TRANSPARENT MOTION PERCEPTION AS DETECTION OF UNBALANCED MOTION SIGNALS .2. PHYSIOLOGY

We investigated how the primate visual system solves the difficult pro blem of representing multiple motion vectors in the same part of the v isual space-the problem of motion transparency. In the preceding compa nion article we reported that displays with locally well-balanced moti on signals in opposite directions are perceptually nontransparent (i.e ., one does not see two coherent moving surfaces) and that transparent displays always contain locally unbalanced motion signals. This is ex emplified by our paired and unpaired dot patterns. Although both types of stimuli contain two sets of dots moving in opposite directions, th e former is locally well balanced and appears like flicker while the l atter gives a perception of two transparent surfaces. In this article we report our physiological recordings from areas V1 and MT of behavin g monkeys, comparing single-cell responses to the paired and the unpai red dot patterns. Although a small proportion of directionally selecti ve V1 cells responded differently to the two types of patterns, the av erage V1 responses could not reliably distinguish between the paired a nd the unpaired stimuli. A large fraction of MT cells, on the other ha nd, responded significantly better to the unpaired dot patterns than t o the paired ones. Furthermore, the average response of all MT cells t o the unpaired dot patterns was significantly higher than that to the paired dot patterns. These results demonstrate a neural correlate of t he perceptual transparency at the level of MT. On the other hand, V1 c ells do not generally discriminate between the transparent and nontran sparent stimuli, indicating that V1 activity is not well correlated wi th the perception of motion transparency. Our results are consistent w ith a two-stage model for motion processing: the first stage measures local motion and the second stage introduces suppression if different directions of motion are present at a local region of the visual field . The first stage is located primarily in V1 and the second stage prim arily in MT. Finally, we found a strong and negative correlation betwe en the degree of the opponent-direction suppression of MT cells and th eir responses to flicker noise stimuli. This result suggests that one of the fundamental roles of the opponent-direction suppression in MT i s noise reduction.