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Stéphane Rainville; Grouping by visual synchrony - separate motion and flicker pathways. Journal of Vision 2007;7(9):407. https://doi.org/10.1167/7.9.407.
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© ARVO (1962-2015); The Authors (2016-present)
PURPOSE: Human vision is extremely skilled in combining synchronous features of the retinal image - features that change contrast (or ‘flicker’) simultaneously tend to form coherent perceptual groups, as do moving features that change direction simultaneously. A key issue is whether the visual system is sensitive to the synchrony of higher-order events (e.g. direction changes) because such events are usually accompanied by confounding lower-order artefacts (i.e. contrast changes). METHODS: Here I present a novel technique by which changes in contrast can be made directional or nondirectional purely through manipulation of their local spatiotemporal phase spectrum. Observers discriminated the synchrony of counterphasing Gabors pairs oscillating at 3.1 Hz in a 2AFC task. The level of directionality in each Gabor was manipulated by varying the spatial phase of a superimposed static Gabor mask. RESULTS: Human observers, while exquisitely sensitive to synchrony among directional changes or among nondirectional changes, are surprisingly blind to synchrony between directional and nondirectional changes, despite the fact that both types of events have identical local and global spatiotemporal spectra. CONCLUSION: This failure of the visual system to encode synchrony between directional and nondirectional events provides compelling evidence that distinct motion and flicker pathways mediate perceptual grouping by synchrony. Results imply that perceptual grouping mechanisms capitalize on the synchrony of higher-order events. These findings challenge existing notions on grouping-by-synchrony and strongly motivate computational research on the nature of synchrony in dynamic natural scenes as well as physiological and behavioural research on their perceptual-grouping correlates.
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