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Jesse B. schallek, Benjamin Masella, Jennifer J. Hunter, David R. Williams; Stimulus-dependent Changes In Capillary Blood Velocity Revealed With Adaptive Optics Scanning Laser Ophthalmoscopy. Invest. Ophthalmol. Vis. Sci. 2011;52(14):6029.
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It has been reported that inner retinal circulation flow rates increase in response to visual stimulation as measured by the blue field entopic phenomenon and laser Doppler flowmetry. However, an objective measure of stimulus-evoked flow has not been observed in individual capillaries. Here we use an adaptive optics scanning laser ophthalmoscope (AOSLO) to directly measure hematic cell velocities in retinal capillaries in response to visual stimulation.
An AOSLO was used to image parafoveal capillary beds in anesthetized macaque monkeys. The confocal image plane was focused on capillary strata in proximity to the ganglion cell and retinal nerve fiber layers. Movies of retinal reflectance using a 794 nm super luminescent diode were collected at a rate of 25Hz for 120 seconds. Within the acquisition period, a stimulus was presented using an argon/krypton laser tuned to 488 nm (40µW at the cornea). The scanning raster of the AOSLO provided a bright, visible temporal flicker at 25Hz. Stimulation epochs lasted 60 seconds. Eye movements were minimal during acquisition, allowing for repeated measurements of the same capillaries. Motion registration software was used to correct for small translational and warping effects. Then, the velocities of individual leukocyte cells moving through identified capillaries were analyzed for velocity change in response to stimulation.
Cell velocities were variable across capillaries of different depth, shape and width. The velocity of the slowest moving hematic cells could be resolved over a 2.5 degree field at 25Hz temporal acquisition. Mean velocities of slowest cells were measured at 315-505µm/second, consistent with the range of reported leukocyte velocity in capillaries measured in the brain and retina. Hematic cell velocity showed a significant ~6% increase in response to visual stimulation.
Consistent with measurements obtained using laser Doppler flowmetry and blue field entopic technique, our results show a functional increase in retinal blood flow rate in response to visual stimulation. Unlike these prior techniques, adaptive optics retinal imaging allows observation of flow rate changes in individual capillaries. This demonstration enables future studies to investigate the precision of hemodynamic functional control at the capillary level.
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