Functional magnetic resonance imaging reflects changes in brain functioning with sedation

Functional magnetic resonance imaging (fMRI) studies have demonstrated loca lized brain activation during cognitive tasks. Brain activation increases w ith task complexity and decreases with familiarity. This study investigates how sleepiness alters the relationship between brain activation and task f amiliarity. We hypothesize that sleepiness prevents the reduction in activa tion associated with practice. Twenty-nine individuals rated their sleepine ss using the Stanford Sleepiness Scale before fMRI. During imaging, subject s performed the Paced Auditory Serial Addition Test, a continuous mental ar ithmetic task. A positive correlation was observed between self-rated sleep iness and frontal brain activation. Fourteen subjects participated in phase 2. Sleepiness was induced by evening dosing with chlorpheniramine (CP) (8 mg or 12 mg) and terfenadine (60 mg) in the morning for 3 days before the s econd fMRI scan. The Multiple Sleep Latency Test (MSLT) was also performed. Results revealed a significant increase in fMRI activation in proportion t o the dose of CP. In contrast, for all subjects receiving placebo there was a reduction in brain activation. MSLT revealed significant daytime sleepin ess for subjects receiving CP. These findings suggest that sleepiness inter feres with efficiency of brain functioning. The sleepy or sedated brain sho ws increased oxygen utilization during performance of a familiar cognitive task. Thus, the beneficial effect of prior task exposure is lost under cond itions of sedation. Copyright (C) 2000 John Wiley & Sons, Ltd.