Spatial Contrast Sensitivity at Twilight: Luminance, Monocularity, and Oxygenation

CONNOLLY DM. Spatial contrast sensitivity at twilight. luminance, monocularity, and oxygenation. Aviat Space Environ Med 2010; 81:475-83 Introduction. Visual performance in dim light is compromised by lack of oxygen (hypoxia) The possible influence of altered oxygenation on foveal contrast sensitivity under mesopic (twilight) viewing conditions is relevant to aircrew flying at night, including when using night vision devices, but is poorly documented Methods. Foveal contrast sensitivity was measured binocularly and monocularly in 12 subjects at 7 spatial frequencies, ranging from 0 5 to similar to 16 cycles per degree, using sinusoidal Gabor patch gratings Hypoxic performance breathing 14 1% oxygen, equivalent to altitude exposure at 3048 m (10,000 ft), was compared with breathing air at sea level (normoxia) at low photopic (28 cd . m(-2)), borderline upper mesopic (similar to 2 1 cd . m(-2)) and midmesopic (similar to 0 26 cd . m(-2)) luminance Mesopic performance was also assessed breathing 100% oxygen (hyperoxia) Results: Typical 'inverted U' log/log plots of the contrast sensitivity function were obtained, with elevated thresholds (reduced sensitivity) at lower luminance Binocular viewing enhanced sensitivity by a factor approximating root 2 for most conditions, supporting neural summation of the contrast signal, but had greater influence at the lowest light level and highest spatial frequencies (8 26 and 16 51 cpd) Respiratory challenges had no effect Conclusions: Contrast sensitivity is poorer when viewing monocularly and especially at midmesopic luminance, with relevance to night flying The foveal contrast sensitivity function is unaffected by respiratory disturbance when twilight conditions favor cone vision, despite known effects on retinal illumination (pupil size) The resilience of the contrast sensitivity function belies the vulnerability of foveal low contrast acuity to mild hypoxia at mesopic luminance