Auditory Mondrian masks the airborne-auditory artifact of focused ultrasound stimulation in humans

Low-intensity Focused Ultrasound Stimulation (FUS) can modulate neural activity in cortical, subcortical, and deep brain regions, achieving millimeter precision through transcranial ultrasound stimulation (TUS) and can affect behavior [1,2]. However, there is concern that FUS may induce an auditory effect, cortically activating the subject's auditory sensation, thus confounding behavioral and electrophysiological responses during TUS research [3e7]. Several studies have mitigated this auditory artifact through ramping [8] or audio masking [5,6], but prior research has had two main limitations. First, previous studies addressed a limited range of the total FUS parameter space: fundamental frequency (f0), pulse repetition frequency (PRF), duty cycle (DC), sonication duration (SD), and intensity (I) [1,7,9e11]. Second, as these studies used direct stimulation of subjects, they did not distinguish between airborne or tissue conduction effects [5e7]. Here, we evaluate the airborne auditory artifact over a range of FUS parameters through sonographic characterization, the human subject's response to recorded audio clips, and a two-interval forced choice (2IFC) task to test the effectiveness of three mask types: square [5,6], pulsed sine, and randommultitone. The multitone randommask, or Auditory Mondrian, is inspired by the visual Mondrian used in the continuous flash suppression to mask visual targets [12]. We recruited 228 healthy participants for the three online auditory psychophysical experiments (See Supplementary Methods for details). In experiment 1, participants performed a detection task in which they were asked whether they detected a distinct sound while listening to audio recordings of FUS sham and stimulation trials. In experiments 2 and 3, participants performed a two-interval forced choice (2IFC) task in which they chose which interval of a pair contained the FUS stimulation embedded in an auditory mask. Audio clips from the microphone were used without volume (loudness) manipulation and confirmed by experimenters to match the sound produced from the FUS setup. In an artificial environment, we found that the ultrasound transducer is a primary source of airborne auditory artifacts (Fig. S1 A and B). Short-time Fourier transforms (STFT) of the audio recordings of FUS revealed clear frequency bands at the PRF and harmonics, along with additional frequency bands in the human hearing range that did not fit with the corresponding PRF (Fig. 1 A). These additional frequency bands were consistent at approximately 8 and 12 kHz throughout all PRF and even seenwith continuous wave US bursts (Fig. 1 A, yellow arrows) and appeared regardless of the coupling method or the cone and arm setup (Fig. 1 B). The electrical spectrum density showed no peaks at the human frequency range, so it likely did not contribute to the auditory artifact. Based on the above acoustic analyses, we concluded