Khurana, L. | bioRxiv Science

bioRxiv2026

In silico framework for benchmarking optogenetic hearing restoration

Cochlear implants (CI) partially restore hearing in profoundly hearing impaired or deaf people by electrically stimulating the auditory nerve. A bottleneck of electrical CIs is the broad spread of electrical current from each electrode that limits the transfer of spectral information, which might be overcome by future spatially confined optogenetic stimulation. Here we established an in silico framework, FraSCO, to model sound encoding in the human cochlea by an optogenetic CI (oCI) for testing the potential of optogenetic hearing restoration. The biophysical modeling framework combined an optical raytracing model implementing a human cochlea implanted with a waveguide-based oCI with a single compartment model of optogenetically modified spiral ganglion neurons (SGNs). The input was an optogenetic sound coding strategy and the quality of the neural representation was evaluated based on comparison of neurograms evoked by optogenetic and electrical stimulation to the spectrogram of the sound applied. The model aimed for technologically feasible properties of the oCIs with 64 stimulation channels. The biophysical modeling framework successfully captured essential physiological features of optogenetic SGN stimulation with a minimal set of ion channel types expressed in the SGN soma. Working with a sample of 1000 SGNs distributed along the tonotopic axis to represent sound encoding, we found that improved spectral selectivity more than compensates for lower temporal fidelity of current implementations of optogenetic stimulation. The established computational framework enables in silico investigation and benchmarking of sound encoding in the cochlea by future oCI and state-of-the-art eCI. The results indicate that optogenetic sound encoding has potential to improve speech understanding in noisy environments for CI users.

neuroscience↗

bioRxiv2023

Is Inappropriate Pulse Timing Responsible for Poor Binaural Hearing with Cochlear Implants?

Cochlear implants (CIs) have restored enough of a sense of hearing to around one million severely hearing impaired patients to enable speech understanding in quiet. However, several aspects of hearing with CIs remain very poor. This includes a severely limited ability of CI patients to make use of interaural time difference (ITD) cues for spatial hearing and noise reduction. A major cause for this poor ITD sensitivity could be that current clinical devices fail to deliver ITD information in a manner that is accessible to the auditory pathway. CI processors measure the envelopes of incoming sounds and then stimulate the auditory nerve with electrical pulse trains which are amplitude modulated to reflect incoming sound envelopes. The timing of the pulses generated by the devices is largely or entirely independent of the incoming sounds. Consequently, bilateral CIs (biCIs) provide veridical envelope (ENV) ITDs but largely or entirely replace the "fine structure" ITDs that naturally occur in sounds with completely arbitrary electrical pulse timing (PT) ITDs. To assess the extent to which this matters, we devised experiments that measured the sensitivity of deafened rats to precisely and independently controlled PT and ENV ITDs for a variety of different CI pulse rates and envelope shapes. We observed that PT ITDs completely dominate ITD perception, while the sensitivity to ENV ITDs was almost negligible in comparison. This strongly suggests that the confusing yet powerful PT ITDs that contemporary clinical devices deliver to biCI patients may be a major cause of poor binaural hearing outcomes with biCIs. Significance StatementCIs deliver spectro-temporal envelopes, including speech formants, to severely deaf patients, but they do little to cater to the brains ability to process temporal sound features with sub-millisecond precision. CIs "sample" sound envelope signals rapidly and accurately, and thus provide information which should make it possible in principle for CI listeners to detect envelope ITDs in a similar way to normal listeners. However, here we demonstrate through behavioral experiments on CI implanted rats trained to detect sub-millisecond ITDs that pulse timing ITDs completely dominate binaural hearing. This provides the strongest confirmation to date that the arbitrary pulse timing widely used in current clinical CIs is a critical obstacle to good binaural hearing through prosthetic devices.

neuroscience↗