Abstract
Individually moving elements in the visual scene may be perceptually grouped together into a coherently moving object leading to a global motion percept. In the current experiment, we show that the perceived speed of a particular global motion percept is not completely dictated by the speed of the local moving elements. We investigate a specific stimulus that leads to bi-stable percepts in which local and global motion may be perceived in alternating fashion. Four rotating dot-pairs, when arranged into a square-like configuration may be perceived either locally, as independently rotating dot-pairs, or globally, as two large squares translating along overlapping circular trajectories. Using a modified version of this stimulus, we demonstrate that the perceptually grouped squares appear to move more slowly than the locally perceived rotating dot-pairs.
Further experiments investigate several possible explanations as to why the effect occurs, including a) the hypothesis that the illusory squares in the global percept are treated as veridical objects by the motion processing system, where larger objects are perceived as moving slower, and b) the hypothesis that the rotational motion of the dots in the local percept stimulate rotation-specific motion detectors sub-optimally, leading to an imprecise measure of speed.
These data give further evidence that speed perception is not determined solely on the basis of local speed-tuned mechanisms, but rather must receive input from operations that carry out a global analysis of form, and specifically address how certain properties of the stimuli, such as rotation or illusory lines, might contribute to the analysis of speed. This result therefore draws a clear distinction between stages of processing involved in the detection of visual information explicitly represented in the image and stages of perceptual construction that can extract and even create information that is not explicit in the image. Stuart Anstis