tDCS of the primary motor cortex improves the detection of semantic dissonance

Does the involvement of motor cortex in embodied language comprehension stand on solid ground? A p-curve analysis and test for excess significance of the TMS and tDCS evidence

According to the embodied cognition view, comprehending action-related language requires the participation of sensorimotor processes. A now sizeable literature has tested this proposal by stimulating (with TMS or tDCS) motor brain areas during the comprehension of action language. To assess the evidential value of this body of research, we exhaustively searched the literature and submitted the relevant studies (N = 43) to p-curve analysis. While most published studies concluded in support of the embodiment hypothesis, our results suggest that we cannot yet assert beyond reasonable doubt that they explore real effects. We also found that these studies are quite underpowered (estimated power < 30%), which means that a large percentage of them would not replicate if repeated identically. Additional tests for excess significance show signs of publication bias within this literature. In sum, extant brain stimulation studies testing the grounding of action language in the motor cortex do not stand on solid ground. We provide recommendations that will be important for future research on this topic.

Electrifying discourse: Anodal tDCS of the primary motor cortex selectively reduces action appraisal in naturalistic narratives

Non-invasive stimulation of the primary motor cortex (M1) modulates processing of decontextualized action words and sentences (i.e., verbal units denoting bodily motion). This suggests that language comprehension hinges on brain circuits mediating the bodily experiences evoked by verbal material. Yet, despite its relevance to constrain mechanistic language models, such a finding fails to reveal whether and how relevant circuits operate in the face of full-blown, everyday texts. Using a novel naturalistic discourse paradigm, we examined whether direct modulation of M1 excitability influences the grasping of narrated actions. Following random group assignment, participants received anodal transcranial direct current stimulation over the left M1, or sham stimulation of the same area, or anodal stimulation of the left ventrolateral prefrontal cortex. Immediately afterwards, they listened to action-laden and neutral stories and answered questions on information realized by verbs (denoting action and non-action processes) and circumstances (conveying locative or temporal details). Anodal stimulation of the left M1 selectively decreased outcomes on action-relative to non-action information -a pattern that discriminated between stimulated and sham participants with 74% accuracy. This result was particular to M1 and held irrespective of the subjects' working memory and vocabulary skills, further attesting to its specificity. Our findings suggest that offline modulation of motor-network excitability might lead to transient unavailability of putative resources needed to evoke actions in naturalistic texts, opening promising avenues for the language embodiment framework.

Time to Face Language: Embodied Mechanisms Underpin the Inception of Face-Related Meanings in the Human Brain

In construing meaning, the brain recruits multimodal (conceptual) systems and embodied (modality-specific) mechanisms. Yet, no consensus exists on how crucial the latter are for the inception of semantic distinctions. To address this issue, we combined electroencephalographic (EEG) and intracranial EEG (iEEG) to examine when nouns denoting facial body parts (FBPs) and nonFBPs are discriminated in face-processing and multimodal networks. First, FBP words increased N170 amplitude (a hallmark of early facial processing). Second, they triggered fast (~100 ms) activity boosts within the face-processing network, alongside later (~275 ms) effects in multimodal circuits. Third, iEEG recordings from face-processing hubs allowed decoding ~80% of items before 200 ms, while classification based on multimodal-network activity only surpassed ~70% after 250 ms. Finally, EEG and iEEG connectivity between both networks proved greater in early (0-200 ms) than later (200-400 ms) windows. Collectively, our findings indicate that, at least for some lexico-semantic categories, meaning is construed through fast reenactments of modality-specific experience.

Beyond the target area: an integrative view of tDCS-induced motor cortex modulation in patients and athletes

Transcranial Direct Current Stimulation (tDCS) is a non-invasive technique used to modulate neural tissue. Neuromodulation apparently improves cognitive functions in several neurologic diseases treatment and sports performance. In this study, we present a comprehensive, integrative review of tDCS for motor rehabilitation and motor learning in healthy individuals, athletes and multiple neurologic and neuropsychiatric conditions. We also report on neuromodulation mechanisms, main applications, current knowledge including areas such as language, embodied cognition, functional and social aspects, and future directions. We present the use and perspectives of new developments in tDCS technology, namely high-definition tDCS (HD-tDCS) which promises to overcome one of the main tDCS limitation (i.e., low focality) and its application for neurological disease, pain relief, and motor learning/rehabilitation. Finally, we provided information regarding the Transcutaneous Spinal Direct Current Stimulation (tsDCS) in clinical applications, Cerebellar tDCS (ctDCS) and its influence on motor learning, and TMS combined with electroencephalography (EEG) as a tool to evaluate tDCS effects on brain function.

tDCS to premotor cortex changes action verb understanding: Complementary effects of inhibitory and excitatory stimulation

Processing the meaning of action language correlates with somatotopic activity in premotor cortex (PMC). A previous neurostimulation study supported a causal contribution of PMC activity to action verb understanding, but the direction of the effect was unexpected: inhibiting PMC made participants respond faster to action verbs. Here we investigated the effects of PMC excitation and inhibition on action verb understanding using tDCS. Right-handed participants received tDCS stimulation with the anodal electrode (presumed to be excitatory) and cathodal electrode (presumed to be inhibitory) placed over left and right PMC, respectively, or with the reverse configuration. After completing the stimulation protocol, participants made lexical decisions on unimanual action verbs (e.g., throw) and abstract verbs (e.g., think). tDCS configuration selectively affected how accurately participants responded to unimanual action verbs. When the anode was positioned over left PMC we observed a relative impairment in performance for right-hand responses (i.e. the hand with which these participants typically perform unimanual actions). By contrast, when the cathode was positioned over left PMC we observed a relative improvement. tDCS configuration did not differentially affect responses to abstract verbs. These complementary effects of excitatory and inhibitory tDCS clarify the functional role of premotor hand areas in understanding action language.

Semantic discrimination impacts tDCS modulation of verb processing

Motor cortex activation observed during body-related verb processing hints at simulation accompanying linguistic understanding. By exploiting the up- and down-regulation that anodal and cathodal transcranial direct current stimulation (tDCS) exert on motor cortical excitability, we aimed at further characterizing the functional contribution of the motor system to linguistic processing. In a double-blind sham-controlled within-subjects design, online stimulation was applied to the left hemispheric hand-related motor cortex of 20 healthy subjects. A dual, double-dissociation task required participants to semantically discriminate concrete (hand/foot) from abstract verb primes as well as to respond with the hand or with the foot to verb-unrelated geometric targets. Analyses were conducted with linear mixed models. Semantic priming was confirmed by faster and more accurate reactions when the response effector was congruent with the verb's body part. Cathodal stimulation induced faster responses for hand verb primes thus indicating a somatotopical distribution of cortical activation as induced by body-related verbs. Importantly, this effect depended on performance in semantic discrimination. The current results point to verb processing being selectively modifiable by neuromodulation and at the same time to a dependence of tDCS effects on enhanced simulation. We discuss putative mechanisms operating in this reciprocal dependence of neuromodulation and motor resonance.

Non-invasive brain stimulation for the treatment of brain diseases in childhood and adolescence: state of the art, current limits and future challenges

In the last decades interest in application of non-invasive brain stimulation for enhancing neural functions is growing continuously. However, the use of such techniques in pediatric populations remains rather limited and mainly confined to the treatment of severe neurological and psychiatric diseases. In this article we provide a complete review of non-invasive brain stimulation studies conducted in pediatric populations. We also provide a brief discussion about the current limitations and future directions in a field of research still very young and full of issues to be explored.

Cortico-spinal embodiment of newly acquired, action-related semantic associations

BACKGROUND

Behavioral and neurophysiological studies indicate that the semantic derivation of the motor skills of a given model (e.g., famous tennis or soccer athlete) modulates the reactivity of arm and leg cortico-spinal representations of an onlooker who performs a categorization task. Information on the possible plastic nature of the sensorimotor mapping of action-related knowledge is still lacking.

OBJECTIVE/HYPOTHESIS

Here we explored the time course of any cortico-spinal excitability modulation induced by the creation of arbitrary associations between a personal name and tennis- or soccer-related motor skills.

METHODS

We recorded the amplitude of Transcranial Magnetic Stimulation (TMS) Motor Evoked Potentials (MEPs) from arm and leg muscles during a categorization task concerning names that were learned in association with either soccer players, tennis players or control, non-motor, identities (actors). We stimulated the cortico-spinal system and recorded the MEPs at three different time points (0-24-72 h) after the association learning.

RESULTS

Coherently with previous literature we found a relative dissociation of leg muscles MEPs during reading of soccer-associated personal names with respect to tennis ones. Importantly this modulation was measured only 72 h after having learned the association. This effect was not found in the arm muscle.

CONCLUSION

The results suggest that for the process of embodying semantic associations in the motor system to take place, the strength of the association itself needs to rise above some level of consolidation.

Temporal accuracy and variability in the left and right posterior parietal cortex

Several brain-imaging and lesion studies have suggested a role for the posterior parietal cortex (PPC) in computing interval-timing tasks. PPC also seems to have a key role in modulating visuospatial mechanisms, which are known to affect temporal performance. By applying transcranial direct current stimulation (tDCS) over the left and right PPC, we aimed to modulate timing ability performance in healthy humans performing a cognitively controlled timing task. In two separate experiments we compared time-processing abilities of two groups of healthy adults submitted to anodal, cathodal or sham tDCS over right or left PPC, by employing a supra-second time reproduction task. Cathodal stimulation over the right PPC affected temporal accuracy by leading participants to overestimate time intervals. Moreover, when applied to the left PPC, it reduced variability in reproducing temporal intervals. No effect was reported for anodal stimulation. These results expand current knowledge on the role of the parietal cortex on temporal processing. We provide evidence that the parietal cortex of both hemispheres is involved in temporal processing by acting on distinct components of timing performance such as accuracy and variability.