A large study from the Semel Institute for Neuroscience at UCLA recently looked at the brain-related changes that cause deficiencies of certain behavioral activities as humans age.
The researchers tested subjects, aged 20-80, on a number of behavioral tasks. The fine finger movement task required subjects to rotate a pin between their forefinger and thumb with first one and then two hands. The digit symbol task required subjects to fill in boxes with symbols that corresponded to single digit numbers as quickly as possible. Subjects were measured for “ataxia” battery by measuring how long they could stand in various positions with eyes closed. The researchers also gathered tons of white matter data using diffusion tensor imaging, a useful technique that relies upon the direction of water in individual axons.
Although the authors found very few sex related effects in brain regions, they did find many effects due to aging. The superior lateral systems of the brain in particular were highly affected, and the diffusivity increase was more pronounced in the transverse (across) than the longitudinal (down) regions. Since the corpus callosum that connects the two hemispheres would affect transverse regions, and because the callosum has lots of white matter, the increased diffusivity in the transverse regions implicates diminished myelin integrity.
Age was found to significantly correlate negatively with each of the three behavioral measures (meaning that on average the older the subject, the lower the performance). When the researchers performed multiple regression analysis on this data they found that all of this correlation in the fine finger movement task could be attributed to fiber integrity as measured by the diffusion tensor imaging. For the digital symbol task, most of the variation could be explained by age, but the diffusivity in the temporal callosal sector also contributed. Finally, for the ataxia test all of the variance between subjects could be explained by age alone.
These results lead to a number of interesting conclusions. The authors suggest that since the diffusivity in the collosum is highly indicative of myelin decay, the fact that variance in the digital symbol task could be explained by differences in this region suggests that a decay in myelination could also be driving its decline. Both grey and white matter is probably involved with the decline in behavioral functions as we age, but pinpointing exactly how and why this occurs is of importance in the study of Alzheimer’s and aging in general.
Sullivan EV, Rohlfing T, Pfefferbaum A. 2008 Quantitatitive fiber tracking of lateral and interhemispheric white matter systems in normal aging: Relations to timed performance. Neurobiology of Aging. doi:10.1016/j.neurobiolaging.2008.04.007