Transcranial alternating current stimulation (tACS) is a non-invasive brain stimulation technique that allows interaction with endogenous cortical oscillatory rhythms by means of external sinusoidal ...potentials. The physiological mechanisms underlying tACS effects are still under debate. Whereas online (e.g., ongoing) tACS over the motor cortex induces robust state-, phase- and frequency-dependent effects on cortical excitability, the offline effects (i.e. after-effects) of tACS are less clear. Here, we explored online and offline effects of tACS in two single-blind, sham-controlled experiments. In both experiments we used neuronavigated transcranial magnetic stimulation (TMS) of the primary motor cortex (M1) as a probe to index changes of cortical excitability and delivered M1 tACS at 10 Hz (alpha), 20 Hz (beta) and sham (30 s of low-frequency transcranial random noise stimulation; tRNS). Corticospinal excitability was measured by single pulse TMS-induced motor evoked potentials (MEPs). tACS was delivered online in Experiment 1 and offline in Experiment 2. In Experiment 1, the increase of MEPs size was maximal with the 20 Hz stimulation, however in Experiment 2 neither the 10 Hz nor the 20 Hz stimulation induced tACS offline effects. These findings support the idea that tACS affects cortical excitability only during online application, at least when delivered on the scalp overlying M1, thereby contributing to the development of effective protocols that can be applied to clinical populations.
Imperceptible transcranial alternating current stimulation (tACS) changes the endogenous cortical oscillatory activity in a frequency-specific manner. In the human motor system, tACS coincident with ...the idling beta rhythm of the quiescent motor cortex increased the corticospinal output. We reasoned that changing the initial state of the brain (i.e., from quiescence to a motor imagery task that desynchronizes the local beta rhythm) might also change the susceptibility of the corticospinal system to resonance effects induced by beta-tACS. We tested this hypothesis by delivering tACS at different frequencies (theta, alpha, beta, and gamma) on the primary motor cortex at rest and during motor imagery. Motor-evoked potentials (MEPs) were obtained by transcranial magnetic stimulation (TMS) on the primary motor cortex with an online-navigated TMS-tACS setting. During motor imagery, the increase of corticospinal excitability was maximal with theta-tACS, likely reflecting a reinforcement of working memory processes required to mentally process and "execute" the cognitive task. As expected, the maximal MEPs increase with subjects at rest was instead obtained with beta-tACS, substantiating previous evidence. This dissociation provides new evidence of state and frequency dependency of tACS effects on the motor system and helps discern the functional role of different oscillatory frequencies of this brain region. These findings may be relevant for rehabilitative neuromodulatory interventions.
Different corticothalamic brain modules intrinsically oscillate at a "natural frequency" in a topographically organized manner. In "quiescent" human sensorimotor regions, the main detectable ...oscillatory activity peaks at ∼20 Hz, and partly contributes to determine the state of corticospinal excitability. Here, we showed that the transcranial application of an imperceptible, short-lasting (90 s) electric field oscillating at a physiological range increases corticospinal excitability online, with well defined frequency dependence and regional specificity. Indeed, the size of motor evoked potentials (MEPs) induced by navigated single-pulse TMS over the motor cortex significantly increased only during the local application of transcranial alternating current stimulation (tACS) at 20 Hz (β range). Other tACS frequencies (5, 10, and 40 Hz) applied on the motor cortex did not impact MEPs' size. Moreover, tACS applied on a control site (parietal cortex) and on a peripheral site (ulnar nerve) also failed to modulate MEPs. These results help clarifying the functional significance of the 20 Hz idling β rhythm of sensorimotor regions and suggest potential clinical applications of this approach.
In the past decade, several studies have examined the effects of transcranial direct current stimulation (tDCS) on long-term episodic memory formation and retrieval. These studies yielded conflicting ...results, likely due to differences in stimulation parameters, experimental design and outcome measures.
In this work we aimed to assess the robustness of tDCS effects on long-term episodic memory using a meta-analytical approach.
We conducted four meta-analyses to analyse the effects of anodal and cathodal tDCS on memory accuracy and response times. We also used a moderator analysis to examine whether the size of tDCS effects varied as a function of specific stimulation parameters and experimental conditions.
Although all selected studies reported a significant effect of tDCS in at least one condition in the published paper, the results of the four meta-analyses showed only statistically non-significant close-to-zero effects. A moderator analysis suggested that for anodal tDCS, the duration of the stimulation and the task used to probe memory moderated the effectiveness of tDCS. For cathodal tDCS, site of stimulation was a significant moderator, although this result was based on only a few observations.
To warrant theoretical advancement and practical implications, more rigorous research is needed to fully understand whether tDCS reliably modulates episodic memory, and the specific circumstances under which this modulation does, and does not, occur.
•We conducted four meta-analyses to assess the effects of tDCS on episodic memory.•We examined the effects of anodal and cathodal tDCS.•We found no effects of tDCS on episodic memory accuracy or response times.•Specific stimulation parameters moderated the effects of tDCS.
Abstract
In this study, we provide causal evidence that the dorsolateral prefrontal cortex (DLPFC) supports the computation of subjective value in choices under risk via its involvement in ...probability weighting. Following offline continuous theta-burst transcranial magnetic stimulation (cTBS) of the DLPFC subjects (N = 30, mean age 23.6, 56% females) completed a computerized task consisting of 96 binary lottery choice questions presented in random order. Using the hierarchical Bayesian modeling approach, we then estimated the structural parameters of risk preferences (the degree of risk aversion and the curvature of the probability weighting function) and analyzed the obtained posterior distributions to determine the effect of stimulation on model parameters. On a behavioral level, temporary downregulation of the left DLPFC excitability through cTBS decreased the likelihood of choosing an option with higher expected reward while the probability of choosing a riskier lottery did not significantly change. Modeling the stimulation effects on risk preference parameters showed anecdotal evidence as assessed by Bayes factors that probability weighting parameter increased after the left DLPFC TMS compared to sham.
Neocortical structures of the left frontal lobe, middle frontal gyrus (MFG) in particular, have been suggested to be linked to the processing of punishing and unpleasant outcomes in decision tasks. ...To assess the role of left MFG (lMFG) in communicative decisions, we used repetitive transcranial magnetic stimulation (rTMS) to inhibit its function during communicational exchanges under two types of social contexts: formal and informal. Three groups of participants received an offline 1-Hz inhibitory rTMS of lMFG, right MFG as an active control site, or lMFG sham/placebo TMS as a passive control condition. Participants' task included answering difficult general-knowledge questions, rating their confidence in their answers' correctness, and, finally, deciding if they would report or withhold these answers in formal and informal social contexts. There were significantly more reported than withheld answers in the informal context in all groups. The formal context showed no differences between reported and withheld answers in both control conditions, while, crucially, real rTMS of lMFG produced a different pattern, with more withheld than reported answers. Thus, lMFG inhibition seems to result in more rational decisions made only in formal communication contexts, where there is a perception of a certain pressure or possible negative outcomes. In informal social contexts and in the absence of negative consequences the pattern of answers did not change, regardless of the reporting strategy or the TMS protocol used. These results suggest selective context-dependent involvement of the lMFG in decision-making processes during communicational exchanges taking place under social pressure.
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