The effects of beta sympathetic drugs on positive accommodation (accommodation for near vision) were studied in vervet monkeys. Isoproterenol, a beta sympathetic stimulator, depressed the positive accommodation function. It was not a complete antagonist since 3 to 4 diopters of accommodation remained. Only atropine, a parasympathetic blocker, could abolish these 3 to 4 D as well as the entire function. In many of the experimental animals, positive accommodation function was biphasic. The initial component had a flatter slope, lower amplitude, and lower threshold of stimulation than the second. The transition point of the components was between 2 and 4 D. Beta sympathetic stimulation appeared to abolish selectively the second component while parasympathetic inhibition abolished both components. It appeared that peripherally, parasympathetic inhibition was a more potent and thus more important mechanism in antagonizing positive accommodation than beta sympathetic stimulation.
Hurwitz BS, Davidowitz J, Chin NB, Breinin GM. The Effects of the Sympathetic Nervous System on AccommodationI. Beta Sympathetic Nervous System. Arch Ophthalmol. 1972;87(6):668-674. doi:10.1001/archopht.1972.01000020670011