Doctors at Peking Union Medical College Hospital in Beijing fixed ultrasound probes to both sides of three men's heads and asked them to do something entirely ordinary: turn, tilt, and in one case, open wide and yawn. In all three, blood flow to the back of the brain collapsed within seconds.
Writing in BMC Neurology in March 2026, the team reported that flow velocity in both posterior cerebral arteries fell to between 10% and 30% of baseline during the provoking movement, a drop of 70% to 90%. Dizziness arrived at the same moment. When the men returned their heads to neutral, flow rebounded to between 110% and 140% of baseline, a surge the researchers described as a reactive hyperemic response.
