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Jay A. Edelman, Mariya V. Cherkasova, Ken Nakayama; A spatial memory system for the guidance of eye movements in crowded visual scenes. Journal of Vision 2002;2(7):572. doi: 10.1167/2.7.572.
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© ARVO (1962-2015); The Authors (2016-present)
The primary purpose of saccadic eye movements is to bring the image of visual objects of interest onto the fovea. However, this does not imply that all purposive saccades in a visual environment are visually guided, since visual crowding can render less salient objects in a visual scene invisible if they are located far from present gaze. Moreover, after repeatedly fixating such a non-salient element in a visual scene one senses that one's eye lands right on the target, despite its invisibility. We sought to confirm this phenomenological finding by recording saccades of 3 subjects using an S-R Eyelink video eyetracker (250 Hz) while they made saccades in a complex visual scene displayed on a computer monitor (∼40 × ∼30 deg). Prior to recordings, subjects freely viewed the display and were told the identity of possible saccade targets in the scene. In the invisible task, initial fixation position was varied trial-by-trial in a region from which all possible targets were invisible due to crowding and distance. Target was held constant within a block of 16 trials, and performance on the last 8/16 trials was analyzed. Saccade endpoint errors and peak velocities in the invisible task were compared with those in visually guided and memory-guided delayed tasks. Phenomenology was confirmed: metrics in the invisible task were comparable to the visually guided task, whereas memory-guided saccades had greater endpoint errors and lower peak velocities. Additional experiments demonstrated that up to 3 targets could be stored simultaneously in this memory system. These results suggest that a scene-based memory system enables the generation of saccades to invisible targets as if they actually were visible. Such a system could prove useful both for the continuous monitoring of non-salient objects in a crowded visual scene and for refixating more salient stimuli without having to tax peripheral visual processing.
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