Color vision in the twilight zone: an unsolved problem

Abstract

Mesopic vision (-2 to 1 log cd/m2 ), where rods and cones are responsive simultaneously, lies between rod-driven scotopic and cone-driven photopic vision. Due to the technical and psychophysiological challenges of assessing mesopic vision, the visual system is not well quantified over the mesopic luminance range. Mesopic vision combined with hypoxia is of particular interest in applications where Army aviators must rely on color-coded visual information displayed under mesopic conditions at altitude. Results are presented from three vision tests (the Lanthony Desaturated-15, the Cone Contrast Test, and the Color Assessment and Diagnosis system) demonstrating that normobaric hypoxia (12% oxygen, approximating 14,000 feet) disrupts mesopic color vision; but not all tests were equally sensitive to this phenomena. Additionally, respiratory data showed that Army aviators increased their tidal volume under normobaric hypoxia, thus respiratory rate assessments alone are insufficient for assessing physiologic adaptation to hypoxia. Under mesopic hypoxia conditions, the known high oxygen demand of rods may reduce the retinal oxygen available for cones thereby diminishing color sensitivity as well as other cone functions. Color vision under mesopic conditions warrants further examination, with particular attention to hypoxia as an important confound in high-terrain, aviation, and aerospace applications.