Optic nerve reflectance variations in the near‐infrared during neural function

Purpose Diffuse luminance flicker induces optic nerve head (ONH) reflectance changes, ChRonh, in humans, when Ronh is measured at visible wavelengths (Crittin and Riva, 2004). Intrinsic signal imaging in macaque retina and ONH has revealed also flash‐induced infrared (840‐900 nm) reflectance changes (Hanazono et al., 2007). Our purpose was to determine a) whether flicker‐induced near‐infrared ChRonh are detectable from the human ONH; b) is there a spatial distribution of ChRonh. Methods Ronh was measured at 770 nm with a fundus camera based reflectometer. Neural activity was evoked by 535‐nm diffuse (50 deg) flicker (various frequencies below 20Hz). 7 successive recordings were performed at 9 temporal sites, each consisting of a 20s baseline, a 60s flicker period and a 40s recovery. ChRonh (%) was defined as 100 x (Ronh,fl ‐ Ronh,bl)/Ronh,bl, where Ronh,bl is the average response during the baseline (no flicker) and Ronh,fl is the response, averaged over time slots of 2s, during flicker. ONH regions containing visible vessels were avoided. Results Flicker induced significant decreases in Ronh at most sites. These decreases presented a strong oscillatory behavior (period of about 8s), reached a maximum in less than 40s of flicker and were more marked at the rim. ChRonh was found to be frequency‐dependent. Ronh during recovery also showed marked oscillations Conclusion Flicker induces significant frequency‐dependent decreases in near‐infrared (770 nm) reflectance in the temporal region of the human ONH, confirming previous infrared optic nerve intrinsic signal reflectance findings in monkeys. Comparison with flicker‐induced blood flow increases in the ONH, these decreases are most probably due to changes in blood volume during neural activity.