Abstract
When one visual object moves behind another, it provides a compelling sense of which object is closer to the viewer and which object is farther in depth. This percept is referred to as depth-order from motion (DFM). The neural mechanisms of DFM are largely unclear, including the relative roles of the first-order (i.e., luminance-based) vs. the second-order (i.e.,non-luminance-based) motion processing mechanisms, and the relative contributions of the two know types of DFM cues, the accretion-deletion (AD) cue and the common motion (CM) cue. We performed a whole-brain fMRI scan using a mixed (i.e., events-within-blocks) design, which allowed us to compare the responses across blocks as well as across individual trials. Depending on the stimulus block, subjects were shown either stimuli that elicited depth-order percepts, or stimuli that did not. Stimuli that elicited the depth-order percept contained both types of motion and both types of DFM cues. During each trial of each stimulus block, subjects reported the perceived depth-order using a button press. We found significantly greater responses to depth-order stimuli relative to non-depth-order stimuli in several early retinotopic regions, including V1, V2, V3, V3A, and V4v. The response in V3A reliably reflected, on a trial-to-trial basis, whether the subjects perceived depth-order (logistic regression; group data, N = 5; p <0.05). However, we were unable to find any region that was differentially responsive to near vs. far depth-order stimuli, or to the corresponding percepts. Importantly, neither V5/MT+ nor the kinetic occipital region (KO) showed significant differential responsiveness to the DFM stimuli across subjects (p > 0.05), although the responses in both regions showed a slight response suppression by the depth-order stimuli in some subjects. Together, these results identify specific brain regions may play an important role in DFM cue processing and mediate DFM perception.
Supported by Medical College of Georgia.