Electrify your brain: can a controversial technique enhance recovery of function after brain damage?

Abstract

The number, connectivity and effectiveness of synapses in the adult human brain can be modified by learning across the entire lifespan. We all recall numerable examples of how members of the so called older generation develop amazing creativity to enhance their mental capacities by learning novel skills until very old age. The processes and mechanisms induced by learning on a cellular level are similar to those to be found during restoration of function after brain damage. Functional imaging of the brain has shown that there is widespread reorganization within various cortical areas of both the affected and unaffected hemispheres after brain damage, irrespective of the particular function that is lost. These changes in neural activity, although most commonly regarded to represent beneficial plasticity, may also represent maladaptive plasticity. Maladaptive plasticity in neural circuits may deteriorate the brain’s intrinsic potential for recovery of function and therefore may interfere with physical therapy. Non-invasive techniques of brain stimulation, such as transcranial direct current stimulation (tDCS) and repetitive transcranial magnetic stimulation (rTMS), may be used to study beneficial and/or maladaptive plasticity of the cerebral cortex after brain lesion. Depending on the stimulation parameters applied tDCS can either inhibit or facilitate cortical excitability and these effects outlast the time of stimulation. The exact mechanisms underlying the changes of cortical excitability induced by tDCS are unknown, but are thought to reflect alterations in synaptic connectivity. Based on these lasting effects, brain stimulation may also be used to modify neural activity in an attempt to shift maladaptive plasticity towards beneficial plasticity. The therapeutic usefulness and efficiency of brain stimulation, however, is still a matter of ongoing debate. Several initial studies suggest that noninvasive brain stimulation may be a valuable adjunct in neurological rehabilitation. In this issue, Schabrun reviews the pertinent literature regarding the potential of tDCS to assist physical therapy in rehabilitation of motor performance of the affected hand after stroke or in Parkinson’s disease and to reduce pain scores in chronic pain syndromes. When the application follows the current safety guidelines tDCS appears to be a safe technique with only minor side effects. As highlighted in the review by Schabrun, only a handful of quite heterogeneous studies tested the effectiveness of tDCS in small samples of patients with stroke, Parkinson’s disease or chronic pain, among other disease entities. From these proof-of-principle studies we learn that several problems persist that need to be addressed prior to a more widespread application of the technique within phase II or III study designs. Future studies must be well-designed as well as feasible, which includes blinding of patients and assessors, and long-term follow-up. Among others the following questions must be addressed: (1) which stimulation parameters (time of stimulation, intensity) are most effective? (2) shall we stimulate over a longer period of time (several days or weeks or months)? (3) which hemisphere and brain area should be stimulated in a given clinical condition? (4) what is more effective: inhibition or facilitation of neural tissue? (5) at what time point or time period in the course of the disease is brain stimulation effective? (6) what are the long-term outcomes (and side effects) of brain stimulation? (7) should brain stimulation combined with conventional training therapies, such as physical therapy? If these questions remain unanswered, tDCS cannot be recommended for widespread adjunctive use in physical therapy practice.