Trans-spinal direct current stimulation for the modulation of the lumbar spinal motor networks

Trans-spinal Direct Current Stimulation (tsDCS) is a noninvasive neuromodulatory tool for the modulation of the spinal neurocircuitry. Initial studies have shown that tsDCS is able to induce a significant and lasting change in spinal-reflex- and corticospinal information processing. It is therefore hypothesized that tsDCS may be a useful tool in the rehabilitation of spinal cord dysfunctions or injuries. However, to efficiently utilize tsDCS as a tool in neurorehabilitation, more knowledge is necessary about its mechanisms of action, as well as how tsDCS needs to be applied to ensure the desired outcome. This dissertation focuses on the use of tsDCS for the modulation of the lumbar spinal motor circuitry, for a possible application in spinal cord injury rehabilitation. This is investigated using theoretical as well as experimental techniques. Chapter 2 focusses on simulating the electric field (EF) generated during tsDCS and its interaction with the targeted neural structures. This includes visualization and analysis of the generated EF as well as the identification of the most likely neural target. Furthermore, a comparison with existing human tsDCS studies and the possible effects of electrode misplacement during application are discussed. After having established a theoretical basis of some of the underlying mechanisms of action, the following two chapters deal with experimentally assessing the effects of tsDCS for different protocol variations. Chapter 3 deals with experimentally assessing the effects of tsDCS applied with different EF directions, as well as the repeatability of results previously obtained by others. The central question was to assess whether the tsDCS outcome is dependent on EF direction. Chapter 4 compares the effects of tsDCS during active movement and rest, to investigate during which of the two conditions the application of tsDCS leads to larger modulatory effects. The underlying hypothesis is, that the modulatory effect of tsDCS can be significantly increased when paired with ongoing neural activity. Lastly, chapter 5 investigates important safety aspects, when tsDCS is applied in the presence of metallic spinal implants. The presence of metallic implants in the body is still a safety concern, in connection with electrical stimulation procedures. Since spinal implants are expected to be present in at least part of the targeted population with spinal cord injury, it is necessary to explore the safety and application specific consequences of tsDCS with the presence of a spinal metallic implant