Neurovascular coupling: the signal fUS reads
Functional ultrasound (fUS) does not sense spikes or membrane voltage. Like fMRI, it reads a hemodynamic proxy: when a local population becomes active, neurovascular coupling increases local cerebral blood volume a second or so later. fUS measures that blood-volume change with sound. This is a strength — it images a whole slice at depth, non-genetically — and a hard limit — it is indirect and slow.
The measured blood-volume signal s(t) is the neural activity a(t) convolved with a slow hemodynamic response h(τ). Because h is a low-pass filter (seconds wide), fUS can never be faster than the vasculature it watches.
Ultrafast plane-wave power Doppler
The enabling trick is ultrafast imaging. Instead of scanning focused beams line by line, fUS transmits unfocused plane waves and reconstructs the whole image computationally, reaching thousands of frames per second. That huge frame rate lets a power Doppler estimate — the energy of the Doppler signal, proportional to the number of moving red cells — detect the tiny blood-volume changes that track activity.
The Doppler shift from blood moving at velocity v. Power Doppler sums the signal's energy rather than its mean shift, making it far less angle-dependent and sensitive to slow flow in small vessels — exactly what activity imaging needs.
From imaging to a BCI
The step to a fUS-BCI is to decode intent from these blood-volume maps. In non-human primates, movement intentions (for example, planned eye or reach direction) have been decoded from cerebral blood volume in parietal cortex, and even predicted before the movement — evidence that the hemodynamic signal, slow as it is, still carries usable intent. Because the probe images a whole plane at once, a single acquisition sees many areas, which is attractive for decoding distributed states.
A complementary frontier is ultrasound localization microscopy (ULM): injected microbubbles are localized one at a time and accumulated over minutes, resolving the microvasculature below the diffraction limit. ULM trades temporal resolution for spatial detail — a super-resolution map of vessels rather than a fast movie of activity.