A: Whole-slice averaged signal time course (selected by a mask) during 8 heart cycles. Subject had extra head fixation and was instructed to breath-hold during the period. In contrast to signals in the veins (not shown here), zigzag signal showed no response to CO2 activity other than an immediate lengthening of the zigzag. B: Whole-slice averaged signal time course during normal breathing first. At 60 s, the subject was instructed to hyperventilate. The inlet shows the total time course with 3 hyperventilation periods and the selected time interval in red.