LAUSR

In activated brain regions, the deoxygenated hemoglobin (deoxy-Hb) concentration decreases despite an increase in oxygen consumption. This is attributed to the fact that the cerebral blood flow (CBF) induced by neuronal activation exceeds the accompanying increase in the cerebral metabolic rate of oxygen (CMRO2). The discrepancy between large CBF and disproportionately small CMRO2 responses provides the basis for detecting the hemodynamic correlates of neuronal activities by functional magnetic resonance imaging (fMRI). However, this implies that if the supply of oxygen is made smaller than the oxygen consumed by the suppression of stimulus-induced CBF, the polarity of signals would be reversed. We used near-infrared spectroscopy (NIRS) to search for a condition wherein a marked decrease in the stimulus-evoked oxygenated Hb (oxy-Hb) concentration change was accompanied by an increase in the deoxy-Hb concentration in the human brain. We found that when a specific brain region was activated by two working memory (WM) task blocks in rapid succession, the local change in the deoxy-Hb concentration evoked by the second task block was reversed to an increase due to the refractory effect in the hemodynamic response. The result suggests that the polarity of the blood oxygenation level-dependent (BOLD) signal could change during repetitive neuronal activation, and thus caution must be taken in the interpretation of the BOLD signal under such situations.