Motion transparency occurs when multiple object velocities are present within a local region of retinotopic space. Transparent signals can carry information useful in the segmentation of moving objects and in the extraction of three-dimensional structure from relative motion cues. However, the physiological substrate underlying the detection of motion transparency is poorly understood. Direction tuned neurons in area MT are suppressed by transparent stimuli, suggesting that other motion sensitive areas may be needed to represent this signal robustly. Recent neuroimaging evidence implicated two such areas in the macaque superior temporal sulcus. We studied one of these, FST, with electrophysiological methods and found that a large fraction of the neurons responded well to two opposite directions of motion and to transparent stimuli containing those same directions. A linear combination of MT-like responses qualitatively reproduces this behavior and predicts that FST neurons can be tuned for transparent motion containing specific direction and depth components. We suggest that FST plays a role in motion segmentation based on transparent signals.

Temporal interactions in direction-sensitive complex cells in area 18 and the posteromedial lateral suprasylvian cortex (PMLS) were studied using a reverse correlation method. Reverse correlograms to combinations of two temporally separated motion directions were examined and compared in the two areas. A comparison to the first-order reverse correlograms allowed us to identify nonlinear suppression or facilitation due to pairwise combinations of motion directions. Results for area 18 and PMLS were very different. Area 18 showed a single type of nonlinear behavior: similar directions facilitated and opposite directions suppressed spike probability. This effect was most pronounced for motion steps that followed each other immediately and decreased with increasing delay between steps. In PMLS, the picture was much more diverse. Some cells exhibited nonlinear interactions, that were opposite to those in area 18 (facilitation for opposite directions and suppression for similar ones), while the majority did not show a systematic interaction profile. We conclude that nonlinear second-order reverse correlation characteristics reveal different functional properties, despite similarities in the first-order reverse correlation profiles. Directional interactions in time revealed optimal integration of similar directions in area 18, but motion opponency—at least in some cells—in PMLS.