Non invasive brain stimulation : modeling and experimental analysis of transcranial magnetic stimulations and transcranial DC stimulation as a modality for neuropathology treatment

This thesis will explore the use of Transcranial Magnetic Stimulation (TMS) and Transcranial DC Stimulation (tDCS) as modalities for neuropathology treatment by means of both experimental and modeling paradigms. The first and primary modality that will be analyzed is Transcranial Magnetic Stimulation (TMS). TMS is a technique that uses the principle of electromagnetic induction to focus induced currents in the brain and modulate cortical function. These currents can be of sufficient magnitude to depolarize neurons, and when these currents are applied repetitively (repetitive Transcranial Magnetic Stimulation (rTMS)) they can modulate cortical excitability, decreasing or increasing it, depending on the parameters of stimulation. This thesis will explore important facets of the electromagnetic field distributions and fundamental electromagnetic interactions to lay the foundation for future development of a more complete neural model and improved stimulation techniques. First, TMS will be analyzed as a technique used in normal healthy subjects. Finite element modeling (FEM) studies will be explored for realistic healthy human head models with a particular focus placed on the TMS induced cortical currents and their dependency on coil position, normal tissue anatomy, and the electromagnetic tissue properties. This component of the thesis will also include experimental work focused on exploring the in-vivo tissue conductivity and permittivity values used in TMS studies and their impact on stimulation (including a detailed literature review). The next component of the thesis will explore the use of TMS in subjects suffering from various pathologies. The first pathological condition that will be analyzed is cortical stroke. FEM studies will be evaluated and compared to the healthy head models to assess how the cortical modifications brought on at an infarction site can alter the TMS induced current densities. We will also include a laboratory study that assesses the efficacy of TMS in stroke treatment, where repetitive TMS (rTMS) was applied to the unaffected hemisphere to decrease inter-hemispheric inhibition of the lesioned hemisphere and improve motor function in stroke patients. Next, the use of TMS in conditions of brain atrophy will be assessed through modeling analyses. This component will also include an evaluation of the clinical work in the field and ways in which the current density alterations caused by the atrophy have led to clinical misconceptions. Transcranial DC Stimulation (tDCS) will be the second modality analyzed through modeling and experimental work. In tDCS, the cerebral cortex is stimulated through a weak dc current in a non-invasive and painless manner and can modulate cortical excitability like TMS. We will define finite element head models of tDCS for both normal and pathologic cases and evaluate the use of tDCS in the clinic in a stroke treatment experiment (analogous to the one completed with TMS). Finally, we will assess and compare these forms of brain stimulation to other forms of neurological treatment and conclude with proposed future improvements to the field of non-invasive brain stimulation. Table of