In an article published in the journal Neurology, scientists report that longitudinal changes in certain brain structures negatively affect cognition. Specifically, a decrease in the size of the hippocampus over time was associated with a decline in memory, and longitudinal changes in multiple brain structures impaired executive function. Executive function is a set of cognitive abilities that we use to plan and execute goal-directed behaviour.
Scientists used MRI scans to measure various structures in the brain; for example, the size of the hippocampus, which is an important structure for memory. They also measured the volume of grey matter in the cerebral cortex (the outer layer of the brain) which has many functions including, memory, attention, and planning. MRI scans occurred at least one year apart, and on average, there were about three years between the first and last MRI scan of study participants.
Researchers also carried out various psychological tests of memory and executive function; for example, delayed and immediate recall tests, and a verbal fluency test where participants were asked to come up with as many words as they were able to that started with the letter ‘F’ in a fixed period of time. At least two psychological assessments were completed for each participant, and on average, about five years elapsed between the first and last psychological test. The researchers then used statistical modelling to estimate the effects of brain structure (both at baseline and longitudinal changes) on two aspects of cognition, memory and executive function.
There were over one hundred participants in the study, and they had an average age of 74 years. Participants with differing cognitive abilities were recruited into the study to fill three different groups. Participants in the first group were cognitively normal at the study baseline. Participants in the second group were cognitively impaired at baseline, and those in the third group had dementia at the study baseline.
THE FINDINGS
At the beginning of the study, there were fifty-eight cognitively normal participants in the first group. As the study progressed, fourteen of them became cognitively impaired, and one developed dementia. Initially, there were thirty-four participants in the cognitively impaired group. As the study advanced, four became cognitively normal, and twelve developed dementia. Finally, at study baseline, there were eleven people who had dementia in the third group. Amoung them, there were different dementia diagnoses: six had Alzheimer’s disease, three had vascular dementia, and two had a combination of Alzheimer’s disease and vascular dementia.
Memory and executive function differed between cognitive groups
At baseline, memory scores differed considerably between the three cognitive groups. The people in the cognitively normal group had the highest memory score. People in the cognitively impaired group had a lower memory score, and people who had dementia had the lowest memory score. For executive function at baseline, there was no significant difference between scores for people in the cognitively impaired group and individuals with dementia. However, the executive function scores for those who were cognitively impaired and for those who had dementia were significantly different from the cognitively normal individuals.
The average annual rate of change in memory for individuals with dementia was statistically significant. These individuals had a decline in memory function. The average annual rate of change of memory between the normal, impaired, and dementia groups was not significantly different. The average annual rate of change in executive function was significantly different for people with dementia. This difference was also significantly different from the average annual rate of change in executive function observed in the cognitively normal and the cognitively impaired groups.
Brain structure differed between cognitive group
At baseline, the size of the hippocampus differed between groups. The hippocampus was largest in the cognitively normal group, it was smaller in the cognitively impaired group, and it was the smallest in the group of individuals with dementia. At study baseline, the volume of cortical grey matter was greatest in all groups. At the last measurement, the volume of cortical grey matter had declined in all groups.
The rate of change of the volume of the hippocampus and the rate of change of cortical grey matter volume was significantly different for all cognitive groups. The rate of change of both hippocampal and greymatter volumes for each group was different from the rate of change of the other two groups. For hippocampal volumes, the annual decrease was about 1% in the cognitively normal group, 2.4% in the cognitively impaired group, and about 3% in the group of people with dementia. For cortical grey matter volumes, the annual decrease was about 0.5% in the cognitively normal group, about 2% in the cognitively impaired group, and almost 3% in the group of people with dementia.
The effects of brain structure on memory and executive function
Researchers used statistical modelling to explore associations between baseline levels and annual changes in brain structures (measured by MRI) and memory and executive function. They report that changes in memory were associated with baseline hippocampal volumes and annual changes in hippocampal volumes. Changes in executive function were associated with baseline volumes of cortical grey matter and annual changes in cortical grey matter volumes, among other variables.
RELEVANCE OF THE STUDY
This study addresses a gap in the literature because while it is known that a decline in the volume of the hippocampus can predict the transition from mild cognitive impairment to dementia, broader relationships between various brain structures and cognitive decline were unknown. This study further advances our knowledge because while several previous studies have examined the effects of baseline measurements of brain structures on baseline and longitudinal cognitive function, this study provides evidence of the effects of longitudinal changes in brain structure on cognitive function.
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