   The zigzags as shown in Figure~\ref{499388}A were achieved using additional cushions inside the coil and by breath-holding (without taking a deep breath). Breath-holding showed an immediate increase in zigzags (up to 4-5 peaks) but no delayed response to the PCO$_2$ challenge as we found  in the blood vessels (not shown here).   Under normal condition, the zigzags usually only contain two or three peaks as shown in Figure~\ref{499388}B (during the first 10 s from 50 s to 60 s).  At 60 s, the volunteer was instructed to hyperventilate with the result that the zigzags immediately disappeared.   