In recent years, sleep deprivation has been shown to impair high-level, executive thinking. Now, a series of brain-imaging studies provides new evidence that even one night of sleep deprivation causes not only poorer performance on tasks requiring high-level processing--such as working memory and language--but also, for some types of cognitive tasks, intriguing changes in the areas of the brain responsible for this thinking, particularly the prefrontal and temporal lobes.
But the research, led by clinical psychologist Sean P.A. Drummond, PhD, psychiatrist J. Christian Gillin, MD, and neuropsychologist Gregory G. Brown, PhD, all of the University of California, San Diego (UCSD), also indicates that other brain regions at least partially compensate for the effects of lost sleep.
The findings suggest that "the brain does not react in a uniform manner to sleep deprivation," says Drummond. "In some circumstances, the brain can adapt and continue to perform--although not as well as after normal sleep."
"The fact that there is some kind of fallback position during sleep loss suggests that the brain is more flexible than we've previously thought," comments James A. Horne, PhD, a professor of psychophysiology at Loughborough University in the United Kingdom and one of the world's experts on the neuropsychological effects of sleep loss. "In my view, that is a remarkable finding."
In addition to its theoretical importance, gaining a better understanding of the brain's ability to adapt to sleep deprivation has considerable practical benefits, says Drummond. For example, such insight might guide decisions about the kinds of jobs that people should be allowed to do while sleep deprived, point toward appropriate work-rest schedules for shift workers and others whose work disrupts their sleep, and help researchers develop better strategies for combating sleepiness.
Mapping performance and brain function
To investigate the effects of lost sleep on brain activity and performance, the UCSD researchers conducted a series of experiments in which sleep-deprived and rested participants performed complex cognitive tasks while a functional magnetic resonance imaging (fMRI) scanner measured their brain activity.
In their initial experiment, published in the journal Neuroreport (Vol. 10, No. 18) in December 1999, participants performed an arithmetic working-memory task while in the fMRI scanner, once after 35 hours of sleep deprivation and once after a normal night's sleep.
As the researchers expected, participants performed more poorly on the math task when they were sleep deprived than when they were rested. And consistent with the notion that sleep deprivation impairs working-memory functioning in the prefrontal cortex, the fMRI data revealed less activity in the prefrontal cortex while participants were performing the math task after sleep deprivation than after a normal night's sleep.
In a second study, published in Nature (Vol. 403) in February 2000, the investigators examined performance and brain activity during a verbal learning test. As before, sleep deprivation led to poorer performance--although only slightly so, in this case.
In contrast to the results of the math task, during the verbal learning task, prefrontal areas of the brain remained active after sleep deprivation, and parietal lobe activity actually increased. However, activity in the left temporal lobe--a classic language-processing area--decreased during sleep deprivation. The researchers also found that better performance on the verbal learning task after sleep deprivation was correlated with increased parietal lobe activity.
Drummond and his colleagues interpret these findings as evidence that the brain--specifically, the parietal region, in this instance--is able to at least partially compensate for sleep deprivation.
A third study, published in the Journal of Sleep Research (Vol. 10, No. 2) in June, further supports that notion. In that investigation, Drummond and his colleagues presented participants with a "divided-attention" task, in which they completed both an arithmetic and a verbal-learning task within the same block of experimental trials.
As in the verbal-learning study, the results again revealed poorer performance, depressed brain activation in the left temporal region and heightened activation in prefrontal and parietal regions after sleep deprivation. In addition, the team found increased activation in areas of the brain that are involved in sustained attention and error monitoring, suggesting that these areas may also be deployed in the brain's effort to adapt to sleep deprivation.
More research needed
Why did sleep deprivation lead to different patterns of brain activation for arithmetic and verbal tasks?
"That's the big question right now," says Drummond. "Right now, the best we can say is that different cognitive demands seem to elicit different cerebral responses to sleep deprivation."
To follow up, he and his colleagues will further investigate how different cognitive tasks respond to sleep deprivation and probe how cognitive performance and brain activity are affected by factors such as length and type of sleep deprivation, time of day, and individual variables such as age, sex and education.
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