Abstract
Millisecond changes in the optical properties of the human brain during stimulation were detected in five volunteers using noninvasive frequency-domain near-infrared spectroscopy. During a motor stimulation task we found highly significant signals, which were directly related to neuronal activity and exhibited much more localized patterns than the slow hemodynamic signals that are also detected by the near-infrared method. We considerably reduced the noise in the instrumental system and improved data analysis algorithms. With the achieved signal-to-noise ratio, single subject measurements were feasible without the requirement of particularly strong stimuli and within a reasonable period of measurement of 5 min at a mean signal-to-noise ratio of 3.6. The advantage of this noninvasive technique with respect to electrical recording is that it is able to detect neuronal activity with the relatively high spatial resolution of 8 mm.
Highlights
Brain activity is associated with physiological changes of the optical properties of the tissue that can be measured by noninvasive near-infrared spectroscopy (NIRS)
The activated area of the brain consumes more oxygen and glucose, which leads to an increase in blood flow, which is accompanied by an increase in oxyhemoglobin concentration (O2Hb) and a decrease in deoxyhemoglobin concentration (HHb), which are both detected by NIRS
The optical signal is directly related to neuronal activity, as in EEG or MEG, which is in contrast to conventional functional NIRS, fMRI (BOLD signal) or PET, which detect only the slow hemodynamic response to neuronal activity
Summary
Brain activity is associated with physiological changes of the optical properties of the tissue that can be measured by noninvasive near-infrared spectroscopy (NIRS). The activated area of the brain consumes more oxygen and glucose, which leads to an increase in blood flow, which is accompanied by an increase in oxyhemoglobin concentration (O2Hb) and a decrease in deoxyhemoglobin concentration (HHb), which are both detected by NIRS This signal has been demonstrated by many authors (Maki et al, 1995; Hirth et al, 1996; Obrig et al, 1996; Villringer and Chance, 1997, Colier et al, 1999; Franceschini et al, 2000; Toronov et al, 2000, 2001) and corresponds to the BOLD signal, which is the basis of the fMRI technique. The second signal consists of fast changes in the optical properties of cerebral tissue, which appear in the range of milliseconds after stimulation. The optical signal is directly related to neuronal activity, as in EEG or MEG, which is in contrast to conventional functional NIRS, fMRI (BOLD signal) or PET, which detect only the slow hemodynamic response to neuronal activity
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