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Gordon Logan, Jane Zbrodoff, Xingshan Li; Do the eyes count? The role of eye movements in visual enumeration. Journal of Vision 2008;8(6):115. doi: 10.1167/8.6.115.
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© ARVO (1962-2015); The Authors (2016-present)
Enumeration of displays that contain five or more objects is notoriously difficult. It depends on a serial counting algorithm in which each object is indexed and each indexing event increments a counter. The algorithm continues until every object has been indexed. Then the value in the counter equals the numerosity of the display. This theory predicts a linear increase in response time (RT) with numerosity. Typical data confirm this prediction, showing a slope of 300 ms/object. The role of eye movements in visual enumeration tasks is not understood. The displays are exposed until subjects report the numerosity, so RT can be 3000 ms or more, allowing several eye movements. We investigated the role of eye movements in enumeration, measuring eye movements while subjects enumerated displays of 5–12 objects. We tested the hypothesis that eye movements play a critical role in visual enumeration: Eye movements are the indexing events that drive increments of the counter. This hypothesis predicts that subjects should move their eyes once for each object in the display, so that the number of fixations should increase linearly with the number of objects with a slope approaching 1.0. The data were largely consistent with this prediction. RT increased linearly with the number of objects in the display (r = .991) with a slope of 262 ms/object. The number of fixations also increased linearly with the number of objects (r = .991) with a slope of .835 fixations/object. Preliminary analyses suggest that subjects may not have fixated each object directly (few fixations fell directly on the objects) but nevertheless they made about one fixation for each object. We suggest that eye movements are not made to foveate the objects, but rather, to provide motor outflow that is synchronized with visual indexing and drives the increments of the counter.
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