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Lawrence Cormack; Dynamics of Motion Induced Position Shifts Revealed by Continuous Tracking. Journal of Vision 2019;19(10):294c. doi: https://doi.org/10.1167/19.10.294c.
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When a Gabor with a drifting carrier is viewed peripherally, the entire Gabor patch appears to move – a compelling phenomenon called the motion induced position shift (MIPS). Indeed, an entire global form composed of many such Gabors appears to move despite remaining fixed (see the 2016 illusion-of-the-year winner “Motion Integration Unleashed”). Here, subjects attempted to keep a global form (a large circle comprising 16 Gabor patches) centered on a fixation mark using a trackpad while an unseen force attempted to drive it in a random walk. The first main stimulus condition was ”zebra motion,” in which each Gabor had a fixed phase and the entire circle of Gabors was driven in a 2D random walk. The second was the MIPS-inducing “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 directions, but the responses parallel to the carrier stripes were slower than the orthogonal responses (as one might expect based on the direction of contrast energy). 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 by pushing the Gabor positions in the opposite direction). Moreover, the response latencies to MIPS were shorter than for the orthogonal “real” motion, and very similar to those for the fast component of the response to zebra motion. Hence, MIPSs are tracked as though they are real global motion, and a continuous tracking paradigm (or the nulling variant used here), can be used to parametrically explore and (quickly) quantify the MIPS percept.
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