Spatio-temporal characteristics of neurovascular coupling in the anesthetized cat and the awake monkey

Understanding of the spatio-temporal characteristics of the sensory-evoked cortical blood-volume and oxygenation changes is important from the physiological perspective as well as for the interpretation of results obtained by various neuroimaging techniques, such as optical imaging, PET and f-MRI, and for their improvement. The detailed picture, however, has remained elusive for more than a century. We investigated the blood-volume and oxygenation changes in anesthetized cats and awake monkeys using intrinsic imaging at isosbestic and other wavelengths, laser Doppler, imaging spectroscopy, phosphorescence quenching and fluorescence imaging of activity-dependent responses of intravenously injected extrinsic probes. We found that the onset of blood-volume changes was delayed (>300 ms) with respect to a fast decrease in blood oxygenation. Thus, the blood-volume effects cannot merely explain the “initial dip”. 570-nm measurements and high-resolution imaging (80 ms, 7 μm) of a fluorescent tracer injected into the blood circulation facilitated the resolution of the responses of different microvascular compartments. Preliminary results show that the arterioles led the blood volume increase, rapidly spreading towards the other microvascular compartments. Veins lagged behind. Functional maps of stimulus vs. blank (single condition maps) in a conscious macaque and an anesthetized cat were obtained only during the early deoxygenation phase at 605 nm. At later times or at 570 nm, the vessel artifacts dominated. These results indicated a stronger co-localization of oxygen consumption and electrical activity as compared to the subsequent volume and flow increase, which are not well regulated at the cortical column level.