Both sides of the bell curve: Base rates of high and low scores in cognitively unimpaired and impaired older adults and their relationship to biomarkers of Alzheimer's disease.

Objective: To further investigate the "other side of the bell curve" hypothesis, the current study examined the number of low and high scores on a neuropsychological battery: 1) in cognitively unimpaired or impaired older adults, 2) as they relate to biomarkers of Alzheimer's disease (AD), and 3) as they relate to traditional scores on this battery.

Method: In 68 cognitively unimpaired and 97 cognitively impaired participant, the number of low (i.e., ≤ 16^th percentile) and high (i.e., ≥ 75^th percentile) scores on the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS) were calculated, compared between the two groups, and related to biomarkers of AD (i.e., amyloid deposition, hippocampal volumes, ε4 alleles of Apolipoprotein E (APOE)) and RBANS Total score.

Results: In this cognitively diverse sample, low and high scores were common, with approximately 75% having at least one low score and 86% having at least one high score. Unimpaired participants had significantly more high scores and fewer low scores than their impaired counterparts. The number of low scores was significantly related to more amyloid deposition, smaller hippocampal volume, and having one or more copies of the ε4 allele of APOE. The number of high scores was similarly related with these biomarkers. Low/high scores were comparable to traditional scores on the RBANS in identifying cognitively impaired participants.

Conclusions: Support for the "other side of the bell curve" hypothesis was equivocal in these analyses, with both sides of the bell curve appearing to provide relevant information in a cognitively diverse sample.