The composition of central programs subserving horizontal eye movements in man

A hypothesis is presented which describes, in biomechanical terms, the central programs underlying horizontal eye movements in man. It is suggested that eye movements are produced by means of programmed shifts of the so-called invariant muscle characteristics (static force vs angle phi of gaze). These shifts lead to a change of the equilibrium point resulting from the interaction of agonist and antagonist muscles and, as a consequence, to movement and the attainment of a new position of gaze. A reciprocal or a coactivation command to agonist and antagonist muscles occurs when their characteristics shift with respect to the coordinate phi in the same or opposite directions, respectively. It is proposed that during pursuit and saccadic eye movements a superposition of the both central commands occurs. During a saccade, the reciprocal command develops evenly up to a certain level. The initial and final levels of the reciprocal command dictate the respective position of gaze and therefore the size of the saccade. The coactivation command develops to a maximum level and is slowly switched off when the new position of gaze has been achieved. The magnitude of the coactivation command seems to be not connected with an absolute position of gaze. It provides probably a stability of the movement and, in particular, prevents overshoot and oscillation during the saccade. The same timing of these commands occurs during pursuit movements, but the magnitude of the coactivation command and the rates of the development of the both commands are less in this case and correlate with the velocity of the movement. This hypothesis enables the tension changes in the muscle during saccadic and pursuit movements to be simulated in qualitative accordance with unique experimental data obtained by Collins et al. (1975). The functional significance of superposition of these motor commands and similarity in the efferent organization of eye and limb movements are discussed. Analysis of limb movements in man and animals has allowed one to formulate some concepts concerning the motor control. For instance, it has been suggested and experimentally confirmed that central commands are adequately expressed in terms of shifts of muscle static length - force characteristics and specify an equilibrium point resulting from the interaction of agonist and antagonist muscles (Asatryan and Feldman, 1965; Felman, 1966a, 1974, 1979, 1980a, b; Bizzi et al., 1976; Kelso, 1977; Polit and Bizzi, 1978, 1979; Houk, 1979; Kelso and Holt, 1980). Experimental observation have also shown that two central commands, i.e. reciprocal and unidirectional activation of agonist and antagonist muscles are usually combined by the nervous system in a proper manner depending on the motor task (Feldman, 1979, 1980a, b). The present, theoretical report is designed to show that these concepts are consistent with available experimental data concerning oculomotor control.