Stability of tonic vergence.

Tonic vergence was monitored at 30 min intervals over a 10 hr period of a single day and at intervals of approximately 14 days over a period of 19 weeks. Measurements were also made during a continuous 112 min period in total darkness. Tonic vergence showed neither significant systematic variation nor large random fluctuations during the course of either a day or weeks, demonstrating stability under naturally occurring viewing conditions. During the period in total darkness, however, tonic vergence exhibited a significant decreasing trend, ie, eye position became less convergent with time. Stability was therefore poorer in the absence of visual stimuli.