Noninvasive brain stimulation: from physiology to network dynamics and back

Abstract

Noninvasive brain stimulation techniques have been widely used for studying the physiology of the CNS, identifying the functional role of specific brain structures and, more recently, exploring large-scale network dynamics. Here we review key findings that contribute to our understanding of the mechanisms underlying the physiological and behavioral effects of these techniques. We highlight recent innovations using noninvasive stimulation to investigate global brain network dynamics and organization. New combinations of these techniques, in conjunction with neuroimaging, will further advance the utility of their application.

Figure 1

Typical NIBS setups. (a) A standard figure-eight TMS coil placed on the scalp; here, over dorsolateral prefrontal cortex. (b) Bipolar tDCS electrode configuration, with one electrode over left dorsolateral prefrontal cortex and a reference electrode over the contralateral supraorbital region. Human head model from http://www.ir-ltd.net/. Used by Creative Commons license.

Figure 2

TMS protocols. (a) TMS is commonly applied in single pulses (spTMS), multiple pulses or repetitively (rTMS, applied in low or high frequencies). An emerging form of rTMS is theta-burst stimulation (TBS), in which three 50-Hz pulses are applied at 5 Hz for 20–40 s (continuous TBS, cTBS) or each burst is applied for 2 s and repeated every 10 s for 190 s (intermittent TBS, iTBS). In a third variant, intermediate TBS (imTBS), 5-s burst trains are repeated every 15 s for a total of 110 s (ref. 31). (b) MEPs recorded from the first dorsal interosseous (FDI) muscle using surface electromyography (EMG) after spTMS to M1 and paired-pulse TMS to ventral premotor cortex (PMv) and M1.

Figure 3

Probing cortical network dynamics with NIBS. (a) Alpha-frequency rhythmic TMS applied to presumed generators of endogenous alpha oscillations in the posterior parietal cortex (inset, top; shown in red) entrains local endogenous oscillations that are specific to that frequency. This results in progressive increase in alpha power in early (w1) and late (w2) time windows (bottom). Modified from ref. with permission. (b,c) rTMS protocols for investigating the role of nodes and connections in brain network dynamics. (b) A conventional rTMS protocol application that targets a specific brain region, or network node. In combination with functional neuroimaging, the effects of this stimulation on overall network dynamics can be assessed. (c) A recently developed repetitive paired-pulse TMS that seems to be capable of targeting a specific functional connectivity pathway (red). This will enable investigation of the roles of connections in network dynamics.

Figure 4

Stimulation focality of tDCS. (a) Cortical electric fields induced by a conventional tDCS electrode configuration. (b) Cortical electric fields induced by a 4 × 1 ring electrode configuration. Modified from ref. with permission.