Motion transparency from opposing luminance modulated and contrast modulated gratings

Two luminance gratings of identical orientation and opposite directions of motion are seen as moving across one another (i.e. moving transparently) only if they differ in spatial frequency (SF) by a factor of four or more. Identical SF gratings produce counter-phase flicker. This suggests that opposite motions cancel each other at the level of motion detection. Here we show that motion transparency is perceived with two gratings of the same SF and orientation moving in opposite directions, when one grating is a first-order, luminance modulated (LM) stimulus and the other is a second-order, contrast modulated (CM) stimulus. Participants were presented with various combinations of LM and CM gratings. In experiment 1, the test stimulus contained the summation of oppositely moving LM and CM gratings. In order to assess the simultaneous perception of both motions, we used a paradigm where observers were required to discriminate the direction of motion of each component from counter-phase flicker. Results show that observers can accurately discriminate both LM and CM directions of motion in a transparent configuration. We next measured the effect of varying the contrast/modulation depth of LM and CM gratings on the perception of transparency. The perception of motion transparency depends upon the relative contrast/modulation depth of the component gratings: raising the contrast of the LM component necessitates a greater modulation depth for the CM component if motion transparency is to be perceived. Our results are consistent with a motion system comprised of two separate, but not wholly independent, pathways for the encoding of LM and CM signals. We hypothesise that the observed contrast dependence is the result of contrast gain control mechanisms that receive inputs from separate motion systems.