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Kelly Shen, Martin Paré; Selection and timing of gaze fixations in visual conjunction search. Journal of Vision 2008;8(6):1075. doi: 10.1167/8.6.1075.
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© ARVO (1962-2015); The Authors (2016-present)
Vision is an active process that requires a sequence of saccadic eye movements interrupted by periods of fixation during which details of the foveal image are analyzed and the next detail in the periphery is selected. To understand how the brain regulates this visual behavior, we developed a task in which monkeys could freely search for a target amongst 15 distractor stimuli spaced by 10 deg on a grid. Monkeys were given [[gt]]5 s to locate the search target by foveating it for [[gt]]500 ms. Each target was a unique conjunction of color (red/green) and form (circle/square), and distractors could share either one feature with the target (same-color/same-form) or none (opposite). The monkey made a single targeting saccade in 20% of trials but a total of 4 saccades, on average, were made in a single trial (range: 1–15). The great majority of saccades (94%) were directed to the nearest stimuli (10 & 14 deg), and fewer saccades were made when the target was one of these stimuli than when located remotely (3.5 vs. 5.5). Saccade probability to the different distractors was variable, with most being directed towards same-color distractors, fewer towards same-form and even fewer to opposite distractors (57, 16, 6%). Fixation duration for these distractors co-varied with the probability of targeting them, being longer for same-color distractors, shorter for same-form, and even shorter for opposite distractors (118, 53, 29 ms). These short fixation durations, along with the observation that they were significantly shorter than the initial response time following display onset (200 ms), indicate that saccade programming and visual processing occurred in parallel. Beyond showing that top-down influences guide visual search by biasing both the processes regulating where and when to look next, this study promises to be a valuable approach to simultaneously investigate the neural mechanisms underlying these processes.
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