Perceptual Degradation Affects Stop-Signal Performance in Normal Healthy Adults

During stop-signal task performance, little is known how the quality of visual information of the ‘go’ stimuli may indirectly affect the interplay between the ‘go’ and ‘stop’ processes. In this study, we assessed how perceptual degradation of the visual ‘go’ stimuli affect response inhibition. Twenty-six healthy individuals (mean age 33.34 ± 9.61) completed a modified 12-minute stop-signal task, where ‘V’ and ‘Y’ letters were used as visual ‘go’ stimuli. The stimuli were subjected to four levels of perceptual degradation using Gaussian smoothing, to parametrically manipulate stop difficulty across low, intermediate-1, intermediate-2 and high difficulty conditions. On 33% of trials, the stop-signal (50ms audio tone) followed a ‘go’ stimulus after a stop-signal delay, which was individually adjusted for each participant. As predicted, we found that with increased level of stop difficulty (little perceptual degradation), reaction times on ‘go’ trials and the proportion of successful behavioural inhibitions on ‘stop’ trials (P(i)) decreased in normal healthy adults. Contrary to our predictions, there was no effect of increased stop difficulty on the number of correct responses on ‘go’ trials and reaction times on ‘stop’ trials. Overall, manipulation of the completion time of the ‘go’ process via perceptual degradation has been partially successful, whereby increased stop difficulty differentially affected P(i) and SSRT. These findings have implications for the relationship between the ‘go’ and ‘stop’ processes and the horse-race model, which may be limited in explaining the role of various cortico-basal ganglia loops in modulation of response inhibition.HighlightsManipulation of the completion time of the ‘go’ process is partially successfulPerceptual degradation differentially affects stop-signal performanceIncreased stop difficulty (easy ‘go’) results in lower P(i)Increased stop difficulty (easy ‘go’) has no effect on SSRTHorse-race model does not fully explain basal ganglia involvement in inhibition

The role of general executive functions in receptive language switching and monitoring

We assessed language switch and mixing costs in a language-general semantic categorization task and examined how these costs relate to general inhibition and set shifting capacities. The participants were 51 native Finnish subjects with English as L2. The results showed significant symmetric language switch costs and, unexpectedly, a mixing advantage in L2: reaction times were faster in the mixed language block than in the single language block. The interactions with the general executive functions showed no consistent overall pattern. We argue that the L2 mixing advantage stems from statistical facilitation in line with a horse race model, or from opportunistic planning as suggested by the Adaptive Control hypothesis. We argue that the results overall indicate that lexical access in language reception is non-selective.

Stopping and Restarting an Unfolding Action at Various Times

The ability to inhibit an unfolding action is usually investigated using a stop signal (or go—stop) task. The data from the stop-signal task are often described using a horse-race model whose key assumption is that each process (i.e., go, stop) exhibits stochastic independence. Using three variations of a coincident-timing task (i.e., go, go—stop, and go—stop—go) we extend previous considerations of stochastic independence by analysing the go latencies for prior effects of stopping. On random trials in the go—stop—go task the signal sweep was paused for various times at various distances before the target. Significant increases in latency errors were reported on those trials on which the signal was paused (p <.005). Further analyses of the pause trials revealed significant effects for both the stopping interval (p <.001) and the pause interval (p <.05). Tukey post hoc analyses demonstrated increased latency errors as a linear function of the stopping interval, as expected, and decreased latency errors as a nonlinear function of the pause interval. These latter results indicate that the latencies of the go process, as reflected in the latency errors, may not exhibit stochastic independence under certain conditions. Various control mechanisms were considered in an attempt to explain these data.