ABSTRACT

The neurophysiology of the saccade has been approached by starting at the saccadic trajectory itself and working back into the brain sequentially from the forces that move the eye, to the motoneurons that cause the muscles to contract, to the possible neural structures that provide inputs to the motoneurons. The forces that move the eye have been very elegantly described by Robinson (1964) who used a tightly fitting suction contact lens both to apply horizontal external forces to the eye and to measure forces exerted by the eye during a variety of eye movements. If a step of force is applied (Fig. 3A, dashed

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1.4 THE NEUROPHYSIOLOGY OF SACCADES 43

curve), the eye, to a first approximation, responds along an exponential trajectory with a time constant of 150 msec (Fig. 3A, solid curve). Clearly, a step in net force is not adequate to provide an actual saccade (Fig. 3A, dotted curve). By further applying a variety of isometric and isotonic constraints to the tracking eye, Robinson (1964) was able to construct a model that predicted the net force on the eye during saccades (Fig. 3B). Just prior to a 10° movement (lowest two traces), there is a rapid increase in force to a level (43 g) much greater than that required to keep the eye in its deviated position (15 g). The duration of the force is approximately equal to the duration of the saccade. During a 200 saccade (Fig. 3B, upper traces), the net force is greater and is applied for a longer time appropriate to the longer duration of the larger saccade. Once again the net force during the saccade exceeds that required to keep the eye deviated. However, the excess force is approximately equal (23-25 g) for both saccades, suggesting that a separate group of muscle motor units fire during saccades to provide a pulse of constant force whose duration reflects the duration of the saccade (Robinson, 1964). Histological evidence indicates that the rectus muscles are composed of a variety of muscle fiber types ranging from very fast singly innervated fibers to very slow multiply innervated types. While it is tempting to ascribe the excess pulse of force to contraction of the fast twitch fibers, very little physiological evidence is available to support such a hypothesis.