Haptic alignment of the hands in three dimensions.

The problem of constancy of perception over variations in sensory input applies as much to proprioception as it does to vision and audition. The authors conducted an experiment in which blindfolded participants (N = 6) attempted to produce a constant relation between the left and the right hands' attitudes (roll, pitch, and yaw) at different distances and directions relative to the body. The measure of perceptual constancy over the changing 3-dimensional configurations of the arms' 14 joint degrees of freedom was the hands' deviations from the required attitudes when participants perceived that they satisfied the required attitudes. Two slant spinors quantified the attitudes of the hands relative to the body or the laboratory reference frame. A vergence spinor quantified the attitude of the right hand relative to that of the left hand. The scalar components of the spinors were found to be idiosyncratic to, and reasonably constant for, individual participants, as were the bivector components (revealed through spherical graphical and statistical procedures). Achieving particular attitudinal relations to the body and between contralateral limb segments are individual-specific perception-action skills. They are constrained by haptic invariants approximated by the slant and vergence spinors.