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Stéphane J. Rainville; Simultaneous acceleration is key in grouping by visual synchrony. Journal of Vision 2006;6(6):755. doi: 10.1167/6.6.755.
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© ARVO (1962-2015); The Authors (2016-present)
PURPOSE: Perceptual grouping is highly sensitive to temporal synchrony between moving Gabor elements. Here I tested whether grouping by synchrony depends on instantaneous carrier position, velocity, or acceleration.
METHODS: Observers discriminated the synchrony of Gabor pairs oscillating at 5 Hz in a 2AFC task. The phase of each Gabor's carrier evolved according to its own independent time-varying function f(t) = k1 + k2t + k3cos(ωt + k4). The k1 and k2 terms acted as position and speed pedestals while the k3 and k4 terms determined the oscillation's amplitude and phase lag respectively. Similarity between the k4 terms of the two Gabors determined their degree of synchrony.
RESULTS: Synchrony thresholds were immune to mismatches either in instantaneous carrier position or speed between the two Gabors.
CONCLUSION: The position and speed pedestal terms (k1 and k2) do not appear in the second derivative (i.e. acceleration) of time-varying phase profiles. Results therefore imply that simultaneous acceleration between Gabor elements - not simultaneous position or speed - drives grouping by visual synchrony. Newton's second law (F = ma) states that forces involved in scene dynamics are only reflected in acceleration. Findings from this study lead to the simple yet remarkable conclusion that grouping by simultaneous acceleration can succeed where lower-derivative grouping rules such as good continuation (0th time derivative) or common fate (1st time derivative) necessarily fail.
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