Near infrared light
Absorption of light by blood
Cerebral Cortex
fMRI (functional Magnetic Resonance Imaging) is one of the facilities attached to the conventional MRI, which is used for detecting deoxy-hemoglobin. Deoxy-hemoglobin is one of the paramagnetic substances, which disturbs the surrounding magnetic field, and makes T2* ( effective Spin-Spin Interaction ) shorten (BOLD effect). When the brain activates, and the regional blood volume increases, oxy-hemoglobin concentration is overwhelmed with that of deoxy-hemoglobin, then the shield effect is releasing. So, MRI signal of deoxy-hemoglobin increases. Auditory stimulation of the brain is not feasible because of acoustic noise of MRI, while visual/motor/somatosensory tasks have no difficulty. However, the measurement of high-degree brain activities is apt to be interfered by the background noise.