it has been found that in humans CBFV in the middle cerebral artery decreased substantially during
lower body negative pressure (LBNP) and head-up tilt in the absence of systemic hypotension, which suggests the presence of cerebral vasoconstriction associated with augmented sympathetic nerve activity during orthostatic stress.
IEEE Eng Med Biol Mag. 2009 Nov-Dec;28(6):54-62.