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Pascal Mamassian; Grouping impairs motion direction perception. Journal of Vision 2009;9(8):659. doi: 10.1167/9.8.659.
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© ARVO (1962-2015); The Authors (2016-present)
The literature on motion perception abounds with contextual effects, such as the induced effect where a stationary object is seen moving when surrounding objects are moving. We are here interested in contextual effects where no additional moving objects are present in the image. More specifically, we are interested in motion perception when the moving elements are grouped together. In contrast to previous work on the effects of image segmentation on speed discrimination (Verghese & Stone, 1996, Nature; Verghese & McKee, 2006, Vision Research), we are interested in grouping moving elements without changing their spatial arrangement.
Stimuli consisted in low contrast vertical lines moving left or right. Two such lines were placed on either side of a fixation point and moved either towards or away from each other. Two configurations were used: either (1) only the two vertical lines were shown, or (2) the vertical lines were connected by two horizontal lines so as to form a rectangular figure. The task of the observer was to report the direction of motion of the vertical lines (towards or away from each other). The method of constant stimuli was used to measure thresholds in both configurations.
We found that connecting two moving lines significantly increased the threshold to correctly identify their direction of motion. In other words, motion was more easily perceived when it was split across two objects than when it belonged to a single object. We propose a model based on the propagation of object motion uncertainty towards the object features to explain these results.
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