Threshold position control signifies a common spatial frame of reference for motor action and kinesthesia

Abundance of muscle spindles is most likely related to gradual recruitment and functional specialization of motor units, as well as to their fundamental role in reflex intermuscular interaction and cooperation with other sensory systems. Spindle afferents per se usually convey ambiguous kinesthetic information to the brain. Experimental data indicate that the nervous system cannot use efferent copies, i.e., pre-programmed imitations of motor commands to muscles to overcome this ambiguity and form adequate position sense. Instead, position sense becomes adequate when proprioceptive signals are interpreted in reference to the threshold limb position set by the brain. By resetting the threshold position, the nervous system not only appropriately adjusts kinesthetic sense but also causes motor action. This brief analysis illustrates not only that action and perception are coupled [J.J. Gibson, The Senses Considered as Perceptual Systems. George Allen and Unwin Ltd., London, 1968; W.H. Warren, The dynamics of perception and action. Psychol. Rev. 113 (2006) 358-89] but also that they are accomplished in the same spatial frame of reference selected and manipulated by the brain.