Electrophysiological indices of response inhibition in the stop-signal task

Response inhibition is vital for the performance of everyday tasks, allowing us to stop and adjust inappropriate behaviour in accord with our external environment and our own internal directions, while a deficiency in this process is believed to lead to impulsive behaviours. The main aim of this thesis was to investigate the nature of response inhibition in adults using the stop-signal task, a laboratory paradigm which allows estimation of the latency of the unobservable inhibitory response. This was achieved through: (a) a comparison of inhibition in the typical (simple) stop-signal task with other forms of inhibition, (b) a within-subject manipulation of stop-signal probability, and (c) an examination of response inhibition in populations characterised by impulsivity. Furthermore, throughout this thesis, event-related potentials (ERPs) were examined to provide an insight into the electrophysiological nature of response inhibition. Therefore, a secondary aim of this thesis was to further elucidate the functional significance of inhibition-related ERPs. The comparison of the simple stop-signal and go/nogo tasks revealed that inhibition manifests more centrally when an ongoing response is inhibited successfully, relative to the frontal distribution of inhibition when a response is merely in preparational stages, reflecting the differential sites of inhibitory control acting on response processing between the two tasks. However, when the stop-signal inhibitory response was manipulated by including an additional stimulus discrimination, creating a stop/no-stop (selective) form of inhibitory control, the latency of the inhibitory response and the manner in which responses were inhibited remained unaffected. Thus, inhibiting an ongoing response, whether it be in a simple or selective context, was associated with a fast, centrally-located inhibitory action. The electrophysiological index of this �urgent� stop-signal inhibition process was found to be reflected in the P3 component on successful stop trials, while the small auditory N2 was believed to be associated with a frontally-located, deliberate form of response selection or control, a process which may be by-passed by subjects when a more urgent form of inhibitory control is required. The N2 and P3 for failed stop trials, however, were shown to be overlapped by response-related components reflecting erroneous processing. The manipulation of stimulus probabilities revealed that larger ERP component amplitudes for rare stop-signals may not necessarily reflect greater inhibitory activation compared to frequent stop-signals, but rather may reflect stimulus probability �oddball� effects. In the final phase of this thesis, an examination of response inhibition in impulsive populations revealed that non-clinical adults who showed high degrees of the impulsiveness personality trait did not suffer from an inhibitory deficit, but rather, were characterised by an over-active response process. In contrast, adults with Attention-deficit Hyperactivity Disorder, despite similar overt performance with non-clinical groups, showed a slower and under-active inhibition process that was compensated for through a reduction of response activation. These findings suggest that a deficiency in the stop-signal inhibition process is not associated with impulsive behaviours in general, but with a more clinical form of impulsivity. Together the findings from this thesis have presented an electrophysiological view of response inhibition in the stop-signal task that has: (a) furthered our understanding of the manifestation of inhibition in different inhibitory contexts and populations, (b) clarified the functional significance of the N2 and P3 components in the stop-signal task, and (c) provided an insight into the relationship between stop-signal inhibition and impulsivity.