Transcranial ultrasound stimulation (TUS) is a new and exciting means of non-invasively modulating neural circuits in the intact human brain, offering far greater spatial resolution and depth of stimulation than is possible with transcranial magnetic stimulation (TMS) or transcranial electrical stimulation (tES).
For instance: transcranial electrical stimulation is shunted by the skull, meaning that only a small amount of the total current applied actually reaches the cortex successfully. And while transcranial magnetic stimulation does offer the ability to reveal the chronometry of neural processes with a reasonable degree of spatial resolution, it suffers from a depth-accuracy tradeoff whereby stimulating at greater depths comes at the cost of a reduction in spatial resolution1.
Transcranial focused ultrasound neuromodulation, on the other hand, offers a new means of modulating neural circuits using an acoustic mechanism. tFUS generates an acoustic wave via piezoelectricity — a technique in which electricity ﬂows through a crystal, causing it to vibrate — and then releases this energy through the ultrasound transducer. The shape of the ultrasound transducer enables the acoustic energy to be acutely focused to one point in space: in the case of transcranial focused ultrasound stimulation, the acoustic wave can be focused at depths beneath the cortical surface without any loss of spatial resolution.
Applications and Effects of TUS
The ability to stimulate beneath the cortical surface has enabled new sites to become candidates for non-invasive brain stimulation. In one example, tFUS has enabled the causal functional relevance of the anterior cingulate cortex (ACC), the thalamus and amygdala, to be investigated. Such investigations have revealed that the ACC is involved in representing the interval value of choices that have not been made immediately, but could be made in the future, and whether these representations were used to select a different choice in the future2.
In addition, another study applying tFUS for 40 seconds found an effect that reduced the ACC’s connectivity pattern with interconnected regions, lasting for approximately one hour after the application of stimulation3. Critically, the spatial distribution of the effects measured by functional magnetic resonance imaging (fMRI) corresponded to the points in space where the acoustic wave was estimated to be greatest on computed tomography (CT) scans.
The effects of tFUS have also been observed during electroencephalography (EEG) when focused ultrasound neuromodulation is estimated to have an acoustic focus on the position of the ventro-posterior lateral (VPL) nucleus of the thalamus4. A speciﬁc component of EEG — the P14 — is sensitive to the VPL when the median nerve is stimulated5. When tFUS was applied to the thalamus, the amplitude of the P14 was reduced: consistent with the theory that tFUS can stimulate sites deep beneath the cortical surface, with effects that can be measured non-invasively from the scalp.
- Coil design considerations for deep transcranial magnetic stimulation.. Deng Z., Lisanby S H., Peterchev A V.. Clinical Neurophysiology, 125.. (June 2014), pp. 1202-1212.
- The macaque anterior cingulate cortex translates counterfactual choice value into actual behavioural change.. Fouragnan E F., Chau B H K., Folloni D., Kolling N., Verhagen L., Klein-Flügge M., Tankelevitch L., Papageorgiou G K., Aubry J., Sallet J., Rushworth M.. Nature Neuroscience, 22.. (April 2019), pp. 797-808.
- Offline impact of transcranial focused ultrasound on cortical activation in primates.. Verhagen L., Gallea C., Folloni D., Constans C., Jensen D E A., Ahnine H., Roumazeilles L., Santin M., Ahmed B., Lehericy S., Klein-Flügge M C., Krug K., Mars B R., Rushworth M F S, Pouget P, Aubry J., Sallet J.. eLife.. (February 2019).
- Neuromodulation with single-element transcranial focused ultrasound in human thalmus.. Legon W., Ai L., Bansal P., Mueller J K.. Human Brain Mapping, 39.. (May 2018), pp. 1995 - 2006.
- Somatosensory evoked potentials from the thalamic sensory relay nucleus (VPS) in humans: correlations with short latency somatosensory evoked potentials recorded at the scalp.. Katayama Y., Tsubokawa T.. Electroencephalography and Clinical Neurophysiology/Evoked Potentials Section, 68.. (May 1987), pp. 187-201.
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