Neural basis of functional ultrasound signals
Functional ultrasound imaging (fUSI) is a popular method for measuring blood flow and thus infer brain activity, but it relies on the physiology of neurovascular coupling and requires extensive signal processing. To establish to what degree its trial-by-trial signals reflect neural activity, we performed simultaneous fUSI and neural recordings with Neuropixels probes in awake mice. fUSI signals strongly correlated with the slow (<0.3 Hz) fluctuations in local firing rate, and were closely predicted by the smoothed firing rate of local neurons, particularly putative inhibitory neurons. The optimal smoothing filter had width ~3 s, matched the hemodynamic response function of awake mouse, was invariant across mice and stimulus conditions, and similar in cortex and hippocampus. fUSI signals also matched neural firing spatially: firing rates were as highly correlated across hemispheres as fUSI signals. Thus, hemodynamic signals measured by ultrasound bear a simple and accurate relationship to neuronal firing.