Applying the stop signal task to speech: neural and behavioural investigations of proactive and reactive inhibition

It is interesting to investigate how the brain executes a response such as a spoken word or a manual action but perhaps even more intriguing to study how such responses are deactivated. This thesis focuses on response inhibition with a particular focus on inhibition of vocal responses. There are two types of response inhibition: proactive inhibition refers to withholding a response when a stop signal is imminent; in contrast, reactive inhibition requires countermanding an already initiated response at the appearance of a stop signal. In this thesis I examine the hypotheses (1) that greater level of proactive inhibition enhances reactive inhibition; (2) that manual and vocal responses are controlled by common central generators for response inhibition; (3) and that reactive and proactive inhibitions are less efficacious in the vocal compared to the manual effector system. In chapter 1 I review the literature on response inhibition, clinical and non-clinical deficiencies in response inhibition and provide evidence for differences in the efficacy of response inhibition across the ocular, vocal and limb effector systems. In chapter 2 I compare both reactive and proactive inhibition across vocal and manual responses, the relationship between proactive and reactive inhibition and the effect of excitatory transcranial direct current stimulation (anodal tDCS) on response inhibition. In chapter 3 I develop a new task to explore proactive inhibition where response time is controlled within a sensorimotor synchronisation task. In chapter 4 I explore magnetoencephalographic (MEG) indices of vocal response inhibition in younger and older adults. In chapter 5 I measure the effect of proactive on reactive inhibition by varying the stop signal probability and explore the relationship between response inhibition and both functional and dysfunctional impulsivity trait measures. The main findings in this thesis are threefold. First, greater level of proactive inhibition enhanced reactive inhibition. Second, vocal and manual reactive inhibitions are controlled by common central generators. Third, vocal relative to manual responding shows weaker reactive inhibition. These findings contribute to our understanding of vocal response inhibition and response inhibition enhancement, findings that could potentially contribute to the treatment of clinical and non-clinical response inhibition deficiencies. I address the implications of these findings in Chapter 6.