Image-guided, transcranial application of FUS to the V1, along with simultaneous acquisition of fMRI, revealed strong evidence in humans that FUS activates the sonicated brain area and concurrently elicits the associated efferent sensory perception in the form of phosphene. Successful stimulation of the V1 was also supported by the presence of cortical evoked EEG potentials having similarities with the classical VEP generated by photic stimulation. The stimulatory phenomena were transient and reversible without causing any discomfort or adverse effect across the study participants.

We found that pulsed application of the FUS, using similar parameters as those from our previous studies10,14,17 (i.e., TBD of 1 ms, PRF of 500 Hz, sonication duration of 300 ms), elicited the phosphene in more than half of the subjects (N = 11 responsive, N = 2 partially-responsive) during the fMRI and in all subjects during the EEG measurement sessions (N = 10). The sensation was described mostly as non-colored, non-patterned, amorphous phosphene, which occurred diffusely over the entire visual field without retinotopical arrangements. From three of the responsive subjects (‘h5’, ‘h9’, ‘h10’), patterned visual responses (‘vibrating’ or ‘line’) or colored-phosphene were reported. These types of phosphene sensations bear similarities to those reported in previous studies of TMS stimulation of the V127,28,29, whereby the perception of weak lights having a pale white/gray or sometimes an ‘unsaturated color’ was reported, sometimes even accompanying the presence of textured visual patterns30,31. Although retinotopic or topographic localization in the visual responses was not seen in the present study, further systemic examination of the localized stimulation of sub-regions in the V1 would be advantageous to reveal the spatial propagation of FUS stimulation at the cortical surface as well as in depth with column organizations.

The concurrent acquisition of BOLD-contrast based fMRI revealed that the site of activation in the V1 area was spatially aligned with the sonication target (Fig. 2, evident from the individual activation maps). The subject’s perception of the phosphene, supported by the electrophysiological data, indicates that the majority of the effects may indeed stem from the actual activation from the visual area that was sonicated. This observation is in good agreement with previous functional imaging (including fMRI and PET) investigations in small animals, whereby the localized neuromodulatory effects took place at the site of the sonication focus13,18,19. However, acoustic pressure-mediated mechano-vascular coupling and its manifestation in BOLD signal32,33 cannot be completely ruled out as a contributing factor. Further functional mapping investigations in humans using different image-modalities, such as PET (which examines the degree of glucose metabolic activity of the brain), would help revealing the stimulatory effects of the FUS in the brain. Besides the sonicated V1 area, neural substrates in the visual pathway (such as the LGN) and visual association areas (such as precuneus, fusiform, and ITG) were activated23,34,35, thus sharing the neural substrates that were activated during the photic stimulation. In contrast, sham sonication did not activate any of these areas (neither from the responsive nor the partially-/non-responsive group of subjects), suggesting that neuromodulatory effects only occur as a result of sonication.

Our fMRI data suggest that the sonication not only activated the visual circuits, but also activated other remote areas of the brain (Figs 2 and 3 and Supplementary Table S1). Although definite causes for the involvement of these brain areas are difficult to ascertain, a few conjectures can be made. For example, the activations in the fronto-parietal (including IFG, MFG, mSFG, and precuneus/IPL) and temporal areas (i.e., STG and MTG), together with the ACC and PCC, which all sub-serve the attention networks for cognitive processing24,25,26, may indicate potential engagement of attentional processes during the perception of phosphene in response to sonication27. In this context, the cerebellum (including vermis), departing from its traditional role in motor coordination/planning, might have been activated for the perception of phosphene due to its role in attentional processes36 and motion discrimination37. We also found that the PHG and thalamic AN/MD were bilaterally activated by the FUS stimulation, whereby these brain areas are known to participate in visual information processing, such as memory/navigation38 and recognition39,40. Specifically, the thalamic MD is an important substrate for the attention/intention circuits41 as well as executive functions42 as a part of cortico-striato-thalamo-cortical connections43. These data implicate that FUS stimulation increased neural activity from the targeted V1 area and from the network of regions involved in visual and higher-order cognitive processes, which is also supported by existing literature on TMS-induced phosphene perception27,28,29. Interestingly, the activations in some of these remote brain regions, e.g., frontal (MFG or mSFG), temporal (MTG), and cingulate gyri (PCC), including the thalamic area (i.e., VL), have also been reported from TMS stimulation of the V1 in humans44. This finding also indicates that the common neural substrates are activated even when different stimulation modalities are employed.

Direct comparisons between different experimental conditions (i.e., FUS versus photic stimulation and FUS versus sham sonication) showed that FUS and photic stimulation resulted in similar activation patterns among responsive individuals. On the other hand, the FUS condition, when compared to the sham condition, showed a higher level of activation from thalamic areas, fusiform, and frontal gyri (Supplementary Table S3). These areas were identified as being activated in the FUS condition only. On the other hand, the partially-/non-responsive group showed greater activation in the V1 and bilateral fusiform areas during photic stimulation when compared to the FUS condition, suggesting that V1 is less likely to be activated among the individuals who did not experience phosphene.

In addition to the fMRI observation from the sonicated brain, EEG measured during the acoustic stimulation showed a distinctive positive evoked potential peak at a latency of 100 ms after the onset of sonication (Fig. 4), bearing striking similarities with the P100 component detected during the photic stimulation (associated with activation of the extrastriate cortex45). The FUS also elicited the N55 component, which occurred a little earlier than the N70, believed to be derived from V1 activation during photic stimulation45. This may suggest that the observed N55 peak stems from the neural activity at the sonicated V1 area. Differences may exist in terms of the temporal sequence of neural activation, compared to the direct sensory stimulations. The N150/P250 components detected during the photic stimulation, reported to be related to the activation of occipito-parietal areas away from the V145,46, were not seen during acoustic stimulation. Based on these observations, along with fMRI data, we conclude that the FUS successfully stimulated the targeted visual area.

Our findings suggest that FUS can stimulate the human V1, resulting in the perception of phosphene and associated evoked potentials, while also showing a network of activated brain regions typically involved in visual and higher-order cognitive processes. The current data, therefore, does not support the spatially-selective activation of the sonicated V1 area, mostly due to the concomitant activations that were detected across the brain. Unlike functional imaging studies on anesthetized animals that showed spatially-selective activation at the sonicated region13,19, the cognitive processes from awake humans would prevent proper isolation of the brain region that was stimulated primarily by the FUS. Stimulation of unilateral somatosensory/somatomotor areas with a specific somatotopic arrangement (along with simultaneous fMRI mapping) or retinotopic creation of visual phenomena by sonicating a specific sub-region of the visual system will corroborate the spatial-selectivity of the method. Demonstration of selective stimulation of the deep brain area that is beyond the reach of the TMS technique (for example, subcortical thalamic nuclei) and the safety of the method awaits further investigations.

Based on the numerical simulation of acoustic intensity distribution around the sonicated V1 area, we found that the use of 270 kHz fundamental frequency, lower than that used in our previous small animal studies10,13, provided adequate acoustic transmission of the sonication through the skull. In most cases, the estimated location of the FUS focus was closely aligned with the target-of-interest (Table 1). It is interesting, however, to note that a few individuals (‘h12’–‘h16’, including partially responsive subjects, N = 5) did not report robust and repeated perception of the phosphene even though spatially-accurate sonication was given at an acoustic intensity level (i.e., 2.7–6.6 W/cm2 I sppa ) that was comparable to that given to the responsive individuals. We conjecture that this seemingly inconsistent perception of phosphene events may be attributed to the existence of individual variabilities/differences in the threshold acoustic intensity level for successful FUS stimulation, whereby a similar tendency has also been observed in previous studies involving both humans17 and animals11,14,47. The variability of the threshold in eliciting neural responses was also seen in TMS-mediated phosphene in humans (see Deblieck et al.48). To explain the large individual variability in responses that was observed, a measure of subject responses to different FUS intensities/parameters would be desirable. Alternatively, ‘active’ control conditions (for example, sonicating different locations within the V1) would have been valuable and constitutes a subject for future investigation. While the ultrasound-mediated alteration of cellular transmembrane capacitances/potentials49 or involvement of the glial systems50 has been hypothesized to be associated with the stimulatory phenomena, unveiling the underlying mechanism of acoustic neural stimulation would provide the key to explain these variabilities, or even perhaps reduce them. Further study on temporal sequence of cell-level neural activation with respect to the acoustic stimulus may also help to elucidate the mechanism.

Among a subset of subjects who did not report any visual sensations (i.e., ‘h17’–‘h19’, N = 3), the simulation revealed a rather large spatial error in sonication (ranging 7.2–16.1 mm from the targeted V1, Table 1). We conjecture that the differences in local skull shapes, such as osseous grooves often seen in the inner wall of the occipital-cerebellar junction of the skull, which may be present in the sonication path, might have resulted in an alteration of the propagation of the acoustic waves through the skull. In this context, guidance of the sonication path using on-site acoustic simulation with acoustic aberration corrections51 would further improve the spatial accuracy of transcranial FUS.

Through the neurological and neuroimaging examinations, the sonication parameters used in the present study appear to be safe, with a maximum estimated acoustic intensity of 11.6 W/cm2 I sppa (experienced in ‘h5’). The corresponding MI was 1.2, which is much lower than the US Food and Drug Administration (FDA) safety guideline limit of 1.9 for soft tissue sonication52. The estimated I spta at the target, averaged across the subjects, was on the order of 1.5 ± 0.9 W/cm2 (based on the 50% duty cycle of 3.0 ± 1.7 W/cm2 I sppa ; Table 1). This was lower than 3 W/cm2 I spta , which is in compliance with the international electrotechnical commission (IEC) 60601 part 2 standard for therapeutic equipment52. These results, along with the accumulating safety records from previous studies10,13,14,15,16,17, cast a promising feasibility for the safe administration of neuromodulatory FUS in humans. However, caution is needed to avoid the use of sonication at extremely high repetitions (>500 times and more) in short intervals (e.g., given every second) since this may cause microhemorrhages in the brain parenchyma, based on our recent observation from ovine experiments14.

Our results suggest that FUS can serve as a novel non-invasive stimulation technique in conscious humans. The emerging capabilities of FUS as a non-invasive stimulation modality of region-specific areas in the human brain may lead to many applications, for example, assessment of the regional brain functions and their functional connectivity to different parts of the brain. Along with its ability to modulate levels of neurotransmitters53,54, FUS may also be able to provide a new mode of neurotherapeutics for remedying various types of pathological conditions associated with aberrant neurotransmissions. Optimization of acoustic parameters for effective and safe sonication as well as further systemic investigations for revealing the detailed mechanism of acoustic neuromodulation would help to realize these potential applications.