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Abstract

Neuronal activation is coupled to localised changes in regional cerebral blood flow, blood oxygenation and metabolism (Leninger-Follert et al. 1979, Frostig et al. 1990). On this basis it is possible to detect and localise activated brain areas by the use of functional imaging methods like PET and fMRI (Phelps et al. 1985, Fox et al. 1986, Belliveau et al. 1991). The high spatial resolution of these imaging methods allows to characterise and localise hemodynamic and metabolic changes of activated brain areas on an anatomical basis. Near infrared spectroscopy noninvasively detects changes in the concentration of oxy-Hb, deoxy-Hb and Cyt-O2 by measuring changes in absorption at specific wavelength of light in the near infrared region. The technique in the first instance was used to detect global changes in cerebral hemodynamics (Jöbsis 1977, Elwell 1994) and was recently introduced to assess hemodynamic response induced by functional brain activation (Hoshi et al. 1993, Villringer et al. 1993, Obrig et al. 1995, Kato et al. 1993, Meek et al. 1995), The high temporal resolution and the ability to assess several oxygenation parameters simultaneously provides information about temporal dynamics of oxygenation changes in response to functional stimulation. Reasons for using this technique to investigate functional brain activation lie in some advantages compared to traditionally used functional imaging methods. Near infrared spectroscopy is completely non-invasive low expensive and can be used with high flexibility. NIRS allows repeated measures and administration of exogenous tracers is not required. The technique is therefore suited for assessment of brain function in clinical settings as a bedside technique.