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Biological tissue is relatively transparent to light in the near-infrared range between 700 and 1000 nm. This is due to the fact that water absorption and hemoglobin absorption are relatively small within this wavelength region. Several kinds of methods have been used to investigate human brain activity such as positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). Recent research has shown that brain activity is associated with changes in the optical properties of brain tissue.1 The activity of nerve and brain cells has been reported to be associated with changes in the optical properties of the nervous system, cell cultures, and bloodless brain slices, as well as in intact cortical tissues. Optical signals have been used to map brain function after surgical exposure of cortical tissue in animal and human subjects. Chance et al. measured the brain responses to functional activation with near infrared spectroscopy in 1993. Tamura et al. analyzed the spatio-temporal changes in the oxygenation states through simultaneous near-infrared spectroscopy (NIRS) measured at five typical locations on the brain. Maki et al. mapped the NIR topography on the human brain’s motor cortex by measuring the changes in oxygenation caused by motor activity induced by finger stimulation.
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