Tonic adaptation in symptomatic and asymptomatic subjects.

Tonic accommodation and tonic vergence were measured in symptomatic and asymptomatic subjects before, during, and after 45 min of binocular reading at near (5 D). Reading produced significant increases in both tonic levels, with the magnitude of change being similar in the two groups. After reading, both groups exhibited rapid loss of the tonic vergence effect, but showed no evidence for decay of the tonic accommodation effect. Differences between the groups were observed in the rate of adaptation during the course of reading, with both tonic components increasing more slowly for the symptomatic subjects than for the asymptomatic subjects. In addition, baseline tonic accommodation was significantly higher in the symptomatic group than in the asymptomatic group. The results suggest that although the failure to undergo tonic oculomotor adaptation is not a causal factor in asthenopia, the initial steady-state level and/or subsequent temporal changes of tonic accommodation and tonic vergence may be related to nearwork symptomatology.