Relations between accommodation and vergence in darkness.

Prior studies reported that the resting postures of accommodation and vergence are uncoupled in darkness, calling into question whether accommodation-vergence synkinesis persists when both systems are deprived of stimulation. We tracked the postures of accommodation and vergence of 20 normal subjects for 15 min in darkness, using a Vernier optometer and dichoptic nonius lines. Consistent with previous reports, the mean resting postures of accommodation and vergence correlated weakly (r = 0.19); spontaneous fluctuations of accommodation and vergence in darkness also exhibited little correlation (median r = 0.15). However, resting vergence postures could be predicted (r = 0.65) from the clinically measured distance phoria and the resting posture of accommodation, when subjects' AC/A ratios were taken into account. We conclude that the relaxation of accommodation and vergence to separate resting postures in darkness is compatible with the persistence of synkinesis; independent noise in the two systems may reduce apparent coupling under open-loop conditions.