Voluntary effort as a stimulus to accommodation and vergence.

Voluntary effort can be used to elicit changes in accommodation and vergence. We investigated the potential cross-coupling interactions of accommodative vergence/accommodation (AC/A) and vergence accommodation/vergence (CA/C) in response to voluntary effort. The responses of accommodation and vergence were measured simultaneously with a dual Purkinje image eye tracker and infrared optometer while subjects viewed a Maltese cross monocularly through a pinhole pupil and made voluntary efforts to imaginary changes in target distance. The accommodation and vergence evoked by voluntary efforts occurred in a ratio that was similar to the response AC/A ratio and different from the response CA/C ratio. One exceptional subject was found who could voluntarily accommodate with one eye occluded without the usual concomitant change in vergence. These results indicate that it is primarily the focusing response that is adjusted during voluntary efforts in the near response.