Examining Inter- And Intra-Individual Differences In The Neurobiological Mechanisms Associated With Inhibitory Control

Adolescence is an ideal time to measure the development of the neural mechanisms associated with inhibitory control because this age period is marked by impulsive and risk taking behaviors. Maturational brain changes in the prefrontal cortex that are associated with the emergence of inhibitory control are thought to occur during this age. With knowledge of how this system develops, it may be possible to identify the development of disorders that arise from poor inhibitory control such as attention deficit hyperactivity disorder (ADHD) and substance use. The goal of the current dissertation is to examine the neurobiological correlates associated with individual differences in inhibitory ability, and examine the age-related changes in neurobiological mechanisms of inhibitory control. This report will be the first of its size (n = 538) to examine within-subject changes longitudinally over five years of adolescent development (age 14 to 19). Furthermore, we supplement the longitudinal data with findings from a split-brain patient on the lateralization of inhibitory control, and we explore a subtle nuance that may have large implications on how to best measure inhibition-related brain activity. In the second chapter of the dissertation, we examine the lateralization of inhibitory control by measuring hemispheric differences in the ability to inhibit a motor response in a split-brain patient. Here, we found patient J.W.’s right hemisphere performed better than his left hemisphere on three different inhibitory control tasks. Interestingly, although inferior to the performance of the right hemisphere, the left hemisphere still performed relatively well on the three tasks, suggesting the left hemisphere can perform response inhibition independently. The third chapter examines both the functional correlates of Stop Signal Task performance, and the age-related differences in the functional mechanisms of response inhibition. At age 14 and age 19, similar patterns of activation were associated with performance, however relatively little overall activity exhibited performance-related effects. Superior performance was associated with greater right inferior frontal gyrus (rIFG) activation, as well as greater activation in a set of regions potentially involved with a stimulus-detection and attention-orienting system. However, at age 14 performance was also negatively associated with default mode network activity, and at age 19 performance was also positively associated with left amygdala activity. In the absence of within-subject differences in performance between ages 14 to 19, there were significant decreases in functional activation associated with successful inhibition. The potential mechanisms by which activity decreases over time while performance remains stable are discussed. The fourth chapter of the dissertation examines the effect of objective task difficulty on the magnitude of activation associated with successful inhibition. The Stop Signal Task employs an adaptive algorithm that alters task difficulty to meet participants’ abilities. Typically, when capturing functional activation associated with response inhibition, activation is extracted from all successful trials. Here, we find that individual differences in activation are expanded when using the activation from the extreme, rather than average, aspects of task performance variables. Individual differences in performance may best be captured by examining the maximum difficultly at which a participant is able to inhibit a response, rather than the average of all successful inhibitions. These results also lend support to the minimal activity associated with performance in Chapter 3, and we discuss how improving the measure of stop-related activity may help explain both inter- and intra-individual differences in inhibitory control