The dynamics of small saccadic eye movements.

1. The mechanical characteristics of the system comprising the eyeball and its attachments have been determined, by applying rotational forces to the eyeball, and using an accelerometer on a contact lens to measure the resulting movement.2. The angular acceleration-versus-time curves of small saccades have been recorded. Some of these saccades are made whilst the eye is being vibrated by external forces.3. A mechanical model of the orbital system has been formulated. This is considered to bear a relationship to the structures in the orbit.4. The model has been used to deduce the force pattern of the active component in the extraocular muscles during the execution of a saccade. It is concluded that a saccade is initiated by a rapid rise of tension. After a short time, the tension falls to a lower level, which is the new steady-state level. The present findings are therefore basically different from those of previous workers, in that we deduce a change in the force pattern during the course of the movement.5. In the execution of saccades of various sizes, both the strength and duration of the brief heightened tension are found to vary. For a 4 degree saccade, the transient tension is approximately 36 g, lasting for approximately half of the duration of the saccade.6. Characteristic features of saccades are interpreted in terms of variations in the duration of the transient excess tension.7. Differences in the time course of horizontal and vertical saccades are attributed to differences in the elasticity of the muscles involved in the movements.8. It is suggested that, in view of the similarity of the acceleration-versus-time wave forms, the muscular force pattern which is responsible for rapid head movements is of a similar type to that which has been deduced for saccades.