Stability of human binocular alignment in the dark and under conditions of nonfixation

Purpose: To evaluate the stability of human binocular alignment under conditions of altered fixation and luminance. Methods: Horizontal binocular alignment in 8 healthy orthotropic subjects was measured using infrared video-oculography (VOG) under conditions of binocular fixation and luminance change. Each testing condition was preceded by a binocular fixation period in room light (475 lux) to define the baseline binocular alignment. Binocular alignment was then measured in darkness without fixation, in room light through a semitranslucent filter that precluded fixation, and in darkness with a distant fixational target. We used the signed rank test to determine statistically whether these experimental conditions induced significant binocular alignment change from each baseline binocular alignment. Results: The mean horizontal binocular alignment in the dark without fixation was similar to baseline binocular alignment (0.2° ± 2.8°; . P = 0.4). The mean horizontal binocular alignment without fixation in room light was also similar to baseline binocular alignment (-1.4° ± 1.6°; . P = 0.08). The mean horizontal binocular alignment in the dark when a fixational target was provided showed an exodrift compared to baseline alignment (2.3° ± 1.0°; . P = 0.0004). Conclusions: The human brain does not require visual input to maintain binocular alignment on a short-term basis. The resilience of binocular alignment probably reflects the presence of phoria adaptation.