Auditory confounds can drive online effects of transcranial ultrasonic stimulation in humans
Benjamin R Kop, Yazan Shamli Oghli, Talyta C Grippe, Tulika Nandi, Judith Lefkes, Sjoerd W Meijer, Soha Farboud, Marwan Engels, Michelle Hamani, Melissa Null, Angela Radetz, Umair Hassan, Ghazaleh Darmani, Andrey Chetverikov, Hanneke EM den Ouden, Til Ole Bergmann, Robert Chen, Lennart Verhagen
eLife 2024, 12 · 10.7554/elife.88762
Abstract
Transcranial ultrasonic stimulation (TUS) is rapidly emerging as a promising non-invasive neuromodulation technique. TUS is already well-established in animal models, providing foundations to now optimize neuromodulatory efficacy for human applications. Across multiple studies, one promising protocol, pulsed at 1000 Hz, has consistently resulted in motor cortical inhibition in humans (Fomenko et al., 2020). At the same time, a parallel research line has highlighted the potentially confounding influence of peripheral auditory stimulation arising from TUS pulsing at audible frequencies. In this study, we disentangle direct neuromodulatory and indirect auditory contributions to motor inhibitory effects of TUS. To this end, we include tightly matched control conditions across four experiments, one preregistered, conducted independently at three institutions. We employed a combined transcranial ultrasonic and magnetic stimulation paradigm, where TMS-elicited motor-evoked potentials (MEPs) served as an index of corticospinal excitability. First, we replicated motor inhibitory effects of TUS but showed through both tight controls and manipulation of stimulation intensity, duration, and auditory masking conditions that this inhibition was driven by peripheral auditory stimulation, not direct neuromodulation. Furthermore, we consider neuromodulation beyond driving overall excitation/inhibition and show preliminary evidence of how TUS might interact with ongoing neural dynamics instead. Primarily, this study highlights the substantial shortcomings in accounting for the auditory confound in prior TUS-TMS work where only a flip-over sham and no active control was used. The field must critically reevaluate previous findings given the demonstrated impact of peripheral confounds. Furthermore, rigorous experimental design via (in)active control conditions is required to make substantiated claims in future TUS studies. Only when direct effects are disentangled from those driven by peripheral confounds can TUS fully realize its potential for research and clinical applications.
Abstract via europepmc.
Exposures
Exposure 1: Experiment I & III: on-target TUS to left-hemispheric hand motor area, 500 kHz
Target: primary motor cortex — “left-hemispheric hand motor area”
Device: Sonic Concepts · Sonic Concepts Inc, Bothell, WA, USA ✓
| Waveform | pulsed | |
|---|---|---|
| Fundamental frequency (kHz) | 500 | ✓✓✓ |
| Pulse duration (ms) | 0.3, 0.1swept | ✓✓✓ |
| Pulse repetition frequency (Hz) | 1,000 | ✓✓✓ |
| Duty cycle (%) | 30, 10swept | ✓✓✓ |
| Sonication duration (s) | 0.1, 0.5swept | ✓✓✓ |
| Free-field pressure (kPa) | 1,020, 1,440, 540swept | ✓✓✓⚑ |
|---|---|---|
| Free-field Isppa (W/cm²) | 32.5, 65, 9.26swept | ✓✓✓ |
| Free-field Ispta (W/cm²) | not reported | |
| In-situ estimate | not reported | |
| In-situ pressure (kPa) | not reported | |
| In-situ Isppa (W/cm²) | not reported | |
| In-situ Ispta (W/cm²) | not reported |
Ultrasonic stimulation was delivered at two pulse train durations (100/500 ms) and at two intensities (32.5/65 W/cm2 Isppa) in Experiment I to probe a dose-response effect; Experiment III used a 10% duty cycle and 500 ms pulse train duration with a single intensity (9.26 W/cm2). Table 1 reports PD=0.3/0.1 ms, PRF=1000 Hz, DC=30%/10%, PTD=100/500 ms and 500 ms respectively.
Exposure 2: Experiment II: on-target TUS to left-hemispheric hand motor area, 250 kHz
Target: primary motor cortex — “left-hemispheric hand motor area”
Device: Sonic Concepts · Sonic Concepts Inc, Bothell, WA, USA ✓
| Waveform | pulsed | |
|---|---|---|
| Fundamental frequency (kHz) | 250 | ✓✓✓ |
| Pulse duration (ms) | 0.3 | ✓✓✓ |
| Pulse repetition frequency (Hz) | 1,000 | ✓✓✓ |
| Duty cycle (%) | 30pulse duration × PRF gives 30% | ✓✓✓ |
| Sonication duration (s) | 0.5 | ✓✓✓ |
| Free-field pressure (kPa) | 450, 780swept | ✓✓✓ |
|---|---|---|
| Free-field Isppa (W/cm²) | 6.35, 19.06swept | ✓✓✓ |
| Free-field Ispta (W/cm²) | not reported | |
| In-situ estimate | not reported | |
| In-situ pressure (kPa) | not reported | |
| In-situ Isppa (W/cm²) | not reported | |
| In-situ Ispta (W/cm²) | not reported |
A preregistered replication/extension of Experiment I; a 2x2x2 design of stimulation site, intensity (6.35/19.06 W/cm2), and auditory masking, applied exclusively at an effective 500 ms pulse train duration; participants completed 25 trials per condition (250 trials total).
Exposure 3: Experiment IV: on-target TUS to white matter ventromedial to hand motor area (inactive control site), 250 kHz
Target: corticospinal tract — “white matter ventromedial to the hand motor area (corticospinal white matter)”
Device: Sonic Concepts · Sonic Concepts Inc, Bothell, WA, USA ✓
| Waveform | pulsed | |
|---|---|---|
| Fundamental frequency (kHz) | 250 | ✓✓✓ |
| Pulse duration (ms) | 0.3 | ✓✓✓ |
| Pulse repetition frequency (Hz) | 1,000 | ✓✓✓ |
| Duty cycle (%) | 30pulse duration × PRF gives 30% | ✓✓✓ |
| Sonication duration (s) | 0.4 | ✓✓✓ |
| Free-field pressure (kPa) | 370, 530, 580swept | ✓✓✓ |
|---|---|---|
| Free-field Isppa (W/cm²) | 4.34, 8.69, 10.52swept | ✓✓✓ |
| Free-field Ispta (W/cm²) | not reported | |
| In-situ estimate | not reported | |
| In-situ pressure (kPa) | not reported | |
| In-situ Isppa (W/cm²) | not reported | |
| In-situ Ispta (W/cm²) | not reported |
TUS applied to an inactive control site (white matter ventromedial to the hand motor area) at three intensities (4.34/8.69/10.52 W/cm2), i.e. three auditory confound volumes, both with and without a continuous auditory masking stimulus, to isolate the effect of auditory confound volume on motor excitability.
Flags from extraction
n_subjects— Paper reports four independent experiments (n=12,27,16,12) at three institutions; no combined total is stated, so group sizes are listed rather than summed.exposures[0].free_field.pressure_kpa— Table 1 only explicitly states that Isppa ('sppa') is measured in free water; the pressure ('P') column's domain is not explicitly stated in the table footnote, so free-field placement is inferred from being in the same row/measurement context.exposures[0]— Experiments I and III share the same target (hand motor area) and frequency (500 kHz) and were merged into one exposure with list-valued timing/intensity parameters per the one-exposure-per-target-x-frequency rule, even though they were run as nominally separate experiments at different institutions with different depths.n_sessions_per_subject— Paper does not explicitly state the number of sessions per participant; multi-block single-visit designs are described but no explicit session count is given.