Proprioceptive sensory codes mediating movement trajectory perception: human hand vibration-induced drawing illusions.

To study the organization of the proprioceptive sensory codes subserving movement trajectory perception, complex hand drawing illusions were elicited using various vibration patterns applied to the wrist muscles of nine human subjects. It was established that it is possible to elicit kinesthetic illusions involving spatially oriented lines and geometrical shapes such as rectilinear or curvilinear figures by activating four groups of muscle tendons at the wrist level. The vibration sequences specifically evoking each shape were determined by varying the vibration frequency, the duration of each stimulus applied, and the vibrator onsets, and by applying the vibrations either successively or simultaneously. The proprioceptive coding of a trajectory can be modelled in terms of a series of vectors, the direction of which depends on the anatomical sites of the muscles that are stretched and shortened during the movement. The vector giving the spatial path of a movement is the sum vector of the vectors determined on the basis of the proprioceptive inputs originating from each muscle, and the modulus of the resulting vector is the instantaneous velocity of the movement. In line with previous cortical data, our results suggest that the perception of the spatial paths of limb segment movements is coded in joint space kinematic coordinates on the basis of the relevant ongoing proprioceptive information. It therefore emerges from the results of this study that muscle proprioception is able to generate spatiotemporal afferent patterns that may mediate complex cognitive operations such as those involved in the memorizing and recognition of motor forms.