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Remy Allard, Jocelyn Faubert; First- and second-order motion processing are separate at low temporal frequencies but common at high temporal frequencies. Journal of Vision 2007;7(9):1006. doi: 10.1167/7.9.1006.
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There is an ongoing debate on whether first- and second-order stimuli are processed by common or separate mechanisms. Some authors suggest that they are initially processed by separate mechanisms and that a rectification within the second-order pathway enables post-rectification mechanisms to process both types of stimuli. Others suggest that nonlinearities within a common motion mechanism enable the detection of both types of stimuli.
In the present study, observers were asked to discriminate the direction of drifting luminance- (LM, first-order) and contrast-modulated (CM, second-order) signals embedded in LM or CM dynamic noise. The signals were drifting at either a low (2 Hz) or high (8 Hz) temporal frequency.
At low temporal frequencies, the results showed no cross-modal interaction: LM noise affected LM discrimination thresholds but had no or little impact on CM discrimination, and CM noise affected CM discrimination thresholds but had no or little impact on LM discrimination. This double dissociation implies that, at low temporal frequencies, LM and CM stimuli are processed, at least at some point, by separated mechanisms.
At high temporal frequencies, the results showed a complete cross-modal interaction: LM noise affected both the LM and CM discrimination thresholds in similar proportions, and CM noise also affected both the LM and CM discrimination thresholds in similar proportions. This complete cross-modal interaction suggests that, at high temporal frequencies, LM and CM stimuli are processed by common mechanisms.
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