Second-order motion processed by the first-order motion system at high carrier contrasts R Allard, J Faubert |
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Visual Psychophysics and Perception Lab, Université de Montréal, QC, Canada
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| Previous studies have shown that contrast-defined motion is processed by a feature tracking motion or an energy-based motion system depending on whether the carrier contrast is low or high, respectively. The fact that global distortion products could not explain the energy-based processing of contrast-defined motion has been taken as evidence of a dedicated second-order motion system. However, we [in press, Journal of Vision] recently revealed the existence of nonlinearities that have not been considered before and can enable the first-order system to process contrast-defined motion by introducing residual distortion products (i.e. local luminance artifacts of both polarities). Here we evaluated the impact of a static luminance pedestal on luminance- and contrast-defined motion processing. For various carrier contrasts, the contrast of the luminance pedestal was adjusted to affect luminance-defined motion thresholds by a factor of about 2. The luminance pedestal was found to affect contrast-defined motion thresholds by a factor near 2 when the carrier contrast was high, but had little impact when it was low. We conclude that contrast-defined motion with high contrast carriers was processed by the first-order motion system, not a second-order motion system. Our results question the existence of a dedicated second-order motion system. |
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