Nonselective motor-level changes associated with selective response inhibition: evidence from response force measurements

Proactive cues facilitate faster action reprogramming, but not stopping, in a response-selective stop signal task

The ability to stop simple ongoing actions has been extensively studied using the stop signal task, but less is known about inhibition in more complex scenarios. Here we used a task requiring bimanual responses to go stimuli, but selective inhibition of only one of those responses following a stop signal. We assessed how proactive cues affect the nature of both the responding and stopping processes, and the well-documented stopping delay (interference effect) in the continuing action following successful stopping. In this task, estimates of the speed of inhibition based on a simple-stopping model are inappropriate, and have produced inconsistent findings about the effects of proactive control on motor inhibition. We instead used a multi-modal approach, based on improved methods of detecting and interpreting partial electromyographical responses and the recently proposed SIS (simultaneously inhibit and start) model of selective stopping behaviour. Our results provide clear and converging evidence that proactive cues reduce the stopping delay effect by slowing bimanual responses and speeding unimanual responses, with a negligible effect on the speed of the stopping process.

Exploring stop signal reaction time over two sessions of the anticipatory response inhibition task

Various behavioural tasks measure response inhibition encompassing the ability to cancel unwanted actions, evaluated via stop signal reaction time (SSRT). It is unclear whether SSRT is an unchangeable inherent measure of inhibitory network integrity or whether it can improve with repetition. The current study explored if and how SSRT changed over two sessions for the Anticipatory Response Inhibition Task (ARIT), and how this compared with the Stop Signal Task (SST). Forty-four participants repeated the ARIT and SST over two sessions. SSRT and its constituent measures (Go trial reaction time, stop signal delay) were calculated. SSRT reflecting non-selective response inhibition was consistent between sessions in the ARIT and SST (both p > 0.293). Reaction time and stop signal delay also remained stable across sessions in the ARIT (all p > 0.063), whereas in the SST, reaction time (p = 0.013) and stop signal delay (p = 0.009) increased. SSRT reflecting behaviourally selective stopping on the ARIT improved (p < 0.001) over two sessions, which was underpinned by changes to reaction time (p < 0.001) and stop signal delay (p < 0.001). Overall, the maximal efficiency of non-selective inhibition remained stable across two sessions in the ARIT. Results of the SST confirmed that non-selective inhibition can, however, be affected by more than inhibitory network integrity. Behaviourally selective stopping on the ARIT changed across sessions, suggesting the sequential neural process captured by the SSRT occurred more quickly in session two. These findings have implications for future studies that necessitate behavioural measures over multiple sessions.

The role of interhemispheric communication during complete and partial cancellation of bimanual responses

Precise control of upper limb movements in response to external stimuli is vital to effectively interact with the environment. Accurate execution of bimanual movement is known to rely on finely orchestrated interhemispheric communication between the primary motor cortices (M1s). However, relatively little is known about the role of interhemispheric communication during sudden cancellation of prepared bimanual movement. The current study investigated the role of interhemispheric interactions during complete and partial cancellation of bimanual movement. In two experiments, healthy young human participants received transcranial magnetic stimulation to both M1s during a bimanual response inhibition task. The increased corticomotor excitability in anticipation of bimanual movement was accompanied by a release of inhibition from both M1s. After a stop cue, inhibition was reengaged onto both hemispheres to successfully cancel the complete bimanual response. However, when the stop cue signaled partial cancellation (stopping of one digit only), inhibition was reengaged with regard to the cancelled digit, but the responding digit representation was facilitated. This bifurcation in interhemispheric communication between M1s occurred 75 ms later in the more difficult condition when the nondominant, as opposed to dominant, hand was still responding. Our results demonstrate that interhemispheric communication is integral to response inhibition once a bimanual response has been prepared. Interestingly, M1-M1 interhemispheric circuitry does not appear to be responsible for the nonselective suppression of all movement components that has been observed during partial cancellation. Instead such interhemispheric communication enables uncoupling of bimanual response components and facilitates the selective initiation of just the required unimanual movement.NEW & NOTEWORTHY We provide the first evidence that interhemispheric communication plays an important role during sudden movement cancellation of two-handed responses. Simultaneously increased inhibition onto both hemispheres assists with two-handed movement cancellation. However, this network is not responsible for the widespread suppression of motor activity observed when only one of the two hands is cancelled. Instead, communication between hemispheres enables the separation of motor activity for the two hands and helps to execute the required one-handed response.

Classification of Movement and Inhibition Using a Hybrid BCI

Brain-computer interfaces (BCIs) are an emerging technology that are capable of turning brain electrical activity into commands for an external device. Motor imagery (MI)—when a person imagines a motion without executing it—is widely employed in BCI devices for motor control because of the endogenous origin of its neural control mechanisms, and the similarity in brain activation to actual movements. Challenges with translating a MI-BCI into a practical device used outside laboratories include the extensive training required, often due to poor user engagement and visual feedback response delays; poor user flexibility/freedom to time the execution/inhibition of their movements, and to control the movement type (right arm vs. left leg) and characteristics (reaching vs. grabbing); and high false positive rates of motion control. Solutions to improve sensorimotor activation and user performance of MI-BCIs have been explored. Virtual reality (VR) motor-execution tasks have replaced simpler visual feedback (smiling faces, arrows) and have solved this problem to an extent. Hybrid BCIs (hBCIs) implementing an additional control signal to MI have improved user control capabilities to a limited extent. These hBCIs either fail to allow the patients to gain asynchronous control of their movements, or have a high false positive rate. We propose an immersive VR environment which provides visual feedback that is both engaging and immediate, but also uniquely engages a different cognitive process in the patient that generates event-related potentials (ERPs). These ERPs provide a key executive function for the users to execute/inhibit movements. Additionally, we propose signal processing strategies and machine learning algorithms to move BCIs toward developing long-term signal stability in patients with distinctive brain signals and capabilities to control motor signals. The hBCI itself and the VR environment we propose would help to move BCI technology outside laboratory environments for motor rehabilitation in hospitals, and potentially for controlling a prosthetic.

Cortical control and performance monitoring of interrupting and redirecting movements

Voluntary behaviour requires control mechanisms that ensure our ability to act independently of habitual and innate response tendencies. Electrophysiological experiments, using the stop-signal task in humans, monkeys and rats, have uncovered a core network of brain structures that is essential for response inhibition. This network is shared across mammals and seems to be conserved throughout their evolution. Recently, new research building on these earlier findings has started to investigate the interaction between response inhibition and other control mechanisms in the brain. Here we describe recent progress in three different areas: selectivity of movement inhibition across different motor systems, re-orientation of motor actions and action evaluation.This article is part of the themed issue 'Movement suppression: brain mechanisms for stopping and stillness'.

An Activation Threshold Model for Response Inhibition

Reactive response inhibition (RI) is the cancellation of a prepared response when it is no longer appropriate. Selectivity of RI can be examined by cueing the cancellation of one component of a prepared multi-component response. This substantially delays execution of other components. There is debate regarding whether this response delay is due to a selective neural mechanism. Here we propose a computational activation threshold model (ATM) and test it against a classical "horse-race" model using behavioural and neurophysiological data from partial RI experiments. The models comprise both facilitatory and inhibitory processes that compete upstream of motor output regions. Summary statistics (means and standard deviations) of predicted muscular and neurophysiological data were fit in both models to equivalent experimental measures by minimizing a Pearson Chi-square statistic. The ATM best captured behavioural and neurophysiological dynamics of partial RI. The ATM demonstrated that the observed modulation of corticomotor excitability during partial RI can be explained by nonselective inhibition of the prepared response. The inhibition raised the activation threshold to a level that could not be reached by the original response. This was necessarily followed by an additional phase of facilitation representing a secondary activation process in order to reach the new inhibition threshold and initiate the executed component of the response. The ATM offers a mechanistic description of the neural events underlying RI, in which partial movement cancellation results from a nonselective inhibitory event followed by subsequent initiation of a new response. The ATM provides a framework for considering and exploring the neuroanatomical constraints that underlie RI.

Does response inhibition have pre- and postdiagnostic utility in Parkinson's disease?

Parkinson's disease (Pd) is the second most prevalent degenerative neurological condition worldwide. Improving and sustaining quality of life is an important goal for Parkinson's patients. Key areas of focus to achieve this goal include earlier diagnosis and individualized treatment. In this review the authors discuss impulse control in Pd and examine how measures of impulse control from a response inhibition task may provide clinically useful information (a) within an objective test battery to aid earlier diagnosis of Pd and (b) in postdiagnostic Pd, to better identify individuals at risk of developing impulse control disorders with dopaminergic medication.

Effect of foreknowledge on neural activity of primary "go" responses relates to response stopping and switching

Being able to stop (or inhibit) an action rapidly as in a stop-signal task (SST) is an essential human ability. Previous studies showed that when a pre-stimulus cue warned of the possible need to stop a response in an upcoming trial, participants' response time (RT) increased if the subsequent trial required a "go" response (i.e., "go" RT cost) relative to a trial where this uncertainty was not present. This increase of the "go" RT correlated with more efficient response stopping. However, it remains a question whether foreknowledge of upcoming inhibition trials given prior to the task is sufficient to modulate neural activity associated with the primary "go" responses irrespective of whether stopping an overt response is required. We presented three task conditions with identical primary (i.e., "go") response trials but without pre-stimulus cues. Participants were informed that Condition 1 had only "go" trials (All-go condition), Condition 2 required a "stop" response for some trials (Stop condition), and Condition 3 required a response incongruent with the primary response (i.e., Switch response) for some trials (Switch condition). Participants performed the tasks during functional magnetic resonance imaging (fMRI) scans. Results showed a significant increase in the "go" RT (cost) in the Stop and Switch conditions relative to the All-go condition. The "go" RT cost was correlated with decreased inhibition time. fMRI activation in the frontal-basal-ganglia regions during the "go" responses in the Stop and Switch conditions was also correlated with the efficiency of Stop and Switch responses. These results suggest that foreknowledge prior to the task is sufficient to influence neural activity associated with the primary response and modulate inhibition efficiency, irrespective of whether stopping an overt response is required.

The fall and rise of corticomotor excitability with cancellation and reinitiation of prepared action

The sudden cancellation of a motor action, known as response inhibition (RI), is fundamental to human motor behavior. The behavioral selectivity of RI can be studied by cueing cancellation of only a subset of a planned response, which markedly delays the remaining executed components. The present study examined neurophysiological mechanisms that may contribute to these delays. In two experiments, human participants received single- and paired-pulse transcranial magnetic stimulation while performing a bimanual anticipatory response task. Participants performed most trials bimanually (Go trials) and were sometimes cued to cancel the response with one hand while responding with the other (Partial trials). Motor evoked potentials were recorded from left first dorsal interosseous (FDI) as a measure of corticomotor excitability (CME) during Go and Partial trials. CME was temporally modulated during Partial trials in a manner that reflected anticipation, suppression, and subsequent initiation of a reprogrammed response. There was an initial increase in CME, followed by suppression 175 ms after the stop signal, even though the left hand was not cued to stop. A second increase in excitability occurred prior to the (delayed) response. We propose an activation threshold model to account for nonselective RI. To investigate the inhibitory component of our model, we investigated short-latency intracortical inhibition (sICI), but results indicated that sICI cannot fully explain the observed temporal modulation of CME. These neurophysiological and behavioural results indicate that the default mode for reactive partial cancellation is suppression of a unitary response, followed by response reinitiation with an inevitable time delay.

Voluntarily-generated unimanual preparation is associated with stopping success: evidence from LRP and lateralized mu ERD before the stop signal

According to the race models of the stop-signal paradigm, stopping success (successful vs. unsuccessful stopping) is attributed to the finishing times of a go and a stop process. In addition to those factors involving processing times, in the present study we sought to use electrophysiological measures to find factors involving activations that could affect stopping success. We hypothesized that voluntarily-generated unimanual preparation would be a factor. To assess voluntarily-generated unimanual preparation in the stop-signal paradigm, we used a selective-stopping task without any precue. The selective-stopping task also allowed us to assess reaction times (RTs) even when stopping was successful. We demonstrated shorter RTs in signal-respond (i.e., unsuccessful stopping) than in signal-inhibit (successful stopping) trials, as is predicted by the race models. More importantly, we also demonstrated different pre-signal lateralized readiness potentials between the two types of trials and larger lateralized mu ERD in signal-respond than in signal-inhibit trials, suggesting that voluntarily-generated unimanual preparation affects stopping success. In addition to what is described in the race models of the stop-signal paradigm, the present results therefore demonstrated measures of pre-signal activations that could influence stopping success.

Signal-related contributions to stopping-interference effects in selective response inhibition

In our ability to selectively inhibit a subset of concurrent response tendencies, referred to as selective response inhibition, stopping-interference (SI) effects have been found and attributed to global inhibitory processes. In the standard stop-signal paradigm, the stop signal might not only signal stopping but also produce other effects simply by virtue of being an additional signal. Therefore, we investigated whether previously observed SI effects reflect not only selective response inhibition but also other effects caused by the appearance of the stop signal. In Experiment 1, we controlled for the possible extra influences of the stop signal and still found SI effects, allowing a more confident attribution of SI effects to global inhibitory processes. Furthermore, the extra signal affected the motor system, as revealed by a reduction in SI effects on response force after the improved control. Using the lateralized readiness potential, Experiment 2 showed that the extra signal affected relatively central motor processing. The findings thus advance our knowledge about the distinction between signal-related and motor-inhibitory effects in stop-signal tasks.

Current advances and pressing problems in studies of stopping

The stop-signal task probes agents' ability to inhibit responding. A well-known race model affords estimation of the duration of the inhibition process. This powerful approach has yielded numerous insights into the neural circuitry underlying response control, the specificity of inhibition across effectors and response strategies, and executive processes such as performance monitoring. Translational research between human and non-human primates has been particularly useful in this venture. Continued progress with the stop-signal paradigm is contingent upon appreciating the dynamics of entire cortical and subcortical neural circuits and obtaining neurophysiological data from each node in the circuit. Progress can also be anticipated on extensions of the race model to account for selective stopping; we expect this will entail embedding behavioral inhibition in the broader context of executive control.

Uncoupling response inhibition

The ability to prevent unwanted movement is fundamental to human behavior. When healthy adults must prevent a subset of prepared actions, execution of the remaining response is markedly delayed. We hypothesized that the delay may be sensitive to the degree of similarity between the prevented and continued actions. Fifteen healthy participants performed an anticipatory response inhibition task that required bilateral index finger extension or thumb abduction with homogeneous digit pairings, or a heterogeneous pairing of a combination of the two movements. We expected that the uncoupling of responses required for selective movement prevention would be more difficult with homogeneous (same digit, homologous muscles) than heterogeneous pairings (different digits, nonhomologous muscles). Measures of response times (and asynchrony between digits) during action execution, stopping performance, and electromyography from EIP (index finger extension) and APB (thumb abduction) were analyzed. As expected, selective trials produced a delay in the remaining movement compared with execution trials. Successful performance in the selective condition occurred via suppression of the entire prepared response and subsequent selective reinitiation of the remaining component. Importantly, the delayed reinitiation of motor output was sensitive to the degree of similarity between responses, occurring later but at a faster rate with homogeneous digits. There were persistent aftereffects from the selective condition on the motor system, which indicated greater levels of inhibition and a higher gain were necessary to successfully perform selective trials with homogeneous pairings. Overall, the results support a model of inhibition of a unitary response and selective reinitiation, rather than selective inhibition.