Figure 2

(a) Demonstrates the "drift" phenomenon in all three perpendicular directions with both types of head support after smoothing. The "drift" appears to increase as the scan progresses but then seems to decrease towards the end. This could be due to the individual falling asleep during the scan but becoming more alert towards the end. Whereas, in (b), there is a gradual "drift" away from baseline until the end of the scan. In this example, the phenomenon may be explained by increased muscle tension at the start; individuals are generally tense and anxious prior to the scan but once the scan has commenced, they start to relax and so muscle tension decreases resulting in the observed "drift".