Normal brain function is critically dependent on moment-to-moment adjustment of cerebral blood flow to match demands of activated neurons. This process is known as neurovascular coupling (NVC) and recent in vivo studies demonstrate that impairment of NVC responses is associated with worse cognitive performance. Several methods are available to measure NVC responses in human subjects, including transcranial Doppler (TCD), functional near infrared spectroscopy (fNIRS), and dynamic retinal vessel analysis (DVA). Although all these methodologies aim to measure hemodynamic changes in the brain vasculature in response to cognitive, motor, or visual stimulation, the responses are evaluated on the different levels of cerebral vasculature including microvasculature (fNIRS), large cerebral vessels such as middle cerebral artery (TCD), or in the arterioles and venules of the retina (DVA). Currently, there are limited data available on the simultaneous assessment of NVC responses using these methodologies.~Caffeine is the most commonly used stimulant drug with well documented effects on cerebral vasculature. Caffeine is known to non-specifically bind to adenosine receptors in the brain and to reduce resting blood flow while improving attention and cognitive function, which suggests that it may allow a more efficient dynamic blood flow regulation through neurovascular coupling. This study will use standardized dose of caffeine to test its effect on NVC responses in cerebral and retinal arterioles.~This study is designed to establish the direct link between reactivity in the cerebral and retinal micro- and macrovasculature. To achieve this goal, a prospective, single-blinded, placebo controlled, cross-over study will be employed to evaluate changes in the NVC responses measured simultaneously with DVA and TCD, or DVA and fNIRS before and after administration of 100mg of incapsulated caffeine or placebo pill.
Caffeine is the most commonly used stimulant drug with well documented effects on cerebral vascula-ture. Caffeine is known to non-specifically bind to adenosine receptors in the brain and to reduce resting blood flow while improving attention and cognitive function, which suggests that it may allow a more efficient dynamic blood flow regulation through neurovascular coupling. This study will use standardized dose of caffeine to test its effect on NVC responses in cerebral and retinal arterioles.