Motion Repulsion Research Articles

Motion repulsion arises from stimulus statistics when analyzed with a clustering algorithm

Motion repulsion is the perceived enlargement of the angle between the directions of motion of two transparently moving patterns. An explanation of this illusion has long been sought for in the neural circuitry of the brain. We show that motion repulsion already arises from the statistical properties of the motion transparency problem when analyzed with a clustering algorithm.

Effects of attention on motion repulsion

Motion repulsion involves interaction between two directions of motion. Since attention is known to bias interactions among different stimuli, we investigated the effect of attentional tasks on motion repulsion. We used two overlapping sets of random dots moving in different directions. When subjects had to detect a small speed-change or luminance change for dots along one direction, the repulsive influence from the other direction was significantly reduced compared with the control case without attentional tasks. However, when the speed-change could occur to either direction such that subjects had to attend both directions to detect the change, motion repulsion was not different from the control. A further experiment showed that decreasing the difficulty of the attentional task resulted in the disappearance of the attentional effect in the case of attention to one direction. Finally, over a wide range of contrasts for the unattended direction, attention reduced repulsion measured with the attended direction. These results are consistent with the physiological finding that strong attention to one direction of motion reduces inhibitory effects from the other direction.

Motion repulsion is monocular

Motion repulsion is the illusory enlargement of the angle between objects moving in two different directions of motion. Previous work suggests that motion repulsion occurs under dichoptic conditions, and therefore is binocular. In reference repulsion the direction of motion is misperceived even if only a single direction of motion is presented. In an experiment I show that repulsion under dichoptic conditions is correlated with reference repulsion, but not with binocular motion repulsion. This suggests that motion repulsion proper, which occurs over and beyond reference repulsion, does not occur under dichoptic conditions, implying that motion repulsion is monocular.

Motion rivalry impairs motion repulsion

In their classic study on motion repulsion, Marshak and Sekuler (Science 205 (1979) 1399) reported a repulsion of up to 10° when two different directions of motion were presented dichoptically. However, subjects in that study did not experience binocular rivalry, presumably because of the brief presentation time. In the present study, we measured repulsion during binocular rivalry by requiring subjects to dichoptically view the stimuli until one direction of motion appeared to exclusively dominate the other (Blake, Yu, Lokey, & Norman (1998). J. Cogn. Neurosci., 10, 46–60). We found that motion repulsion was significantly reduced during exclusive dominance. Indeed, after controlling for reference repulsion—the misjudgment of a single direction of motion (Rauber & Treue (1998). Perception, 27, 393–402)—we found no significant motion repulsion during exclusive dominance. These data suggest that motion repulsion may require the perception, rather than merely the physical presence, of multiple directions.

The dependence of motion repulsion and rivalry on the distance between moving elements

We investigated the extent to which motion repulsion and binocular motion rivalry depend on the distance between moving elements. The stimuli consisted of two sets of spatially intermingled, finite-life random dots that moved across each other. The distance between the dots moving in different directions was manipulated by spatially pairing the dot trajectories with various precisions. Data from experiment 1 indicated that motion repulsion occurred reliably only when the average distance between orthogonally moving elements was at least 21.0 arc min. When the dots were precisely paired, a single global direction intermediate to the two actual directions was perceived. This result suggests that, at a relatively small spatial scale, interaction between different directions favors motion attraction or coherence, while interaction at a somewhat larger scale generates motion repulsion. Similarly, data from experiment 2 indicated that binocular motion rivalry was significantly diminished by spatially pairing the dots, which moved in opposite directions in the two eyes. This supports the recent proposal that rivalry occurs at or after the stage of binocular convergence, since monocular cells could not have directly responded to our interocular pairing manipulation. Together, these findings suggest that the neural mechanisms underlying motion perception are highly sensitive to the fine spatial relationship between moving elements.

Revisiting motion repulsion: evidence for a general phenomenon?

Previous studies have found large misperceptions when subjects are reporting the perceived angle between two directions of motion moving transparently at an acute angle, the so called motion repulsion. While these errors have been assumed to be caused by interactions between the two directions present, we reassessed these earlier measurements taking into account recent findings about directional misperceptions affecting the perception of single motion (reference repulsion). While our measurements confirm that errors in directional judgements of transparent motions can indeed be as big as 22° we find that motion repulsion, i.e. the interaction between two directions, contributes at most about 7° to these errors. This value is comparable to similar repulsion effects in orientation perception and stereoscopic depth perception, suggesting that they share a common neural basis. Our data further suggest that fast time scale adaptation and/or more general interactions between neurons contribute to motion repulsion while tracking eye movements play little or no role. These findings should serve as important constraints for models of motion perception.

A model for motion coherence and transparency.

A recent model for two-dimensional motion processing in MT has demonstrated that perceived direction can be accurately predicted by combining Fourier and non-Fourier component motion signals using a vector sum computation. The vector sum direction is computed by a neural network that weights Fourier and non-Fourier components by the cosine of the component direction relative to that of each pattern unit, after which competitive inhibition extracts the signals of the most active units. It is shown here that a minor modification of the connectivity in this network suffices to predict transitions from motion coherence to transparency under a wide range of circumstances. It is only necessary that the cosine weighting function and competitive inhibition be limited to directions within +/- 120 deg of each pattern unit's preferred direction. This network responds by signaling one pattern direction for coherent motion but two distinct directions for transparent motion. Based on this, neural networks with properties of MT and MST neurons can automatically signal motion coherence or transparency. In addition, the model accurately predicts motion repulsion under transparency conditions.

Phase Transitions in Monomolecular Layer Films Physisorbed on Crystalline Surfaces

The weakest form of interaction between two electrically neutral mole­ cules is the one due to induced dipoles. It has been known for a long time that rare gas atoms interact with each other via this mechanism, known as the van der Waals force. For all rare gases except He, the attraction among atoms is nevertheless sufficiently strong that at suit­ able low temperatures they will condense into liquid and solid phases. For 3He and 4He the quantum zero point motion repulsion is large compared to the weak van der Waals attraction, so their absolute zero temperature phase is a liquid rather than a solid. In physical adsorption (physisorption), individual atoms or molecules are attracted toward the surfaces of liquids or solids of a different chemical species by a weak force whose origin lies in the van der Waals attraction between the constituents of the solid (or liquid) and the individual molecule. The attractive force has to be weak enough for the adsorbed molecule (adsorbate) to retain its molecular identity and not to transform into a different chemical species. This consideration, not important for rare gas atoms, is relevant for molecular adsorbates (02' CH4, etc). Provided that physisorption occurs, it is possible to study how many molecules can be accommodated next to the adsorber surface (first layer) and what structural, thermal, and dynamic properties the first layer will have. For the past decade a great deal of effort has been