Accommodative hysteresis. Fundamental asymmetry in decay rate after near and far focusing.

The resting level of accommodation, or dark focus (DF), was compared before and after subjects maintained 8-min of clear focus on targets either at the far point (FP) of accommodation or at an equivalent dioptric distance on the near side of the DF. Although both conditions showed significant shifts in tonic accommodation in the direction of their respective fixation targets, only after near fixation was the shift maintained in darkness for 24-min postfixation. After FP viewing the aftereffect decayed in darkness with a significant linear trend (P less than 0.05). Thus, maintaining focus on a near target is more likely to induce an enduring adaptive shift in tonic accommodation than is far-target viewing, for reasons associated with the systems controlling far and near accommodation, respectively. Additional analyses provided further evidence that the degree of separation of the target from the DF is a highly significant factor controlling individual differences in initial adaptation levels.