The role of body centre of mass on haptic subjective vertical.

We examined to which extent proprioceptive information involved in the perception of subjective vertical corresponded to mechanical mass-based axes of the body. Blindfolded subjects (n=14) estimated their subjective vertical in conditions of deviation of the centre of mass (CM) of (1) the head-trunk unit or (2) the head segment alone. Verticality estimates (provided in a haptic modality) were significantly altered by the deviation of the head-trunk CM (by either 4 degrees or 7 degrees), indicating that the subjects used mass-based proprioceptive information stemming from the trunk. Modifying the perception of body orientation by shifting the trunk CM sideways while keeping the body vertical, deviates the subjective vertical towards the opposite side in a similar way to the so-called 'E-effect'. The induced deviations of head CM (9.33 degrees) did not affect the subjective vertical, suggesting that when the balance of the body is not threatened, proprioceptive or vestibular information is prioritized to estimate gravitational direction.