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Dual mode brain near infrared spectroscopy and electroencephalography hardware design and signal processing Shahbaz, Askari


Electroencephalography (EEG) and cerebral near-infrared spectroscopy (NIRS) are both well-known monitoring methods for analyzing cerebral neurophysiology and hemodynamics. Neuronal activity in the gray matter of the brain requires energy and thus a high metabolic rate, which is related to oxygen consumption. The blood regulatory system operates to ensure sufficient spatial and temporal distribution of oxygen and energy substrates to supply neuronal activity. In this study, we designed and developed a prototype NIRS/EEG instrument for recording electrophysiological activity and hemodynamic changes in the human forehead. This novel probe, combining both EEG and NIRS technologies, consists of Ag/AgCl EEG electrodes positioned between NIRS optodes. As ambient light is capable of contaminating the NIRS signal, a novel amplitude modulation methodology was incorporated into the NIRS/EEG device for multiplexing NIRS light sources and eliminating the interfering noise signal produced by ambient light. This method is based on the modulation of each specific NIR source by its specific carrier frequency. The summation of all sources and ambient interference is measured at the receiver site. A bandpass filter separates each source based on the carrier frequency. Three experiments were conducted using the aforementioned NIRS/EEG instrument. The experiments were performed on five healthy human subjects. The initial experiment was conducted to evaluate the functionality of the developed NIRS/EEG prototype. The association between gamma-band oscillations and total hemoglobin during subjective pain (Cold Pressor Test, CPT) was demonstrated. The increase in gamma-band oscillations, as measured by EEG electrodes on the forehead, and the increase of total hemoglobin, as measured by NIRS optodes on the forehead, have been recorded and reported accordingly. Conventional EEG recording is conducted in the 0.16 to 70 Hz frequency range. However, ultra-low-frequency EEG, found in the range of 0.015 to 4 Hz, is highly informative and allows discovery of the general state of neurons. The genesis of low-frequency EEG signals may be non-neuronal. In the last experiment, the changes of pCO₂ and low-frequency EEG are measured concurrently and the results are presented accordingly.

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