Studies on cortical excitability regulation and systemic interference effects of transcranial direct current stimulation

The past decade has witnessed a significant improvement in neuroimaging technologies and analysis methods in order to observe and measure brain activity. The advances in the understanding of how the brain is organized is the key to progress in fighting neurological disorders. The efforts to find ways to detect and diagnose these impairments depend largely on the data that we are able to obtain about cerebral hemodynamics, fluctuations, structural changes and the associated cortical activities in the brain. The aim of this thesis is to develop innovative methodologies for examining cortical excitability regulation particularly for stroke patients during transcranial direct current stimulation (tDCS) with the help of Near Infrared Spectroscopy (NIRS) and Electroencephalography (EEG) combined brain imaging. EEG is an electrophysiological method that measures the electrical neural activity directly while NIRS is a method that measures the brain activity indirectly through hemodynamic responses associated with neuron behavior. Transcranial direct current stimulation (tDCS) is a non-invasive technique of modulating brain activity that delivers low intensity direct current to cerebral cortex. It is widely used for stimulating or inhibiting spontaneous neuronal activity. The alterations in cerebrovascular hemodynamics caused by tDCS can be studied through functional NIRS. A low cost tDCS device was developed and integrated with the custom built NIRS data acquisition device to study the effects of electrical stimulation on brain in patients for prediction of neurological disorders such as ischemic stroke. The NIRS – tDCS device was taken for clinical validation at Institute of Neurosciences, Kolkata and it proved to be a good predictor of cerebral vascular status. It can be used for possible identification of acute stroke. The tDCS – NIRS protocol was then further extended to tDCS-NIRS-EEG protocol. It was further investigated that the application of tDCS on cerebral site causes systemic interference in the superficial layers of the head. Due to this, there were some fluctuations found in NIRS measurements. The anterior temporal artery tap technique is discussed in the thesis to remove these fluctuations, which may also be able to classify carotid stenosis, external carotid artery stenosis, and internal carotid artery stenosis patients using the laterality in the hemodynamic response evoked by anodal tDCS both at the brain as well as at the superficial layers. These findings may have important implications for both prognosis and rehabilitation of patients with intracranial stenosis. The EEG and NIRS have different spatial and temporal resolutions, the online tracking of the relation between EEG and NIRS data acquired simultaneously during tDCS has not been investigated earlier. This could provide a sensitive means to monitor the tDCS neuro-modulatory effect. A Kalman filter based approach for online parameter estimation of an autoregressive model to track the relation between tDCS induced cortical neural activity leading to changes in EEG power spectrum and oxy Hemoglobin signals. This new online NIRS EEG tracking method may allow quantitative assessment of the existence of a coupling relationship between electrophysiological and hemodynamic response to tDCS, which could be used to monitor tDCS neuro-modulatory effect in health and disease.