Dwindling brain specialization with increasing age
What happens in our brains as we get older? We probably all know what happens at the behavioral level. Most notable is the changes in memory, and the ability to couple information together. Remembering a name, or mixing names on people is a frequent effect. Forgetting what happened when and who did what are well-known memory problems of aging.
But what causes these problems? And are they caused at the stage of learning or retention? Or may the changes even occur during preparation or rehearsal?
In a study now in press in Neurobiology of Aging (download PDF copy here), we studied the effects of healthy aging on how the brain processes different kinds of visual information. Based on prior work showing that visual attention towards objects predominantly recruited regions of the medial temporal lobe (MTL), compared to attention towards positions, we tested whether this specialisation would wither with increasing age.
Basically, we tested the level of brain specialization by comparing the BOLD fMRI signal directly between object processing and position processing. We looked at each MTL structure individually by analyzing the results in each individual brain (native space) rather than relying on spatial normalization of brains, which is known to induce random and systematic distortions in MTL structures (see here and here for PDF of conference presentations I’ve had on this).
Running the test with functional MRI, we found that several regions showed a change in specialization. During encoding, the right amygdala and parahippocampal cortex, and tentatively other surrounding MTL regions, showed such decreases in specialization.
During preparation and rehearsal, no changes reached significance.
However, during the stage of recognition, more or less the entire MTL region demonstrated detrimental changes with age. That is, with increasing age, those regions that tend to show a strong response to object processing compared to spatial processing, now dwindle in this effect. At higher ages, such as 75+, the ability of the brain to differentiate between object and spatial content is gone in many crucial MTL structures.
This suggests that at least one important change with increasing age is its ability to differentiate between different kinds of content. If your brain is unable to selectively focus on one kind of information (and possibly inhibit processing of other aspects of the information), then neither learning or memory can operate successfully.
One important feature of this study is that it provides a new means to study age-related disorders such as Alzheimer’s Disease. It is well known that this disorder initiates in the MTL region, most likely the trans-entorhinal region, and it does so long before the clinical symptoms of Alzheimers or even its predecessor, Mild Cognitive Impairment. Hence, the search for ways to assess changes in this region has become a growing field of interest. One possibility could be to employ the methods developed in this project to assess early functional and morphological changes in the MTL region, and possibly improve early detection of Alzheimer’s and related disorders.
Another interesting option would be to explore to what extent healthy aging is related to changes in everyday functions, such as shopping behaviour, learning and remembering movie contents and other complex kinds of information.