Orthostatic intolerance is defined by debilitating upright symptoms that are relieved by sitting or lying. Symptoms include upright lightheadedness, fatigue, confusion, and decreased memory called 'Brain Fog' by patients. The most common chronic form is Postural Tachycardia Syndrome (POTS), characterized by excessive upright tachycardia without hypotension. Of note, >85% of POTS patients are female. The proposal that Brain Fog was caused by reduced cerebral blood flow (CBF) has been disproven, because graded incremental upright tilt failed to demonstrate difference in mean CBF compared to healthy volunteers. Nevertheless, memory task performance deteriorates with angle of tilt as does task-related neurovascular coupling (NVC), which links neural activity to an increase in CBF known as "functional hyperemia". The investigators have previously observed that large low frequency (0.07-0.13 Hz) oscillations in BP (OBP), which entrained and amplified oscillations in CBF (OCBF), increased with tilt angle and were associated with impaired working memory and reduced functional hyperemia.