Functional significance of age-related differences in motor activation patterns.

Recent functional MRI (fMRI) studies have revealed an increased task-related activation in older subjects during a variety of cognitive or perceptual tasks, which may signal beneficial compensatory activity to counteract structural and neurochemical changes associated with aging. Under the assumption that incremental movement rates are associated with an increased functional demand on the motor system, we used fMRI and acoustically paced movements of the right index finger at six different frequencies (2.0, 2.5, 3.0, 4.0, 5.0 and 6.0 Hz) to investigate the behavioral significance of additionally recruited brain regions in a group of healthy, older subjects (mean age 66 +/- 8 years) compared with a group of young (mean age 23 +/- 7 years) subjects. The actual tapping frequency (F(1,14) = 0.049, P = 0.829), the tapping interval (F(1,14) = 0.043, P = 0.847), and the error rates (F(1,14) = 0.058, P = 0.743) did not differ significantly between both groups, whereas there was a significant increase in reaction time in the older subjects (F(1,14) = 281.786, P < or = 0.001). At all frequencies, the older subjects demonstrated significant overactivation within the ipsilateral sensorimotor and premotor cortex. However, we did not observe an increased age-related overactivation during higher movements rates in these or other motor regions. Moreover, the magnitude of the hemodynamic response in overactivated regions remained constant across all frequencies. In contrast to cognitive tasks, these findings indicate that an age-related overactivation within the motor system is not related to the functional demand and does not necessarily reflect reorganization to compensate for the neurobiological changes of aging.