What motion distributions yield global transparency and spatial segmentation?

We have examined the ability-of observers to parse bimodal local-motion dis tributions into two global motion surfaces, either overlapping (yielding tr ansparent motion) or spatially segregated (yielding a motion boundary). The stimuli were random dot kinematograms in which the direction of motion of each dot was drawn from one of two rectangular probability distributions. A wide range of direction distribution widths and separations was tested. Th e ability to discriminate the direction of motion of one of the two motion surfaces from the direction of a comparison stimulus was used as an objecti ve test of the perception of two discrete surfaces. Performance for both tr ansparent and spatially segregated motion was remarkably good, being only s lightly inferior to that achieved with a single global motion surface. Perf ormance was consistently better for segregated motion than for transparency . Whereas transparent motion was only perceived with direction distribution s which were separated by a significant gap, segregated motion could be see n with abutting or even partially overlapping direction distributions. For transparency, the critical gap increased with the range of directions in th e distribution. This result does not support models in which transparency d epends on detection of a minimum size of gap defining a bimodal direction d istribution. We suggest, instead, that the operations which detect bimodali ty are scaled (in the direction domain) with the overall range of distribut ions. This yields a flexible, adaptive system that determines whether a gap in the direction distribution serves as a segmentation cue or is smoothed as part of a unitary computation of global motion. (C) 1999 Elsevier Scienc e Ltd. Al rights reserved.