Background: The aim of this study is to describe and compare the course of the score of two psychometric tests during a five-year period in a cohort of non-demented elderly subjects. We studied a timed test of simple logical reasoning (the Digit Symbol Substitution Test of Wechsler) and a test of short-term visual memory (the Benton Visual Retention Test). Then we compared the course of these two tests with the course of the Mini-Mental State Examination (MMSE) which evaluates the general cognitive functioning and which has been studied previously.
Methods: The sample consisted of 2792 subjects aged 65 years and older living at home at the beginning of the study (the Paquid cohort in Gironde). Subjects were interviewed at home by a psychologist at the baseline visit and one year, 3 years and 5 years after the baseline visit. All subjects who were diagnosed as demented at any of the four examinations were excluded from this study (218 subjects). The statistical analysis was performed using a mixed effects linear model and an additional analysis was performed to study sensitivity of the results to hypotheses regarding missing data mechanism.
Results: Among the three tests studied, only the score of the Digit Symbol Substitution Test declined over the five years (by -1.15 points for subjects aged 65 years to -3.4 points for subjects aged 85 years). We observed an improvement of the score between the first two visits which is sharper for the test of Benton and the MMSE than for the Digit Symbol Substitution Test and which reflects a stress effect at the baseline visit or a learning effect at the second visit. The difference between the cross-sectional measure of age effect and the longitudinal measure of time effect suggests either a cohort effect or a practice effect which would persist during the five years.
Conclusions: The cognitive change in a non-demented elderly population is different according to the psychometric test used. Among the three tests studied, only the score of the Digit Symbol Substitution Test which has a a speed component declined over the five years.