Hearing Restoration using focused ultrasound neurostimulation – HEAR-US

Cochlear implants bypass the malfunctioning auditory periphery of profoundly-deaf people to electrically stimulate their auditory nerve. Although many implant listeners can follow auditory-only conversations in quiet environments, their speech recognition performances are much poorer in real-life scenarios where the sound they listen to is mixed with other sounds, such as noise. A commonly-acknowledged reason for this limitation is the poor spatial selectivity of the electrical stimulation. The current delivered by each electrode spreads widely across the cochlea and excites a large population of spiral ganglion neurons (SGNs). The Hear-US project aims to investigate ultrasound (US) neurostimulation as an alternative method to restore hearing to the deaf with an improved spatial selectivity compared to electrical stimulation. Focused US is a recently-explored method for neurostimulation. Evidence of direct neural excitation by US has been obtained for a wide range of stimulus parameters and for different types of neurons. The mechanisms of US stimulation remain, however, unclear and there is, to our knowledge, no demonstration that US can directly excite auditory neurons in mammals. Here we propose:
(1) To understand the physical and molecular mechanisms underlying US-evoked neural responses. A custom experimental set-up will be designed to measure the effects of focused US on neurons cultured in vitro using calcium imaging. Numerical simulations and experiments will be conducted to evaluate which US-induced phenomena (acoustic radiation force, streaming, heating) activate the neurons in our in vitro set-up. The molecular mechanisms will be investigated by first identifying the type of dorsal root ganglion (DRG) neurons responding to US stimulation. The RNA contents of US-responding and non-responding neurons will then be subjected to RNA deep sequencing and compared together to identify candidate US-responsive genes. The genes expressed in both the DRG and SGNs will be selected and their contribution to US responses further validated in an in-vitro gain and loss-of-function experiment.
(2) To investigate US stimulation at different levels of the auditory system in order to understand if and how US waves can be transduced into neural activity. These experiments will be performed in normal hearing and in deafened guinea pigs. Neural activity will be measured either at the level of the inferior colliculus or at the level of the auditory cortex using multichannel recording. The US parameters yielding neural excitation will be compared to those found in the in vitro measurements.
(3) To evaluate if US can produce more spatially-selective excitation than electrical stimulation. This hypothesis will be investigated both in vitro using cultured SGNs placed on micro-electrode arrays and in vivo using deafened guinea pigs.
The Hear-US project brings together three laboratories with complementary skills: the LMA with an expertise in cochlear implant design and US, the IBDM in molecular and cellular biology and the LNSC in auditory neurophysiology. This research aims to provide in the near future a new-generation of US cochlear and/or midbrain implants based on multi-element phased array, by analogy with multi-electrode cochlear implants. As previously mentioned, focused US will allow to selectively target subpopulations of SGNs which is not currently possible with standard cochlear implants. A particular interest in multi-element phased array would be the possibility to continuously steer the excitation along the cochlea, similar to what is being achieved in the normal auditory system, for acoustic stimulation. Our methodology combining in vitro and in vivo experimental set-ups, as well as computer simulations will also contribute to the understanding of US neurostimulation. Importantly, we believe that the present project is also timely for other applications that require focal stimulation, such as for blind or paralyzed subjects.