Abstract
The existence of a second-order motion pathway, distinct from both the first-order luminance pathway and the high-level feature tracking motion system, remains controversial. Two key arguments support its existence: second-order motion is perceived at temporal frequencies too high to be tracked and sensitivity is independent of the relative phase between superimposed luminance- and contrast-modulated gratings. The later argument is typically used to reject the hypothesis that the contrast-modulation contribution to motion is due to early nonlinearities introducing luminance artifacts. But this argument is only valid if the nonlinearities are homogenous across luminance motion detectors; it does exclude the possibility that compressive nonlinearities precede some luminance motion detectors and expansive nonlinearities precede others. In the current study, to neutralize the impact of such opposing nonlinearities, we superimpose a luminance-modulated grating with a high contrast contrast-modulated grating so that some luminance artifacts would sum with the luminance-modulated grating and others would subtract resulting in no net gain. The gratings were drifting in the same direction at a temporal frequency too high to be tracked (15Hz) and their relative phase was systematically varied. Observers were asked to null the net perceived motion by adjusting the contrast of another luminance-modulated grating drifting in the opposite direction. The contrast-modulation contribution to motion was estimated as the contrast difference between the two opposing luminance gratings when no net motion was perceived. Results showed no effect of phase and the contrast-modulation significantly contributed to motion when the contrast of the superimposed grating was low, but not when it was high (i.e. no net motion was perceived when both luminance gratings had the same contrast). We conclude that our sensitivity to contrast-modulated motion at temporal frequencies too high to be tracked is due to early opposing nonlinearities differing across luminance motion detectors, not to a second-order motion pathway.
Meeting abstract presented at VSS 2012