Do Temporally Predictable Stop Signals Affect Behavioural or Neural Measures of Inhibitory Control in the Stop Signal Task?

The stop signal task (SST) is a popular paradigm for assessing response inhibition, namely the ability to cancel or withhold irrelevant or inappropriate behaviours. On a subset of trials, a stop signal follows the go signal after the stop signal delay (SSD), urging the participants to cancel their already initiated response to the go signal. The staircase method adjusts the SSD to be longer in the next stop trial if inhibition was successful, or shorter if it was unsuccessful. Although this tracked SSD approach dominates in the literature, it remains unknown whether humans are sensitive to the temporal predictability of the stop signals induced with these systematic SSD adjustments. If so, participants may strategically postpone making their responses until the anticipated moment the stop signal would occur has passed, resulting in potentially misleading estimates of inhibitory capacity. The present study therefore sought to investigate whether the tracked procedure, as compared to a novel, more random SSD procedure, results in differences in the behavioural (the stop signal reaction time; SSRT) and the proposed neural (the frontocentral P3 event-related potential) markers of stopping latency. Furthermore, the suggestion that the relationships among SST-derived measures are driven by an underlying monitoring-related process was assessed under the assumption that temporal predictability could induce differential monitoring behaviours. To this end, 34 participants completed the two versions of the SST while undergoing electroencephalography recordings. It was hypothesised that participants would exhibit faster SSRTs and more response slowing over both longer and shorter timescales, and that the correlations among the SST-derived measures would be stronger in the tracked version. The analyses (n = 25) revealed no significant differences between the task versions but indicated that the P3 does not fulfil the requirements to be considered a direct neural marker of inhibition. In addition, the examination of behavioural adjustments over shorter timescales indicated an effect of recent trial history on reaction times. This study should serve as an inspiration to further examine the highly relevant questions posed here with improved statistical power