Феурра Маттео

State-Dependent Effects of Transcranial Oscillatory Currents on the Motor System during Action Observation

2019 · ARTICLE · en

We applied transcranial alternating current stimulation (tACS) to the primary motor cortex (M1) at different frequencies during an index-thumb pinch-grip observation task. To estimate changes in the corticospinal output, we used the size of motor evoked potentials (MEPs) obtained by transcranial magnetic stimulation (TMS) of M1 using an online MRI-guided simultaneous TMS-tACS approach. The results of the beta-tACS confirm a non-selective increase in corticospinal excitability in subjects at rest; an increase was observed for both of the tested hand muscles, the first dorsal interosseous (FDI) and the abductor digiti minimi (ADM). However, during action observation of the pinch-grip movement, the increase of corticospinal excitability was only observed for the prime mover FDI muscle and took place during alpha-tACS, while gamma-tACS affected both the FDI and control muscle (ADM) responses. These phenomena likely reflect the hypothesis that the mu and gamma rhythms specifically index the downstream modulation of primary sensorimotor areas by engaging mirror neuron activity. The current neuromodulation approach confirms that tACS can be used to induce neurophysiologically detectable state-dependent enhancement effects, even in complex motor-cognitive tasks

Intensive language-action therapy combined with anodal tDCS leads to verb generation improvements in non-fluent post-stroke aphasia

2019 · CHAPTER · en

Investigating and Modulating Physiological and Pathological Brain Oscillations: The Role of Oscillatory Activity in Neural Plasticity

2019 · ARTICLE · en

There is accumulating evidence that oscillatory activity plays a significant role in regulating brain function. Rhythmic phenomena are routinely observed during perception, motor, and cognitive tasks and have been implicated in altered functions across a broad range of diseases. Several studies suggest that the alpha rhythm gates information flow, beta inhibits changes in motor activity and is responsible for the maintenance of the current sensorimotor or cognitive state, and gamma reflects intracortical local synchronization. However, so far, understanding of the contribution of these rhythms to human behaviour and the manifestation of symptoms in disease states is limited. Moreover, the relationship between brain oscillations and neural plasticity is not clear, although recent evidence supports a link. For instance, it has been demonstrated that resonant rhythms in sensorimotor areas modulate motor learning and enhanced high-gamma activity in the primary motor cortex influences LTP/LTD-like plastic mechanisms. As such we find ourselves in an era where we are rapidly garnering the tools to not only observe brain activity but also alter neural processes in a circumscribed manner.

A systematic review and meta-analysis of the effects of transcranial direct current stimulation (tDCS) on episodic memory

2019 · ARTICLE · en

Background: 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. Objectives: In this work we aimed to assess the robustness of tDCS effects on long-term episodic memory using a meta-analytical approach. Methods: 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. Results: 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. Conclusions: 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.

No effect of the right posterior parietal cortex tDCS in dual-target visual search

2018 · ARTICLE · en

«Subsequent search misses» represent a decrease in accuracy at detecting a second target in a visual search task. In this study, we tested the possibility to modulate this effect via inhibition of the right posterior parietal cortex trough transcranial direct current stimulation (tDCS). The target stimuli were T-shapes presented among L-shaped distractors. The participant’s task was to detect targets or to report their absence. For each trial, targets could be represented by one high-salient target, one low-salient target, two different targets (one high salient and one low salient), two high salient targets, two low salient targets, or no targets at all (catch-trials). Offline tDCS was applied over the right (target site) or left (control site) posterior parietal cortex. Sham stimulation over the right posterior parietal cortex was included as a control (placebo). Stimulation lasted for 10 minutes. Afterwards, participants were asked to perform the experiment. Our findings suggest that stimulation did not modulate any of the task conditions, suggesting potential limitation of the study: either tDCS was not enough powerful to modulate the task performance or the task was too easy to be modulated by stimulation.

Modulating the interhemispheric balance in healthy participants with transcranial direct current stimulation: No significant effects on word or sentence processing

2018 · ARTICLE · en

Patient studies and brain stimulation evidence suggest that language processing can be enhanced by altering the interhemispheric balance: namely, preferentially enhancing left-hemisphere activity while suppressing right-hemisphere activity. To our knowledge, no study has yet compared the effects of such bilateral brain stimulation to both logically necessary control conditions (separate left- and right-hemisphere stimulation). This study did so in a between-group sham-controlled design, applying transcranial direct current stimulation over Broca’s area and/or its homologue in 72 healthy participants. The effects were measured not only in a single-word-level task but also in a sentence-level task, rarely tested previously. We did not find either any significant overall effects of stimulation or greater stimulation effects in the bilateral compared to control groups. This null result, obtained in a large sample, contributes to the debate on whether tDCS can modulate language processing in healthy individuals.

Midfrontal theta transcranial alternating current stimulation modulates behavioural adjustment after error execution

2018 · ARTICLE · en

Cognitive control during conflict monitoring, error processing, and post-error adjustment appear to be associated with the occurrence of midfrontal theta (MFϴ). While this association is supported by correlational EEG studies, much less is known about the possible causal link between MFϴ and error and conflict processing. In the present study, we aimed to explore the role of band-specific effects in modulating the error system during a conflict resolution. In turn, we delivered transcranial alternating current stimulation (tACS) at different frequency bands (delta δ, theta θ, alpha α, beta β, gamma γ) and sham stimulation over the medial frontal cortex (MFC) in 36 healthy participants performing a modified version of the Flanker task. Task performance and reports about the sensations (e.g. visual flickering, cutaneous burning) induced by the different frequency bands, were also recorded. We found that online θ-tACS increased the response speed to congruent stimuli after error execution with respect to sham stimulation. Importantly, the accuracy following the errors did not decrease because of speed-accuracy trade off. Moreover, tACS evoked visual and somatosensory sensations were significantly stronger at α-tACS and β-tACS compared to other frequencies. Our findings suggest that theta activity plays a causative role in modulating behavioural adjustments during perceptual choices in a stimulus-response conflict task.

Selective Social Context-Dependent Role of the Left Medial Prefrontal Cortex in Communication Exchanges

2018 · CHAPTER · en

Previous research showed that under uncertainty (when we are not sure about what the correct answer is) in informal contexts such as chatting with friends, we tend to provide any retrieved information indiscriminately. However, in more formal contexts, like a job interview, we apply a more conservative threshold and balance reporting some information while withholding other answers as a way to provide some information while keeping the accuracy high, if possible. The left medial prefrontal cortex (lmPFC) was suggested as one of the areas linked with self-referential processing in metacognitive decisions. In the present research, we used repetitive transcranial magnetic stimulation (rTMS) to assess the involvement of the lmPFC in two different types of social contexts: formal and informal. Three groups of participants were exposed for 15 minutes to an offline 1-Hz rTMS stimulation of either: (1) lmPFC, (2) control site (rmPFC) or (3) sham (placebo stimulation). Afterwards, participants answered difficult general knowledge questions and rated their confidence in the correctness of their answers. Finally, they decided if they would report or withhold those answers in a formal (job interview) and in informal (chatting with friends) contexts. There were significantly more reported than withheld answers in the informal context for all three groups. However, in the formal context, there were more withheld than reported answers in the lmPFC group, with no differences in the other two groups. No differences in confidence ratings between groups were found. These results suggest a selective involvement of the lmPFC in self-monitoring in formal contexts; its inhibition seems to highlight the need of accuracy in our answers in a job interview over the socially more acceptable behaviour of always providing an answer when asked.

Combined CIAT-tDCS Therapy in Chronic Post-Stroke Aphasia Improves Neural Speech Processing: Neurophysiological and Behavioral Outcomes

2018 · CHAPTER · en

Intensive speech therapy was shown to lead to plastic changes reflected by lexical mismatch negativity (MMN) responses in aphasic patients (Mohr et al., 2016). Since many studies point to the role of the right hemisphere (RH) in aphasia recovery, noninvasive brain stimulation techniques (TMS, tDCS; Shah et al. 2013) usually target RH to facilitate this process. However, individual variance in stimulation-induced neural changes and resulting therapy outcomes are not understood. Here, 14 patients with chronic non-fluent mild-to-moderate aphasia caused by a left middle-cerebral artery stroke underwent Intensive Language-Action Therapy (ILAT, Pulvermüller, 2008) combined with anodal tDCS over the left IFG. To assess speech comprehension mechanisms, lexical MMN paradigm (120 deviant words vs. 487 standard pseudowords) was applied in MEG, before and after therapy for patients and once for a group of 10 controls. To improve the SNR of the responses, we applied a noise bootstrapping procedure with individual noise level computation. We found that the average power of the MMN response in the LH was significantly higher in the patients comparing to the healthy controls (p=0.01). Moreover, the variance of the MMN power increased significantly in the patient group after the treatment. This increase correlated with the behavioural improvements measured as a change of accuracy in a verb generation task (p=0.002). These changes were also associated with clinical improvements in speech comprehension measured by Russian Aphasia Test: scores in a sentence repetition subtest significantly increased after therapy (p = 0.035). No effects were found for the RH. Our results point to the compensatory role of the left hemisphere in the chronic stage of aphasia. They also highlight the role of individual variability in recovery of the language function, suggesting considerable individual differences in the recovery potential. The study continues to investigate these effects in larger samples.

Transcranial alternating current stimulation modulates risky decision making in a frequency controlled experiment

2017 · ARTICLE · en

In this study, we investigated the effect of transcranial alternating current stimulation (tACS) on decision making under risk and executive control in humans. Stimulation was delivered online at 5, 10, 20, and 40 Hz on the left and right DLPFC while subjects performed a modified risky decision making task. This task allowed subjects to voluntarily switch between risky and safe options associated with potential gains or losses while simultaneously measuring the cognitive control component (voluntary switching) of decision making. Our results revealed a frequency- and hemisphere-specific effect of 20Hz tACS delivered on the left DLPFC that significantly increased risk-taking. These results suggest a modulatory role of 20 Hz neural oscillations on the left DLPFC in risk-taking perhaps by activating the brain’s reward system.