In an article published in the journal Intelligence, the study author reported that whether longitudinal studies of cognition measured an increase or a decrease in cognitive ability depended on a number of factors: the area of cognition under study, the length of time between cognitive tests, and the age and overall cognitive ability of the participant.
In traditional longitudinal studies, cognitive tests are administered at the study baseline, then the tests are administered again at another time point weeks or years later. In this study, the researcher used a longitudinal study design that differed from traditional longitudinal cognitive studies. At study baseline, participants were administered parallel versions of cognitive tests at three separate visits, with less than one week, on average, between visits. Then between one and eleven years later, participants were again administered parallel versions of cognitive tests at three separate visits with an average of less than one week between visits. There were over 2000 participants in the study, and just under half of them were administered a third set of cognitive tests during three separate visits with less than a week between visits on average, and from about one to eight years after the second set of tests. The schedule for the nine visits is shown in the graphic below.
Participants in the study were aged 18 to 80 years old, and those who may have had dementia, as identified by their performance on a cognitive test, were excluded from the study. The researcher administered sixteen tests to examine five aspects of cognition. For example, memory was probed with word recall tests. Speed of perception was tested by asking participants to match symbols to numbers. Vocabulary was tested by asking participants to give definitions for various words. Spatial visualization was tested by a paper-folding test. Reasoning was examined with a matrix reasoning test where participants were shown a series of patterned figures with one figure missing, which participants were asked to fill in. All tests were performed in the same order at each visit. The study researcher created a composite score for each cognitive ability by combining test results for each aspect of cognition.
THE FINDINGS
Using statistical analyses, the study author examined the effects of the longer time intervals between visits on participants’ cognitive test scores. The study author reported that the longer the time interval between visit 3 and visit 4, the lower participants scored on tests of memory and speed of perception. The researcher also reported that the longer the time interval between visit 6 and visit 7, the lower participants scored on tests of memory. These decreases in performance were statistically significant.
Differences in cognitive test performance between age groups
The researcher plotted composite test scores for each of the five cognitive abilities under investigation, separately plotting scores for three different age groupings: 18 to 39, 40 to 59, and 60 to 80 years old. For cognitive tests of memory, speed of perception, spatial visualization, and reasoning, participants aged 18 to 39 had the highest scores, participants aged 40 to 59 had lower scores, and participants aged 60 to 80 had the lowest scores.
For cognitive tests of vocabulary, this trend was reversed. Participants aged 60 to 80 scored the highest, participants ages 40 to 59 scored lower, and participants aged 18 to 39 scored the lowest. This trend in cognitive performance on vocabulary tests was observed in all four vocabulary tests, not only in the composite vocabulary score. At younger ages, participants saw greater gains in vocabulary scores across shorter time intervals (e.g., from visit 1 to visit 3).
In general, cognitive performance tended to increase over the shorter time intervals (e.g., from visit 1 to visit 3). Older study participants and participants with lower cognitive ability tended to see the greatest gains over these shorter intervals. Cognitive performance over the longer time intervals (e.g., from visit 3 to visit 4) tended to decline, particularly for older study participants.
Associations between short term changes and long-term changes
The study investigator wondered whether changes in cognitive performance across the shorter time intervals (e.g., visit 1 to visit 3) could predict changes in cognitive performance over the longer intervals (e.g., visit 3 to visit 4). Contrary to what the researcher expected, study participants who had the largest increases in performance from visits 1 to 2, from visits 4 to 5, and from visits 7 to 8 had the largest decreases in performance from visit 3 to visit 4and from visit 6 to visit 7.
RELEVANCE OF THE STUDY
Having previous experience with a cognitive test can affect participants’ performance on the test. Often, performance improves. This paper addresses a gap in the literature because the study was designed to examine the effects of having prior experience with cognitive tests. There are very few studies in the scientific literature that account for the effects of experience in the study design. The results of this research study extend our knowledge by providing evidence of the effects of experience, amoung other factors, on cognitive test performance.
The results of this study have a number of important implications. One implication is that an initial cognitive test may not be fair to older people and people with lower cognitive abilities. These individuals saw the greatest benefit from having prior experience with the cognitive tests, as they had the largest increases in cognitive performance scores from an initial to a second cognitive test.
Reports from previous scientific studies on the effects of cognitive ability on cognitive performance over shorter time intervals have been inconsistent. There have been a number of studies that have reported an increase in cognitive performance over short time periods in high cognitive ability individuals. There have been some reports of no association between cognitive ability and cognitive performance, and some studies have observed improved cognitive performance over short time intervals in lower-ability participants.
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