Where is my hand in space? The internal model of gravity influences proprioception

Abstract

Knowing where our limbs are in space is crucial for a successful interaction with the external world. Joint position sense (JPS) relies on both cues from muscle spindles and joint mechanoreceptors, as well as the effort required to move. However, JPS may also rely on the perceived external force on the limb, such as the gravitational field. It is well known that the internal model of gravity plays a large role in perception and behaviour. Thus, we have explored whether direct vestibular-gravitational cues could influence JPS. Participants passively estimated the position of their hand while they were upright and therefore aligned with terrestrial gravity, or pitch-tilted 45° backwards from gravity. Overall participants overestimated the position of their hand in both upright and tilted postures; however, the proprioceptive bias was significantly reduced when participants were tilted. Our findings therefore suggest that the internal model of gravity may influence and update JPS in order to allow the organism to interact with the environment.

Keywords: gravity; joint position sense; proprioception; vestibular system.

Figure 1.

(a) Setup and body postures. A three-dimensional-printed platform supported the hand. An Oculus Rift CV1 showed references for hand location. (b) Raincloud plot [24] indicating each participant's CE at each target angle in upright (i) and tilted (ii) body postures. Target angles against gravity are shown above the horizontal line, while targets with gravity are shown below the line. Long horizontal lines in each target angle indicate means, while pink stars indicate the actual target angle. (c) CEs in upright (pink and light grey) and tilted (teal and dark grey) body postures. Coloured bars indicate target angles against gravity, while grey bars indicate target angles with gravity. Points indicate individual estimates, while error bars reflect standard error. Diamonds indicate the overall means in each posture across all target angles. (d) VEs in upright and tilted body postures. Colours and legend as figure 1c.