Haptic size perception is influenced by body and object orientation

Changes in body orientation impacts our perception of visual size. This has been attributed to the involvement of the vestibular system in constructing and maintaining a representation of space. Here we investigate how body orientation might influence haptic size perception. Blindfolded participants estimated the felt length of a rod and then adjusted it back to its previously felt size after it had been set to a random size. Participants could feel and adjust the rod in the same posture (standing or supine) or after changing posture. They held the rod either aligned with the long axis of their body such that its orientation relative to gravity changed with body tilt, or they held it laterally across the chest where its orientation relative to gravity was constant. Based on past literature, perceived size should be larger when the body is supine. In support of this hypothesis, our results indicate an expansion of perceived haptic size when supine but only for a rod held aligned with the body. This suggests that vestibular cues not only influence visual but also haptic size perception and the perception of the space around us.