Abstract
Purpose: Determining how moving dots mask the motion of a point light walker can elucidate what factors are important in perceiving biological motion. We sought to determine whether absolute or relative mask density is important by varying the size of the point light walker and the relative density of the masking dots. Method: Twenty-four naive participants (twelve in each experiment) viewed artificially created point light walker sequences with masks for approximately one second. In the first experiment, the number of dots in the mask remained constant, while the size of the point light walker varied (100, 200, or 400%). In the second experiment, the number of dots in the mask varied, such that the density of the dots within the immediate walker area was constant. The same point light walker sizes were used. In both experiments, each participant completed 60 trials for each walker size and the total display area remained constant regardless of the size of the walker. Results: In the first experiment, the walker was more difficult to detect as the size of the walker increased (as measured by d'). In fact, the largest walkers were undetectable by our participants. In the second experiment, the walker still became more difficult to detect as its size increased, but the increase in difficulty was much smaller (the largest walker was detectable). Conclusions: The density of the mask does play a role in masking biological motion, but does not explain all the variability in detectability. The size of the walker also matters, with smaller walkers easier to detect than larger walkers. The important factor may be that the visual system must integrate over large distances between walker dots as the size of the walker increases.