Summary

A systematic review of 230 human low-intensity focused ultrasound experiments found that acoustic settings associated with neural facilitation or suppression differed between cortical and subcortical targets. The medRxiv preprint introduces SonoMap, an interactive resource for comparing these parameter-effect patterns.

A systematic review of human experiments suggests that low-intensity focused ultrasound neuromodulation cannot be designed around one set of acoustic rules for the whole brain. The medRxiv preprint, authored by researchers at the University of Michigan, found different parameter-effect patterns for cortical and subcortical targets and identifies the anatomical target as a central factor in protocol design.

The review analysed 230 experiments from 136 articles. It included studies targeting the human cerebrum that reported enough information about their acoustic settings and neuromodulatory outcomes to support comparison. The paper also introduces SonoMap, described as an open, interactive web resource for examining the evidence.

Contents

What the review examined

Low-intensity focused ultrasound, or LIFU, uses focused acoustic energy to modulate activity in neural tissue without an invasive procedure. One reason it is being studied for neuromodulation is that it can be directed at both superficial and deep brain structures.

A LIFU protocol is defined by several acoustic parameters. Pulse repetition frequency describes how often ultrasound pulses or bursts are repeated. Duty cycle describes the fraction of time the ultrasound is active during a treatment period. Intensity and dose describe other aspects of the delivered acoustic exposure.

The researchers compared how these parameters were associated with reported outcomes at different brain targets. The review therefore examined patterns across human experiments rather than testing one new ultrasound protocol in a single participant group.

Different brain targets showed different patterns

For cortical targets, the review found a nonmonotonic pattern involving pulse repetition frequency and duty cycle. In practical terms, the relationship did not move steadily in one direction as a parameter increased. Instead, the parameter space contained distinct zones associated with facilitation, suppression, or null and ambiguous effects.

Facilitation refers to reported outcomes indicating increased neural responsiveness or activity, while suppression refers to reduced activity or responsiveness. The presence of separate zones means that changing an acoustic setting can be associated with a different outcome depending on where the protocol sits in the broader parameter landscape.

Subcortical outcomes followed a different pattern. They varied predominantly with duty cycle, shifting from suppression at lower duty-cycle values towards facilitation at higher values. This contrast between cortical and subcortical targets is the review's central result.

The analysis also found that intensity and dose did not provide a consistent, target-independent explanation for whether an outcome would be facilitatory or suppressive. A setting that appears meaningful in one anatomical context therefore cannot automatically be treated as a general rule for another target.

Why the findings matter for ultrasound protocols

The results place the location of stimulation alongside the acoustic settings used to deliver it. Rather than asking only which intensity, frequency or dose produces a desired effect, protocol design may need to consider how those settings interact with the specific cortical or subcortical structure being targeted.

That distinction matters for efforts to use LIFU as a research tool or as a possible treatment platform. A protocol developed for one brain region may not produce the same type of neuromodulation when redirected elsewhere. SonoMap is intended to make these target-dependent relationships easier to inspect and support a more systematic approach to selecting parameters.

The findings also provide a more precise interpretation of the field's current challenge: LIFU can modulate human brain activity, but predicting the direction and persistence of the effect from acoustic settings remains difficult. The review specifically states that current evidence is insufficient to identify parameters that promote lasting effects.

Evidence status and practical boundary

This study is a medRxiv preprint rather than a peer-reviewed journal publication. Its evidence comes from a systematic synthesis of 230 human experiments, not from a single standardised clinical trial with one intervention and comparator. The included experiments also had to report sufficient acoustic information and neuromodulatory outcomes, tying the analysis to the studies that met those reporting criteria.

The review establishes target-dependent associations in the available human literature and offers a framework for organising them. It is not itself evidence that a particular LIFU protocol provides a clinical benefit for a specific disease. The authors' conclusion is instead focused on improving the design and interpretation of future human neuromodulation studies.

Sources