Identifying Neural and Performance Markers of Subjective Cognitive Decline During Fine Motor Dual Tasks

Abstract

Subjective cognitive decline (SCD) refers to older adults who self-report persistent cognitive deficits in the absence of objective cognitive impairment. By clinical standards, older adults with SCD are considered healthy but may be at a greater risk of progressing to dementia than older adults without SCD (i.e., non-SCD). Despite this, neuropsychological tests are not sensitive enough to detect individuals at risk of future cognitive impairment since SCD relies on self-reported symptoms. This emphasizes a need for the early identification of markers that may be used to track or detect future cognitive impairment.

The dual-task paradigm has been previously used to identify differences in cognitive and motor performance in healthy older adults but its use in SCD has been limited. Based on the capacity model of attention, performing a cognitive and a motor task simultaneously can exceed an individual's processing capacity, leading to performance decrements on one or both tasks. The dual-task paradigm is thus a useful tool for detecting subtle cognitive deficits that may not be evident during single tasks.

Further, evidence suggests that changes in brain activity may precede observable changes in cognitive performance, including increased recruitment of neural resources (i.e., compensation), to support task performance. Neuroimaging techniques such as functional near-infrared spectroscopy (fNIRS) and electroencephalography (EEG) may, therefore, offer insights into the neural mechanisms underlying SCD. This highlights the potential use of neuroimaging to identify markers that distinguish between older adults with and without SCD during dual tasks.

To address this knowledge gap, this dissertation includes four studies aimed at identifying whether neural markers and cognitive and motor dual-task performance may be used to differentiate between older adults with and without SCD.

The first study (Chapter 2) included a systematic review and meta-analysis to quantitatively synthesize the dual-task literature on cognitive, motor, and neural outcomes in older adults with SCD compared to healthy controls, older adults with motoric cognitive risk syndrome (MCR), and objective cognitive impairment, including mild cognitive impairment (MCI) and dementia. The findings revealed decrements in gait performance between MCI and SCD but no differences between healthy older adults with and without SCD. In addition, few studies used neuroimaging to examine the underlying neural mechanisms of SCD, indicating a need for further research on markers associated with SCD during dual-task performance.

The second study (Chapter 3) used the knowledge gained from Chapter 2 by designing a dual-task paradigm consisting of walking or finger tapping with a cognitive task. In line with the literature, fNIRS findings revealed greater changes in cerebral oxygenation in the SCD compared to non-SCD group. This was observed during the finger tapping dual task, suggesting that it may be more sensitive to detecting differences in cerebral oxygenation than dual task walking. Taken together with performance findings, older adults with SCD may be demonstrating compensatory mechanisms to maintain cognitive performance, whereas the non-SCD group may prioritize walking over the cognitive task to maintain motor performance.

The third study (Chapter 4) expanded on Chapters 2-3 by investigating the effects of combining finger tapping with an n-back or double number sequence working memory task on cerebral oxygenation. The findings revealed that cognitive and motor outcomes declined with increasing cognitive load. In addition, greater changes in cerebral oxygenation were only observed in the SCD group compared to the non-SCD group during the double number sequence task. Taken together with performance findings, older adults with SCD may demonstrate compensation and neural inefficiency, in which increased brain activation may support task performance during the n-back task but not during the double number sequence task. The hemodynamic response may be sensitive to detect subtle differences in cognitive processing within the cognitive domain of working memory.

The fourth study (Chapter 5) used EEG, which has not been previously used to examine dual-task outcomes in SCD, to investigate P2 and P3 mean amplitude in the time-domain as well as power density in the delta, theta, alpha, beta, and gamma bands in the frequency domain. Findings revealed a larger P2 and P3 mean amplitude during the single task compared to the dual task, reflecting a greater availability of cognitive resources. In the frequency domain, theta and alpha power were higher in the SCD compared to non-SCD group, suggesting compensatory activation. In addition, the frequency domain may be more sensitive for detecting group differences in working memory processes compared to mean amplitude.

Four main outcomes emerged from this dissertation: 1) Research on dual-task outcomes in SCD is still emerging, with most studies focusing on dual-task walking and few studies examining the neural mechanisms underlying SCD. 2) Overall, older adults with and without SCD demonstrate similar levels of cognitive and motor dual-task performance. 3) The hemodynamic and electrophysiological response may be useful in detecting subtle differences in cognitive processing during dual tasks in older adults with and without SCD. 4) Longitudinal evidence is needed to better understand the trajectory of cognitive decline following SCD and whether early neural markers or performance outcomes can be used to predict the progression of cognitive impairment over time.