Abstract
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.
Highlights
Among tES, transcranial direct current stimulation (tDCS) is the most widely used and studied and, this review focuses on tDCS only. tDCS induces a subthreshold polarization of cortical neurons and acts by changing neuronal excitability, by inducing modifications in the resting membrane potentials, and in the postsynaptic activity of the stimulated neurons, without directly affecting action potentials. tDCS can alter the spontaneous firing rate, leading to changes in synaptic activity [12–15]
We propose that participants’ expectations present before or during the stimulation session could have a role in shaping the effect of Non-invasive brain stimulation (NIBS) on the brain and on behavior, partially accounting for the inconsistencies found in the literature
Recent work by Flanagan et al [29] investigated whether active intermittent theta burst stimulation, which is a type of repetitive TMS, could be distinguished from sham TMS with regards to visual, acoustic, and tactile sensations
Summary
In the last two decades, non-invasive brain (NIBS) stimulation techniques have been extensively applied in clinical and cognitive neuroscience, making a significant contribution to a better understanding of the neurophysiological correlates of several cognitive functions. The reported polarity-dependent effects are, not consistent, being mainly described in the motor domain and to a lesser extent in cognitive investigations [16] The effects of both TMS and tDCS on the brain and on motor and cognitive functions may depend on a variety of characteristics, such as position of the coil or electrodes, direction and intensity of the current (and frequency and duration in the case of tDCS) [9,17,18], properties of the stimulated brain tissue [19], demographic variables of the stimulated individual (e.g., gender and age) [20–22], and the cognitive state of the stimulated brain area [23,24].
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