Abstract
Abstract: :
Purpose: To obtain non–invasively 2–dimensional functional maps of the human optic nerve and peripapillary region based on reflectance changes induced by diffuse luminance flicker. Methods: The measurements were performed in 6 normal subjects with a fundus camera–based reflectometer. A 30° field centred at the optic disc was illuminated with light at 569 (isobestic) and 600 nm. Using an image splitter, pairs of fundus images at these wavelengths were acquired simultaneously with a computer controlled ICCD video camera. The measuring protocol consisted of 3 phases of each 60–90 s. The dual band illumination was maintained steady during phase 1 (baseline) and phase 3 (recovery). It was modulated at 8 Hz during phase 2 (flicker). This cycle was repeated 3–4 times consecutively. After applying flat field correction and image registration, the images at each wavelength obtained during the last 30 s of each phase were averaged. Maps of reflectance changes [ΔR(%) = 100 x (Rf – Rb) / Rb] were derived from these images. Rb and Rf are the averages of R over the last 30 s of baseline and during the last 30 s of flicker, respectively. From these maps, average ΔR (ΔRav) over the disc area and ΔRav based on 4 sites of the peripapillary region (180 x 180 µm2) were determined, avoiding visible retinal vessels. Results:Using the first cycle of measurements for each subject, the group mean ΔRav decreased significantly (paired t–test, p < 0.05) by 10.7 ± 4.0 % at 569 nm and 3.5 ± 1.9 % at 600 nm (mean ± SD) for the optic disc tissue and by 3.6 ± 2.6 % at 569 nm and 1.7 ± 1.4 % at 600 nm for the peripapillary retina. No significant changes were found between the baseline and the recovery period for all regions. The group mean (n = 6) coefficient of variation of ΔRav for all cycles was 13% and 30% at 569 and 600 nm, respectively for the tissue of the optic disc. Conclusions: Based on reflectance measurements from the brain cortex at 569 and 600 nm in response to increased neural activity, our data strongly suggest that the observed activity–induced decreases in R at the optic disc and peripapilla region mainly represent an increase in blood volume.
Keywords: imaging/image analysis: non–clinical • optic disc • retina