Chapter 2. Prehension

140 THE PHASES OF PREHENSION
tire body and receives proprioceptive information from the joint cap-
sules, muscle spindles, cutaneous receptors, and tendon organs. The
spinocerebellum (sending output to the magnocellular red nucleus) re-
ceives commands from somatosensory and motor cortices. Receiving
this efference copy of the intended motor command u and feedback
about the evolving movement, it can regulate the periphery by making
error-correcting compensations for small variations in loads and
smooth out the physiological tremor. Kawato suggested that this sys-
tem provides an approximated prediction, x*, of the actual movement,
x, since it has access to the motor command itself. To do this, the
Spinocerebellum/Magno red nucleus system has an internal neural
model of the musculoskeletal system’s forward dynamics. The pre-
dicted movements error, u-x*, is sent to motor cortex and to the mus-
cles. Slowly as learning proceeds, this internal feedback pathway re-
places the external feedback path for feedback control.
At the same time, Kawato et al. (1987) suggested another area of
the cerebellum is involved in predicting the motor command using an
inverse dynamical model. The cerebro-cerebellum (having its output
to the parvocellular part of the red nucleus) monitors the desired motor
pattern, Xd, and the motor command, u. There is neural evidence for
this, since the cerebro-cerebellum has outputs to the thalamus and
motor and premotor cortex, and has inputs from the pontine nuclei
which relays information from cerebral cortical areas that include the
somatosensory, motor, premotor and posterior parietal area. It has no
inputs from the periphery, and the parvocellular part of the red nucleus
does not contribute to the rubrospinal tract, meaning that it does not
have a direct output to the musculo-skeletal system the way the
magnocellular part does. Kawato et al. suggested that the cerebro-
cerebellum has an internal model of the system’s inverse-dynamics,
thus in effect modelling motor cortex. In simulating the inverse
dynamics, Kawato et al. used a feedforward and feedback control
model, such that:
T(t) = Ti(t) + Tf(t) M
u = u*+Kx (6)
where T(t) is the total torque at time t, Ti(t) is the torque computed by
the inverse-dynamics model, and Tf(t) is the feedback torque. The
Cerebrocerebellum/Pao Red Nucleus computes a motor command
u* from the desired trajectory xd, sending its results to the motor cor-
tex.