NIRS measurement of hemodynamic evoked responses in the primary somatosensory cortex by finger stimulation

To investigate the relationship between neuronal activity and hemodynamics, we carried out an NIRS study (near-infrared spectroscopy) to measure the regional changes of hemoglobin concentration associated with cortical activation in the primary somatosensory cortex (SI) to electrical stimuli. We examined the hemodynamic evoked responses to the electrical stimuli applied on the fingers (right thumb and ring finger, respectively). NIRS measurements were performed on 6 healthy right-handed volunteers using block paradigms and we analyzed both the spatial/temporal features and the magnitude of the optical signal induced by cerebral activation during these paradigms. Based on the experiment data, we consistently found an increase/decrease in the cerebral concentration of oxygenated hemoglobin [oxy-Hb]/deoxygenated hemoglobin [deoxy-Hb] at the cortical side contralateral to the stimulated side. Firstly, we found that, using NIRS, we can distinguish the local distribution of fingers on SI, whose result is in good accordance with the anatomical arrangement of hand area in SI. Secondly, we investigated the effects of electrical stimuli intensity on the hemodynamic evoked responses in SI. We found that, after raising the electrical stimuli intensity, an obvious increment of [oxy-Hb] in SI can be observed. Our findings based on NIRS optical imaging are in agreement with results in positron emission tomography (FET), functional magnetic resonance imaging (fMRI) and EEG (Electroencephalogram).