Based on already published large evidence, non-invasive brain stimulation (NIBS) techniques like tdCS represent very important approach for the improvement of abnormal brain functions in various conditions (psychiatric and neurological). NIBS can induce temporary changes of neural oscillations and performance on various functional tasks. One of the key-points in understanding a mechanism of NIBS is the knowledge about the brains response to current stimulation and underlying brain network dynamics changes. Until recently, concurrent observation of the effect of NIBS on multiple brain networks interactions and most importantly, how current stimulation modifies these networks remained unknown because of difficulties in simultaneous recording and current stimulation. Recently, in Neuroelectrics wireless hybrid EEG/tCS 8-channel neurostimulator system has been developed that allows simultaneous EEG recording and current stimulation. Now, a relatively new imaging technique called magnetoencephalography (MEG) has emerged as a procedure that can bring new inside into brain dynamics. In this context, our group conducted a successfully proof of concept test to ensure the feasibility of concurrent MEG recording and current stimulation using Starstim and a set of non-ferrous electrodes (Figure 1). But first of all, what actually is MEG? Magnetoencephalography (MEG) is a noninvasive recording method of the magnetic flux from the head surface. Magnetic flux is associated with intracranial electrical currents produced by neural activity (the neural currents are caused by a flow of ions through postsynaptic dendritic membranes). From Maxwell equations, magnetic fields are found whenever there is a current flow, whether in a wire or a neuronal element. Hence, MEG detects these magnetic fields generated by spontaneous or evoked brain activity.
Following an ischemic insult within the motor cortex, one or more body parts contralateral to the infarct result impaired or paretic. The degree of the motor impairment depends on many factors, such as the extent of the infarct, the identity of the damaged region(s) and the effectiveness of the early medical care. Substantial functional recovery can occur in the first weeks after stroke, mainly due to spontaneous mechanisms (Kwakkel et al., 2004; Cramer, 2008; Darling et al., 2011; Ward, 2011; Grefkes and Fink, 2014). About 26% of stroke survivors are able to carry on everyday activities (Activity of Daily Living or ADLs, i.e., eating, drinking, walking, dressing, bathing, cooking, writing) without any help, but another 26% is forced to shelter in a nursing home (Carmichael, 2005). Impairments of upper and lower limbs are particularly disabling as they impact on the degree of independence in ADLs. Overall, a significant percentage of the patients exhibit persistent disability following ischemic attacks. Therefore, it is critical to increase our knowledge of post-stroke neuroplasticity for implementing novel rehabilitative strategies. In this review we summarize data about plastic reorganizations after injury, both in the ipsilesional and contralesional hemisphere. We also describe non-invasive brain stimulation (NIBS) techniques and robotic devices for stimulating functional recovery in humans and rodent stroke models.
Neuroplasticity After Stroke
The term brain plasticity defines all the modifications in the organization of neural components occurring in the central nervous system during the entire life span of an individual (Sale et al., 2009). Such changes are thought to be highly involved in mechanisms of aging, adaptation to environment and learning. Moreover, neuronal plastic phenomena are likely to be at the basis of adaptive modifications in response to anatomical or functional deficit or brain damage (Nudo, 2006). Ischemic damage causes a dramatic alteration of the entire complex neural network within the affected area. It has been amply demonstrated, by many studies, that the cerebral cortex exhibits spontaneous phenomena of brain plasticity in response to damage (Gerloff et al., 2006; Nudo, 2007). The destruction of neural networks indeed stimulates a reorganization of the connections and this rewiring is highly sensitive to the experience following the damage (Stroemer et al., 1993; Li and Carmichael, 2006). Such plastic phenomena involve particularly the perilesional tissue in the injured hemisphere, but also the contralateral hemisphere, subcortical and spinal regions.