During neural activity, increases in glucose and oxygen consumption and release of vasoactive neurotransmitters cause a local increase in cerebral blood fl ow (CBF)  and cerebral blood volume (CBV). Increases in oxygen consumption are signifi-cantly lower than increases in CBF and as a result we see a net increase in the amount of oxygen in the blood and tissue . Typical measures of neural activity in adults with near-infrared spectroscopy (NIRS) show a local increase in oxy-hemoglobin concentration (HbO) and a decrease in deoxy-hemoglobin concentration (HbR), which corresponds to a local increase in BOLD signal measured with fMRI. In many neonatal functional studies inversions of these hemoglobin signals have been reported, across visual [3, 4], olfactory , sensory-motor  and auditory  cortices. In general, the inversion starts at a few weeks of age.
The reason for such an inversion in the functional hemodynamic signals is not yet understood. We hypothesize that changes in hematocrit during the transition from fetal to adult hemoglobin and the consequent period of low hematocrit cause such an inversion. To test this hypothesis, we performed a longitudinal auditory functional study in premature infants.