The idea of corollary discharge signals, signals from one central pattern generator to another, or from one ganglion to another, that inform the recipient of the actions of the sender, does much to explain many aspects of behaviour that our model previously was unable to incorporate into its predictions.

The phenomenon of the "phantom limb" has very many interesting observations, and many equally interesting conclusions can be drawn from them. It allows us to practice our methodology of determining function by effect of absence, without involving the experimenters in ethical issues. The information that amputees provide, that they feel the presence of the limb even in its absence, even when they cognitively know that it is no longer there, and that they often feel discomfort in the limb, implies that most of the motor patterns that involve movement of the particular limb are the results of central pattern generators, and that somehow these CPGs still exist after amputation. Also, they are able to communicate well with other CPGs and with the brain, including the I- function of self- awareness, that makes the "person" aware of the feeling of substance of a limb that no longer exists. Corollary discharge patterns fit nicely into the communications niche; the person hence knows where the limb is without sensory input because of the still- functioning firing system of corollary discharge signals.

Corollary discharge signals, coupled with central pattern generation, indicate the functioning of a higher level of organisation within the nervous system. They can be thought of as communications pathways, or methods, or processes for and between central pattern generators that are inclusive of the universality of the basic structural unit (the neuron) and the basic functional unit (the action potential), and the variations observed in behaviours that have the same inputs. They can sometimes serve as the input itself into CPGs, that the immediately modifies the interpretation by the nervous system of the state of the outside world. Hence interpretations of exactly the same sentence or word can be widely varying between two different individuals.

What happens, for example, when one walks off a treadmill upon which one has been walking for a while at a reasonably rapid pace, faster than normal walking gait? You still feels as if the body is moving through space in the smooth, rapid manner as the pace set by the machine, while at the same time your feet are telling your brain that they are not really moving as fast as before! Your feet try to move at the same rate as before, but the ground simply won't provide the same give as the treadmill does, i.e., the treadmill moves along with your legs, and so you are able to take large strides and not feel the effect of dragging your body against gravity in tune to those large strides. Off the treadmill, you has no such luxury, and gravity catches up with you, although your brain has not yet realized it.

In other words, the central pattern generator still continues to fire and create a motor symphony of large strides, while sensory inputs inform the brain that the ground isn't moving as fast anymore- there is a disjunction between the sensory inputs and the corollary discharge signals that the central pattern generator sends to the other parts of the body and the I- function, and so you feel like you're still moving through space at the same rate as you were before. Thus the feeling of sudden physical disorientation one experiences as a result of perturbations of the inertia of motion could be explained by incomparable inputs and corollary discharge signals from the central pattern generators that initiate the motor symphonies in question.

Sophisticated review, with nice extension. Yes, lots of situations in which we "notice" a mismatch between input and expectation. Might even, in fact, be a general lesson about the "I-function" there, one we'll try and come back to. PG