An exploration of tDCS Transcranial direct current stimulation (tDCS) is a form of non-invasive brain stimulation that uses low currents and low voltages (eg. <2mA) applied to the scalp to modulate brain activity. Generally, when correctly used, it is considered a safe intervention with adverse effects limited to skin tingling, itching etc. The sessions are generally short, ~20min daily and research indicates greater time periods may not yield improved outcomes. There has been some preliminary indication that coupling tDCS with cognitive training, aerobic exercise or even neurotrophic pharmacological interventions may augment the effectiveness. There is marked differences seen in brain activity when the stimulation is applied during a task, vs at rest. The general effect at rest is to suppress default mode network activity, whilst during a task, the salience network is also enhanced. In general, the behavioural effect of an acute tDCS session can last 90min post session, with lasting effects of up to months after multiple sessions. Brain function under the anodal electrode site during tDCS is enhanced 20-40% above a certain point of current density. tDCS can also be used to accelerate learning and memory. Short durations of stimulation produced large physiological effects almost immediately. Large changes in brain activity were seen after seconds, rather than minutes, of stimulation. Some noted outcomes from recent tDCS research: - tDCS opens exciting possibilities for enhancing brain function and its synergy with pharmacological agents holds promise for improving mental health and cognitive performance. It has been suggested that neurotrophic compounds may augment tDCS-induced neuroplasticity by promoting synaptogenesis and neuronal survival. - tDCS has the capacity to enhance processing speed, working memory, and executive functions in patients with mood and schizophrenia-spectrum disorders. - the majority of studies have shown that tDCS targeting at DLPFC can significantly improve depression symptoms for a month or longer. - Some studies suggest that enhancing cortical excitability in specific brain regions through tDCS can enhance overall inhibitory control ability - tDCS improved both positive syndromes and negative syndromes in patients with schizophrenia. There seems to be some dependence on dopaminergic signaling for efficacy in psychotic spectrum disorders, with people on low affinity dopaminergic antagonists seeing better results - tDCS can help prevent and ameliorate stress, with effects not only on emotion, cognition, and behaviour but also on the cardiovascular and autonomic nervous systems 1 - tDCS can significantly relieve the symptoms of addictions - the effects of tDCS are optimal when combined with behavioural strategies, such as exercise, cognitive training, mindfulness techniques, and Virtual Reality training. There has been growing research into potential clinical applications of the simple technology, including but not limited to treatment resistant depression, psychotic spectrum disorders, neurodegenerative (AD etc), addictive, ADHD and neurodevelopmental disorders. There has also been consideration of using the simple technology as neuroenhancement, for cognitive deficits including improving attention, for enhancing creativity, or even for moral enhancement. The general mechanism of action is said to be that brain activity, including long-term potentiation (LTP), or strengthening of a pattern of synaptic activity, is enhanced near the site of the anode, while supressed at the site of the cathode. The excitatory effect of the anodal stimulation modulates various neurotransmission processes, including glutamate (particularly via NMDARs), dopamine and GABA, with downstream effects on neuroplasticity including those mediated by BDNF. 2 Whilst the weak electrical field generally stimulates outer layers of the cortex, it has been noted that modulation of the outer cortical layers can impact subcortical processes, this effect is particularly noted with regard to dopaminergic transmission where via mesocortical pathways, DLPFC stimulation can impact subcortical mesolimbic striatal dopaminergic transmission modulating things like attentional prcesses, or even the nucleus accumbens, modulating reward. With regard to the modulation of dopaminergic processes, bifrontal tDCS seems to modulate the mesocorticolimbic pathways: Bifrontal tDCS to the left and right dorsolateral prefrontal cortex (DLPFC) induces dopamine release in the ventral striatum associated with improved accuracy on an attentional control task, possibly underlying efficacy for disorders like ADHD where it also seems to improve reward processing deficits and frontal stimulation increases dopamine release in the nucleus accumbens reward centre, the latter possibly underlying the efficacy seen for reducing anhedonia in depression, psychotic spectrum disorders, PTSD and SUDs. Similarly, dopaminergics (L-dopa) seem to enhance the duration of tDCS effects whilst high affinity D2 blockade seems to attenuate effects in psychotic disorders. The mesocorticolimbic pathway may be involved in the application of prefrontal tDCS as a moderator of subcortical dopamine activity and this mesocortical modulation may play a role in the efficacy of bifrontal tDCS for negative symptoms of psychotic disorders 3 Specific 'montages' or electrode placements are used to generate a weak polarised electrical field across the cortex. An example of a specific montage to enhance left DLPFC activity, whilst supress right OFC activity is pictured below. 4 An example of the polarised electrical field generated (anodal stimulation of rDLPFC) is shown below The regional specificity of the targeted stimulation of different regions of the brain, whilst 5 allowing supression of other areas allows different cortical regions to be specifically modulated. Perhaps the most targeted region is the dorsolateral prefrontal cortex (DLPFC) which can be modulated on either the left or right side of the cortex. In general, the DLPFC can be stimulated to enhance prefrontal cognitive, emotional or social processes. The left DLPFC is often a site for mood enhancement, where some of the c processes that can be modulated via anodal tDCS stimulation of left DLPFC include decreased negative emotions and improved cognitive control, along with a variety of cognitive and emotional processes like working memory (temporarily holding and manipulating information for cognitive tasks), executive functioning (planning, problem- solving, and decision-making), attention control, language production, emotional judgement, emotional memory encoding and emotion regulation. In general, the dlPFC is part of a network of brain regions involved in instantiating motivational processes. Specifically, the left dlPFC has been associated with approach tendencies. On the other hand, the right dlPFC has been linked to behavioural inhibition and avoidance tendencies The rDLPFC has been stimulated to enhance creativity, but also in addictions to target things like impulsivity and response inhibition. Stimulation of the DLPFC (both left and right) seems to be beneficial for the cognitive effects of neurostimulation in drug addiction and the right DLPFC has been targeted to improve measures of cognitive control in SUDs like working memory, response inhibition, cognitive control/flexibility, and risk-taking behaviour, and that these effects were also associated with significantly reduced craving. Social processes can also be modulated where rDLPFC stimulation is said to inhibit selfish actions whilst promoting greater social norm-compliance. In terms of moral neuroenhancement, various studies have been undertaken. An initial study found just 20 min of prefrontal stimulation was able to reduce agressive and immoral trends in thinking in a sample by 50%, see: https://penntoday.upenn.edu/news/brain- stimulation-decreases-intent-commit-physical-sexual-assault Non-invasive brain stimulation alters specific neuropsychological processes contributing to normal moral behavior and can also influence altruism, trust, cooperation, and other prosocial behaviors. - honesty can be strengthened by noninvasive interventions - excitatory brain stimulation to the right DLPFC reduced aggression and increased adherence to social norms. - Inhibition of the right DLPFC reduces the influence of harm on decision-making, both when deciding to perform a harmful action oneself, and when punishing harmful or unfair behaviors in others. This reduces decisions to punish or disapprove of potentially harmful personal moral violations in others. However, it also increased the likelihood of performing a harmful action oneself, such as lying or reacting 6 aggressively. - stimulation of the left DLPFC may also results in increased cooperation and increased consideration of harm violations in moral dilemmas. - the right TPJ presumably exerts a more fundamental role in modulating the role of beliefs and intentions in moral judgments. This may influence the degree to which a harmful action is perceived as moral (e.g., intended) vs. non-moral (e.g., an accident). References: Li Qing, Fu Yu, Liu Chang, Meng Zhiqiang. Transcranial Direct Current Stimulation of the Dorsolateral Prefrontal Cortex for Treatment of Neuropsychiatric Disorders Frontiers in Behavioral Neuroscience 16, 2022 https://doi.org/10.3389/fnbeh.2022.893955 Ciullo, Valentina, et al. Transcranial direct current stimulation and cognition in neuropsychiatric disorders: systematic review of the evidence and future directions. 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