Time course and decay of effects of near work on tonic accommodation and tonic vergence.

Changes in tonic accommodation and tonic vergence were monitored before, during and immediately after a 45 min period of reading adult level text displayed on either a computer monitor or hard copy at 20 cm under binocular viewing conditions. Both tonic accommodation and tonic vergence gradually increased during the reading period, with changes in tonic accommodation occurring more rapidly. The average increases in tonic accommodation and tonic vergence following reading were 0.35 dioptres and 0.38 metre angles, respectively. Decay of both tonic components to the baseline in either light or total darkness occurred within the first 3 min. These results suggest that a moderate period of interrupted reading at a close distance can produce a transient increase in the tonic oculomotor levels.