Reduced hippocampus size, mild memory loss in healthy older adults
As people age, it is common to experience memory loss. However, older adults who have symptoms of forgetfulness often score within the normal range on psychological tests even though the memory loss can become progressively worse. It is unknown whether these symptoms are an early sign of an underlying brain condition.
The authors of the present research study wondered whether mild memory loss could occur in an otherwise healthy older individual, where mild memory loss has been found to be characteristic of reduced size in the hippocampus, the part of the brain responsible for memory and which has been noted as a site for degeneration as a result of aging.
Scientists from the New York University Aging and Dementia Research Center used two techniques that are equally sensitive to detecting changes in the size of the hippocampus: CT scans and MRI scans. They examined two kinds of memory: immediate memory and recent memory. To probe immediate memory, scientists asked study participants to immediately repeat back a string of numbers that was read to them. To probe recent memory, participants were read a paragraph or shown cards with designs and numbers on them. They were then asked to recall as much of the paragraph contents or as many of the number-design pairs as they were able. Scientists performed statistical analyses to examine whether having a reduced hippocampus size affected participants’ performance on the memory tests.
The researchers performed a comprehensive set of additional tests to ensure that all 154 participants included in the study were healthy from a medical perspective, and that they were cognitively unimpaired. Participants were excluded from the study if they had, for example, cardiovascular, endocrine, or psychiatric disorders, or if they had a history of depression or excessive alcohol consumption.
Having a decline in the size of the hippocampus and mild memory loss (though otherwise cognitively healthy) may indicate an increased risk of developing dementia.
The effect of age on size of the hippocampus
Participants in the study were aged 55 to 88 years old. The older the participant, the greater the prevalence of having a reduced hippocampus size. Among the participants aged 55 to 65, about one tenth had a reduction in the size of the hippocampus. For those aged 66 to 76, the prevalence increased to one third. In the oldest age group, 77 to 88 years, over half of the participants had a reduced hippocampus size.
The effect of sex on size of the hippocampus
A greater number of men had a reduced hippocampus size than women. The percentage of men who had a reduced hippocampus size was nearly double that of women. The authors report that age was not a factor for this difference, since the average age of the men in the study was not statistically different from the average age of the women. Additionally, the increased prevalence of having a reduced hippocampus size with age, as discussed in the previous section, appears to be more pronounced in men than in women.
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The effect of hippocampus size on memory performance
Participants who had reduced hippocampus size had impaired performance on the paragraph-recall tests of recent memory than those with a normal-sized hippocampus. While those who had a reduced hippocampus size also showed impaired performance on the non-verbal, design-number matching tests of recent memory, the differences just failed to reach statistical significance. Researchers found no difference in performance on the immediate memory tests between study participants who did and who did not have a reduced hippocampus size. The study authors controlled for age and education in their statistical analyses, as these factors were found to influence performance on the memory tests.
RELEVANCE TO ALZHEIMER’S DISEASE AND DEMENTIA
The point at which a healthy, aging brain transitions to a brain showing the earliest stages of Alzheimer’s disease is under active investigation by the research community. There are several lines of investigation that suggest that the hippocampus may be the site of early degeneration in aging and Alzheimer’s disease.
Previous research studies have indicated that older participants with reduced hippocampus size and who have a mild cognitive impairment have an increased risk of being diagnosed with Alzheimer’s disease. The authors of the present study suggest that having a decline in the size of the hippocampus and mild memory loss (though otherwise cognitively healthy) may indicate an increased risk of developing dementia. Further studies are required to examine this possibility.