Flicker-Evoked Changes in Red Blood Cell Flow and Distribution in Human Retinal Capillaries

Purpose: To quantify flicker-induced changes in capillary flow and red blood cell (RBC) distribution in the human retina. Methods: Capillary blood flow in the parafoveal region of five healthy subjects was imaged using high-speed flood-illumination adaptive optics at 300 fps. Measurements were obtained before and after 30 seconds of 10-Hz flicker stimulation (≥5 repeats). Changes in capillary diameter, RBC velocity, hematocrit, and transit time were quantified. Results: Flicker stimulation increased mean RBC velocity by 11% (P < 0.001) but reduced pulsatility by 7.1% (P < 0.001). Mean capillary diameter increased by 3.5% (P < 0.01). Average transit time for individual capillary segments was reduced by 10.0% (P < 0.001), as was the standard deviation (SD) across segments (12.5%; P < 0.001), such that the coefficient of variation (CV) was preserved (P > 0.05). Average RBC flow increased (21.6%; P < 0.001), as did SD across segments (17.1%; P < 0.05), with no significant change in CV (P > 0.05). Despite variability in capillary diameter and RBC flow responses, no clear baseline-dependent relationship was observed for either parameter, and fractional RBC flow distribution at branches was preserved (P > 0.05). A subset of segments lacked RBC perfusion at both baseline and following stimulation. Conclusions: Within the sampled post-stimulation window, the flicker-evoked retinal capillary flow response was characterized primarily by broadly distributed, approximately proportional scaling across segments, with no evidence of homogenization. Some segments consistently lacked measurable flow, and their physiological role remains unclear.