Demonstrated vs aspirational
Keep the ledger honest. Demonstrated: functional ultrasound imaging and even fUS decoding of intent in animals and in intraoperative/neonatal humans; battery-free neural-dust recording of peripheral nerve/muscle at millimetre scale; clinical high-intensity FUS ablation; FUS blood-brain-barrier opening in trials. Emerging / contested: reliable, mechanism-clear low-intensity neuromodulation; mammalian sonogenetics. Aspirational: dense untethered mote swarms for cortical recording, human sonogenetic therapy, and any notion of whole-brain acoustic read/write.
The mechanism problem
The deepest open problem in acoustic modulation is that we cannot yet predict, from parameters alone, whether a given dose will excite or inhibit a given target — and the auditory confound showed how easily an indirect effect is mistaken for a direct one. Progress needs validated biophysical models (cavitation, capacitance, radiation force), rigorous non-auditory controls, and dose-response maps. Until then, treat parameter choices as empirical, not principled.
Fundamental tradeoffs
Some limits are physics, not engineering. Focal volume shrinks with wavelength while penetration shrinks with attenuation, and both push on frequency in opposite directions — you cannot simultaneously get a tiny focus and deep reach with a fixed aperture.
The core tension in one line: a smaller focus wants high frequency (V ∝ f⁻³), but reach wants low frequency (z ∝ f⁻¹). Aperture and skull aberration set where the achievable compromise lands.
On the recording side, the backscatter channel is intrinsically low-rate. Its capacity is bounded like any channel, and the acoustic carrier plus modulation bandwidth limits how much each mote can send — the acoustic analogue of the wireless telemetry bottleneck and part of the broader fundamental bandwidth bounds on any interface.
A Shannon ceiling on the per-mote data rate: bandwidth B (a fraction of the carrier) times log of one-plus-SNR. Powering, backscatter SNR and safe insonation jointly cap what a swarm can stream.
Where it fits — an honest roadmap
Acoustic neurotech is unlikely to replace electrical or optical interfaces; it is likelier to own specific niches where its physics is decisive: deep non-invasive modulation (reaching targets TMS and tDCS cannot focus), untethered distributed recording where wires are the enemy, and deep functional imaging where light cannot go. Its natural allies are genetics (sonogenetics for specificity) and other wireless-power physics such as magnetoelectric nanoparticles.