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Duje Tadin, Kristin K. Grdinovac, Bjorn P. Hubert-Wallander, Randolph Blake; Both simple and choice reaction times reveal suppressive center-surround interactions in motion perception. Journal of Vision 2007;7(9):97. doi: 10.1167/7.9.97.
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Increasing the size of a high-contrast moving object can make its motion substantially more difficult to perceive (Tadin et al., Nature, 424, 312–5, 2003). Based on converging lines of evidence, we attributed this counterintuitive finding to the involvement of motion-sensitive neurons with suppressive center-surround receptive fields. Here we used reaction times (RTs) to reveal the effects of center-surround interactions on neural processing speed of moving stimuli.
As expected, at low contrast (2.3%), RTs to a motion onset decreased as stimulus size increased from 2.4deg to 12deg. This was true for both simple (369ms to 306ms) and choice (439ms to 399ms) RTs. At high contrast (93%), however, as the stimulus size increased, choice RTs increased from 350ms to 384ms. Interestingly, increasing the size of a moving stimulus almost completely eliminates the well-established contrast-dependency of RTs. This paradoxical increase in RT, found in both naïve and experienced observers, is consistent with impaired motion processing resulting from surround suppression. Moreover, we found a parallel result for simple RTs; increasing the size of a high-contrast stimulus significantly increased simple RTs, a remarkably different result from simple RTs to stationary stimuli of varying size. Thus, even a task for which motion direction is irrelevant yields performance changes consistent with center-surround suppression. This same overall pattern of results was observed regardless of whether the motion stimulus abruptly appeared or if the stimulus was a stationary grating that abruptly moved.
These increases in both choice and simple RTs with increasing size extend the effect of center-surround interactions to the neural processing speed of motion. Additionally, we found that these effects were dependent on the predictability of the stimulus event evoking RTs, with a paradoxical RT improvement for large, unpredictable stimuli. This counterintuitive finding may reflect attentional modulation of center-surround interactions; a hypothesis that we are currently investigating.
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