The myriad ways in which the brain changes with age are in some ways very well explored, but overall still a dark forest, little understood in fine detail. One approach to gain greater understanding of the processes that cause declining cognitive function with age is to compare people with good function and people with poor function, first categorizing, and then secondly assessing the properties of the brain, as best researchers are able to do so, given limited access to the inside of the cranium. Today's research materials are an example of this sort of research, focused on trying to better understand why some older people retain a good memory function, while their peers decline.
There are numerous possibilities, even looking at broad categories of potential mechanisms. This could be a matter of slower degeneration, that some people make good lifestyle choices throughout life and as a result take longer to reach critical thresholds of damage and dysfunction that impact memory. One might look at recent research that suggests the hippocampus is running right at the upper limit of its supply of nutrients, and thus better maintained physical fitness into later life ensures that blood supply remains sufficient. Alternatively, some people may be more resilient to specific mechanisms of damage that impact areas of the brain, such as the hippocampus, that are important to memory. Some individuals have a high burden of amyloid-β in the brain, but little to no sign of neurodegeneration, for example.
Lastly, it is possible that some people exhibit a better set of age-related compensatory changes in the brain. This is perhaps driven by a greater pace of neurogenesis, allowing the creation of new neural networks and the replacement of damaged or dead neurons in existing networks. Neurogenesis in the hippocampus is meaningfully affected by exercise and gut microbiome, via mechanisms that include those related to expression of BDNF. The storage and processing of memories in the brain appears quite dynamic, and it is possible to envisage processes whereby memory is continually shuffled around and preserved through the loss of specific neurons or alternations to neural networks.
As we age, our brains typically undergo a slow process of atrophy, causing less robust communication between various brain regions, which leads to declining memory and other cognitive functions. But a rare group of older individuals called "superagers" have been shown to learn and recall novel information as well as a 25-year-old. The superagers are participants in an ongoing longitudinal study of aging. "Using MRI, we found that the structure of superagers' brains and the connectivity of their neural networks more closely resemble the brains of young adults; superagers had avoided the brain atrophy typically seen in older adults."
In the new study, the investigators gave 40 adults with a mean age of 67 a very challenging memory test while their brains were imaged using functional magnetic resonance imaging (fMRI), which, unlike typical MRI, shows the activity of different brain areas during tasks. Forty-one young adults (mean age of 25) also took the same memory test while their brains were imaged. While the participants were in the scanner, the researchers paid close attention to the visual cortex, which is the area of the brain that processes what you see and is particularly sensitive to aging.
In the visual cortex, there are populations of neurons that are selectively involved in processing different categories of images, such as faces, houses or scenes. During aging, this selectivity, called neural differentiation, diminishes and the group of neurons that once responded primarily to faces now activates for other images. The brain now has difficulty creating unique neural activation patterns for different types of images, which means it is making less distinctive mental representations of what the person is seeing. That's one reason older individuals have trouble remembering when they may have seen a television show, read an article, or eaten a specific meal.
But in the fMRI study, the superagers' memory performance was indistinguishable from the 25-year-olds', and their brains' visual cortex maintained youthful activity patterns. An important question that researchers still must answer is whether "superagers' brains were always more efficient than their peers, or whether, over time, they developed mechanisms to compensate for the decline of the aging brain.
Superagers are older adults who maintain youthful memory despite advanced age. Previous studies showed that superagers exhibit greater structural and intrinsic functional brain integrity, which contribute to their youthful memory. However, no studies, to date, have examined brain activity as superagers learn and remember novel information. Here, we analyzed functional magnetic resonance imaging data collected from 41 young and 40 older adults while they performed a paired associate visual recognition memory task. Superaging was defined as youthful performance on the long delay free recall of the California Verbal Learning Test. We assessed the fidelity of neural representations as participants encoded and later retrieved a series of word stimuli paired with a face or a scene image.
Superagers, like young adults, exhibited more distinct neural representations in the fusiform gyrus and parahippocampal gyrus while viewing visual stimuli belonging to different categories (greater neural differentiation) and more similar category representations between encoding and retrieval (greater neural reinstatement), compared with typical older adults. Greater neural differentiation and reinstatement were associated with superior memory performance in all older adults. Given that the fidelity of cortical sensory processing depends on neural plasticity and is trainable, these mechanisms may be potential biomarkers for future interventions to promote successful aging.