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Yuning Song, Curtis L. Baker; A common mechanism underlying neuronal processing of contrast envelopes and illusory contours. Journal of Vision 2004;4(8):66. doi: 10.1167/4.8.66.
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We have recently demonstrated that cortical neurons' responses to illusory contours formed by abutting gratings can be mediated by a nonlinear mechanism acting across two spatial scales (Song and Baker, 2003). Previous studies demonstrated that a similar mechanism could account for neuronal responses to contrast envelopes (periodic moving contours defined by local contrast). To address the question of whether the same neuron could respond to both contrast envelope and illusory contour stimuli, we tested responses of single neurons in cat A18 to both. Results showed that around half of the neurons responded to both of these kinds of moving contours, with similar orientation and direction selectivity. Moreover, a given neuron exhibited similar selectivity for spatial frequency of the carrier (fine, stationary grating). The neurons' preferred envelope spatial frequencies (coarse, moving gratings) for contrast envelopes were similar to those for phase-reversals of illusory contours. Both simple and complex type cells showed all these similarities in their responses. Considering the differences in image features of the two kinds of stimuli, these results are more consistent with an energy-based (filter-rectify-filter) rather than a feature-based processing model. These results also suggest that cue-invariant selectivity for moving contours originates at an early stage of cortical processing.
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