We investigated the variability of absolute brood flow velocity (BFV) and t
ask-induced BFV change over consecutive cognitive tasks and compared two me
thods of baseline determination that are used to calculate relative BFV cha
nges. Bilateral transcranial Doppler ultrasonography recordings of BFV in t
he middle cerebral arteries was performed in 90 right-handed volunteers dur
ing 13 cognitive tasks and their preceding rest periods. Both absolute BFV
and elicited BFV changes between rest and activation significantly decrease
d over successive tasks. Instead of calculating an averaged baseline value,
our results suggest that the rest phase immediately preceding the activati
on phase should be selected for baseline measurement. The between-task habi
tuation effect could be due to a fading of attentional resources during the
sustained and demanding performance. (C) 1999 Lippincott Williams & Wilkin
s.