The mechanism of accommodation in primates.

OBJECTIVE: To study the accommodative mechanism in primates using monkeys, in light of a recently proposed novel accommodative mechanism in primates and a concomitant controversial surgical procedure for the reversal of presbyopia,

DESIGN: Experimental study.

METHODS: Accommodation was induced by stimulation of an electrode surgically implanted in the midbrain and by topical ocular application of muscarinic agonists. Pharmacologic disaccommodation was achieved by topical application of a muscarinic antagonist. Movements of the lens equator and the ciliary body were imaged during accommodation and disaccommodation using ultrasound biomicroscopy and goniovideography, and the images were analyzed to determine the direction and the extent of the movements.

RESULTS: Despite the systematic eye movements occurring with electrical stimulation and the nonsystematic eye movements occurring with pharmacologic stimulation, in all instances the ciliary body and the lens equator moved away from the sclera during accommodation.

CONCLUSIONS: Movement of the accommodative structures is consistent with the classic mechanism of accommodation described by Helmholtz, and contrary to that recently proposed by Schachar.