Our second experiment was designed to probe for a motion analogue of this depth-gated collinear facilitation: Depth-gated
co-directional facilitation (see
Figure 4, top). To illustrate, the 50% signal stimulus used in
Experiment 1 would be expected to excite direction-selective units tuned to up-left as well as those tuned to up-right. We postulate that these directly stimulated units facilitate like-tuned units that lie along the motion path consistent with the preferred direction (hence
co-directional facilitation). In the case of the stimuli of
Experiment 1, these facilitated units might include those with receptive fields positioned over one of the four panels surrounding the diamond-shaped aperture. We assume, however, that this facilitation is blocked where depth-ordering cues do not support the existence of occluded motion so that only neurons with receptive fields positioned over the
Near panels are facilitated (hence
gated co-directional facilitation). This assumption is consistent with the findings of McDermott, Weiss, and Adelson (
2001), who demonstrated that the perceived motion of occluded features depends critically upon the extent that
amodal propagation is supported by occlusion cues. As illustrated in
Figure 4 (top), the directional preferences of the facilitated neurons should be those consistent with the “co-directional” propagation of the visible motion underneath the
Near panels. Our final assumption, then, is that the neurons with receptive fields over the abutting panels (thus encoding
amodal motion) interact with the neurons that encode the (visible) motion in the central aperture. This would tilt the perceptual balance in favor of motion in the direction of the
Near panels.