Movement control is faced with a number of computational
problems that arise from kinematic and kinetic redundancy-the
multitude of ways in which a given motion can be executed.
Spinal microstimulation studies indicate that the frog
spinal cord might be organized into modules that produce
force-field primitives.
Based on these initial findings, Bizzi, Giszter, and
Mussa-lvadi proposed that diverse and redundant movements
arise from combinations of these primitives. Giszter went
on to describe studies in which he and colleagues examined
how the frog uses the force-field primitives to construct
the correction response during wiping reflex. They recorded
the force field generated by the frog during wiping reflex
with cutaneous feedback intact and after cutaneous deafferentation.
By subtracting force field under these two conditions,
they got the corrective force field.
There are two lines of evidence that the frog generates
the corrective response by summing the corrective force
field with the normal wiping reflex force field: first,
the correlation of the force field across different time
points is high; second, the rise and fall of force magnitude
at each location is unimodal. In addition to this, they
also identified the muscle synergy responsible for the
corrective response. In conclusion, force-field primitives
may be used as building blocks by biological systems to
deal with the kinematic and kinetic redundancy in movement
control.
It is important to note, however, that the situation
may not be as simple when it comes to higher vertebrates,
especially humans. For example, the influence of gravity
may differ when the arm is in different configurations.
Therefore, the formation of a trajectory cannot be explained
solely by the force field. Nevertheless, the concept of
force-field primitives may provide a new framework in
which to understand psychophysical studies of human movement.
