Prior Knowledge Biases the Perception of Body Postures

Body postures provide information about others’ actions, intentions, and emotional states. However, little is known about how postures are represented in the brain’s visual system. Considering our extensive visuomotor experience with body postures, we hypothesized that priors derived from this experience may systematically bias visual body posture representations. We examined two priors: gravity and biomechanical constraints. Gravity pushes lifted body parts downwards, while biomechanical constraints limit the range of possible postures (e.g., an arm raised far behind the head cannot go down further). We probed participants’ (N = 246) memory of briefly presented postures using change discrimination and adjustment tasks. Results showed that lifted arms were misremembered as lower and as more similar to biomechanically plausible postures. Inverting the body stimuli eliminated both biases, implicating holistic body processing. Together, these findings show that knowledge shapes body posture representations, reflecting modulation from a combination of category-general and category-specific priors. Statement of relevance The human visual system has built up prior knowledge about the environment that shapes our perception and memory. This knowledge includes general physical rules of the world, for example, gravity and motion laws, as well as specific properties of one category of objects. For example, we know that body parts can only move within certain ranges. In this study, we show that visual body representation is biased by both the general knowledge of gravity and the specific knowledge of biomechanical constraints: Lifted arms were remembered as lower and more similar to biomechanically possible postures. These effects suggest that our brain integrates priors of both the physical world and the specific object category to inform the processing of current stimuli. One-sentence summary In two perceptual judgment tasks, body postures were misremembered as lower, reflecting knowledge of gravity, and as more similar to possible postures, reflecting knowledge of biomechanical constraints.