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Noah J. Steinberg, Zvi N. Roth, J. Anthony Movshon, Elisha P. Merriam; Neural correlates of the double-drift illusion. Journal of Vision 2019;19(10):43c. doi: https://doi.org/10.1167/19.10.43c.
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© ARVO (1962-2015); The Authors (2016-present)
In the “double-drift” illusion, local motion within an eccentrically viewed moving contrast window alters the window’s perceived path. The illusion is strong even when the eyes track a target whose motion matches the window so that it remains stable on the retina, suggesting that the illusion engages position encoding not tied to retinal coordinates. We reasoned that different responses for different illusory motion paths, given identical retinal input, might identify the basis for this encoding. The stimulus consisted of a vertically-oriented Gabor that contained both local and global motion. The Gabor envelope moved vertically up and down across the screen as the carrier grating drifted to the left or right. Under these conditions, the perceived stimulus’ motion path differed from its actual motion path by several degrees. Observers pursued a fixation dot that moved smoothly and predictably alongside the Gabor, so that the Gabor remained at a constant retinal location throughout the experiment. We measured BOLD fMRI responses (7T, 1.2 × 1.2 × 1.2 mm voxels) under three stimulus conditions which shared the same physical motion path but differed in perceived motion. In two conditions, the conjunction of local and global motion produced illusory motion paths that were rotated to the left or right of vertical. In the third, control condition, the direction of local motion was randomized every 250 ms – this did not produce an illusory motion path. Several cortical areas had larger BOLD responses during the illusory conditions than during the no-illusion control condition, potentially reflecting attentional enhancement or responses to local motion. But multivariate pattern analysis revealed a number of cortical areas that could discriminate between the two illusory directions. This result is not easily explained by low level factors and may reveal the cortical loci of non-retinal position computation.
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