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High-Level Cognitive Functions in Healthy Subjects

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Abstract

This chapter presents and discusses contemporary cutting-edge brain stimulation techniques that have been successful in enhancing high-level cognitive functions in healthy individuals. It focuses primarily on major advances related to language, cognitive control, numerical cognition, planning, learning, and memory research using transcranial direct current stimulation (tDCS), transcranial alternating current stimulation (tACS), and transcranial random noise stimulation (tRNS). A methodological section then specifies the considerations involved in designing and evaluating a tDCS experiment in the context of cognitive research. The conclusion examines the implications of recent data and suggests avenues for experimental research into future uses of brain stimulation.

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... Poor cognition is a hallmark of these disabilities, and cognition can be defined as "the processes an organism uses to organize information. This includes acquiring information (sensation and perception), selecting (attention), communicating (language, numbers), representing (understanding), and retaining (memory) information, and using it to guide behavior (reasoning and coordination of motor outputs)" (Sela and Lavidor, 2014). While reviews focused on children with neurodevelopmental disabilities report differences in gut microbiome composition (Bundgaard-Nielsen et al., 2020;Iglesias-Vázquez et al., 2020), many studies included are crosssectional or observational, making it difficult to determine whether other symptomology (i.e., picky eating, commonly observed in ASD) impact the gut microbiome, rather than the reverse. ...
... Indeed, work has already begun in this area; in a recent review of the effects of pre and probiotic supplementation and fecal microbiota transplantation in adults, five studies out of eight observed improvements in cognitive function (Baldi et al., 2021). In children, much of the human research on gut-microbiomebrain interactions are cross-sectional in nature and focused on infant temperament (Aatsinki et al., 2019) and early childhood behavior (Flannery et al., 2020), both outcomes of cognition (Sela and Lavidor, 2014). Few studies investigate the development of complex cognition directly during childhood (Sordillo et al., 2019;Streit et al., 2021). ...
... "The processes an organism uses to organize information. This includes acquiring information (sensation and perception), selecting (attention), communicating (language, numbers), representing (understanding) and retaining (memory) information, and using it to guide behavior (reasoning and coordination of motor outputs)" (Sela and Lavidor, 2014). ...
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... The most common NIBS types are transcranial magnetic stimulation (TMS) and transcranial electrical stimulation (tES). One of the applications of NIBS is the modulation of critical areas for cognitive processes such as working memory, executive functions, language, or numerical cognition (Sela and Lavidor, 2014). Moreover, the use of NIBS has resulted in reported therapeutic applications within clinical settings (Parkin et al., 2015;Di Lazzaro et al., 2021;Ekhtiari et al., 2019). ...
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Complex numerical cognition is a crucial ability in the human brain. Conventional neuroimaging techniques do not differentiate between epiphenomena and neuronal groups critical to numerical cognition. Transcranial magnetic stimulation (TMS) allows defining causal models of the relationships between specific activated or inhibited neural regions and functional changes in cognition. However, there is insufficient knowledge on the differential effects of various TMS protocols and stimulation parameters on numerical cognition. This systematic review aimed to synthesize the evidence that different TMS protocols provide regarding the neural basis of numerical cognition in healthy adults. We included 21 experimental studies in which participants underwent any transcranial magnetic stimulation such as a single pulse TMS, repetitive TMS, and theta-burst stimulation. The primary outcome measures were any change in numerical cognition processes evidenced by numerical or magnitude tasks, measured with any independent variable like reaction times, accuracy, or congruency effects. TMS applied to regions of the parietal cortex and prefrontal cortex has neuromodulatory effects, which translate into measurable behavioral effects affecting cognitive functions related to arithmetic and numerical and magnitude processing. The use of TMS for the study of the neural bases of numerical cognition allows addressing issues such as localization, timing, lateralization and has allowed establishing site-function dissociations and double site-function dissociations. Moreover, this technique is in a moment of expansion due to the growing knowledge of its physiological effects and the enormous potential of combining TMS with other techniques such as electroencephalography, functional magnetic resonance imaging, or near-infrared spectroscopy to reach a more precise brain mapping.
... [ 51 ]). However, some authors [ 52 ] have also highlighted the possibility that the null effect may have been due to a fundamental problem with their experimental design. Namely, the extensive and prolonged (30 min) activation of the language system in itself may have played a role. ...
Chapter
This chapter provides an overview of the literature concerning the effects of tDCS on high-level cognitive functions in young healthy adults. tDCS has been found to modulate a multitude of components of cognition, but here we place a particular focus on studies that have examined working memory, attention, language, numerical cognition, general learning and memory. We additionally devote latter portions of the chapter to evaluating two other pertinent topics: the neurocognitive effects of tDCS in the healthy older brain and individual differences in the context of tDCS outcomes. Based on the studies reviewed, we conclude that tDCS holds substantial promise as a tool for exploring novel theoretical hypotheses, as well as for improving cognitive functions in both young and older healthy adults. However, the coherence of the evidence base and the translational potential of these findings is currently constrained by a number of factors, including pervasive inter-individual differences in response to tDCS, heterogeneity of tDCS protocols across studies and inadequate knowledge about the longevity of the effects.
... I will therefore focus on two areas, working memory (WM) and numerical cognition, as examples of such abilities. The interested reader is referred to the recent review by Sela and Lavidor (2014), which covers studies of additional capacities like language, problem-solving, memory and cognitive control. ...
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A new line of research opens the possibility of modulating and enhancing human cognition using mild and painless transcranial electrical stimulation (tES), which includes transcranial direct current stimulation (tDCS), transcranial alternating current stimulation (tACS) and transcranial random noise stimulation (tRNS). Such initial findings trigger excitement as well as scepticism. The current review aims to provide a guideline for those who are interested in expanding their research into this field. I will therefore discuss: (1) the principles of tES and its putative mechanisms; (2) its potential to modulate and enhance cognitive abilities; (3) the misconceptions on which scepticism about this method is based; and (4) possible directions for the advancement of this field in which psychologists in general and cognitive psychologists in particular should in my view play a key role. I will conclude that this nascent field, which has been neglected by psychologists, requires their contribution in order to lead to basic and translational advancements on human behaviour.
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Researchers debate whether domain-general cognitive control supports bilingual language control through brain regions such as the dorsolateral prefrontal cortex (DLPFC). Transcranial direct current stimulation (tDCS) is a method to alter brain activity, which can lead to causal attribution of task performance to regional brain activity. The current study examined whether the DLPFC enables domain-general control for between-language switching and non-linguistic switching and whether the control enabled by DLPFC differs between bilinguals and monolinguals. tDCS was applied to the DLPFC of bilingual and monolingual young adults before they performed linguistic and non-linguistic switching measures. For bilinguals, left DLPFC stimulation selectively worsened non-linguistic switching, but not within-language switching. Left DLPFC stimulation also resulted in higher overall accuracy on bilingual picture-naming. These findings suggest that language control and cognitive control are distinct processes in relation to the left DLPFC. The left DLPFC may aid bilingual language control, but stimulating it does not benefit non-linguistic control.
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Key points Application of 2 mA cathodal transcranial direct current stimulation for 20 min results in cortical excitability enhancement instead of inhibition. Longer or more intensive stimulation does not necessarily increase its efficacy. Short intracortical inhibition and facilitation are shifted towards excitability enhancement after both 2 mA anodal and cathodal stimulation. I‐waves, input–output curves and cortical silent period are unaffected immediately after 2 mA stimulation. Abstract Transcranial direct current stimulation (tDCS) of the human motor cortex at an intensity of 1 mA with an electrode size of 35 cm ² has been shown to induce shifts of cortical excitability during and after stimulation. These shifts are polarity‐specific with cathodal tDCS resulting in a decrease and anodal stimulation in an increase of cortical excitability. In clinical and cognitive studies, stronger stimulation intensities are used frequently, but their physiological effects on cortical excitability have not yet been explored. Therefore, here we aimed to explore the effects of 2 mA tDCS on cortical excitability. We applied 2 mA anodal or cathodal tDCS for 20 min on the left primary motor cortex of 14 healthy subjects. Cathodal tDCS at 1 mA and sham tDCS for 20 min was administered as control session in nine and eight healthy subjects, respectively. Motor cortical excitability was monitored by transcranial magnetic stimulation (TMS)‐elicited motor‐evoked potentials (MEPs) from the right first dorsal interosseous muscle. Global corticospinal excitability was explored via single TMS pulse‐elicited MEP amplitudes, and motor thresholds. Intracortical effects of stimulation were obtained by cortical silent period (CSP), short latency intracortical inhibition (SICI) and facilitation (ICF), and I wave facilitation. The above‐mentioned protocols were recorded both before and immediately after tDCS in randomized order. Additionally, single‐pulse MEPs, motor thresholds, SICI and ICF were recorded every 30 min up to 2 h after stimulation end, evening of the same day, next morning, next noon and next evening. Anodal as well as cathodal tDCS at 2 mA resulted in a significant increase of MEP amplitudes, whereas 1 mA cathodal tDCS decreased corticospinal excitability. A significant shift of SICI and ICF towards excitability enhancement after both 2 mA cathodal and anodal tDCS was observed. At 1 mA, cathodal tDCS reduced single‐pulse TMS‐elicited MEP amplitudes and shifted SICI and ICF towards inhibition. No significant changes were observed in the other protocols. Sham tDCS did not induce significant MEP alterations. These results suggest that an enhancement of tDCS intensity does not necessarily increase efficacy of stimulation, but might also shift the direction of excitability alterations. This should be taken into account for applications of the stimulation technique using different intensities and durations in order to achieve stronger or longer lasting after‐effects.
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Transcranial direct current stimulation (tDCS) is able to generate a long-term increase or decrease in the neuronal excitability that can modulate cognitive tasks, similar to repetitive transcranial magnetic stimulation. The aim of this study was to explore the effects of tDCS on a language task in young healthy subjects. Anodal, cathodal and sham tDCS were applied to the left dorsolateral prefrontal cortex (DLPFC) before two picture naming experiments, a preliminary study (i.e., experiment 1) and a main study (i.e., experiment 2). The results show that anodal tDCS of the left DLPFC improves naming performance, speeding up verbal reaction times after the end of the stimulation, whereas cathodal stimulation had no effect. We hypothesize that the cerebral network dedicated to lexical retrieval processing is facilitated by anodal tDCS to the left DLPFC. Although the mechanisms responsible for facilitation are not yet clear, the results presented herein implicate a facilitation lasting beyond the end of the stimulation that imply cortical plasticity mechanisms. The opportunity to non-invasively interact with the functioning of these plasticity mechanisms will surely open new and promising scenarios in language studies in basic and clinical neuroscience fields.
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Impulsivity is a personality trait within the normal population, but also a feature of many psychiatric disorders that have been associated with poor inhibitory control. The aim of the present study was to examine the relation between subjective impulsivity, theta/beta EEG ratio, and inhibitory control in healthy individuals. In 15 high and 14 low impulsive healthy volunteers (as assessed by the I(7) questionnaire), resting state EEG was recorded during an eyes open condition to obtain estimates for theta and beta activity. Subsequently, a stop-signal task was presented where participants responded to go-signals and had to stop their initiated response to stop-signals. Stopping performance and EEG activity were compared between the impulsive groups as well as between high vs. low theta/beta ratio groups. Results showed that subjective impulsivity was not related to stopping behavior or to theta/beta ratio. In contrast to our expectations that individuals with high theta/beta ratios would show relatively long stopping reaction times, analyses revealed that the low theta/beta ratio group had longer stopping reaction times. Given that increased theta/beta ratio may reflect reduced cortical inhibition over subcortical drives, it is proposed that healthy individuals with relative high theta/beta ratios are more motivated to maximize inhibition-related performance.