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Farid I. Kandil, Manfred Fahle; The notion of ‘purely time-based figure-ground segregation’ is still justified. Journal of Vision 2002;2(7):88. doi: 10.1167/2.7.88.
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© ARVO (1962-2015); The Authors (2016-present)
Can our visual system bind elements together if their only shared feature is synchronicity of change?
Lee and Blake found evidence in favor of this idea using (a)synchronous motion reversals of shifting Gabor patches. Motion reversals in both, figure and ground, followed independent random processes, thus achieving synchronicity within each area and asynchronicity between them. Farid and Adelson replaced the Gabor patches by randomly distributed dots which moved back and forth within small apertures. They demonstrated that figure-ground segregation does not rely on the (a)synchronicity itself but on a contrast artifact resulting from low-pass filtering: long runs of dots produce low contrasts whereas repetitive reversals result in temporally high contrast.
Here, we report about two new experiments using the same spatial stimuli but different temporal protocols. In the first one, we found that (a)synchronous motion reversals of dots on a CRT-screen were insufficient to segregate figure from ground, thus ruling out the involvement of motion detectors as an underlying mechanism for segregation. In contrast, introducing short breaks of about 30 ms before each reversal rendered the figure visible. Since small breaks also lead to temporally higher contrasts, this experiment is supportive of Farid and Adelson's hypothesis.
In a second experiment, we used the same random process for both, figure and ground, but with a constant temporal offset between them. Hence, contrast modulations in the figure appeared in the ground a few frames later. Segregation was possible with offsets longer than 30 ms.
We conclude that, whatever the integration time of the low-pass filter proposed by Farid and Adelson (typically around 100 ms), its output has to be read by a spatio-temporal mechanism able to detect offsets with a high temporal resolution in order to achieve segregation in these displays. Hence, the notion of a ‘purely time-based figure-ground segregation’ still seems justified.
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