Velocity– not Perceived as such: The Role of Perceived Mass on Motion Estimation

In physics, velocity is conventionally defined as the first derivative of an object's position with respect to time. However, a longstanding assumption within vision science has equated the visual hierarchy of object motion with the principles of Newtonian mechanics– treating velocity as a unitary and fundamental variable, akin to its role in physics. This assumption has persisted despite compelling evidence suggesting that the human visual system is biased in perceiving velocity, influenced by factors such as viewing distance and angle. Contrary to the presumed universality of velocity as a visual primitive, numerous studies have revealed pronounced biases in human observers' ability to discern object motion, particularly influenced by the size of moving objects. Notably, larger objects are consistently perceived as moving more slowly. Building upon these findings, our study investigates the impact of perceived mass on the perceived speed of moving objects. Through a 2-AFC paradigm involving objects of varying sizes and materials, our findings unveil a discernible trend: as objects increase in perceived mass due to size or apparent density, their perceived speed decreases. In light of these observations, we challenge the prevalent assumption that velocity operates as a visual primitive. Instead, we contend that momentum, rather than velocity, may offer a more suitable conceptual framework within object motion perception, acknowledging the inherent integration of information about mass in the visual processing of motion.