Abstract
Background/Objective: Non-invasive neuromodulation techniques, such as repetitive Transcranial Magnetic Stimulation (rTMS) and transcranial Direct Current Stimulation (tDCS), have increasingly been investigated for their potential as treatments for neurological and psychiatric disorders. Despite widespread dissemination of these techniques, the underlying therapeutic mechanisms and the ideal stimulation site for a given disorder remain unknown. Increasing evidence support the possibility of non-invasive neuromodulation affecting a brain network rather than just the local stimulation target. In this article, we present evidence in a clinical setting to support the idea that non-invasive neuromodulation changes brain networks.Method: This article addresses the idea that non-invasive neuromodulation modulates brain networks, rather than just the local stimulation target, using neuromodulation studies in tinnitus and major depression as examples. We present studies that support this hypothesis from different perspectives.Main Results/Conclusion: Studies stimulating the same brain region, such as the dorsolateral prefrontal cortex (DLPFC), have shown to be effective for several disorders and studies using different stimulation sites for the same disorder have shown similar results. These findings, as well as results from studies investigating brain network connectivity on both macro and micro levels, suggest that non-invasive neuromodulation affects a brain network rather than just the local stimulation site targeted. We propose that non-invasive neuromodulation should be approached from a network perspective and emphasize the therapeutic potential of this approach through the modulation of targeted brain networks.
Highlights
The use of invasive and non-invasive neuromodulation for the treatment of neurological and psychiatric disorders has grown exponentially in recent years, with increasing interest in non-invasive neuromodulation
As opposed to repetitive Transcranial Magnetic Stimulation (rTMS), the electrical currents delivered by transcranial Direct Current Stimulation (tDCS) are not strong enough to fire an action potential (Radman et al, 2009)
We have presented support from different perspectives to demonstrate that non-invasive neuromodulation techniques, such as rTMS and tDCS modulate brain networks rather than just local stimulation targets
Summary
The use of invasive and non-invasive neuromodulation for the treatment of neurological and psychiatric disorders has grown exponentially in recent years, with increasing interest in non-invasive neuromodulation. Several factors can influence excitatory/inhibitory changes of brain stimulations (Filmer et al, 2014), including the state of the brain during stimulation (at rest or paired with a task; Horvath et al, 2014), any intake of substances such as nicotine (Thirugnanasambandam et al, 2011) and even the time of the day (Sale et al, 2007) This sliding of the modification threshold for increased excitation (or long-term potentiation, LTP) and decreased excitation (or LTD), depending on the previous history of neural activity is referred as ‘‘metaplasticity’’ or ‘‘homeostatic plasticity’’ (Abraham and Bear, 1996; Desai, 2003; Lang et al, 2004; Siebner et al, 2004; Abraham, 2008; Cosentino et al, 2012; Hulme et al, 2013; Bocci et al, 2014). We present further evidence in clinical studies demonstrating possible effects on the functional connectivity of the brain for rTMS and tDCS on different stimulation sites and different disorders by: (1) presenting various effective stimulation sites for one disorder; and (2) presenting one effective stimulation site for different disorders
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