Optic Nerve Reflectance Variations During Neural Activity

PurposeChanges in optic nerve reflectance in response to visual stimulation have been demonstrated in the visible wavelengths (Crittin and Riva, 2004) and in presence of flash stimulation in monkeys (Hanazono et al., IOVS 2007). The purpose of the study was to determine whether diffuse flicker-induced reflectance changes in the human optic nerve could be detected using near-infrared light. MethodsSix 770-nm reflectance measurements were carried out on a healthy, informed and consenting volunteer, by means of a PC-controlled ocular fundus reflectometer based on a commercial fundus camera and modified for the purpose of the study. Neural activity was evoked by 535-nm diffuse square wave flicker stimulation at 12Hz. The measurement protocol consisted of 6 successive reflectance registrations at 770nm, each consisting of a 20s baseline followed by 60s flicker. We investigated two different measuring sites in the optic nerve papilla deprived of visible vessels, near the temporal rim. The reflectance changes measured in a control region outside the optic nerve were subtracted from the reflectance variations obtained from the two investigation sites, to cancel out the effects of involuntary eye and head movements. ResultsFlicker induced a significant decrease in near-infrared reflectance, which reached a maximum value at approximately 60s of stimulation. The decrease with respect to control amounted to -2.6% {+/-} 1.4% (mean {+/-} 1 standard error) for measuring site 1 and -2.4% {+/-} 1.4% for measuring site 2. ConclusionsFlicker stimulation induces a significant decrease in near-infrared reflectance in the temporal region of the human optic nerve papilla, confirming previous near-infrared reflectance findings in monkeys. This decrease is most probably due to changes in blood volume during neural activity.