The Glenn A. Fry award lecture: adaptive regulation of accommodative vergence and vergence accommodation.

The tonic resting level of accommodation, measured with an objective infrared optometer in an open-loop state (pinhole pupil or empty field), was increased by more than 1.75 D from baseline measures of resting focus after adapting monocularly for 1 min to a 2 D minus lens. The accommodative aftereffect disappeared in darkness, but returned when a visible stimulus reappeared in the open-loop state. Stimulation of disparity vergence with 10 delta, while accommodation was in an open-loop state, also increased the resting focus of accommodation. Similarly, the tonic resting level of vergence became more esophoric after adapting for less than 1 min to base-out prism or to a minus lens presented while the vergence loop was opened. The effects of tonic accommodation on accommodative vergence and the effects of tonic vergence on vergence accommodation were investigated with the temporal frequency responses of the AC/A and CA/C ratios to sinusoidal variations in blur (2 D) and disparity (10 delta), respectively. Accommodative vergence was unresponsive to low temporal frequency sinusoidal variations in blur (less than 0.1 Hz). Similarly, vergence accommodation was unresponsive to low temporal frequency sinusoidal variations in disparity. However, accommodative vergence and vergence accommodation were responsive to higher temporal frequency stimuli (up to 0.5 Hz). When negative feedback to the stimulated system (accommodation or vergence) was cancelled electronically, the low temporal frequency response increased for the AC/A and CA/C ratios, respectively. There was also a nonlinear increase of both AC/A and CA/C ratios as stimulus amplitude increased. It is hypothesized that the nonlinearity resulted from limitations of adaptation to small stimuli. An inverse complementary relation is suggested between the amplitude of the AC/A ratio and adaptable tonic accommodation, and between the amplitude of the CA/C ratio and adaptable tonic vergence. This model predicts that in this complementary relation, adaptable tonic elements would sustain motor responses of accommodation and vergence that were initiated by phasic elements and cross-link interactions. It also predicts that the AC/A and CA/C ratios would decrease in time as dynamic control shifted from the phasic to the adaptable tonic control mechanisms of accommodation and vergence, respectively.