Disjunctive eye movements

Eye movements may be described in terms of two components: conjugate and disjunctive. A pure conjugate eye movement is one in which the two eyes move in a parallel fashion, while in a disjunctive movement changes occur in the orientation of one eye with respect to the other. A typical situation calling for a disjunctive movement is the change in binocular observation from a far to a near target, when the fixation lines of the two eyes have to converge. Recordings of eye movement responses to the presentation of targets requiring convergence show the movements to be slow compared with their reaction time (Westheimer &AMitchell, 1956). In this respect they differ from the most commonly occurring conjugate eye movements, the saccadic movements. It is clear now that saccadic eye movements are not guided, in that no adjustments in response to visual clues are made during the movement (Westheimer, 1954). The time characteristics of disjunctive movements, on the other hand, would theoretically allow correction of movements during their course on the basis of visual information. That the elements for such a control system exist is evidenced by the following two facts: (1) the sensory system on which such a control system would depend is capable of making very fine discriminations-retinal disparities of less than a minute of arc can be detected-and (2) the muscular system is quite fast and capable of making fine gradations in response. The experiments reported in this paper fall into two groups. Those describing the eye movement responses to simple types of convergence stimuli are given in Part I, and those in which responses are studied to special stimulus situations designed to elucidate the properties of the control mechanism for disjunctive eye movements are contained in Part II.