Cybernetics of ocular movement.

CYBERNETICS has been defined as "the control of machines by machines instead of by men" (Stanley-Jones, 1957). Its meaning embraces also the pattern of that control. A cardinal notion is the concept of feed-back. This may be simply explained by reference to the most familiar example of cybernetic control, the thermostat of an electric iron. An electric iron is a machine for converting electricity into heat. It has accordingly, like all machines, an input and an output. The thermostat is the little machine that controls the big machine. It is actuated by the output, that is the temperature of the iron, switching on and off as this falls or rises. It acts on the input. This pattern of cybernetic control, whereby changes in the output (temperature) are made to cause corresponding but inverse changes in the input (current), is known as feed-back, or, more elegantly, as refection. The refection in the thermostat of an electric iron is so designed that the output or working-temperature may be controlled within pre-set limits. It is a pattern of control the purpose of which is the maintenance of stability. It may therefore be called stabilizing refection ("negative feed-back"). A thermostatic apparatus could be constructed with the wiring reversed, so that, in the terminology of the engineer, the feed-back becomes positive. Suppose the connexions of an electric iron were reversed, so that the current was switched on when the heating-surface passed the upper limit of temperature and switched off as it cooled. Two possibilities arise, depending upon the initial setting. If the thermostat is switched on, the iron continues to heat up until it has passed the upper safety-limit. The thermostat, its wiring reversed, fails in its stabilizing duties: the current remains on. The iron gets hotter and hotter, and in due course may become red-hot, set fire to the ironing-cloth and then to the whole house, and so end by causing its own destruction. If the thermostat, on the other hand, is switched off, and the temperature is falling, the iron will lose heat until it falls below its proper workingtemperature. The thermostat fails in its task of switching on, and the iron will cool to room-temperature and remain there. These two sequels of positive feed-back are known as "runaway": runaway to maximum, runaway to zero. Both are due to a reversal of the normal pattern of stability-control or refection. Although the end-results are so