Unbalanced Motion Research Articles

Transparent motion perception as detection of unbalanced motion signals. II. Physiology

We investigated how the primate visual system solves the difficult problem of representing multiple motion vectors in the same part of the visual space--the problem of motion transparency. In the preceding companion article we reported that displays with locally well-balanced motion 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 exemplified by our paired and unpaired dot patterns. Although both types of stimuli contain two sets of dots moving in opposite directions, the former is locally well balanced and appears like flicker while the latter gives a perception of two transparent surfaces. In this article we report our physiological recordings from areas V1 and MT of behaving monkeys, comparing single-cell responses to the paired and the unpaired dot patterns. Although a small proportion of directionally selective V1 cells responded differently to the two types of patterns, the average V1 responses could not reliably distinguish between the paired and the unpaired stimuli. A large fraction of MT cells, on the other hand, responded significantly better to the unpaired dot patterns than to the paired ones. Furthermore, the average response of all MT cells to the unpaired dot patterns was significantly higher than that to the paired dot patterns. These results demonstrate a neural correlate of the perceptual transparency at the level of MT. On the other hand, V1 cells do not generally discriminate between the transparent and nontransparent stimuli, indicating that V1 activity is not well correlated with the perception of motion transparency. Our results are consistent with 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 primarily in MT. Finally, we found a strong and negative correlation between the degree of the opponent-direction suppression of MT cells and their responses to flicker noise stimuli. This result suggests that one of the fundamental roles of the opponent-direction suppression in MT is noise reduction.

Transparent motion perception as detection of unbalanced motion signals. III. Modeling

In the preceding two companion articles we studied the conditions under which transparent motion perception occurs through psychophysical experiments, and investigated the underlining neural mechanisms through physiological recordings. The main finding of our perceptual experiments was that whenever a display has finely balanced motion signals in all local areas, it is perceptually nontransparent, and that transparent displays always contain motion signals in different directions that are either spatially unbalanced, or unbalanced in their disparity or spatial frequency contents. In the physiological experiments, we found two stages in the processing of transparent stimuli. The first stage is located primarily in area V1. At this stage motion measurements are made and V1 cells respond well to both the balanced, nontransparent stimuli and the unbalanced, perceptually transparent stimuli. The second stage is located primarily in area MT. MT cells show strong suppression between opposite directions of motion. The suppression for the unbalanced, transparent stimuli is significantly less than that for the balanced, nontransparent stimuli. Therefore, the activity in the second, MT stage correlates better with the perception of motion transparency than the first, V1 stage, which does not distinguish reliably between transparent and nontransparent motion. The above experiments suggest a two-stage model of motion perception with a motion measurement stage in V1 and an opponent-direction suppression stage in area MT. In this article we explicitly test this model through analysis and computer simulations, and compare the response of the model to the perceptual and physiological results using the same balanced and unbalanced stimuli we used in the experiments. In the first stage of the computational model, motion energies in different spatial frequency and disparity ranges are extracted from each local region. Similar to V1, this stage does not distinguish between the balanced and unbalanced stimuli. In the subsequent stage motion energies of opposite directions but with same spatial frequency and disparity contents suppress each other using subtractive or divisive inhibition. This stage responds significantly better to the transparent stimuli than to the nontransparent ones, in agreement with MT activity.

Transparent motion perception as detection of unbalanced motion signals. I. Psychophysics

Our visual system can solve the difficult problem of representing multiple motions in the same part of the visual space, the motion transparency problem. We investigated the conditions under which transparent motion perception occurs through psychophysical observations, using a series of visual displays composed of two simple patterns moving in opposite directions. We found that whenever a display has finely balanced opposing motion signals in all local regions, it is perceptually nontransparent. The displays that appeared transparent always contain locally unbalanced motion signals, with some local regions having net motion signals in one direction and some other regions in the opposite direction. These interdigitating net motion signals in both directions appear to be integrated separately to form two overlapping transparent surfaces. Displays that were spatially balanced could be made perceptually transparent if the two components moving in opposite directions were at different stereo depth planes or had different spatial frequency contents. Our results can be explained by proposing a disparity- and spatial frequency-specific suppression stage in the motion pathway, at which motion signals of different directions, but of the same disparity and spatial frequency contents, locally inhibit each other. Such a mechanism would suppress noise input to the motion system, which generally activates several direction channels simultaneously, and would still not eliminate activity evoked by transparent surfaces that are at different depths or have different textures.

The Adjustment of a Non-balanced Velocity Field towards Geostrophic Equilibrium in a Stratified Fluid

The previous work by Rossby and Cahn on the adjustment of a non-balanced velocity field towards geostrophic equilibrium has been generalized to incorporate the corresponding process in a stratified but incompressible fluid. It is shown that the character of the process essentially depends upon the width of the current, the velocity profile in the vertical and the vertical density gradient. As long as the vertical density gradient is small the adjustment of the mean motion of the fluid ( ) exactly corresponds to the one described by Cahn. On the other hand vertical variations of the unbalanced motion will set up internal oscillations which have considerable larger (vertical) amplitudes than is found for the adjustment of the mean motion. Furthermore, the speed of propagation of the gravity inertia waves thus generated (c K ) is considerably smaller than that of the waves generated during the adjustment of the mean motion (c 0 ). For values of the vertical density gradient as found in the oceans c K < 0.05 c 0 . In the final state of equilibrium the mean motion of the fluid will be approximately the same as before the adjustment, while a considerable smoothing of the vertical gradients of the velocity field will take place, the more the broader the original current is. DOI: 10.1111/j.2153-3490.1953.tb01068.x

The Adjustment of a Non-balanced Velocity Field towards Geostrophic Equilibrium in a Stratified Fluid

The previous work by ROSSBY and CAHN on the adjustment of a non-balanced velocity field towards geostrophic equilibrium has been generalized to incorporate the corresponding process in a stratified but incompressible fluid. It is shown that the character of the process essentially depends upon the width of the current, the velocity profile in the vertical and the vertical density gradient. As long as the vertical density gradient is small the adjustment of the mean motion of the fluid (∫0D0Udz) exactly corresponds to the one described by CAHN. On the other hand vertical variations of the unbalanced motion will set up internal oscillations which have considerable larger (vertical) amplitudes than is found for the adjustment of the mean motion. Furthermore, the speed of propagation of the gravity inertia waves thus generated (cK) is considerably smaller than that of the waves generated during the adjustment of the mean motion (c0). For values of the vertical density gradient as found in the oceans cK &lt; 0.05 c0. In the final state of equilibrium the mean motion of the fluid will be approximately the same as before the adjustment, while a considerable smoothing of the vertical gradients of the velocity field will take place, the more the broader the original current is.