Auditory confounds can drive online effects of transcranial ultrasonic stimulation in humans

Abstract

Transcranial ultrasonic stimulation (TUS) is rapidly emerging as a promising non-invasive neuromodulation technique. TUS is already well-established in animal models, and now stimulation protocols that optimize neuromodulatory efficacy for human application are required. One promising protocol, pulsed at 1000 Hz, has consistently resulted in motor cortical inhibition. At the same time, a parallel research line has highlighted the potentially confounding influence of peripheral auditory stimulation arising from pulsing TUS at audible frequencies. Across four experiments, one preregistered, at three independent institutions, we employed tightly matched control conditions to disentangle direct neuromodulatory effects of TUS from those driven by the salient auditory confound in a combined transcranial ultrasonic and magnetic stimulation paradigm. We replicated motor cortical inhibition following TUS, but showed through both controls and manipulation of stimulation intensity, duration, and auditory masking conditions that this inhibition was driven by peripheral auditory stimulation rather than direct neuromodulation. This study highlights the substantial impact of the auditory confound, invites a reevaluation of prior findings, and calls for appropriate control conditions in future TUS work. Only when direct effects are disentangled from those driven by peripheral confounds can TUS fully realize its potential for neuroscientific research and clinical applications.

Effects of focused ultrasound in a “clean” mouse model of ultrasonic neuromodulation

“…31 Additionally, similar waveforms have also been used in various animal and human UNM experiments. 3,20,22,24,26,[32][33][34][35][36] As expected, when using the 270 kHz FUS, we were able to observe strong auditory confounds and off-target brain activation in normal hearing mice (Thy1-DT). In contrast, in deafened mice (PouThy1-DT), the widespread cortical responses were mostly eliminated when using low peak negative pressure FUS (≤ 500 kPa).…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] However, FUS can activate peripheral auditory pathways and cause off-target activation throughout the brain, including both ipsilateral and contralateral regions, regardless of the specific brain targets being stimulated. [20][21][22][23][24] These auditory confounds have become a constant challenge in ultrasonic neuromodulation (UNM) experiments in rodents. To eliminate these confounds, using deaf animal models is an effective approach.…”

High-throughput ultrasound neuromodulation in awake and freely behaving rats

“…In the thermal simulations, we assigned voxel-wise thermal conductivity and specific heat capacity [ 22 ] to the segmented CT data. For the bone compartment, we used a thermal conductivity of 0.32 W/m/°C and specific heat capacity of 1313 J/kg/°C; for the brain compartment, we used a thermal conductivity of 0.51 W/m/°C and specific heat capacity of 3630 J/kg/°C; for the water compartment, we used a thermal conductivity of 0.6 W/m/°C and specific heat capacity of 4178 J/kg/°C.…”

“…However, its underlying neurobiological mechanisms have remained elusive, and it is unclear how different parameters including pulsing frequency, duty cycle, stimulus duration, and acoustic intensity contribute to the elicited effects [ 2 , 18 , 19 ]. Critically, the neuromodulatory action of ultrasound has been linked to peripheral auditory responses [ 20 , 21 ], highlighting the need for carefully controlled investigations to disentangle direct neural effects from nonspecific auditory or other sensory confounds [ 22 ], particularly in rodent models.…”

Noninvasive intervention by transcranial ultrasound stimulation: Modulation of neural circuits and its clinical perspectives

“…In contrast, 'offline' protocols aim to induce effects lasting minutes to hours, utilizing long PD or trains of pulses lasting ≥20 s. Observations of offline TUS suggest not only local effects but also a network-wide impact on regions connected with the stimulation site, as evidenced by changes in functional connectivity in nonhuman primates with fMRI. 44,45 It is noteworthy that, while online TUS experiments may be compromised by auditory confounds, [46][47][48][49] offline TUS experiments mitigate such risks.…”