Goodman, J. M.
Lee, A. S.
Suresh, A. S.
Hatsopoulos, N. G.
Bensmaia, S. J.
Despite the remarkable complexity of the hands, we are able to use
them effortlessly to grasp and manipulate objects. To achieve dexterous
object manipulation requires not only a sophisticated motor system to
move the hand but also a sensory system to provide proprioceptive
feedback on the consequences of those movements. To investigate hand
proprioception in cortex, we simultaneously record time-varying joint
kinematics of the hand – measured using a camera-based motion tracking
system – and neural activity from somatosensory cortex of rhesus
macaques – using chronically implanted electrode arrays – as they
perform a wide range of natural grasping movements and undergo passive
hand movements. We also record from primary motor cortex (M1) to compare
sensory and motor representations of the hand. We find that, during
active grasping movements, somatosensory representations of kinematics
are very similar to their motor counterparts, with spiking activity from
both preferentially encoding the future changes in posture (rather than
the velocities) of multiple joints spanning the entire hand. However,
during passive manipulation of the hand, somatosensory neurons encode
the postural changes of fewer joints with spiking activity that lags,
rather than leads, those kinematics. That sensorimotor representations
of the hand encode postural changes of the digits stands in contrast to
similar representations of the arm, where velocities are preferentially
encoded, and suggests that the different functions of the arm and hand
give rise to different types of kinematic representations. Moreover,
that somatosensory representations of hand kinematics reflect their
motor counterparts during volitional movements, but not during passive
movements, suggests that proprioceptive somatosensory cortex integrates
both peripheral afferent and central motor inputs.