Abstract:

In daily life, we often encounter situations where we need to stop an ongoing response or suppress a response tendency. This process, termed response inhibition, has long been thought to occur at a response effector only when it interferes with the current task goal. However, a growing number of studies have shown that inhibition is associated with multiple response effectors, suggesting that global inhibition can occur across the entire motor system. In this review, we propose a new taxonomy of response inhibition based on the different response effectors of the motor system, and on the task contexts that engage inhibition. Specifically, we classify response effectors as task-interfering effectors, task-irrelevant effectors, and task-required effectors, where task contexts are classified as response-conflict tasks or non-conflict tasks. Based on this new taxonomy, we summarize evidence from recent studies showing inhibitory effects in the three classes of effectors for both conflict and non-conflict task contexts. The new taxonomy adds to the existing classification models of response inhibition by providing clearly defined boundaries between different kinds of inhibition.
Two alternative models proposed by recent studies are introduced to explain how the inhibition of multiple effectors is implemented in the brain. The dual process model proposes that the inhibitory processes at different effectors are controlled by different brain areas respectively, whereas the spot-light model proposes that they are controlled by a single system. We also examined the degree to which the spot-light model corresponds with the normalization model, which is a fundamental model of neural computation. The normalization model accounts for global inhibition of the motor system, and thus enhances its explanatory and predictive power of multiple cognitive functions such as visual selective attention, multisensory integration, and value-based decision-making. The collective evidence suggests that the normalization model is a common neural computational mechanism underlying a broad range of cognitive phenomena.
As empirical evidence regarding response control is lacking, in this review, we propose specific hypotheses that can be tested in future studies. One hypothesis is that the behavioral cost of inhibition is dependent on the competition strength between different response effectors. The other hypothesis is that the degree of inhibition of a specific response effector is dependent on the representational distance in the brain between this effector and the task-required response effector. In both hypotheses, the factors affecting the inhibitory effect are highly quantifiable and can be specified in the normalization model. In combination with the normalization model, the global inhibition framework can be tested using signals recorded at the molecular, cortical, and behavioral levels. Integrating evidence from multiple levels of neural processing may help to reveal the common mechanisms of response control.
The evidence reviewed here challenges the traditional view of response inhibition and demonstrates the scope of global inhibition within the motor system. The global inhibition framework provides an integrated explanation for the inhibitory effects of multiple effectors and multiple task contexts, and also offers a new perspective for treating response control as a coordinated and integrated process involving multiple response effectors. This new perspective highlights the cognitive flexibility and computational efficiency of the neural system, with significant applications in the clinical and industrial domains. Many mental conditions such as obsessive-compulsive disorder (OCD), attention-deficit hyperactivity disorder (ADHD), and addiction are characterized by deficits in motor inhibition. Dysfunctional global inhibition may be a sensitive biomarker that could be useful in diagnosing and predicting relapses of these mental conditions. Furthermore, according to the framework of global inhibition, the efficiency of response control is more likely to be improved by training schemes that focus on motor flexibility versus inhibition using a specific response effector. This inspires the industrial application to employ training schemes in more ecological situations rather than in highly controlled laboratory settings.

Key words: response control, global inhibition, motor system, cognitive flexibility