Now researchers at the Medical College of Wisconsin in Milwaukee and Veterans Affairs Medical Center in Albuquerque have identified areas in the brain responsible for perceiving the passage of time in order to carry out critical everyday functions.

Their study is the first to demonstrate that the basal ganglia located deep within the base of the brain, and the parietal lobe located on the surface of the right side of the brain, are critical areas for this time-keeping system.

Their results are published in the current issue of Nature Neuroscience. Importantly, the study calls into question a long-standing and widely held belief in the scientific community that the cerebellum is the critical structure involved in time perception.

"We are excited that our findings can also have application to better understand some neurological disorders," says Stephen M. Rao, Ph.D., professor of neurology at the Medical College and principal investigator. "By identifying the area in the brain responsible for governing our sense of time, scientists can now study defective time perception, which has been observed in patients with Parkinson's disease and Attention-Deficit/Hyperactivity Disorder (ADHD), two maladies commonly thought to have abnormal function within the basal ganglia."

Making accurate decisions regarding the duration of brief intervals of time from 300 milliseconds to 10 seconds is critical to most aspects of human behavior. Contemporary theories of short interval timing assume the existence of a timekeeper system within the brain, yet identifying these brain systems has been elusive and controversial.

Using a novel functional magnetic resonance imaging (fMRI) technique that tracks second-by-second changes in brain activity, investigators identified regions within the brain that are critical for this timekeeping system.

Seventeen healthy, young men and women volunteers were imaged while being asked to perceive the duration of time between the presentations of two consecutive tones. One second later, two more tones were presented and subjects were asked to make a judgment as to whether the duration between the tones was shorter or longer than the first two tones.

To make sure that the brain systems associated with time perception were clearly identified, two control tasks were given which involved listening to tones or estimating their pitch, but not making judgments about their duration.

Using this fast imaging technique, the investigators were able to isolate only those areas of the brain activated during presentation of the first two tones -- when subjects are only perceiving and attending to time. Their results conclusively demonstrated that timekeeping functions are governed by the basal ganglia and the right parietal cortex.

Investigators have long suspected, based on indirect evidence, that the basal ganglia might be involved in time perception. The basal ganglia have nerve cells that primarily contain the neurotransmitter, dopamine.

Patients with Parkinson's disease have an abnormal reduction in dopamine within the basal ganglia and commonly experience problems with time perception. These difficulties partially improve when patients are administered a drug that increases dopamine levels in the brain.

Defective time perception has also been observed in patients with Huntington's disease and Attention-Deficit/Hyperactivity Disorder (ADHD), two disorders commonly thought to have abnormal function within the basal ganglia. Animal studies have also demonstrated the importance of dopamine in timekeeping.

Medical College researchers at Froedtert Hospital, a major teaching affiliate of the Medical College, are currently using this new neuroimaging procedure to better understand how the brain enables dopamine replacement drugs and methylphenidate (Ritalin) to normalize time perception in individuals with Parkinson's disease and ADHD, respectively.

An additional study, in collaboration with investigators at the University of Iowa, will examine time perception in the early stages of Huntington's disease, prior to the development of the characteristic movement disorder.

The critical role of the parietal lobes in timekeeping was first suggested by coauthor Deborah L. Harrington, Ph.D., research scientist, Veterans Affairs Medical Center and associate research professor of neurology and psychology, University of New Mexico, Albuquerque, NM. She and her colleagues reported that stroke patients with damage to the parietal cortex on the right but not the left side of the brain experienced impaired time perception.

Patients for the study have been drawn from Froedtert Hospital and the VA Medical Center in Milwaukee. Additionally, the researchers are studying adult ADHD patients who have been seen since childhood at the Medical College.

Coauthor of the study with Drs. Rao and Harrington is Andrew R. Mayer, M.S., graduate student, department of neurology, Medical College of Wisconsin.

The study was supported by grants from the National Institute of Mental Health and the W.M. Keck Foundation to the Medical College, and the Department of Veterans Affairs and National Foundation for Functional Brain Imaging to the Veterans Affairs Medical Center, Albuquerque. - By Toranj Marphetia

[Contact: Toranj Marphetia]

27-Feb-2001