Response inhibition during perceptual decision making in humans and macaques

Response inhibition in stop signal tasks has been explained as the outcome of a race between GO and STOP processes (e.g., Logan, 1981). Response choice in two-alternative perceptual categorization tasks has been explained as the outcome of an accumulation of evidence for the alternative responses. To begin unifying these two powerful investigation frameworks, we obtained data from humans and macaque monkeys performing a stop signal task with responses guided by perceptual categorization and variable degrees of difficulty, ranging from low to high accuracy. Comparable results across species reinforced the validity of this animal model. Response times and errors increased with categorization difficulty. The probability of failing to inhibit responses on stop signal trials increased with stop signal delay, and the response times for failed stop signal trials were shorter than those for trials with no stop signal. Thus, the Logan race model could be applied to estimate the duration of the stopping process. We found that the duration of the STOP process did not vary across a wide range of discrimination accuracies. This is consistent with the functional, and possibly mechanistic, independence of choice and inhibition mechanisms.     

Stopping ability in younger and older adults: Behavioral and event-related potential

This study examines age-related differences in inhibitory control as measured by stop-signal performance. The participants were 24 adults aged 20–30 years and 24 older adults aged 61–76 years. The task blocks were pure choice reaction-time blocks, global stop-signal blocks (with an auditory stop signal), and selective stop-signal blocks (with valid and invalid stop signals). There was a decline in reactive inhibitory control for the older group reflected by greater stop-signal reaction times and reduced P3 peak amplitudes in both global and selective stop-signal task blocks. The decreased reactive inhibitory control might result from speed-accuracy tradeoffs. Conversely, no age-related decline in proactive inhibitory control was observed. This was reflected by slower response times (RTs) and reduced P3 peak amplitudes during GO trials in blocks with stop-signals relative to those in blocks of pure choice reaction-time tasks, and in which the RT and amplitude differences were similar between groups. The results further show age-related compensation responses associated with proactive inhibition, such as increased activation at the frontal site among older participants, resulting in no differences in P3 peak amplitudes between electrode sites, and smaller differences at the Fz site than other sites compared with younger adults. For older adults, the P3 peak amplitude at the Fz site was significantly correlated with the RT of proactive inhibitory control. This shows that larger RT differences were associated with larger reductions in P3 peak amplitudes in the stop-signal blocks relative to the pure choice blocks. These results appear to support age-related compensation hypotheses.     

Nonindependent and nonstationary response times in stopping and stepping saccade tasks

Saccade stop signal and target step tasks are used to investigate the mechanisms of cognitive control. Performance of these tasks can be explained as the outcome of a race between stochastic go and stop processes. The race model analyses assume that response times (RTs) measured throughout an experimental session are independent samples from stationary stochastic processes. This article demonstrates that RTs are neither independent nor stationary for humans and monkeys performing saccade stopping and target-step tasks. We investigate the consequences that this has on analyses of these data. Nonindependent and nonstationary RTs artificially flatten inhibition functions and account for some of the systematic differences in RTs following different types of trials. However, nonindependent and nonstationary RTs do not bias the estimation of the stop signal RT. These results demonstrate the robustness of the race model to some aspects of nonindependence and nonstationarity and point to useful extensions of the model.     

Cancelling discrete and stopping ongoing rhythmic movements: Do they involve the same process of motor inhibition?

Motor inhibition is considered to be an important process of executive control and to be implicated in numerous activities in order to cancel prepared actions and, supposedly, to suppress ongoing ones. Usually, it is evaluated using a “stop-signal task” in which participants have to inhibit prepared discrete movements. However, it is unknown whether other movement types involve the same inhibition process. We therefore investigated whether the inhibition process for discrete movements is involved in stopping ongoing rhythmic movements as well.Twenty healthy adults performed two counterbalanced tasks. The first task was used to estimate the stop-signal reaction time (SSRTd) needed to inhibit prepared discrete key-pressing movements. In the second task, participants drew graphic patterns on a tablet and had to stop the movement when a stop-signal occurred. We calculated the rhythmic stop signal-reaction time as the time needed to initiate stopping such ongoing rhythmic movement (SSRTr) and the same latency relative to the period of the rhythmic movement (relSSRTr). We measured these delays under different movement frequencies and motor coordination conditions and further investigated whether they varied as a function of several parameters of the rhythmic movements (speed, mean and variance of the relative phase, and movement phase at several time events).We found no correlation between inhibition measures in the two tasks. In contrast, generalized linear models showed a moderate yet significant influence of the motion parameters on the inhibition of ongoing rhythmic movements. We therefore conclude that the motor inhibition processes involved in cancelling prepared discrete movements and stopping ongoing rhythmic movements are dissimilar.     

Effects of stop-signal probability in the stop-signal paradigm: the N2/P3 complex further validated

The aim of this study was to examine the effects of frequency of occurrence of stop signals in the stop-signal paradigm. Presenting stop signals less frequently resulted in faster reaction times to the go stimulus and a lower probability of inhibition. Also, go stimuli elicited larger and somewhat earlier P3 responses when stop signals occurred less frequently. Since the amplitude effect was more pronounced on trials when go signals were followed by fast than slow reactions, it probably reflected a stronger set to produce fast responses. N2 and P3 components to stop signals were observed to be larger and of longer latency when stop signals occurred less frequently. The amplitude enhancement of these N2 and P3 components were more pronounced for unsuccessful than for successful stop-signal trials. Moreover, the successfully inhibited stop trials elicited a frontocentral P3 whereas unsuccessfully inhibited stop trials elicited a more posterior P3 that resembled the classical P3b. P3 amplitude in the unsuccessfully inhibited condition also differed between waveforms synchronized with the stop signal and waveforms synchronized with response onset whereas N2 amplitude did not. Taken together these findings suggest that N2 reflected a greater significance of failed inhibitions after low probability stop signals while P3 reflected continued processing of the erroneous response after response execution.     

Motor response inhibition and execution in the stop-signal task: development and relation to ADHD behaviors.

The main aim of this study was to investigate the developmental course of motor response inhibition and execution as measured by the stop-signal task in a population-based sample of 525 4- to 12-year-olds. A further aspiration of the study was to enhance the limited knowledge on how the various stop-signal measures relate to ADHD behaviors in a normal sample. We also wanted to contribute to the theoretical understanding of the various stop-signal measures by examining the relations between the stop-signal measures and performance on tasks reflecting other aspects of response inhibition and execution. Our results showed that the ability to inhibit as well as to execute a motor response as measured by the stop-signal task improved with age during childhood. Of specific interest are the findings suggesting that this task captures the development of motor response inhibition in the late preschool years (age 5 years). Both of the inhibition measures derived from the stop-signal task (i.e., SSRT and probability of inhibition) related significantly to teacher ratings of inattention as well as to performance on tasks tapping other aspects of inhibition. The data provided by this study have thus contributed to the scarce knowledge on early development of motor response inhibition, as well as suggested that the stop-signal task may be a valuable tool for capturing deficient motor response inhibition in ADHD behaviors in normal samples.     

IMPULSIVITY AND INHIBITORY CONTROL

Abstract— We report an experiment testing the hypothesis that impulsive behavior reflects a deficit in the ability to inhibit prepotent responses Specifically, we examined whether impulsive people respond more slowly to signals to inhibit (stop signals) than non-impulsive people In this experiment, 136 undergraduate students completed an impulsivity questionnaire and then participated in a stop-signal experiment, in which they performed a choice reaction time (go) task and were asked to inhibit their responses to the go task when they heard a stop signal The delay between the go signal and the stop signal was determined by a tracking procedure designed to allow subjects to inhibit on 50% of the stop-signal trials. Reaction time to the go signal did not vary with impulsivity, but estimated stop-signal reaction time was longer in more impulsive subjects, consistent with the hypothesis and consistent with results from populations with pathological problems with impulse control.     

Proactive inhibitory control: A general biasing account

Flexible behavior requires a control system that can inhibit actions in response to changes in the environment. Recent studies suggest that people proactively adjust response parameters in anticipation of a stop signal. In three experiments, we tested the hypothesis that proactive inhibitory control involves adjusting both attentional and response settings, and we explored the relationship with other forms of proactive and anticipatory control. Subjects responded to the color of a stimulus. On some trials, an extra signal occurred. The response to this signal depended on the task context subjects were in: in the ‘ignore’ context, they ignored it; in the ‘stop’ context, they had to withhold their response; and in the ‘double-response’ context, they had to execute a secondary response. An analysis of event-related brain potentials for no-signal trials in the stop context revealed that proactive inhibitory control works by biasing the settings of lower-level systems that are involved in stimulus detection, action selection, and action execution. Furthermore, subjects made similar adjustments in the double-response and stop-signal contexts, indicating an overlap between various forms of proactive action control. The results of Experiment 1 also suggest an overlap between proactive inhibitory control and preparatory control in task-switching studies: both require reconfiguration of task-set parameters to bias or alter subordinate processes. We conclude that much of the top-down control in response inhibition tasks takes place before the inhibition signal is presented.     

Post-stop-signal adjustments: inhibition improves subsequent inhibition

Performance in the stop-signal paradigm involves a balance between going and stopping, and one way that this balance is struck is through shifting priority away from the go task, slowing responses after a stop signal, and improving the probability of inhibition. In 6 experiments, the authors tested whether there is a corresponding shift in priority toward the stop task, speeding reaction time to the stop signal. Consistent with this hypothesis, stop-signal reaction time (SSRT) decreased on the trial immediately following a stop signal in each experiment. Experiments 2-4 used 2 very different stop signals within a modality, and stopping improved when the stop stimulus repeated and alternated. Experiments 5 and 6 presented stop signals in different modalities and showed that SSRT improved only when the stop stimulus repeated within a modality. These results demonstrate within-modality post-stop-signal speeding of response inhibition.     

Failures of cognitive control or attention? The case of stop-signal deficits in schizophrenia

We used Bayesian cognitive modelling to identify the underlying causes of apparent inhibitory deficits in the stop-signal paradigm. The analysis was applied to stop-signal data reported by Badcock et al. (Psychological Medicine 32: 87-297, 2002) and Hughes et al. (Biological Psychology 89: 220-231, 2012), where schizophrenia patients and control participants made rapid choice responses, but on some trials were signalled to stop their ongoing response. Previous research has assumed an inhibitory deficit in schizophrenia, because estimates of the mean time taken to react to the stop signal are longer in patients than controls. We showed that these longer estimates are partly due to failing to react to the stop signal (“trigger failures”) and partly due to a slower initiation of inhibition, implicating a failure of attention rather than a deficit in the inhibitory process itself. Correlations between the probability of trigger failures and event-related potentials reported by Hughes et al. are interpreted as supporting the attentional account of inhibitory deficits. Our results, and those of Matzke et al. (2016), who report that controls also display a substantial although lower trigger-failure rate, indicate that attentional factors need to be taken into account when interpreting results from the stop-signal paradigm.     

Inhibitory effects on response force in the stop-signal paradigm

The forcefulness of key press responses was measured in stop-all and selective stopping versions of the stop-signal paradigm. When stop signals were presented too late for participants to succeed in stopping their responses, response force was nonetheless reduced relative to trials in which no stop signal was presented. This effect shows that peripheral motor aspects of primary task responses can still be influenced by inhibition even when the stop signal arrives too late to prevent the response. It thus requires modification of race models in which responses in the presence of stop signals are either stopped completely or produced normally, depending on whether the responding or stopping process finishes first.     

The auditory-evoked N2 and P3 components in the stop-signal task: indices of inhibition, response-conflict or error-detection?

The N2 and P3 components have been separately associated with response inhibition in the stop-signal task, and more recently, the N2 has been implicated in the detection of response-conflict. To isolate response inhibition activity from early sensory processing, the present study compared processing of the stop-signal with that of a task-irrelevant tone, which subjects were instructed to ignore. Stop-signals elicited a larger N2 on failed-stop trials and a larger P3 on successful-stop trials, relative to ignore-signal trials, likely reflecting activity related to failed and successful stopping, respectively. ERPs between fast and slow reaction-time (RT) groups were also examined as it was hypothesised that greater inhibitory activation to stop faster responses would manifest in the component reflecting this process. Successful-stop P3 showed the anticipated effect (globally larger amplitude in the fast than slow RT group), supporting its association with the stopping of an ongoing response. In contrast, N2 was larger in the slow than fast RT group, and in contrast to the predictions of the response-conflict hypothesis, successful-stop N2 and the response-locked error-negativity (Ne) differed in scalp distribution. These findings indicate that the successful-stop N2 may be better explained as a deliberate form of response control or selection, which the slow RT group employed as a means of increasing the likelihood of a successful-stop. Finally, a comparison of stimulus and response-locked ERPs revealed that the failed-stop N2 and P3 appeared to reflect error-related activity, best observed in the response-locked Ne and error-positivity (Pe). Together these findings indicate that the successful-stop N2 and P3 reflect functionally distinct aspects of response control that are dependent upon performance strategies, while failed-stop N2 and P3 reflect error-related activity.     

Stopping,goal-conflict,trait anxiety and frontal rhythmic power in the stop-signal task

The medial right frontal cortex is implicated in fast stopping of an initiated motor action in the stop-signal task (SST). To assess whether this region is also involved in the slower behavioural inhibition induced by goal conflict, we tested for effects of goal conflict (when stop and go tendencies are balanced) on low-frequency rhythms in the SST. Stop trials were divided, according to the delays at which the stop signal occurred, into short-, intermediate-, and long-delay trials. Consistent with goal-conflict processing, intermediate-delay trials were associated with greater 7–8 Hz EEG power than short- or long-delay trials at medial right frontal sites (Fz, F4, and F8). At F8, 7–8 Hz power was linked to high trait anxiety and neuroticism. A separate 4–7 Hz power increase was also seen in stop, relative to go, trials, but this was independent of delay, was maximal at the central midline site Cz, and predicted faster stopping. Together with previous data on the SST, these results suggest that the right frontal region could be involved in multiple inhibition mechanisms. We propose a hierarchical model of the control of stopping that integrates the literature on the neural control of fast motor stopping with that on slower, motive-directed behavioural inhibition.     

The time course effect of moderate intensity exercise on response execution and response inhibition

This research aimed to investigate the time course effect of a moderate steady-state exercise session on response execution and response inhibition using a stop-task paradigm. Ten participants performed a stop-signal task whilst cycling at a carefully controlled workload intensity (40% of maximal aerobic power), immediately following exercise and 30 min after exercise cessation. Results showed that moderate exercise enhances a subjects’ ability to execute responses under time pressure (shorter Go reaction time, RT without a change in accuracy) but also enhances a subjects’ ability to withhold ongoing motor responses (shorter stop-signal RT). The present outcomes reveal that the beneficial effect of exercise is neither limited to motor response tasks, nor to cognitive tasks performed during exercise. Beneficial effects of exercise remain present on both response execution and response inhibition performance for up to 52 min after exercise cessation.     

Redundancy gain in the stop-signal paradigm: implications for the locus of coactivation in simple reaction time

The authors carried out 2 experiments designed to cast light on the locus of redundancy gain in simple visual reaction time by using a stop-signal paradigm. In Experiment 1, the authors found that single visual stimuli were more easily inhibited than double visual stimuli by an acoustic stop signal. This result is in keeping with the idea that redundancy gain occurs prior to the ballistic stage of the stop-signal task. In Experiment 2, the authors found that the response to an acoustic go signal was more easily inhibited by a double than by a single visual stop signal. This result provides conclusive evidence for a redundancy gain in the stop process--in a process that does not involve a motor response but rather its inhibition.     

Postural and focal inhibition of voluntary movements prepared under various temporal constraints

Large disturbances arising from the moving segments (focal movement) are commonly counteracted by anticipatory postural adjustments (APAs). The aim of this study was to investigate how APAs – focal movement coordination changes under temporal constraint. Ten subjects were instructed to perform an arm raising movement in the reactive (simple reaction time) and predictive (anticipation–coincidence) tasks. A stop paradigm was applied to reveal the coordination. On some unexpected trials, a stop signal indicated to inhibit the movement; it occurred randomly at different delays (SOA) relative to the go signal in the reactive task, and at different delays prior to the focal response initiation in the predictive task. Focal movement was measured using contact switch, accelerometer and EMG from the anterior deltoid. APAs were quantified using centre of pressure displacement and EMG from three postural muscles. The inhibition rates as a function of the SOA produce psychometric functions where the bi-serial points allow the moment of the motor "command release" to be estimated. Repeated measures ANOVAs showed that APAs and focal movement were closely timed in the reactive task but distinct in a predictive task. Data were discussed according to two different models of coordination: (1) hierarchical model where APAs and focal movement are the results of a single motor command; (2) parallel model implying two independent motor commands. The data clearly favor the parallel model when the temporal constraint is low. The stop paradigm appears as a promising technique to explore APAs – focal movement coordination.     

Directed forgetting shares mechanisms with attentional withdrawal but not with stop-signal inhibition

To explore the mechanisms underlying the ability to intentionally forget, the present study combined an itemmethod directed forgetting paradigm with tasks that measure stop-signal inhibition (Experiments 1 and 2) and inhibition of return (IOR; Experiment 2). Following each study-phase instruction to remember (R) or forget (F), a target was presented centrally (Experiment 1) or to the left or right in the visual periphery (Experiment 2); the target required a speeded response that was sometimes countermanded by a central stop signal. Although stopsignal reaction times were unaffected by the preceding memory instruction (or relationship with word-target location), F instructions improved stopping and delayed responses. Replicating previous findings in the literature, significant IOR was observed following F instructions but not following R instructions (Experiment 2). These findings suggest that intentional forgetting is an active cognitive process that more likely engages attentional mechanisms related to orienting than those related to stop-signal inhibition.     

Inhibiting responses when switching: Does it matter?

In the present study, cued task-switching was combined with the stop-signal paradigm in order to investigate the interaction between response inhibition and task-switching. In line with earlier findings from Schuch and Koch (2003), the results show that switch and repetition trials following inhibited responses were processed equally fast. This confirms the hypothesis of Schuch and Koch (2003) that after signal-inhibit trials there is less interference, resulting in a disappearance of the switch cost. Furthermore, stopping performance was not affected by task-switching. The estimated stop-signal latencies were similar for switch and repetition trials, while the stop-signal delays were longer for switch compared to repetition trials. This result suggests that response inhibition and the inhibition processes in cued task-switching are not relying upon a common mechanism.     

Horse-race model simulations of the stop-signal procedure

In the stop-signal paradigm, subjects perform a standard two-choice reaction task in which, occasionally and unpredictably, a stop-signal is presented requiring the inhibition of the response to the choice signal. The stop-signal paradigm has been successfully applied to assess the ability to inhibit under a wide range of experimental conditions and in various populations. The current study presents a set of evidence-based guidelines for using the stop-signal paradigm. The evidence was derived from a series of simulations aimed at (a) examining the effects of experimental design features on inhibition indices, and (b) testing the assumptions of the horse-race model that underlies the stop-signal paradigm. The simulations indicate that, under most conditions, the latency, but not variability, of response inhibition can be reliably estimated.     

The duration of response inhibition in the stop-signal paradigm varies with response force

In a previous study, we have found that the speed of stopping a response is delayed when response readiness is reduced by cuing the probability of no-go trials [Acta Psychol. 111 (2002) 155]. Other investigators observed that responses are more forceful when the probability to respond is low than when it is high (e.g. [Quart. J. Exp. Psychol. A 50 (1997) 405]). In this study, the hypothesis was tested that low probability responses are more forceful than high probability responses and that these responses are more difficult to stop. Subjects performed on a choice reaction task and on three tasks with respectively 100%, 80%, and 50% response probabilities. Stop signals were presented on 30% of the trials, instructing subjects to withhold their response. Response force on non-signal (go) trials and the duration of response inhibition on signal (stop) trials increased as response probability decreased. This pattern of findings was interpreted to support the hypothesis predicting that stopping is more difficult when response readiness is low than when it is high.