ABSTRACT
The servomechanisms featured in the preceding chapter were taxes. Another kind of servomechanism, the so-called optokinetic reaction, provides a point of departure for considering reafference and efference copy. In visual taxes, the error signal derives in one way or another from visual angle, that is the angle between the stimulus source and the longitudinal axis of the animal's body. The more this angle departs from some equilibrium, the more the animal attempts to turn back toward the equilibrium orientation. In the optokinetic reaction, the turning tendency depends upon angular velocity rather than upon angle per se. That is, the strength of the turning reaction depends upon the rate at which a patterned field of light sweeps across the photoreceptive surface of the eye.
