MIPS percepts), and often with shorter latencies than for the orthogonal “real” motion. Hence, MIPSs are tracked as though they are real global motion, and such tracking (or the nulling variant used here), can be used to explore and (quickly) quantify the MIPS percept.
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