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A perceptual control system's actions are not specified by its reference signals, or unilaterally determined by any events in its environment. A perceptual control system's actions are driven by its perceptual error signals, e. A perceptual error signal is the difference between the system's perceptual signal and its reference signal, and that difference is "calculated" by the comparator function.
In the brief section about the comparator function, I discussed the idea that comparison is probably like an interaction between excitatory (positive) effects, and inhibitory (negative) effects, acting upon a neuron at the same time. The difference between those two effects determines the perceptual error signal. If that were the case, then the comparator neuron would have a baseline rate of activity (for example, X neural discharges per second). Anything that acts on the comparator neuron and increases its rate of activity would be called "excitatory," and anything that decreases its rate of activity would be called "inhibitory." If both excitatory and inhibitory influences act on the comparator neuron at the same time, then its rate of activity is the sum of its baseline rate, plus the net effect of the positive and negative influences acting on it. In PCT, we think the activity coming out of the comparator neuron is the perceptual error signal, the signal that acts on the output function, which produces the output quantity that drives the system's actions.