Arousal-related changes in dark focus accommodation and dark vergence.

Dual-innervation theory suggests that dark focus (DF; the accommodation in effect in total darkness) reflects a balance between sympathetic and parasympathetic inputs. Since it is assumed that relaxation of accommodation is sympathetically innervated, and that accommodative increases are parasympathetically innervated, stimulation that leads to sympathetic activation should produce lower DF values than is the case when there is greater parasympathetic influence. In addition, the synkinesis between accommodation and vergence, and the known effects of fatigue, injury and drugs on fusion, promote an interest in the effects of arousal on dark vergence (DV; the vergence in effect in total darkness). Twenty-three young adult males experienced a variety of stimulus conditions. The conditions, selected because of their likely autonomic effects, included deep muscle relaxation, cold pressor and mental activity, among others. During each condition, measurements were made of DF (with a laser optometer) and DV (with a Nonius alignment system). Results for DF were consistent with dual innervation theory; manipulations expected to induce sympathetic activation (eg, cold pressor) produced lower DF values than did conditions expected to induce more parasympathetic (or less sympathetic) activation (eg, relaxation). DV tended to be influenced in the opposite direction, in that relaxation led to smaller DV angles than did the other conditions. Several analyses confirmed a lack of synkinesis between accommodation and vergence during darkness.