Previous studies have reported conflicting results on the existence of binocular enhancement of dark vision. This study compares monocular and binocular absolute thresholds of dark adaptation in two different populations (healthy individuals and those with chronic respiratory insufficiency). The study aims to answer the previously unresolved questions regarding binocular summation at absolute threshold. Study Group A consisted of 18 healthy individuals, Group B recruited 13 patients with respiratory failure. All recruits in both groups were examined by computerised dark adaptometry on three separate visit days. Group B received oxygen supplement at visits one and three, but not at visit two. In group A, binocular dark adaptation was significantly more sensitive than monocular adaptation across all three visits. In group B, at visits one and three, binocular dark adaptation was also significantly more sensitive than monocular. However, at visit two, when no oxygen supplement was given, no significant difference was found between binocular and monocular summation. This study goes some way to answering questions regarding binocular summation in reporting that a deficit in oxygen seems to affect the binocular adaptation, and that binocular summation exists at absolute threshold. 

Two eyes are better than one – binocular summation of dark vision in healthy individuals and patients with chronic respiratory disease.
Thylefors J, Havelius U.
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Claire Howard

Salford Royal NHS Foundation Trust, Salford, UK.

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