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Junghyun Park, Madeleine Schlag-Rey, John Schlag; Localization precedes attention-induced acceleration of visual processing. Journal of Vision 2002;2(7):164. doi: 10.1167/2.7.164.
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The temporal order of a pair of visual stimuli presented in the dark just before a saccade determines their perceived spatial relation: the second stimulus in the pair appears to be shifted in the saccade direction. On the other hand, the positions of these stimuli relative to the focus of attention determine their temporal order: the stimulus on which attention is focused appears to be first even when it is presented several tens of ms later. Here we tested whether these illusions depend on the physical properties of stimuli or their perceived properties.
In experiment 1, we presented two 3-ms dots 50ms apart shortly before a 10° rightward saccade and measured perceptions of temporal order and relative position of them simultaneously. One dot was at the site of saccade target where attention would be focused, the other was 3° above it, varying from 1° left to 1° right. The temporal order and relative position of the dots were randomized. The observers' perceived temporal order was inverted when the bottom dot was presented second, but their perceived spatial relation was not affected. This shows that saccadic mislocalization depends on the real order of stimuli. In experiment 2, we changed the dots' location: one dot was at the saccade target position, and the other was at 3° left to it. During the fixation, the right dot, which was presented and perceived at the attentional focus, was almost always perceived to be first. However, when these dots were presented just before the 10° rightward saccade, their perceived position (measured by ocular targeting) shifted to the right so that the left dot was perceived closer to the saccade target position than the right dot. In this situation, the left dot appeared to be first in most trials. This shows that the illusory temporal order depends on the perceived position of the stimulus.
The combined results lead to the conclusion that the brain analyzes the position and the timing of the stimulus serially, and that attention accelerates visual processing after the stage of localization. (Supported by USPHS grant EY-05879)
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