Near-infrared spectroscopy: does it function in functional activation studies of the adult brain?

Changes in optical properties of biological tissue can be examined by near- infrared spectroscopy (NIRS). The relative transparency of tissues includin g the skull to near-infrared light is the prerequisite to apply the method to brain research. We describe the methodology with respect to its applicab ility in non-invasive functional research of the adult cortex A summary of studies establishing the 'typical' response in NIRS vascular parameters, i. e. changes in the concentration of oxygenated and deoxygenated haemoglobin, over an activated area is followed by the validation of changes in the cyt ochrome-oxidase redox state in response to a visual stimulus. Proceeding fr om these findings a rough mapping of this metabolic response over the motio n-sensitive extrastriate visual area is demonstrated. NIRS measures concent ration changes in deoxygenated haemoglobin [deoxy-Hb] which are assumed to be the basis of fMRI BOLD contrast (blood oxygenation level-dependent), The method is therefore an excellent tool to validate assumptions on the physi ological basis underlying the fMRI signal, due to its high specificity as t o the parameters measured. Questions concerning the concept of 'activation' /'deactivation' and that of the linearity of the vascular response are disc ussed. To challenge the method we finally present results from a complex si ngle-trial motor paradigm study testing the hypothesis, that premotor poten tials (contingent negative variation) can be examined by functional techniq ues relying on the vascular response. Some of the work described here has b een 'published elsewhere. (C) 2000 Elsevier Science B.V. All rights reserve d.