Neurovascular Uncoupling: Much Ado about Nothing.

The vast majority of human fMRI studies measure Blood-Oxygen-Level-Dependent (BOLD) contrast, which reflects regional changes in cerebral blood flow (CBF), cerebral blood volume (CBV) and blood oxygenation; all three vascular responses reflect local increases in neural activity (Logothetis and Wandell, 2004). Understanding the exact mechanism (often referred to as neurovascular coupling) by means of which changes in neural activity alter hemodynamics is obviously of paramount importance for the meaningful interpretation of fMRI results. Not surprisingly, over the last decade an increasing number of researchers investigated the neurovascular coupling by combining fMRI with electroencephalography (EEG) or magnetoencephalography (MEG) in humans, e.g. (Dale and Halgren, 2001) as well as with intracortical recordings in animals (Logothetis et al., 2001; Goense and Logothetis, 2008; Logothetis, 2008; Rauch et al., 2008). This neurovascular coupling can also be studied with the optical imaging of intrinsic signals (OIS) (Bonhoeffer and Grinvald, 1996), an excellent invasive method of high spatiotemporal resolution that can measure changes in oxygenation and/or blood volume.