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Jacek Dmochowski, Anthony Norcia; Neural dynamics of fine direction-of-motion discrimination. Journal of Vision 2014;14(10):15. doi: 10.1167/14.10.15.
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The visual system can discriminate small differences in direction of motion despite the relatively broad direction-tuning of motion selective cells in visual cortex. Here we used high-density EEG to probe the neural basis of fine direction discrimination. Fifteen neurotypical adults performed 210-280 trials of a choice reaction time task. The random dot stimulus consisted of 1 second of random motion followed by 1 second of coherent motion. The mean direction of coherent motion was selected from one of seven directions centered on vertical (90deg) ranging from 80 to 100 degrees in increments of 3.4 degrees. Subjects indicated the perceived direction of motion (left or right of vertical) with a button press. We employed a novel technique, "Reliable Components Analysis" (Dmochowski et al., 2012) which computes projections of the EEG that maximize the trial-to-trial reliability/consistency. The first reliable component locked to stimulus onset had a midline-symmetric topography with a maximum over the parietal lobe. The time course of this component exhibited a "ramping" trajectory whose level and slope discriminated between the angular deviations from vertical, but not the absolute directions-of-motion. The second and third reliable components did not depend on offset size and may reflect pre-categorical coherent motion responses. Activity time-locked to the button press exhibited a peak at the time of behavioral response, with a steeper gradient of response leading to the peak for larger offsets for the first component. The second component did not depend on offset and was sharply peaked at response time, suggesting a motor origin. Discrimination of near-threshold-level changes in direction-of-motion can be decoded from electric potentials above parietal cortex. These potentials appear to be the output of a categorization process for direction. The time-course of the differential response is consistent with an integration process that is longer for more difficult discriminations.
Meeting abstract presented at VSS 2014
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