Low-intensity focused ultrasound alters the latency and spatial patterns of sensory-evoked cortical responsesin vivo

Abstract

The use of transcranial, low intensity focused ultrasound (FUS) is an emerging neuromodulation technology that shows promise for both therapeutic and research applications. Compared with other noninvasive neuromodulation approaches, key technical advantages include high lateral resolution of stimulation and deep penetration depth. However, empirically observed effects in vivo are diverse; for example, variations in sonication location and waveform can alternatively elicit putatively inhibitory or excitatory effects. At a fundamental level, it is unclear how FUS alters the function of neural circuits at the site of sonication. To address this knowledge gap, we developed an approach to optically interrogate the spatiotemporal patterns of neural activity in the cortex directly at the acoustic focus, thereby offering a glimpse into the local effects of FUS on distributed populations of neurons in vivo. Our experiments probed electrical activity through the use of voltage sensitive dyes (VSDs) and, in transgenic GCaMP6f mice, monitored associated Ca 2+ responses. Our results directly demonstrate that low-intensity FUS adjusts both the kinetics and spatial patterns of sensory receptive fields at the acoustic focus in vivo. Although our experimental configuration limits interpretation to population activity, the use of VSDs ensures that the detected alterations reflect activity in cortical neurons, unobscured by signals in subcortical or laterally distant cortical regions. More generally, this optical measurement paradigm can be implemented to observe FUS-induced alterations in cortical representation with higher lateral resolution spatial versatility than is practical through more conventional electrodebased measurements. Our findings suggest that reports of FUS-induced sensory modulation in human studies may partly reflect alterations cortical representation and reactivity.

Safety of transcranial focused ultrasound stimulation: A systematic review of the state of knowledge from both human and animal studies

“…Most of the studies testing for cell damage or death [2,19,21,22,27e30,32e40] did not observe harmful effects of TFUS. One recent study [31] observed no differential GFAP expression between the control and sonicated hemisphere for I sppa ¼ 0.69 W/ cm 2 , suggesting the absence of neural trauma. However, an increased number of astrocytes was observed for a control condition with I sppa ¼ 280 W/cm 2 (~1.5 times above the FDA limit).…”

“…It is then tested whether TFUS causes the substance to diffuse into brain tissue. The dyes fluorescein isothiocyanate-dextran (FITCdextran) [27], trypan blue [28e30] or Evans blue [31] have been used for this purpose, and their presence inside the brain was investigated in post-mortem microscopy analyses of brain slices. Alternatively, an MRI contrast agent (a gadolinium chelate) was injected before the stimulation and its penetration into brain tissue was tested by assessing the MRI signal change due to the contrast agent [29].…”

“…[27]. It has been shown that neural trauma causes an abnormal increase in the number of astrocytes [44] that can be detected by the expression of GFAP (glial fibrillary acidic protein) [30,31,39]. One study assessed possible permanent tissue damage after sonication in rats using MRI [39].…”

“…Irreversible changes of neural activity: Recordings after the sonication can determine whether changes in neural activity are reversible and characterize the duration of recovery. The effects on local neural activity in the TFUS target region can be detected directly via invasive recordings or voltage sensitive dyes [31] or Ca 2þ imaging in transgenic mice that express the green fluorescent calcium indicator [31]. TFUS-related changes in extracellular concentrations of excitatory and inhibitory neurotransmitters such as glutamate and l-aminobutyric acid (GABA) can be measured via microdialysis techniques [37].…”

“…BBB opening did not occur in any of the included studies [28e30], except for two cases where it was intentionally provoked in control conditions [27,31], using a high I sppa of 280 W/cm 2 [31] or an ultrasound contrast agent [27].…”