Reliability of triggering inhibitory process is a better predictor of impulsivity than SSRT

The ability to control behaviour is thought to rely on adequately suppressing impulsive responses to external stimuli. However, the evidence for this relationship between response inhibition ability and impulse control is weak and inconsistent. This study investigates the relationship between response inhibition and both self-report and behavioural measures of impulsivity as well as engagement in risky behaviours in a large community sample (N=174) of healthy adolescents and young adults (15-35yrs). Using a stop-signal paradigm with a number parity go task, we implemented a novel hierarchical Bayesian model of response inhibition that estimates stop-signal reaction time (SSRT) as a distribution and also accounts for failures to react to the stop-signal (i.e., “trigger failure”), and failure to react to the choice stimulus (i.e., “go failure” or omission errors). In line with previous studies, the model reduced estimates of SSRT by up to 100ms compared with traditional non-parametric estimation techniques. Although some relationships between behavioural and self-report measures of impulsivity with traditionally estimated SSRT were supported, they did not hold for the model-based SSRT estimates. Instead, the behavioural impulsivity measures were found to be related to trigger failure. We suggest that trigger failure is correlated with impulsivity because it indexes higher order processing related to inhibition, whereas SSRT, when estimated without confounding from trigger failure, indexes more automatic inhibitory processes.