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Mark Harwood, Annabelle Blangero, Josh Wallman; Saccadic motor priority trumps visual salience in a free choice task. Journal of Vision 2012;12(9):1243. doi: 10.1167/12.9.1243.
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© ARVO (1962-2015); The Authors (2016-present)
Decision making experiments and models have concentrated heavily on perceptual processing. For example, one influential idea is that sensory features of objects are integrated into a single measure of conspicuity, salience, which then determines probabilities of a target being selected by attention or by a saccadic eye movement. Larger objects, or those nearer the fovea, are more salient because of their greater cortical representation, and should be targeted more frequently by saccades than smaller more distant objects. In contrast, we have previously shown that at small eccentricities larger targets elicit saccades of double the reaction time of smaller targets. We argued that target movements that are small relative to target size are given a low response priority by the saccade system since their visual information has changed little. Here, we examine the link between salience and priority by giving human subjects a free choice between two targets of differing priority and/or salience. Specifically, twelve subjects were presented with two simultaneously appearing rings (2, 4 or 8 deg diameter) at different eccentricities (0.5-18deg) to the left and right of central fixation. They were instructed to saccade to the center of either target. We inferred the motor priority for a given target size and eccentricity from reaction times to singly presented targets. Subjects did not simply choose the nearer, more salient target. Instead, they were strongly biased to higher priority targets. Choice probabilities were accurately predicted from priority and reaction times, not from salience. We conclude that in our simple task: 1) Motor priority trumps visual salience; 2) Reaction times and choice are not dissociable; 3) More generally, object size and eccentricity form an automatic, obligatory and fundamental constraint on saccadic decision making.
Meeting abstract presented at VSS 2012
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