Differences in adaptation of tonic accommodation with refractive state.

It has recently been demonstrated that intersubject variations in tonic (dark focus) levels of accommodation are related to corrected refractive state (McBrien and Millodot, 1987). The aim of the present study was to investigate the effect of sustained visual tasks on the tonic level of accommodation in different refractive groups. Eleven hyperopes, 16 emmetropes, ten early onset myopes and ten late onset myopes had their tonic accommodation measured with the objective infrared optometer Canon Autoref R-1 before and after a 15 min sustained visual counting task. The post-task tonic accommodation level was monitored for 15 min to assess the decay rate of any observed task-induced changes in tonic accommodation. Subjects repeated the experimental procedure for four task locations (6 m, pre-task tonic position, 37 cm and 20 cm). Late onset myopes showed significant positive (myopic) changes in their tonic level of accommodation at both near viewing distances, which showed no evidence of decay during the 15 min post-task monitoring period. Hyperopes, however, underwent transient "counteradaptive" decreases in their tonic level of accommodation after sustained near viewing. Emmetropes and early onset myopes showed little change in tonic levels at the two near distances. Differences between groups were also obtained at tonic and far viewing distances. Post-task changes in tonic accommodation demonstrated only a weak negative correlation with pre-task tonic accommodation levels at each task distance. It is proposed that the observed differences in adaptation of tonic accommodation among refractive groups may be related to variations in autonomic innervation of the ciliary muscle.