Many situations in our everyday life call for a mechanism deputed to outright stop an ongoing course of action. This behavioral inhibition ability, known as response stopping, is often impaired in psychiatric conditions characterized by impulsivity and poor inhibitory control. Transcranial direct current stimulation (tDCS) has recently been proposed as a tool for modulating response stopping in such clinical populations, and previous studies in healthy humans have already shown that this noninvasive brain stimulation technique is effectively able to improve response stopping, as measured in a stop-signal task (SST) administered immediately after the stimulation. So far, the right inferior frontal gyrus (rIFG) has been the main focus of these attempts to modulate response stopping by the means of noninvasive brain stimulation. However, other cortical areas such as the right dorsolateral prefrontal cortex (rDLPFC) have been implicated in inhibitory control with other paradigms. In order to provide new insight about the involvement of these areas in response stopping, in the present study, tDCS was delivered to 115 healthy subjects, using five stimulation setups that differed in terms of target area (rIFG or rDLPFC) and polarity of stimulation (anodal, cathodal, or sham). The SST was performed 15 min after the offset of the stimulation. Consistently with previous studies, only anodal stimulation over rIFG induced a reliable, although weak, improvement in the SST, which was specific for response stopping, as it was not mirrored in more general reaction time measures.

Retrieving information from long-term memory can result in the episodic forgetting of related material. One influential account states that this retrieval-induced forgetting (RIF) phenomenon reflects inhibitory mechanisms called into play to decrease retrieval competition. Recent neuroimaging studies suggested that the prefrontal cortex, which is critically engaged in inhibitory processing, is also involved in retrieval competition situations. Here, we used transcranial direct current stimulation (tDCS) to address whether inhibitory processes could be causally linked to RIF. tDCS was administered over the right dorsolateral prefrontal cortex during the retrievalpractice phase in a standard retrieval-practice paradigm. Sixty human participants were randomly assigned to anodal, cathodal, or sham-control groups. The groups showed comparable benefits for practiced items. In contrast, unlike both the sham and anodal groups, the cathodal group exhibited no RIF. This pattern is interpreted as evidence for a causal role of inhibitory mechanisms in episodic retrieval and forgetting.

Non-invasive brain stimulation (NIBS) techniques are used in clinical and cognitive neuroscience to induce a mild magnetic or electric field in the brain to modulate behavior and cortical activation. Despite the great body of literature demonstrating promising results, unexpected or even paradoxical outcomes are sometimes observed. This might be due either to technical and methodological issues (e.g., stimulation parameters, stimulated brain area), or to participants’ expectations and beliefs before and during the stimulation sessions. In this narrative review, we present some studies showing that placebo and nocebo effects, associated with positive and negative expectations, respectively, could be present in NIBS trials, both in experimental and in clinical settings. The lack of systematic evaluation of subjective expectations and beliefs before and after stimulation could represent a caveat that overshadows the potential contribution of placebo and nocebo effects in the outcome of NIBS trials.

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