Directional performance in motion transparency

Abstract

Motion transparency provides a challenging test case for our understanding of how visual motion, and other attributes, are computed and represented in the brain. However, previous studies of visual transparency have used subjective criteria which do not confirm the existence of independent representations of the superimposed motions. We have developed measures of performance in motion transparency that require observers to extract information about two motions jointly, and therefore test the information that is simultaneously represented for each motion. Observers judged whether two motions were at 90 degrees to one another; the base direction was randomized so that neither motion taken alone was informative. The precision of performance was determined by the standard deviations (S.D.s) of probit functions fitted to the data. Observers also made judgments of orthogonal directions between a single motion stream and a line, for one of two transparent motions against a line and for two spatially segregated motions. The data show that direction judgments with transparency can be made with comparable accuracy to segregated (non-transparent) conditions, supporting the idea that transparency involves the equivalent representation of two global motions in the same region. The precision of this joint direction judgment is, however, 2-3 times poorer than that for a single motion stream. The precision in directional judgment for a single stream is reduced only by a factor of about 1.5 by superimposing a second stream. The major effect in performance, therefore, appears to be associated with the need to compute and compare two global representations of motion, rather than with interference between the dot streams per se. Experiment 2 tested the transparency of motions separated by a range of angles from 5 degrees to 180 degrees by requiring subjects to set a line matching the perceived direction of each motion. The S.D.s of these settings demonstrated that directions of transparent motions were represented independently for separations over 20 degrees. Increasing dot speeds from 1 to 10 deg/s improved directional performance but had no effect on transparency perception. Transparency was also unaffected by variations of density between 0.1 and 19 dots/deg(2)

Local stimulus disambiguation with global motion filters predicts adaptive surround modulation

“…3) (Marshak and Sekuler, 1979;Braddick et al, 2002). Direction repulsion has also been observed for transparent stimuli (Braddick et al, 2002;Benton and Curran, 2003;Treue et al, 2000), and it has been suggested that it originates from global mechanisms rather than local ones (Benton and Curran, 2003), which is in agreement with our work. In global Fourier space, transparent stimuli share a lot of properties with non-transparent stimuli, and could be treated using the proposed framework by allowing more than one motion to be computed for each point.…”

“…This decrease matches the perceptually measured decay of the motion-repulsion effect for large angles (see Fig. 3) (Marshak and Sekuler, 1979;Braddick et al, 2002). Direction repulsion has also been observed for transparent stimuli (Braddick et al, 2002;Benton and Curran, 2003;Treue et al, 2000), and it has been suggested that it originates from global mechanisms rather than local ones (Benton and Curran, 2003), which is in agreement with our work.…”

“…8 can be safely applied. This may explain motion-repulsion effects that have been observed in psychophysical experiments (Marshak and Sekuler, 1979;Braddick et al, 2002). The overlap between filters in global Fourier space should thus be predictive for the strength of the repulsion effect, and, indeed, a similar dependency on the relative angles has been observed in psychophysical experiments by Marshak and Sekuler (1979) (red diamonds for subject w.m.…”

“…The overlap between filters in global Fourier space should thus be predictive for the strength of the repulsion effect, and, indeed, a similar dependency on the relative angles has been observed in psychophysical experiments by Marshak and Sekuler (1979) (red diamonds for subject w.m. and green circles for the average) and Braddick et al (2002) (blue squares), as illustrated in Fig. 3A, for angles larger than 10 − 20 degrees.…”

“…) and v p = 0.5 pixels/frame. For small angles the overlap is severe, and the most effective way to resolve the problem is to transform the incoming stimuli such that the relative angle between their motion directions is increased, predicting repulsion effects observed in psychophysical experiments (Marshak and Sekuler, 1979;Braddick et al, 2002). For angles smaller than 10 − 20 deg, the stimuli can be considered identical, and no disambiguation is required.…”