Long-term endurance of adaptive shifts in tonic accommodation.

Abstract

After an 8 min inducing period of sustained focusing on a target at the near point (NP) of accommodation, tonic accommodation was monitored at 8 min intervals over a 48 min period in darkness. A laser optometer provided evidence for asymptotically increasing tonic levels. Accordingly, eyes closed in darkness (EC) was not a condition for the relaxation or dissipation of enhanced tonic levels. Furthermore, refixating the NP target for 6 min, 24 min after the initial focusing period, provided no significant enhancement of tonic accommodation over EC. On the other hand, focusing at far point (FP) or walking within a lighted hallway (WH) significantly reduced the tonic after-effect to pre-adaptative levels, but only transiently. Tonic after-effects induced by NP focusing thus appear to be remarkably resistant either to long-term attenuation or enhancement. A model based on the decay of accumulated sympathetic and parasympathetic neurotransmitters was developed to account for the contrasting fact that focusing at near targets for periods up to about 2-3 min as well as sustained focusing at far targets tends to yield complete decay of tonic after-effects.