Intra-individual reaction time variability in sustained attention

Abstract

BACKGROUND: Sustained attention, assessed using the Continuous Performance Test (CPT), is impaired in ‘normal’ ageing and, to a greater degree, in a number of clinical disorders. There are many variants of the CPT, each with different task parameters (e.g., target frequency), and theoretical cognitive demands (e.g., executive functioning). It is unclear how the associated cognitive load of CPTs contributes to measures of attentional impairment, such as intra-individual variability (IIV) in reaction time (RT). There is potential clinical utility in measures of IIV, due to its relationship with increasing age, and brain white matter. Variability can be modelled using the ex-Gaussian distribution, and consists of three parameters: mu (mean RT), and IIV, decomposed into variability across the entire RT distribution (sigma), and characterised by infrequent and long RTs (tau). This thesis aims to examine how the multiple cognitive demands of CPTs contributes to attentional RT/IIV, and how this relationship interacts with age, as well as pathology. The thesis aims are explored in healthy and clinical populations characterised by sustained attention impairment associated with increasing age (‘normal’ ageing and Parkinson’s disease (PD)), or in theoretical ‘accelerated ageing’ (Bipolar disorder (BD) while depressed and in remission). METHODS: Sustained attention was assessed in five cross-sectional studies, using variants of the CPT. Secondary neuropsychological measures of executive functioning, processing speed, and verbal memory were administered. Ex-Gaussian distributional parameters (mu, sigma, and tau) obtained from CPT RTs were analysed. A series of hierarchical regression analyses were examined. RESULTS: (1) In ‘normal’ ageing, better performance on the secondary neuropsychological measures was associated with faster RT (mu) and more consistent responding (sigma, tau), but this varied across CPT. Similar results were obtained for the effect of age on RT and IIV. (2) In PD, better executive functioning was associated with consistent responding (tau), whilst age was associated with slower (mu) and inconsistent (tau) responding. (3) In BD (while depressed), better executive functioning was associated with slower responding (mu), and better processing speed with consistent responding (tau), whilst age did not explain variance in RT or IIV. (4) In BD (in remission), the secondary neuropsychological measures examined did not explain variance in RT or IIV, nor did age. vi CONCLUSIONS: Attentional RT and IIV in ‘normal’ ageing and in clinical populations such as PD and BD, may be supported by secondary neuropsychological processes theorised to be involved in CPT variants. The neuropsychological profile underpinning attentional RT and IIV may reflect secondary cognitive scaffolding mechanisms, engaged depending on the age of participants, rather than the cognitive load of the task per se. The results have implications for our understanding of attentional RT and IIV in ‘normal’ ageing and pathology. Future research would further our understanding on the use of cognitive scaffolding in relation to the CPT, as well as the stability, reliability, and neurobiological origins of RT and IIV.