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Jacob Paul, Robert Reeve, Jason Forte; Investigating eye movements in enumeration using saccade-terminated trials. Journal of Vision 2018;18(10):1009. doi: 10.1167/18.10.1009.
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Recent evidence raises the possibility that numerosity is encoded by dedicated mechanisms in the human visual system, and subsequently processed by a specialized fronto-parietal network. Neural circuits implicated in generating saccadic eye movements partially overlap with this numerosity processing network. The overlap may reasonably imply eye movements play a functional role in enabling exact enumeration of small sets of objects, and underlie computation of visual numerosity. Alternatively, eye movements may reflect obligatory visual processing demands (i.e., object saliency, gaze heuristics, de-crowding). Here we systematically manipulated the opportunity to saccade to precisely specify the role of enumeration eye movements. Fifteen adults (11 naïve, 4 informed) enumerated random dot arrays under three conditions—(1) a novel saccade-terminated design where arrays remained visible until one, two or four fixations occurred; (2) a duration-terminated design where arrays were shown for 250ms, 500ms and 1000ms; and (3) a response-terminated design where arrays remained visible until a response. Enumeration was more accurate for saccade-terminated trials despite similar saccade latencies to duration-terminated trials: saccade-terminated trials lasted, on average [95% BCa CI], 316ms [289ms-356ms] (one fixation), 628ms [592ms-670ms] (two fixations) and 1320ms [1266ms-1380ms] (four fixations), while duration-terminated trials involved 0.46 [0.40-0.51] saccades (250ms), 1.06 [0.98-1.14] saccades (500ms) and 2.20 [2.07-2.34] saccades (1000ms). When participants were informed about how trials would terminate, saccade latencies shifted to match task demands. Fixation distributions were well-characterized by a simple filtering model of visual proximity grouping, while rotating saccade vectors to align with salient image locations accounted for variability in saccade trajectories. Our findings (1) validate the novel saccade-contingent procedure for investigating the functional role of saccades in enumeration, (2) emphasize the importance of simple visual grouping mechanisms for the computation of sets to enumerate, and (3) highlight the importance of incorporating the computational complexities of eye movements into models of numerical cognition.
Meeting abstract presented at VSS 2018
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