Abstract
A great deal of research has been performed with the promise of improving such critical cognitive functions as working memory (WM), with transcranial direct current stimulation (tDCS), a well-tolerated, inexpensive, easy-to-use intervention. Under the assumption that by delivering currents through electrodes placed in suitable locations on the scalp, it is possible to increase prefrontal cortex excitability and therefore improve WM. A growing number of studies have led to mixed results, leading to the realization that such oversimplified assumptions need revision. Models spanning currents to behavior have been advocated in order to reconcile and inform neurostimulation investigations. We articulate such multilevel exploration to tDCS/WM by briefly reviewing critical aspects at each level of analysis but focusing on the circuit level and how available biophysical WM models could inform tDCS. Indeed, such models should replace vague reference to cortical excitability changes with relevant tDCS net effects affecting neural computation and behavior in a more predictable manner. We will refer to emerging WM models and explore to what extent the general concept of excitation-inhibition (E/I) balance is a meaningful intermediate level of analysis, its relationship with gamma oscillatory activity, and the extent to which it can index tDCS effects. We will highlight some predictions that appear consistent with empirical evidence – such as non-linearities and trait dependency of effects and possibly a preferential effect on WM control functions – as well as limitations that appear related to the dynamical aspects of coding by persistent activity.
Highlights
The key operations that support an online representation of the world in order to inform context appropriate behavior are collectively referred to as working memory (WM), encompassing both the simple maintenance of information and cognitive control and the adaptive allocation of cognitive resources (Just and Carpenter, 1992; Goldman-Rakic, 1995; Engle et al, 1999; Miller and Cohen, 2001; Vogel and Machizawa, 2004; Chatham and Badre, 2015)
Empirical studies suggest that prefrontal transcranial direct current stimulation (tDCS) can improve WM under certain conditions, with effects being possibly more prominent with increasing control demand
It is unclear if the E/I balance perspective satisfies this requirement, but this is a testable hypothesis
Summary
The key operations that support an online representation of the world in order to inform context appropriate behavior are collectively referred to as working memory (WM), encompassing both the simple maintenance of information and cognitive control and the adaptive allocation of cognitive resources (Just and Carpenter, 1992; Goldman-Rakic, 1995; Engle et al, 1999; Miller and Cohen, 2001; Vogel and Machizawa, 2004; Chatham and Badre, 2015). It is clear that standard assumptions need to be considered as an oversimplification (Bestmann et al, 2015; Jackson et al, 2016) They fail to explain important aspects of empirical data such as non-linearities, inversions of classical direction effects, polarity neutral effects, state dependency and individual variabilities (Nitsche and Paulus, 2000; Nitsche et al, 2003; Ardolino et al, 2005; Batsikadze et al, 2013; Fresnoza et al, 2014; Esmaeilpour et al, 2018). Let us assume that all relevant variables, subject and state related, are characterized and incorporated in a protocol that successfully leads to an increase of pyramidal cell excitability or other relevant cellular effects as characterized in animal studies A test believed to provide a purer estimate of control functions (Gómez-Ariza et al, 2017; Boudewyn et al, 2018)
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