The neural control of looking

Saccades are the eye movements we make to look at objects. Apart from being astonishingly fast, often lasting little more than 20 milliseconds, they also happen to be the single most frequent movement we make, much commoner than heartbeats: we make two or three saccades every second of our waking lives. Saccades determine what we see, so their importance can hardly be exaggerated. The neural system that generates saccades has a particular interest as well. Because it is relatively simple, with inputs that can easily be manipulated and outputs we can measure with precision, our knowledge about how oculomotor signals are processed by specific neural structures is more quantitative and exact than for any other comparable part of the brain. In addition, it is a system that actually does something, a system with an output as well as an input — a microcosm of the brain itself. Like the brain, this system has a natural functional hierarchy that is reflected in its structure. The hierarchy extends from local control of the forces generated by individual muscles at the lowest levels, through the mechanisms for locating objects and directing movements towards them, to the highest level, at which decisions are made to move or not to move. While the eye is in motion it is effectively blind, so saccades have to be extremely fast. Yet because of friction and internal viscosity, the eyeball and the muscles that move it are intrinsically sluggish. If an oculomotor muscle suddenly increases its tension from one steady level to another, the eye may take almost half a second to reach its new equilibrium position. This means that a saccade as brief as 20 milliseconds cannot be generated simply by a step change in the activity of a muscle or the nerve fibres that innervate it. Recordings from oculomotor nerves during saccades show that something much more sophisticated occurs (Figure 1). During the saccade, they fire in a burst of maximal frequency, throwing the eye into its new position; the firing then settles down to the new rate required to hold it there. This complex pattern of activity is created when signals from two different types of neuron in the neighbouring brainstem meet at the motor neuron. Burst units fire in a rapid burst during saccades in a particular direction, while the activity of tonic units changes from one steady level to another in a way …