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Frans Verstraten, Ryota Kanai, Chris Paffen, Walter Gerbino; What makes local dots turn into moving global surfaces?. Journal of Vision 2003;3(9):171. doi: 10.1167/3.9.171.
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Visual processing involves processing stages in which local features are initially analyzed and combined to construct integrative representations of the visual environment in terms of objects and surfaces. In the case there are multiple cues for integration, conflicts may arise and the visual system has to come to a acceptable solution. Here we are interested in this process in the domain of visual motion perception. For the case of visual motion it is known that at least two distinct — local and global — processing stages are involved. First, motion is analyzed by local motion filters selective for spatio-temporal orientation. Next, these local motions are integrated by combining the signals arising from a common visual object or surface, while segregating those arising from different objects or surfaces. The question is how? We looked at two surface grouping strategies.
Our stimuli consisted of dots which were changing their direction periodically over a short distance at a constant velocity. Oscillating dots were used because they consist of two components, sustained motion signals and transient direction changes (feature-based and synchrony-based grouping strategies, respectively). This stimulus allows us to vary the relative strength of the two grouping strategies.
The results show that when the timing of the direction changes is random (zero synchronicity between the dots), the configuration results in a compelling percept of streaming motion transparency that cannot be distinguished from continuous motion transparency. As the synchronicity of the dots' direction changes increases, the percept of streaming motion transparency changes drastically towards a percept of globally oscillating surfaces, that is, a pair of global surfaces is moving back and forth. The experiments also show that local signals, inconsistent with a surface interpretation, are prevented from further visual processing required for conscious perception of the dots.
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