The Effect of Stimulus Duration on Tonic Accommodation and Tonic Vergence

Changes in tonic accommodation and tonic vergence were studied as a function of stimulus duration and time after near fixation. Thirteen visually normal young adults each participated in experimental sessions comprising each of 12 combinations of stimulus type (accommodative or vergence) and stimulus duration (0.25, 0.5, 1, 2, 4, or 8 min). During each session, measurements were made of either tonic accommodation or tonic vergence 30 s before stimulus onset and at 0.5, 2, 4, 6, 8, 10, 12, and 14 min after stimulus offset. The stimulus (a high contrast reduced Snellen visual acuity chart) was viewed monocularly at the accommodative nearpoint and binocularly through pinholes at the fusional nearpoint on the accommodative and vergence sessions, respectively. For tonic accommodation, stimulus duration had no significant effect on either the magnitude of the aftereffect or its rate of decay. For tonic vergence however, the aftereffect was larger and had a slower rate of decay as stimulus duration increased. The results suggest that the adaptive mechanisms within the two systems have different time constants.