Abstract
Non-invasive brain stimulation (NIBS) has been widely used as a research tool to modulate cortical excitability of motor as well as non-motor areas, including auditory or language-related areas. NIBS, especially transcranial magnetic stimulation (TMS) and transcranial direct current stimulation, have also been used in clinical settings, with however variable therapeutic outcome, highlighting the need to better understand the mechanisms underlying NIBS techniques. TMS was initially used to address causality between specific brain areas and related behavior, such as language production, providing non-invasive alternatives to lesion studies. Recent literature however suggests that the relationship is not as straightforward as originally thought, and that TMS can show both linear and non-linear modulation of brain responses, highlighting complex network dynamics. In particular, in the last decade, NIBS studies have enabled further advances in our understanding of auditory processing and its underlying functional organization. For instance, NIBS studies showed that even when only one auditory cortex is stimulated unilaterally, bilateral modulation may result, thereby highlighting the influence of functional connectivity between auditory cortices. Additional neuromodulation techniques such as transcranial alternating current stimulation or transcranial random noise stimulation have been used to target frequency-specific neural oscillations of the auditory cortex, thereby providing further insight into modulation of auditory functions. All these NIBS techniques offer different perspectives into the function and organization of auditory cortex. However, further research should be carried out to assess the mode of action and long-term effects of NIBS to optimize their use in clinical settings.
Highlights
Non-invasive brain stimulation (NIBS) has recently seen a surge of interest to provide understanding about the functional properties and interactions of cortical systems
In Andoh and Zatorre (2011), using 1-Hz- or 10Hz repetitive TMS (rTMS) applied over the right auditory cortex offline, changes in response time were shown to be differently modulated between time 1 (0–5.5 min) and time 2 (5.5–11 min)
Non-invasive brain stimulation techniques such as transcranial direct current stimulation (tDCS) have been a technique of choice in many clinical and research settings because it is portable, painless, and inexpensive compared with other NIBS techniques. transcranial alternating currentNIBS of Auditory Cortex stimulation (tACS), transcranial random noise stimulation (tRNS), and tDCS have a relatively low spatial precision, due to the large spatial distribution of the electrical current flow produced by the electrodes and is accentuated by the anatomical variability of the targeted brain structures (Parazzini et al, 2015; Alam et al, 2016)
Summary
Non-invasive brain stimulation (NIBS) has recently seen a surge of interest to provide understanding about the functional properties and interactions of cortical systems. Andoh and Paus (2011) showed that rTMS applied over the left temporoparietal area before participants performed an auditory language comprehension task induced changes in neural activity in the right temporoparietal area Such interhemispheric interaction processes are comparable to functional reorganization associated with recovery from language disorders (Saur et al, 2006; Hartwigsen et al, 2013). In order to dissociate task-related compensatory processes from TMS-induced effects, Andoh et al (2015) applied TBS to the auditory cortex immediately before a resting-state fMRI scan, and compared it to a resting-state scan obtained prior to stimulation Such an approach helps to reveal baseline auditory activity and connectivity, avoiding the confound of a cognitive task. This study showed the power of combined imaging and stimulation protocols to demonstrate a causal influence of TMS on behavior, mediated by oscillatory activity in the auditory dorsal network
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