Intra-individual variability of reaction time and subclinical psychosis

Abstract

Clinical and epidemiological studies have indicated that elevated intra-individual variability (IIV) of reaction time (RT) is a trait phenomenon that indexes vulnerability for a number of clinical conditions, including schizophrenia. Persons classified as at-risk of developing psychotic disorder also exhibit increases in IIV, which has led to suggestions that IIV may represent a potentially useful risk marker for psychosis. However, many aspects of this relationship remain unstudied. My aim in presenting this thesis and its component studies was to determine the robustness of the relationship from IIV to two manifestations of schizophrenia liability—psychotic-like experiences (PLE) and schizotypy. In Chapter 2, the predictive validity of IIV for PLE during adolescence was assessed using birth-cohort data. Elevated IIV on a stop-signal task at 15 years of age predicted suspected or definite PLE at 18 years of age, with stronger associations for frequent and severe classifications of PLE. Elevated IIV at age 15 also predicted the presence of more persistent PLE. There were no associations between any measures of IIV at age 13 and PLE classifications or trajectories. Chapter 3 investigated whether elevations in IIV were stronger for PLE or schizotypy, and whether relationships were due to affective dysregulation. Increases in IIV were more strongly associated with PLE than schizotypy, to a greater extent for complex than simple RT tasks. Associations between IIV and PLE did not appear to be due to affective distress. In two studies presented in Chapter 4, I sought to determine whether cognitive control mediated the relationship between IIV and subclinical psychosis. Here, mediation depended on whether self-report or performance-based measures of cognitive control were used. In Study 1, there were consistent relationships between the assessed constructs of self-reported executive dysfunction, schizotypy, and IIV. Self-reported executive dysfunction appeared to mediate the relationship between IIV and schizotypy. In Study 2, elevations in IIV were associated with poorer performance on an executive task battery, and endorsement of PLE. However, cognitive control was not associated with PLE occurrence, nor did it mediate the relationship between IIV and risk status. Throughout the thesis, IIV did not appear to be consistently related to specific expressions of subclinical symptoms. Overall, these findings suggest that IIV is somewhat uniquely able to index liability to psychosis and may represent an interface between cognitive functioning and psychotic symptoms. However, its utility as a clinical marker appears limited.