Response of human retinal blood flow to light and dark.

The laser Doppler technique was used to measure alterations in retinal blood flow in human subjects under dark-adapted conditions compared to light-adapted conditions. Retinal blood flow increased 40 to 70% after the transition from light to dark. We show quantitatively that the measured increases in retinal blood flow provide the appropriate increases of available oxygen for the inner retinal tissue necessary to compensate for increased oxygen consumption in the dark by the photoreceptor-RPE complex. In addition, we show that measured increases or decreases in human retinal blood flow are quantitatively linked to two intrinsic characteristics of the rod photoreceptor: its metabolic response to known, graded light levels, and its rhodopsin-mediated wavelength-dependent light-absorption properties. We also discuss how light-activated blood flow changes may provide a physiologic test for the autoregulatory capacity of the retinal vasculature.