Aggregation and unit formation in the perception of moving collinear lines

The degree to which collinear lines are treated as a unit in resolving rotary motion in depth was investigated with the use of parallel projection to make direction of motion ambiguous. The proportion of time that the collinear lines appeared to rotate in the same direction was used as an index of their perceptual coherence. When the gap between the lines was small, there was strong grouping of the lines with respect to direction of motion as well as appearance of rigidity for the configuration. For larger gaps the grouping for direction of motion was maintained, but the lines appeared to have different axes of rotation leading to an appearance of nonrigidity. It is concluded that line elements can be aggregated for the resolution of certain properties without constituting a unit in any general sense.