Chapter 2. Prehension

Chapter 3 - Serial Order in Prehension 57
army, where the general issues the goal for the troops without specify-
ing the individual movements of all the soldiers. An important point is
that there is a limit to the set of commands that the general has avail-
able; thus, the general may be powerful, but this power exists with re-
spect to a limited collection of troop activities (which in themselves
might be quite complex). While Greene stressed a hierarchical inter-
action between ‘executive’ and ‘low-level systems’, he argued that
their relationship could change; perhaps it would be more appropriate
to view this as a heterarchical or distributed model.
Greene’s model suggests that the executive acts in a feedforward
manner to bring the system into a class of states, or ‘into the right
ballpark’, as seen in Figure 3.5, so that feedback can be used to make
the small remaining corrections. This reduces the complexity of the
control system, but it also reduces generality, because not every
possible response can be made. The executive computes the parame-
ters to tune low-level mechanisms, which can continue to produce rea-
sonable responses even without further communication until the next
update. The executive can run a speeded-up model of the controlled
system’s behavior, and thereby predict the results of actions. Greene
suggested that the brain stores recipes or formulae for generating
functions that map commands into states and transformations of states
for low-level mechanisms. Thus, there is a separation of responsibil-
ity between standard activation and fine-tuning of the motor command.
Greene argued that the executive need not even be aware of the tuning.
In Greene (1972), he argued that the process acted as an approximate
feedforward system, which is corrected by feedback. For example,
when a cat stands on a perturbed platform, its vestibular system must
increase the tension in each supporting muscle so balance is main-
tained. An exact computation would be too slow, but a rule of thumb
would increase the tension in the muscles that are already exerting the
most support. While not exact, feedback can then be used to make
small corrections. However, the vestibular response is not appropriate
when the cat shakes his head; instead of a lack of response in the
vestibular system, its response is just nullified by neck muscle recep-
tors. In grasping, Traub, Rothwell, and Marsden (1980) reported a
‘grab reflex’ or ‘sherry glass response’ whereby, regardless of load-
ing or unloading of the thumb flexor muscle (flexor pollicis longus) by
mechanical perturbations to the wrist, a functional response is to
maintain grasp. This is dependent and adapted to the task and the in-
tent of the individual, which in this case is to maintain the digits in
contact with the object. For example, on board a rolling boat, one in-
stinctively grabs for a falling wine glass. This grab reflex is not vol-