Is intraindividual reaction time variability an independent cognitive predictor of mortality in old age? Findings from the Sydney Memory and Ageing Study.

Nicole A Kochan,John D Crawford,David Bunce,Perminder S Sachdev,Sarah Pont,Henry Brodaty,Stephen D Ginsberg

doi:10.1371/journal.pone.0181719

Abstract

Intraindividual variability of reaction time (IIVRT), a proposed cognitive marker of neurobiological disturbance, increases in old age, and has been associated with dementia and mortality. The extent to which IIVRT is an independent predictor of mortality, however, is unclear. This study investigated the association of IIVRT and all-cause mortality while accounting for cognitive level, incident dementia and biomedical risk factors in 861 participants aged 70–90 from the Sydney Memory and Ageing Study. Participants completed two computerised reaction time (RT) tasks (76 trials in total) at baseline, and comprehensive medical and neuropsychological assessments every 2 years. Composite RT measures were derived from the two tasks—the mean RT and the IIVRT measure computed from the intraindividual standard deviation of the RTs (with age and time-on-task effects partialled out). Consensus dementia diagnoses were made by an expert panel of clinicians using clinical criteria, and mortality data were obtained from a state registry. Cox proportional hazards models estimated the association of IIVRT and mean RT with survival time over 8 years during which 191 (22.2%) participants died. Greater IIVRT but not mean RT significantly predicted survival time after adjusting for age, sex, global cognition score, cardiovascular risk index and apolipoprotein ɛ4 status. After excluding incident dementia cases, the association of IIVRT with mortality changed very little. Our findings suggest that greater IIVRT uniquely predicts shorter time to death and that lower global cognition and prodromal dementia in older individuals do not explain this relationship.

Highlights

In addition to average performance level, there is an increasing focus in ageing research on intraindividual variability or inconsistency in cognitive performance
Participants were drawn from the Sydney Memory and Ageing Study (MAS), a longitudinal study of community dwelling older adults recruited through the Australian electoral roll, aged 70 to 90 years at baseline [16]
The models were estimated without covariates (Model 1), with covariates (Model2) and a backward regression procedure was used with both reaction times (RT) measures included and all covariates to determine the best predictors of mortality (Model 3)

Summary

Introduction

In addition to average performance level, there is an increasing focus in ageing research on intraindividual variability or inconsistency in cognitive performance. A few studies have reported that increased variability predicts mortality up to 19 years before eventual death in older populations [9,10,11] but it is unknown whether this association is independent of general age-related cognitive decline, an established risk factor for mortality [12, 13]. There is preliminary support for the relative importance of IIVRT as a predictor of mortality over the more basic measure of mean RT from the same cognitive task [9, 10] and this is worthy of further investigation in the context of the potential influence of global cognitive level and prodromal dementia on these relationships. IIVRT warrants investigation as a specific predictor of impending death in older age independently of global cognitive level, other mortality risk factors and speed (mean RT from the same task) and prodromal dementia using robust clinical diagnoses in the years before death. The aim of this study was to investigate the association of IIVRT with mortality over 8 years in a large, well-characterised population-based cohort of older adults aged 70 years and over, taking into account general cognitive function as assessed by a battery of psychometric tests, other mortality risk factors including demographics, cardiovascular risk and apolioprotein ε4 status, and dementia diagnosis based on DSM-IV [15]

Participants

Results

Discussion