A recent study published in the journal Cortex highlights differences in brain network organization between healthy aging and semantic dementia. The study found that older adults experience changes in the balance of structural and functional brain connectivity, which are associated with cognitive performance.
These changes tend to be more widespread and diffuse in healthy aging, whereas semantic dementia is associated with localized changes, particularly in temporal and parietal regions. The study suggests that some changes in the brain enhance cognitive function while others impair cognitive performance.
The cognitive function of words, objects, and concepts is affected by semantic dementia, a progressive neurodegeneration that primarily affects the frontotemporal lobar degeneration. The disease is unique because it primarily affects the anterior temporal lobe, not the entire temporal lobe, and may result in a decreased ability to recall the meanings of words, objects, and faces.
It arose from the need to better understand how aging, neurodegenerative diseases such as cognitive dementia and even semantic dementia, can alter cognition: “Semantic dementia causes widespread changes in both structural and functional networks similar to those found in normal aging, with particular damage to the frontal region, but also in temporal and parietal regions but with impairments in a diffuse manner in the temporal regions as well.
These changes raise important questions about whether such reorganization of the brain reorganizes functioning. It is important to recognize the specificities of physiological aging and pathology in order to identify early markers of neurodegeneration. The brain functions as a vast network of interrelated regions that communicate to support thought, memory and behavior.
Both structural connectivity and functional connectivity are commonly used to evaluate this connection. A total of 18 people aged 18 to 30 years participated in the study. They also examined brain imaging using DWI and fMRI techniques. The study focused on 14 young adults (aged 20-30), 19 older adults (aged 51-75), and 12 individuals with semantic dementia (aged 56-80). They also completed cognitive tests assessing memory, executive function, and language abilities.
The researchers used a method called multiplex brain network analysis to assess the alignment of structural and functional networks. They measured network organization using two primary coefficients: the multiplex participation coefficient, which indicates the degree of integration of a brain region across structural and functional networks, and the multiplex clustering coefficient, which indicates the degree of segregation of certain brain regions.
The research found diverse variations in brain connectivity changes in both healthy aging and semantic dementia.
The researchers found that in healthy older adults, the similarity of structural vs. functional networks decreased, with the most significant differences being in frontal regions (not shown in a typical cross-section of the population), and so this low-similarity token meant that ‘a certain degree of structural decline is not necessarily associated with better cognitive performance, but rebalances functional decline with increased pattern recognition and network adaptation’ – however the results showed that older adults also showed an even more troubling pattern of increased network clustering in frontal regions, which was associated with poorer cognition.
The researchers observed that synesthesia was associated with cognitive decline, which was not the case in healthy aging, but was associated with increased similarity between structural and functional networks in temporal and parietal regions of the brain. Furthermore, these individuals also demonstrated increased clustering in temporal regions of the brain, which was also associated with lower cognitive performance.
These results point to the fact that semantic dementia causes the brain to asymmetrically rigidify and reductive pathways resulting in more rigid and less flexible connectivity patterns. The differences in daily life impairment between patients with similar conditions, as noted by Hinault, are due to variations in the impact of the disease on brain structure and communication between brain regions.
The study has a few shortcomings: the sample size was relatively small, particularly for the semantic dementia group, making it unlikely that the findings will generalize, and the study was cross-sectional, meaning it could only take a snapshot of brain connectivity at one specific point in time; more studies are now needed, although the scientists have stated that they do not think the structural/functional changes seen in this study are indicative of future cognitive decline.