Full-field flicker evoked changes in parafoveal retinal blood flow

Abstract

When retinal activity is increased by exposure to dynamic visual stimuli, blood vessels dilate and the flow of blood within vessels increases to meet the oxygen and glucose demands of the neurons. This relationship is termed ‘neurovascular coupling’ and it is critical for regulating control of the human retinal vasculature. In this study, we used a recently developed technique based on a dual-beam adaptive optics scanning laser ophthalmoscope to measure changes in red blood cell velocities, vessel diameter, and flow in interconnected small parafoveal retinal vessels (< 50 µm) of nine healthy participants. A full-field flicker stimulus was presented onto the retina to induce a vascular response to neural activity. Flicker stimulation increased blood velocity, vessel diameter, and therefore flow in arterioles, capillaries, and venules in all nine subjects. ANOVA and post hoc t-test showed significant increases in velocity and flow in arterioles and venules. These measurements indicate that the mechanism of neurovascular coupling systematically affects the vascular response in small retinal vessels in order to maintain hemodynamic regulation in the retina when exposed to visual stimulation, in our case flicker. Our findings may provide insight into future investigations on the impairments of neurovascular coupling from vascular diseases such as diabetic mellitus.

Measuring Temporal and Spatial Variability of Red Blood Cell Velocity in Human Retinal Vessels

“…For arterioles and venules, when measuring RBC velocity responses to flicker stimulation, we have shown that individual capillary RBC velocities vary across capillaries (either increasing or decreasing), differing from arterioles and venules, which show a consistent response. 10 , 24 , 41 Unlike other studies that have found increases in RBC velocity in larger retinal vessels during flicker stimulation, 42 – 44 our capillary measurements in this study showed an increase in RBC velocity in only three out of the five subjects, although from a larger sample we found on average that capillary flow increased by about the same amount as arteriole flow, but the change was not statistically significant, in part due to the variability in capillary measurements. 24 …”

“…The sizes of the variations in RBC velocity were consistent with the results of Experiment 1. Even though flicker stimulation has been shown to cause an overall increase in capillary blood velocity of about 15%, 24 the larger ongoing variability of 17% to 20% in the individual capillaries is consistent with some increasing blood velocity and others decreasing during the flicker stimulation.…”

“… 20 – 22 Using the dual-beam approach, 23 we found an upregulation in blood velocity and ultimately flow in retinal arterioles and venules (<50 µm) during increases in metabolic demand (due to flicker stimulation) across nine subjects. 24 This increase in blood flow was similar in capillaries, yet the measured velocities had larger variations in responses. This led us to question if there is an additional source of variability present in capillaries, perhaps similar to what has been recorded for cortical capillaries.…”

“…There was no significant change in capillary blood velocity during flicker stimulation as in our previous study. 24 Within the 2-minute measurements, the individual capillaries did not respond uniformly to the stimulus, even though all were supplied by the same arteriole. The sizes of the variations in RBC velocity were consistent with the results of Experiment 1.…”

“…During image acquisition periods, a 30° scanning laser ophthalmoscope (SLO) image of the subject's fundus is displayed to the experimenter along with the location of the AOSLO imaging field superimposed onto the image for navigation of the retina during experimentation 30 with flicker stimulus parameters replicated from previous studies. 24 …”