  Transcranial ultrasonic stimulation (TUS) is rapidly gaining traction for
non-invasive human neuromodulation, with a pressing need to establish protocols
that maximise neuromodulatory efficacy. In this review, we aggregate and
examine empirical evidence for the relationship between tunable TUS parameters
and in vitro and in vivo outcomes. Based on this multiscale approach, TUS
researchers can make better informed decisions about optimal parameter
settings. Importantly, we also discuss the challenges involved in extrapolating
results from prior empirical work to future interventions, including the
translation of protocols between models and the complex interaction between TUS
protocols and the brain. A synthesis of the empirical evidence suggests that
larger effects will be observed at lower frequencies within the sub-MHz range,
higher intensities and pressures than commonly administered thus far, and
longer pulses and pulse train durations. Nevertheless, we emphasise the need
for cautious interpretation of empirical data from different experimental
paradigms when basing protocols on prior work as we advance towards refined TUS
parameters for human neuromodulation.
