Kinesthetic estimation of the main orientations from the upright and supine positions.

This work investigated the accuracy of the perception of the main orientations (i.e., vertical and horizontal orientations) with the kinesthetic modality--a modality not previously used in this field of research. To further dissociate the influence of the postural and physical verticals, two body positions were explored (supine and upright). Twenty-two blindfolded participants were asked to set, as accurately as possible, a rod to both physical orientations while assuming one of the two body positions. The horizontal was perceived more accurately than the vertical orientation in the upright position but not in the supine position. Essentially, there were no differences in the supine position because the adjustments to the physical vertical were much more accurate than they were in the upright position. The lower accuracy in the estimation of the vertical orientation observed in the upright position might be linked to the dynamics associated with the maintenance of posture.