The accommodative response, refractive error and mental effort: 1. The sympathetic nervous system.

The accommodative response of 12 emmetropic subjects was measured for an array of numbers located at -1, -3 and -5 dioptres using an objective infra-red optometer. Responses were compared for passive (reading numbers) and active (adding numbers) conditions. The imposition of mental effort induced a significant increase in mean response for the -1 D stimulus, a response equivalent to the passive condition at -3 D and a reduction in response at -5 D. Mental effort induced similar responses for the -1 D and -3 D locations when sympathetic innervation to the ciliary muscle was blocked with the beta-receptor antagonist timolol maleate. Responses for the -5 D location were, however, significantly increased. It is proposed that sympathetic inhibition can modify the effect of mental effort on near accommodative responses.