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Courtney Harman, Harry Haladjian, Deb Aks, Zenon Pylyshyn; Eye movements during an enumerating-by-pointing task enhance spatial compression. Journal of Vision 2011;11(11):254. doi: 10.1167/11.11.254.
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Observers can accurately enumerate and localize sets containing up to six randomly-placed dots when using an “enumerating-by-pointing” method (Haladjian & Pylyshyn, 2010). Analyses of localization errors suggest a form of compression, where location responses are closer to the centroid of the set of dots than their actual locations on the stimulus screen. We address the following questions in the current study: Is this compression stronger around the centroid of the dots or the point of central fixation? Is the frequency of fixations correlated with response accuracy? Is compression of pointing responses linked to eye-movements? We used an EyeLink 1000 eye-tracker to examine the role of eye-movements in our enumerating-by-pointing task. Participants were shown a display with 1–10 randomly-placed black dots (~1° diameter). This gaze-contingent display appeared immediately after participants fixated the center of the screen for one second. After a full-screen mask, participants used a mouse to place markers on a blank screen indicating the perceived locations of the dots. Analyses were performed on enumeration accuracy and localization errors (distance between dots and nearest response marker). Results show strong compression around the centroid of dots, and some compression around fixations (i.e., localization errors are smaller and less variable around the centroid). Stronger compression (on 2/3 of the cases) required at least one fixation to the centroid. More fixations, as well as dots, also strengthened centroid compression. Increased fixation frequency, however, did not improve localization or enumeration performance. Overall, these results suggest that compression is centered on the centroid of a set of stimuli, and eye-movements play a role in perceived shrinkage of the display configuration, but not judgments associated with counting.
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