Neurovascular coupling (the hemodynamic signal)

The physiological chain that ultrasound-based functional imaging actually measures: local neural activity triggers, through astrocytes and signalling molecules, a dilation of nearby arterioles and an increase in cerebral blood volume and flow. fUS power-Doppler is sensitive primarily to blood volume, giving it somewhat different weighting than the blood-oxygenation (BOLD) contrast of fMRI, but sharing the same fundamental indirection.

Frontier work treats neurovascular coupling as a signal to be deconvolved rather than a nuisance: estimating a hemodynamic response function, correcting for its several-hundred-millisecond lag and low-pass character, and questioning its constancy across brain regions, states (anaesthesia, disease), and development. The honest position is that the mapping from spikes to the vascular signal is nonlinear, region-dependent, and only partially understood.