Since the introduction of the fMRI technique a few years ago functional studies of the brain with good localization are now relatively common. One major drawback of the fMRI method is that the BOLD effect, which is at the basis of the method, is sensitive to changes in blood flow and volume rather than to neuronal activity. Furthermore, fMRI has not yet reached the necessary temporal resolution to follow the rapid changes associated with neuronal activation. Near-infrared light can pass through the skull and reach the surface of the brain. It is well established in exposed cortex experiments that brain activity changes the optical properties in the near-ir of the brain surface, due both to changes in blood flow and to scattering from the brain cells. Several researchers have proposed optical methods in the near-infrared spectral region to measure brain function non-invasively with high temporal resolution and good localization. While the detection of slow (in the second time scale) changes of blood flow by the near-ir method is well proven, the detection of optical changes associated with the fast (in the 10-100 ms scale) neuronal signal has been a relatively small field practiced by few experts. Our research has shown that it is possible to increase by at least one order of magnitude the detection of the small changes associated with neuronal activity. Our proposed developments and a new sensor could make this technique widely available and complementary to MRI in a newly proposed multimodality apparatus.
