Saccades reduce latency and increase velocity of ocular accommodation

Horizontal vergence can be stimulated binocularly with disparity (disparity vergence) or monocularly with accommodation (accommodative vergence). The latter results from a neural cross-coupling that causes both horizontal ver gence and accommodation to respond when either one is stimulated [Alpern, M ., & Ellen, P. (1956). American Journal of Opthalmology, 42, 289-303]. The velocity of disparity and accommodative vergence is enhanced when accompani ed by saccades [Enright, J. T. (1984). Journal of Physiology (London) 350, 9-31; Enright, J. T. (1986). Journal of Physiology (London) 371, 69-89]. Ba sed upon the coupling between accommodation and vergence, we predicted that accommodation should also be facilitated by saccades. An SRI Dual Purkinje Eyetracker was used to measure left and right eye position, and the accomm odation of the left eye, in response to stimulation. Horizontal saccades we re stimulated by targets separated by 2-6 degrees and accommodation was sti mulated monocularly over a range of +/-2 diopters (D). When saccades occurr ed within 0-400 ms following a monocular step stimulus to accommodation, la tency of accommodation decreased and the associated accommodative-vergence response was synchronized with the saccade. Saccades also enhanced the velo city of accommodation and accommodative-vergence, and this facilitation inc reased with saccade amplitude. Transient vergence responses that are normal ly associated with saccades [Erkelens, C. J., Steinman, R. M., & Collewijn, H. (1989). Proceedings of the Royal Society of London B. Biological Scienc es, 236, 441-465; Maxwell, J. S., & King, W. M. (1992). Journal of Neurophy siology 68 (4), 1248-1260] did not affect accommodation when it was not sti mulated by defocus. Because saccades and accommodation utilize separate pla nts and final common pathways, the synchronization of saccades and accommod ation and the enhanced velocity of accommodation and accommodative-vergence must occur at more central sites. Possibilities include the superior colli culus, which represents both accommodation and saccades [Nagasaka, Y., & Oh tsuka, K., (1998). Investigative Opthalmology AVRO supplement], vestibular nuclei which project to regions near the oculomotor nuclei [Lang, W., Buttn er-Ennever, J. A., & Buttner, U. (1979). Brain Research, 177, 3-17], and in teractions between omni pause neurons and near response cells of the mesenc ephalic reticular formation (MRF) [Mays, L. E., & Gamlin, P. D. R. (1995a). Current Opinions in Neurobiology, 5, 763-768; Mays, L. E., & Gamlin, P. D. R. (1995b). Lye movement research: Mechanisms, processes and applications. New York: Elsevier] which represent both accommodation and vergence [Judge , S. J., gi Cumming, B. G. (1986). Journal of Neurophysiology 55,915-930; Z hang, Y., Mays, L. E., & Gamli, P. D. R. (1992). Journal of Neurophysiology , 67, 944-960]. (C) 1999 Elsevier Science Ltd. All rights reserved.