Responsiveness to left‐prefrontal tDCS varies according to arousal levels

Abstract

Over the past two decades, the postulated modulatory effects of transcranial direct current stimulation (tDCS) on the human brain have been extensively investigated. However, recent concerns on reliability of tDCS effects have been raised, principally due to reduced replicability and to interindividual variability in response to tDCS. These inconsistencies are likely due to the interplay between the level of induced cortical excitability and unaccounted structural and state-dependent functional factors. On these grounds, we aimed at verifying whether the behavioural effects induced by a common tDCS montage (F3-rSOA) were influenced by the participants' arousal levels, as part of a broader mechanism of state-dependency. Pupillary dynamics were recorded during an auditory oddball task while applying either a sham or real tDCS.The tDCS effects were evaluated as a function of subjective and physiological arousal predictors (STAI-Y State scores and pre-stimulus pupil size, respectively). We showed that prefrontal tDCS hindered task learning effects on response speed such that performance improvement occurred during sham, but not real stimulation. Moreover, both subjective and physiological arousal predictors significantly explained performance during real tDCS, with interaction effects showing performance improvement only with moderate arousal levels; likewise, pupil response was affected by real tDCS according to the ongoing levels of arousal, with reduced dilation during higher arousal trials. These findings highlight the potential role of arousal in shaping the neuromodulatory outcome, thus emphasizing a more careful interpretation of null or negative results while also encouraging more individually tailored tDCS applications based on arousal levels, especially in clinical populations.

Effects of Multisession Transcranial Direct Current Stimulation on Stress Regulation and Emotional Working Memory: A Randomized Controlled Trial in Healthy Military Personnel

“…108,109 Moreover, effects of noninvasive brain stimulation, including tDCS effects on WM performance, have been shown to depend on arousal and stress levels. [110][111][112][113] In this study, central nervous system arousal could have been elevated in particular during the first session owing to the novelty of tDCS and the training task. Altogether, factors such as novel learning processes and arousal could have interacted with the neurophysiological effects of tDCS specifically during the early stages of the tDCS training intervention.…”

Individual response to transcranial direct current stimulation as a function of working memory capacity and electrode montage

“…Yet, our findings suggest, that different electrode montages might accentuate stimulation effects in dependence of initial WM capacity. It has been argued that effects of tDCS vary with the initial recruitment and activity of the stimulated brain areas and that baseline performance could be representative of such underlying neuronal states ( Silvanto et al, 2008 ; Ziemann and Siebner, 2015 ; Hsu et al, 2016 ; Dubreuil-Vall et al, 2019 ; Emonson et al, 2019 ; Esposito et al, 2022 ). As stimulation protocols ( Teo et al, 2011 ; Moos et al, 2012 ; Filmer et al, 2017 ), as well as baseline performance ( Benwell et al, 2015 ; Learmonth et al, 2015 ; Splittgerber et al, 2020 ) influence stimulation effects, they should hence be considered in the analysis and interpretation.…”

Differential effects of remotely supervised transcranial direct current stimulation on recognition memory depending on task order

“…Replication in tDCS studies is often difficult to achieve, as participants often have many individual differences in skull shape/thickness, subcutaneous fat and cerebrospinal fluid densities, as well as anatomical differences in brain regions ( Stagg and Nitsche, 2011 ; Woods et al, 2016 ). Also, differences in participant arousal ( Esposito et al, 2022 ), time of day ( Wong et al, 2018 ), and even task demands ( Chua et al, 2017 ; Stephens et al, 2017 ) can influence how participants respond to brain stimulation. By reporting null tDCS results, a clearer picture of which conditions lead to successful tDCS outcome, and which conditions do not, may emerge.…”