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Kimberly Meier, Marita Partanen, Ryan Lo, Deborah Giaschi; Neural correlates of speed-tuned differences in global motion and motion-defined form perception. Journal of Vision 2013;13(9):360. doi: 10.1167/13.9.360.
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A popular view of the human visual system is that it comprises at least two somewhat parallel pathways: the ventral stream for form and colour, and the dorsal stream for motion. There is also support for a modified view in which slow speeds of motion are processed ventrally. We recently reported speed-tuned psychophysical differences in the typical and atypical development (due to amblyopia) of global motion and motion-defined form perception (Hayward et al., 2011; Narasimhan & Giaschi, 2012). Specifically, immaturities and deficits were reported for slow but not fast speeds. These different developmental patterns may reflect different cortical systems mediating slow and fast motion perception. The current study used functional MRI in adults with normal vision to examine the neural correlates of our psychophysical tasks. The tasks included global motion direction discrimination and motion-defined rectangle orientation discrimination, each at slow (0.1 deg/s) and fast (5 deg/s) speeds. Stimuli were random-walk dot fields presented using a block design with four conditions: high motion coherence, low motion coherence, 0% motion coherence and stationary dots. Coherence levels were adjusted to match the difficulty level across speeds. We examined each task using a whole-brain voxelwise analysis and a 2 (Speed: Fast, Slow) x 3 (Coherence: High, Low, 0%) within-subjects ANOVA. For both tasks, there were significant main effects of Speed and Coherence, and no interaction between the two. On the global motion task, pairwise comparisons between fast and slow speeds showed activation in occipital and parietal regions, with stronger dorsal activation for fast speeds. On the motion-defined form task, comparisons showed activation in occipital regions, with stronger activation in ventral occipital areas for slow speeds. These results demonstrate differences in the cortical systems activated by fast and slow motion, and suggest a role for the ventral stream in the processing of slow speeds.
Meeting abstract presented at VSS 2013
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