Motion information reducing manipulations can bias the discrimination of sex in biological motion perception

The relative importance of motion and form information in biological motion perception has been debated in the biological motion literature. Several techniques have been used to study the role of form information in biological motion perception, including presenting stationary single frames of biological motion to remove motion entirely, or reducing the motion information available in the display by (1) presenting sequential position walkers where the point-lights move on the limbs across frames (Beintema & Lappe, 2002; Beintema, Georg, & Lappe, 2006) or (2) presenting size-changing biological motion where the size of the entire biological motion display varies across frames (Lappe, Wittinghofer, & de Lussanet, 2015). We compared these various methods of investigating the role of form in biological motion perception. Specifically, we compared performance on a sex discrimination task in biological motion in four conditions: (1) normal biological motion, (2) static frame of normal biological motion, (3) sequential position biological motion, and (4) size-changing biological motion. The results showed that discriminability of the sex of the actor, as measured by the slope of the best fitting logistic regression functions, was highest in normal biological motion and significantly lower in the other conditions that remove or reduce motion information. In addition to these expected results, sequential position biological motion and size-changing biological motion also created a significant bias in sex discrimination where the displays were biased to be perceived as more male (as measured by the 50% point of the function). These results suggest that some manipulations of biological motion stimuli may create significant biases in biological motion perception that are not likely due to the removal of motion information per se. Future research is needed to explain the basis of the bias and may lead to greater understanding of the mechanisms of biological motion perception. Meeting abstract presented at VSS 2017