Intraindividual Variability as a Marker of Neurological Dysfunction: A Comparison of Alzheimer's Disease and Parkinson's Disease

Abstract

Individuals with certain neurological conditions may demonstrate greater inconsistency (i.e., intraindividual variability) on cognitive tasks compared to healthy controls. Several researchers have suggested that intraindividual variability may be a behavioral marker of compromised neurobiological mechanisms associated with aging, disease, or injury. The present study sought to investigate whether intraindividual variability is associated with general nervous system compromise, or rather, with certain types of neurological disturbances by comparing healthy adults, adults with Alzheimer's disease (AD), and Parkinson's disease (PD). Participants were assessed on four separate occasions using measures of reaction time and memory. Results indicated that inconsistency was correlated with indices of severity of impairment suggesting a dose-response relationship between cognitive disturbance and intraindividual variability: the more severe the cognitive disturbance, the greater the inconsistency. However, participants with AD were more inconsistent than those with PD, with both groups being more variable than the healthy group, even when controlling for group differences in overall severity of cognitive impairment or cognitive decline. Consequently, intraindividual variability may index both the severity of cognitive impairment and the nature of the neurological disturbance.

Acknowledgments

This research was supported by grants from the Canadian Institutes of Health Research and the Vancouver Health Foundation to D. Hultsch, E. Strauss and M. Hunter, and Natural Sciences and Engineering Research Council and Alzheimer Society of Canada/Canadian Institutes of Health Research Institute of Aging graduate scholarships to C. Burton. Preparation of this manuscript was also supported by a Research Unit Infrastructure award from the Michael Smith Foundation for Health Research to the University of Victoria Centre on Aging.

Notes

^aAge-scaled scores.