Stop Signal Task

Control of the body requires inhibiting complex actions, involving contracting and relaxing muscles. However, little is known of how voluntary commands to relax a muscle are cancelled. Action inhibition causes both suppression of muscle activity and the transient excitation of antagonist muscles, the latter being termed active breaking. We hypothesized that active breaking is present when stopping muscle relaxations. Stop signal experiments were used to compare the mechanisms of active breaking for muscle relaxations and contractions in male and female human participants. In experiments 1 and 2, go signals were presented that required participants to contract or relax their biceps or triceps muscle. Infrequent Stop signals occurred after fixed delays (0-500 ms), requiring that participants cancelled go commands. In experiment 3, participants increased (con-tract) or decreased (relax) an existing isometric finger abduction depending on the go signal, and cancelled these force changes whenever Stop signals occurred (dynamically adjusted delay). We found that muscle relaxations were stopped rapidly, met predictions of existing race models, and had Stop signal reaction times that correlated with those observed during the stopping of muscle contractions, suggesting shared control mechanisms. However, stopped relaxations were preceded by transient increases in electromyography (EMG), while stopped contractions were preceded by decreases in EMG, suggesting a later divergence of control. Muscle state-specific active breaking occurred simultaneously across muscles, consistent with a central origin. Our results indicate that the later stages of action inhibition involve separate excitatory and inhibitory pathways, which act automatically to cancel complex body movements. The mechanisms of how muscle relaxations are cancelled are poorly understood. We showed in three experiments involving multiple effectors that stopping muscle relaxations involves transient bursts of EMG activity, which resemble cocontraction and have onsets that correlate with Stop signal reaction time. Comparison with the stopping of matched muscle contractions showed that active breaking was muscle state specific, being positive for relaxations and negative for contractions. The two processes were also observed to co-occur in agonist-antagonist pairs, suggesting separate pathways. The rapid, automatic activation of both pathways may explain how complex actions can be stopped at any stage of their execution.

All actions, even the simplest like moving an arm to grasp a pen, are associated with energy costs. Thus all mobile organisms possess the ability to evaluate resources and select those behaviours that are most likely to lead to the greatest accrual of valuable items (rewards) in the near or, especially in the case of humans, distant future. The evaluation process is performed at all possible stages of the series of decisions that lead to the building of a goal-directed action or to its suppression. This is because all animals have a limited amount of energy and resources; to survive and be able to reproduce they have to minimize the costs and maximize the outcomes of their actions. These computations are at the root of behavioral flexibility.
Two executive functions play a major role in generating flexible behaviors: i) the ability to predict future outcomes of goal-directed actions; and ii) the ability to cancel them when they are unlikely to accomplish valuable results. These two processes operate continuously during the entire course of a movement: during its genesis, its planning and even its execution, so that the motor output can be modulated or suppressed at any time before its execution. In this review, functional interactions of the extended neural network subserving generation and inhibition of goal-directed movements will be outlined, leading to the intriguing hypothesis that the performance of actions and their suppression are not specified by independent sets of brain regions. Rather, it will be proposed that acting and stopping are functions emerging from specific interactions between largely overlapping brain regions, whose activity is intimately linked (directly or indirectly) to the evaluations of pros and cons of an action. Such mechanism would allow the brain to perform as a highly efficient and flexible system, as different functions could be computed exploiting the same components operating in different configurations.

Adaptive adjustments of strategies are needed to optimize behavior in a dynamic and
uncertain world. A key function in implementing flexible behavior and exerting selfcontrol
is represented by the ability to stop the execution of an action when it is no
longer appropriate for the environmental requests. Importantly, stimuli in our environment
are not equally relevant and some are more valuable than others. One example is
the gaze of other people, which is known to convey important social information
about their direction of attention and their emotional and mental states. Indeed, gaze
direction has a significant impact on the execution of voluntary saccades of an observer
since it is capable of inducing in the observer an automatic gaze-following behavior:
a phenomenon named social or joint attention. Nevertheless, people can exert volitional
inhibitory control on saccadic eye movements during their planning. Little is known about
the interaction between gaze direction signals and volitional inhibition of saccades. To fill
this gap, we administered a countermanding task to 15 healthy participants in which
they were asked to observe the eye region of a face with the eyes shut appearing at
central fixation. In one condition, participants were required to suppress a saccade, that
was previously instructed by a gaze shift toward one of two peripheral targets, when the
eyes were suddenly shut down (social condition, SC). In a second condition, participants
were asked to inhibit a saccade, that was previously instructed by a change in color of
one of the two same targets, when a change of color of a central picture occurred (nonsocial
condition, N-SC). We found that inhibitory control was more impaired in the SC,
suggesting that actions initiated and stopped by social cues conveyed by the eyes are
more difficult to withhold. This is probably due to the social value intrinsically linked to
these cues and the many uses we make of them

Reactive inhibition correlates with the severity of symptoms in paediatric patients with Obsessive-Compulsive Disorder (OCD) though not in those with Tourette syndrome (TS). Here we assessed whether structural alterations in both grey (GM) and white matter (WM) volumes correlate with a measure of reactive inhibition, i.e. the stop-signal reaction time (SSRT), and with clinical scale scores. Nine OCD and 11 TS uncomplicated drug-naïve paediatric patients and 12 age-matched controls underwent 3T magnetic resonance imaging scanning. Between-group differences in GM and WM volumes across the whole brain were assessed. Outside the scanner, patients performed a reaching version of the stop-signal task. Both behavioural inhibitory control and neuroimaging measures were normal in TS patients. By contrast, OCD patients exhibited a significant loss in GM volume in five areas. The GM volume of the left inferior frontal gyrus was inversely correlated with the length of the SSRT, the left mid-cingulate gyrus and the right middle frontal gyrus were inversely correlated with the severity of OCD symptoms, and the left insula and the right medial orbitofrontal gyrus were inversely correlated with both. These results indicate that cortical areas showing GM loss in OCD patients are also involved in the network subserving reactive inhibition.

Typically, the inability to control urges tends to be ascribed to a lack of inhibitory control. Primary complex motor stereotypes (p-CMS), occurring in children with an otherwise typical development, represent a remarkable example of involuntary, complex, repetitive and apparently purposeless movements. However, it has never been tested whether the core of the pathophysiology of p-CMS lies in a deficit of inhibitory control. To fill this gap, we assessed whether children with p-CMS exhibit an impairment of one or both types of inhibition, i.e. reactive inhibition (the ability of subjects to react to a stop-signal) and/or proactive inhibition (the ability of subjects to shape their response strategies according to the context in which subjects are embedded). We compared inhibitory control of 20 drug-naïve patients with p-CMS (mean age ±SD: 7.4±1.1) with that of 20 age- and gender-matched typically developing children (7.5±1.2) via a reaching version of the stop-signal task. We found that while reactive inhibition is significantly impaired, proactive control in children with p-CMS is similar to that of the control group. The deficit in reactive control might explain why patients are unable to inhibit involuntary movements when triggered by states of mind such as stress, fatigue, boredom or excitement. Nevertheless, the absence of a deficit in proactive control suggests that patients are aware of the environmental context and thus they quickly stop the stereotypic movements when their attention is diverted. All in all, our findings might explain two key features of the p-CMS phenotype.

The relationship between handedness, laterality, and inhibitory control is a valuable benchmark for testing the hypothesis of the right-hemispheric specialization of inhibition. According to this theory, and given that to stop a limb movement, it is sufficient to alter the activity of the contralateral hemisphere, then suppressing a left arm movement should be faster than suppressing a right-arm movement. This is because, in the latter case, inhibitory commands produced in the right hemisphere should be sent to the other hemisphere. Further, as lateralization of cognitive functions in left-handers is less pronounced than in right-handers, in the former, the inhibitory control should rely on both hemispheres. We tested these predictions on a medium-large sample of left- and right-handers (n = 52). Each participant completed two sessions of the reaching versions of the stop-signal task, one using the right arm and one using the left arm. We found that reactive and proactive inhibition do not differ according to handedness. However, we found a significant advantage of the right versus the left arm in canceling movements outright. By contrast, there were no differences in proactive inhibition. As we also found that participants performed movements faster with the right than with the left arm, we interpret our results in light of the dominant role of the left hemisphere in some aspects of motor control.

An experimental design is programmed using the presentation tool to investigate the global response inhibition process by quantifying the parameters such as inhibition efficiency, stop-signal delay (SSD) and stop-signal reaction time (SSRT) in the stop-signal paradigm. The aim of this study is to explore the response inhibition mechanisms in the left-hand and right-hand responses by using ERP and ERSP results obtained from the EEG data of different subjects. The inhibition efficiency of the right-hand response and left-hand response appears to be independent of each other as there is no significant difference between them. From these results, the inhibition mechanisms corresponding to these two regions of the brain may be viewed as statistically independent processes. Further, we inferred that the response inhibition mechanisms for both left-hand and right-hand responses have approximately the same spectral power observation analysis and we conclude that these processes are statistically independent of each other.