Non-invasive and invasive neurostimulation have been used to probe and shift cortical excitability, elicit evoked responses, and alter behavior and perception. For instance, neurostimulation can be used to modify perception, by altering frequency or power of ongoing oscillatory brain activity. However, fluctuations of ongoing brain activity (e.g., neuronal oscillations, brain states) and fluctuations within the central and peripheral nervous system (e.g., muscular activity, the gut, etc.), further influence the effect of neurostimulation, as several empirical studies have shown. The relevance of this modulation is two-fold. On the one hand, it may induce potentially large inter- and intra-individual variance when neurostimulation is performed, a nuisance in most applications. On the other hand, it may provide crucial insight into whether ongoing states or fluctuations are behaviorally or otherwise relevant, thus offering highly valuable contributions to understanding the role of these fluctuations.

Yet, despite the clear practical and theoretical importance, we still have no clarity on how and to what extent these fluctuations are relevant in the observed effect of neurostimulation, nor do we have much understanding of when these fluctuations have a modulatory effect. These questions should be confronted from different angles: while simultaneous recording of ongoing brain activity during neurostimulation is becoming more and more common, mechanistic computational models and simulations of the data are crucial to understand (i) the mode of action of different neurostimulation techniques and (ii) potential interactions between ongoing brain activity or ongoing activity of the neuromuscular system and the neuromodulatory input. Here, non-invasive measures of ongoing activity in both animals and humans as well as invasive approaches can further our understanding of the reciprocal impact of endogenous fluctuations and the effect of neurostimulation. Furthermore, using these insights could benefit not only our understanding of neuronal dynamics but crucially inform the development of adaptive neurostimulation techniques and protocols (e.g., closed-loop procedures) and thus lead to improved therapeutic effects.

This Research Topic seeks to provide significant contributions to the field, by inviting primarily empirical studies (non-invasive, invasive), but also reviews and meta-analyses to address these questions. We encourage authors to explore the following themes:

1. The influence of the phase and/or power of ongoing brain activity on excitability and responsiveness to neurostimulation in humans and animal models

2. The influence of fluctuations in BOLD response on excitability and responsiveness to neurostimulation in humans and animal models

3. Closed-loop neuromodulation approaches to alter cognitive processes

4. The impact of fluctuations in ongoing brain activity on investigating causal relationships in cognition via neuromodulation approaches

5. Computational models and simulations to improve the mechanistic understanding of neurostimulation techniques

6. Therapeutic neurostimulation approaches considering the influence of ongoing brain activity on neurostimulation effects

7. The impact of fluctuations in ongoing muscular activity on neuronal excitability and the responsiveness of neurostimulation techniques

8. The influence of fluctuations within the central and peripheral nervous system on the effect of neurostimulation

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Frontiers in Cognition

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