Vergence

Abstract: Animals with binocular single vision use disjunctive (vergence) eye movements to align the two eyes on a visual target Several lines of evidence suggest that conjugate and vergence eye movement commands are generated independently and combined at the medial rectus motoneurons If this were true, then a pure vergence eye-position signal should exist This signal would be proportional to the horizontal angle between the eyes (vergence angle), without regard to the direction of conjugate gaze The purpose of this experiment was to identify and study neurons that carry a pure vergence signal Extracellular unit recordings were made from midbrain and pontine sites in monkeys trained to track visual targets moving in the horizontal, vertical, and depth (or target vergence) planes The most commonly encountered neuron that had a vergence signal was the convergence cell These units had a firing rate that was linearly proportional to the convergence angle; their activity was unaffected by changes in conjugate gaze Changes in convergence cell activity preceded the change in vergence angle slightly Convergence cell activity increased for increased convergence regardless of whether the change was in response to purely accommodative or disparity cues Divergence cells were found far less frequently These cells were similar to convergence cells except that they decreased their firing rate for increases in convergence The activity of divergence cells was unaffected by changes in the direction of conjugate gaze Both convergence and divergence cells were found, intermixed, in the mesencephalic reticular formation must outside the oculomotor nucleus Most cells with a vergence signal were found within 1-2 mm of the nucleus These results support the view that conjugate and vergence signals are generated independently and are combined at the extraocular motoneurons Convergence cells seem ideally suited to provide the vergence signal required by the nearby medial rectus motoneurons

Abstract: 1. We recorded eye movements in four normal human subjects during refixations between targets calling for various combinations of saccades and vergence. We confirmed and extended prior observations...

Abstract: Horizontal binocular eye movements of three subjects were recorded with the scleral sensor coil--revolving magnetic field technique during voluntary shifts of gaze between pairs of stationary, real, continuously visible targets. The target pairs were located either along the median plane (requiring symmetrical vergence), or on either side of the median plane (requiring asymmetrical vergence). Symmetrical vergence was primarily smooth, but it was often assisted by small, disjunctive saccades. Peak vergence speeds were very high; they increased from about 50 degrees s-1 for vergence changes of 5 degrees to between 150 and 200 degrees s-1 for vergence changes of 34 degrees. Differences between convergence and divergence were idiosyncratic. Asymmetrical vergence, requiring a vergence of 11 degrees combined with a version of 45 degrees, was largely saccadic. Unequal saccades mediated virtually all (95%) of the vergence required in the divergent direction, whereas 75% of the vergence required in the convergent direction was mediated by unequal saccades, with the remaining convergence mediated by smooth vergence, following completion of the saccades. Peak divergence speeds during these saccades were very high (180 degrees s-1 for a change of vergence of 11 degrees); much faster than the smooth, symmetrical vergence change of comparable size (14 degrees). Peak convergent saccadic speeds were about 20% lower. This difference in peak speed was caused by an initial, transient divergence, observed at the beginning of all horizontal saccades. The waveform of disjunctive saccades did not have the same shape as the waveform of conjugate saccades of similar size. The smaller saccade of the disjunctive pair was stretched out in time so as to have the same duration as its larger, companion saccade. These results permitted the conclusion that the subsystems controlling saccades and vergence are not independent. Vergence responses were relatively slow and incomplete with monocular viewing, which excluded disparity as a cue. Monocularly stimulated vergence decreased as a function of the increasing presbyopia of our three subjects. Subjects were able to generate some vergence in darkness towards previously seen and remembered targets. Such responses, however, were slow, irregular and evanescent. In conclusion, vergence shifts between targets, which provided all natural cues to distance, were fast and accurate; they appeared adequate to provide effective binocular vision under natural conditions. This result could not have been expected on the basis of previous observations, all of which had been made with severely reduced cues to depth.(ABSTRACT TRUNCATED AT 400 WORDS)

Abstract: This paper reviews the literature on eye movements from the standpoint of automatic control systems. Some general reference to the neurophysiology and neuroanatomy of the oculomotor system is made, but the emphasis is upon those interdisciplinary studies which, either qualitatively or quantitatively, describe eye movements within the framework of control systems theory. Reviewed are the four major subdivisions of eye movement control: the saccadic, the smooth pursuit, the vergence, and the vestibular systems. Analytical descriptions of the mechanics of the eyeball, orbit, and extraocular muscles, which are common to all these systems, are also reviewed.