10.0 Generating Motions

   Twenty years ago, Marr (1969) and Albus (1971) suggested that the circuitry of the
cerebellum resembles the learning machine known as the "perceptron." A perceptron learns how
to assign an appropriate output to each input by obeying the suggestions of its "teacher". The
teacher provides encouragement when the perceptron is successful, and discouragement
otherwise. Marr and Albus proposed that the climbing fibers in the cerebellum play the role of
the teacher, and the mossy fibers play the role of the input to which the perceptron is supposed to
assign output.
   Perceptrons are no longer in vogue. However, the general view of the cerebellum as a learning
machine has received a significant amount of experimental support. For instance, Ito (1984) has
studied the way the brain learns the vestibulo-ocular reflex — the reflex which keeps the gaze of
the eye at a fixed point, regardless of head movement. This reflex relies on a highly detailed
program, but it is also situation-dependent in certain respects; and it is now clear that the
cerebellum can change the gain of the vestibulo-ocular reflex in an adaptive way.
   The cerebellum, in itself, is not capable of coordinating complex movements. However, Fabre
and Buser (1980) have suggested that similar learning takes place in the motor cortex — the part
of the cortex that is directly connected to the cerebellum. In order to learn a complex movement,
one must do more than just change a few numerical values in a previous motion (e.g. the gain of
a reflex arc, the speed of a muscle movement). Sakamoto, Porter and Asanuma (1987) have
obtained experimental evidence that the sensory cortex of a cat can "teach" its motor cortex how
to retrieve food from a moving beaker.
   Asanuma (1989) has proposed that "aggregates of neurons constitute the basic modules of
motor function", an hypothesis which is in agreement with Edelman’s theory of Neural
Darwinism. He goes on to observe that "each module has multiple loop circuits with many other
modules located in various areas of the brain" — a situation illustrated roughly by Figure 10. In
this view, the motor cortex is a network of "schemes" or "programs", each one interacting with
many others; and the most interesting question is: how is this network structured?
Kaynak: A New Mathematical Model of Mind
belgesi-961