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 visual and motor 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). Across three experiments (N = 246) we probed participants’ 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.