TECHNIQUES FOR MEASURING BLOOD OXYGENATION

Neural firing is metabolically demanding and results first in a local increase in deoxygenated hemoglobin (deOxy-Hb), followed immediately by a compensatory sustained increase in oxygenated hemoglobin (Oxy-Hb) and subsequent decrease in deOxy-Hb (see Figure 1). The specific relationship of the circulatory system to local changes in neural activity is referred to as neurovascular coupling.



When a specific pool of neurons is active, the hemodynamic response is believed to be localized within 1–3mmof this neural activity (e.g., Shmuel et al. 2007, using 7T in human occipital cortex, V1) and likely represents the local field potentials rather than spiking activity of clusters of neurons (Goense&Logothetis 2008).However, the hemodynamic response unfolds slowly compared with the rapid change in neural activity. Even after a brief burst of stimulus-evoked neural activity (<1 s), the adult hemodynamic response peaks approximately 6 s later and the concentrations of Oxy-Hb and deOxy-Hb do not return to baseline until more than 10 s after the stimulus was initially presented (Malonek & Grinvald 1996) (see Figure 1). These spatial-temporal aspects of the hemodynamic response greatly constrain the designs and interpretations that can be drawn from neuroimaging methods that rely on neurovascular coupling.