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Joseph S. Lappin, Duje Tadin; Figure-ground segregation by center-surround motion mechanisms. Journal of Vision 2004;4(8):108. doi: 10.1167/4.8.108.
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Background & problem: Center-surround antagonism is a basic characteristic of early motion mechanisms. Tadin et al. (2003, Nature, 424: 312–5) recently described several psychophysical correlates of the center-surround antagonism that characterizes receptive fields of many neurons in cortical area MT. Notably, motion directions of large high-contrast patterns are less discriminable than those of either small high-contrast or large low-contrast patterns. Moreover, these center-surround interactions are contrast-dependent, with suppression at high contrast and spatial summation at low contrast. What visual functions might benefit from such center-surround suppression? Intuitively, center-surround interactions seem likely to foster figure-ground segregation, but this link needs experimental support. Are figure-ground discriminations facilitated by conditions producing greater surround suppression and reduced direction discriminations? Methods & results: A figure-ground segregation task tested discriminations between two motion-defined forms (an ellipse in two orientations) within partially correlated random-pixel arrays. A corresponding direction-discrimination task tested direction discriminations for patterns with uniform directions and no embedded form. Temporal thresholds were measured at contrasts from 9% – 92%. Increasing contrast had opposite effects in the two tasks, raising direction-discrimination thresholds but lowering those for form discrimination by approximately equal amounts (correlation = −0.95). A second study examined joint effects of size and contrast on the two tasks. Spatial summation regions decreased inversely with contrast in both tasks. These results support Braddick's (1993, TINS, 16: 263–8) suggestion that spatial summation and segregation are complementary functions of the same visual motion mechanisms. Additionally, the spatial integration for these two visual functions is contrast-dependent.
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