Motion path misidentification in the periphery

Examination of motion illusions in central and peripheral vision has led to the "feature blur'' hypothesis: the peripheral visual system combines features that the foveal visual system can separate. Others have hypothesized that processes that underlie crowding limit multiple object tracking. Here, I investigate the perception of motion paths of two objects that follow opposite rotational directions; I hypothesized that objects that follow the same path may produce misidentifications that depend on eccentricity. The stimulus consists of two 1-deg disks, one filled with a radial sine-wave pattern (concentric rings), the other with a tangential sine-wave pattern (spokes on a wheel); one disk rotates clockwise around a central point while the other rotates counter-clockwise. When viewed in the periphery, the disks often do not seem to follow circular paths but rather appear as an elliptical jumble of the two disks. In two experiments I measured the critical radius of the circular path (i.e., the size at which there is a transition between the percept of a circular path and jumbled ellipses) at several eccentricities; the results showed that the critical size as a function of eccentricity is similar to Bouma's law of crowding (slope between 0.1 and 0.5). In addition I found that the effect persists when one disk has high internal contrast and the other has low internal contrast---a finding that seems to separate motion path misidentification from standard crowding phenomena. Conclusion: motion path misidentification results may be consistent with a "feature blur" hypothesis; MPM in the visual periphery depends on eccentricity, similar in principle to Bouma's law, but may not share all aspects with visual crowding.