Adaptation of tonic accommodation reduces accommodative lag and is masked in darkness

Tonic aftereffects of accommodation were investigated under three open loop conditions; Maxwellian view (pin hole pupil), empty field, and darkness. Duration of accommodative aftereffects, following 5 sec of adaptation, ranged within several seconds for all three open loop conditions. After adapting for 1 min to a 2-D stimulus, the duration of accommodative aftereffects increased by approximately an order of magnitude for the Maxwellian view and empty field conditions; however, they increased modestly for the dark condition. Tonic aftereffects of accommodation were also stimulated by convergence with base out prism or divergence with base in prism while the accommodative loop was opened. The ratio of open-loop aftereffect amplitude/closed-loop accommodative response to lenses ranged from 0.95 to 0.48, and it became smaller when the accommodative stimulus approached the amplitude of accommodation. The rapid decay of tonic aftereffects of accommodation in darkness was reversable when a visible stimulus was presented while the accommodative loop was opened by Maxwellian view or empty field. Hence, darkness only masked aftereffects that were manifest primarily when there was light stimulation of the retina. The amplitudes of accommodative aftereffects were reciprocally related to the amplitude of the lag of accommodation. During a 2-min adaptation period, accommodative response increased gradually for 30-60 sec as it approached the amplitude of the accommodative stimulus. This resulted in a reduction of accommodative error or lag. Opening the accommodative loop before or after the lag was reduced resulted in a small or large aftereffect of accommodation respectively. Accommodative aftereffects appear to be related to accommodative lag as prism adaptation is related to fixation disparity.