The understanding of brain function is one of the most exciting and difficult scientific challenges. All system neuroscience research suggests that decoding the brain black box will rely on a more precise description of brain tissue at a single-cell level (neuron, microglia, astrocytes, perycites…) or even at a subcellular level or beyond (membrane dynamics, nucleus, mitochondria, DNA, RNA…) but also of visualization of brain circuits at large scale.

To achieve this goal, we need to develop new technologies to probe brain activity at high spatiotemporal resolution, with a large field of view and in depth. The recent use of 2 photon imaging (2Pi) associated with genetically encoded voltage/calcium indicators (GEVI/GECI) has been key for analyzing specific basic brain circuits. Nevertheless, those strategies are limited to preclinical models (mostly rodents) and are hampered by the low penetration of photons due to tissue scattering. Moreover, most 2Pi studies are performed in head restrained conditions that limit the behavior repertoire and thus also limiting our understanding of brain circuits.

Multimodal imaging and circuit interrogation are promising tools for a better understanding of the brain

Therefore, there is a strong need for complementary methods for imaging and/or controlling brain activity, but the choices are limited and governed by the limitation of physics. We believe at NERF that ultrasounds have a strong potential to unravel brain functions because ultrasound waves are well suited to both image and interact with neuronal tissue. Ultrasound is the first clinical imaging modality. The large adoption of ultrasound by medical doctors can be explained by a set of specific benefits including a low cost, a high availability and a non-radiative imaging but the main advantage remains that ultrasound offers real-time analysis of tissues.

Nowadays, our team is breaking the limits of ultrasound technology based on our expertise in ultrasound hardware, software and algorithms. We are developing the most advanced ultrasound imaging platform in the world offering a high resolution (100µm3 voxel size, 100ms temporal resolution) with a large depth of field (several cm) in real-time. This technology is based on a novel generation of ultrasound scanner currently in development associated with a dedicated engine for massively parallel processing. Ultimately, it offers advanced computational capabilities (machine learning, computational neural network).

fUSi has been applied to rodent’s models where for the first time, we have been able to follow brain activation in all relays of a complete circuit from the input (ie a somatosensory cue) to the output (i.e. behavior) in real-time. Currently, we are working in collaboration with a set of selected key opinion leaders all around the world in preclinical research (rodents and primates) to validate the use of this technology in a larger extend and we are involved in several clinical trials (feasibility study) for helping the neurosurgeon identifying eloquent brain areas during resection.

Our team is refining the fUSi technology and we have recently proven its efficiency when combined with minimally invasive implantable flexible electrode array for modifying in real-time (in open or close loop paradigms) brain activity using epidural stimulation during behavioral task as a viable replacement of current invasive electrode array in human. We foresee that functional ultrasound imaging will be a tool of choice to identify what part of the brain is active during brain stimulation using external tools (ie TMS, TDCS,…) or internal tools (ie electrodes used in DBS, in BCI or during surgery in patient, optogenetics in preclinical) to stimulate brain activity.

Moreover, it is important to keep in mind that ultrasound have also proven their efficiency for therapy (high-intensity of focused ultrasound HIFU), BBB opening for drugs targeting (focused ultrasound) and also for modulation of neural activity. Our hardware/software-based platform has been designed to further support those applications with the goal of making digital ultrasound a generic approach to control and image brain activity in real-time.

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Volumetric functional ultrasound imaging to dissect visually-evoked brain-wide activity, by Clément Brunner.

Large scale circuit dynamics during multi-sensory stimulation in awake rodents

The ability to use cues from multiple senses together is a fundamental aspect of brain. Multisensory integration may enhance the physiological salience of external events and is defined experimentally as a statistically significant difference between the response evoked by a cross-modal combination of stimuli versus by the most effective of its components individually. Even if multisensory neurons have been identified in many brain areas (i.e. superior colliculus, cortex and thalamus), the underlying neural circuits of multisensory integration remain poorly understood due to the lack of techniques to probe large scale circuits dynamics with a high sensitivity. We applied functional ultrasound imaging (fUSi) in awake mice in chronic conditions to assess the hemodynamic responses evoked by tactile and auditory stimuli and/or by their combination.

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The automatic detection of the mouse behavior in a droplet reaching task. Video by Dries Kil and analysis by Yun-An Huang.

Role of specific cell populations in the regulation of the NVC

It has been clear for more than a century that the tight coupling between neuronal activity and regional cerebral blood flow is essential for normal brain function. Increases in neuronal activity are associated with an increase of local perfusion, known as functional hyperemia. This activation is controlled by the neuro-glio-vascular unit, composed of terminals of neurons, astrocytes, glial cells, pericytes, blood vessel muscles and more. Despite its importance in both health and disease, the cellular and molecular mechanisms of neurovascular coupling remain poorly understood.

One of the questions the Urban lab aims to address is how activation of the brain triggers functional hyperemia. The answer will help us to better understand diseases such as stroke, migraine or Alzheimer’s disease.

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“Mouse Brain Vasculature Atlas”, by Micheline Grillet. Ultrasound imaging of the whole brain mouse vasculature with a motorized linear ultrasound probe.