Noninvasive near infrared optical imaging of human brain function with subsecond temporal resolution

Our understanding of human brain function can clearly benefit from neurophysiological techniques capable of providing dynamic maps of activity. A series of studies is reviewed indicating that noninvasive near-infrared optical imaging methods can provide a unique combination of spatial and temporal resolution that could be used to derive dynamic maps of human brain activity. The noninvasive NIR optical data reviewed are based on the frequency-domain time-resolved measurement of photon migration parameters (intensity and delay) through brain tissue. These measurements are taken through the intact surface of the head. With these methods, two distinct components of the optical response can be identified: the "slow optical signal" (2-10 s latency), presumably due to hemodynamic and metabolic changes, and the "fast optical signal" (or event-related optical response) occurring as early as 50 to 100 ms from stimulation, and probably due to neuronal activation.