Hearing the light

The electrical cochlear implant (CI) is considered the most successful neuroprosthesis. Implanted in more than 200,000 hearing-impaired subjects worldwide, CIs enable open speech comprehension in the majority of users. A major drawback of current cochlear prostheses is their low frequency resolution due to current spread of electrical stimulation, limiting their use in music enjoyment and prosody. Thus, there is need for spatially confined stimulation to improve frequency resolution. In a recent Journal of Clinical Investigation article, Tobias Moser and his colleagues demonstrate the feasibility of optogenetic activation of the auditory pathway by stimulating the auditory nerve inside the cochlea (Hernandez et al., 2014).

Electrical versus optical stimulation of the cochlea. Left: in electrical CIs usually 12-24 electrodes are used to stimulate SGNs. Current spread leads to activation of a large population of neurons along the tonotopic axis, thereby limiting the frequency resolution and dynamic range of electrical coding. Right: optical stimulation promises spatially confined activation of SGNs allowing for a higher number of independent stimulation channels and, thereby, improving frequency and intensity resolution.

Electrical versus optical stimulation of the cochlea
Left: in electrical CIs usually 12-24 electrodes are used to stimulate SGNs. Current spread leads to activation of a large population of neurons along the tonotopic axis, thereby limiting the frequency resolution and dynamic range of electrical coding.
Right: optical stimulation promises spatially confined activation of SGNs allowing for a higher number of independent stimulation channels and, thereby, improving frequency and intensity resolution.

Two animal models were employed: a transgenic model that expresses ChR2 under control of the Thy1.2 promoter in auditory neurons and an adeno-associated virus-mediated model that expressed the ChR2 variant CatCh (AAV2/6-hSyn-CatCh-YFP). Optogenetic stimulation of spiral ganglion neurons (SGN) evoked auditory brainstem responses both in normal hearing mice and in mouse models of deafness, suggesting the general feasability of cochlear optogenetics for restoring auditory activity.

Approximation of the spatial spread of cochlear excitation by several means, including local field potential recordings in the inferior colliculus in response to suprathreshold optical, acoustic, and electrical stimuli indicated that even unfocused optogenetic stimulation achieves better frequency resolution than monopolar electrical stimulation. Virus-mediated expression of a ChR2 variant with greater light sensitivity in auditory nerve neurons reduced the amount of light required for responses and allowed neuronal spiking following stimulation up to at least 60 Hz. The study demonstrates a strategy for optogenetic stimulation of the auditory pathway in rodents and lays the groundwork for future applications of cochlear optogenetics in auditory research and prosthetics.

Reference
Hernandez VH, Gehrt A, Reuter K, Jing Z, Jeschke M, Mendoza Schulz A, Hoch G, Bartels M, Vogt G, Garnham CW, Yawo H, Fukazawa Y, Augustine GJ, Bamberg E, K├╝gler S, Salditt T, de Hoz, L, Strenzke N, Moser T (2014) Optogenetic stimulation of the auditory pathway. J Clin Investigation, 124(3): 1114-29.

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