The OPTOCODE project focused on establishing frameworks to deliver optimal optical stimulation patterns for the optical cochlear implant (oCI).
Project objectives and focus:
Early in the project, objective O1—developing a non-invasive method to determine the frequency mapping of implanted oCIs—was not pursued further. Preliminary experiments indicated that auditory brainstem responses were too noisy to obtain reliable frequency maps within a feasible timeframe. The data collected during these attempts were analyzed and included in a peer-reviewed publication (Alekseev et al., 2025). Consequently, most project resources were fully directed toward objective O2, optimizing coding strategies using electrophysiology, optogenetics, and computational modeling.
In vivo opsin characterization:
Two opsins, CHREEF and FYTC, were tested in mice and gerbils to determine activation thresholds, kinetics, and suitability for oCI applications. FYTC demonstrated fast kinetics and effective activation with blue-light micro-LEDs, providing a validated stimulation parameter space for future oCI experiments. This characterization was crucial to defining feasible optical stimulation protocols given recording time constraints (Roos et al., 2025).
Acoustic IC recordings and predictive modeling:
A multielectrode recording pipeline was established for efficient measurement of population activity in the inferior colliculus (IC) of Mongolian gerbils. High-quality data from these recordings were used to develop a neural network model capable of predicting IC population responses to acoustic stimuli in near real-time (less than three hours). These results were presented at the Optogenetics Meeting 2025 and received a poster award.
Optical stimulation framework:
While the full cross-modal optimization combining acoustic and optical models has not yet been completed, the framework for optical stimulation has been established. The critical parameter space for optical stimulation, informed by opsin characterization and in vivo optogenetic activation, has been defined, providing a foundation for ongoing experiments.
Prior work integration:
Collaboration with a student supervised prior the OPTOCODE project led to a publication (Kondylidis et al., 2025) exploring fundamental sound preprocessing in the healthy cochlea. Although no optical implant was used in that study, the insights gained informed the predictive modeling framework for the current project.
Project limitations and mitigation:
The main limitation was the delayed delivery of hardware for optical stimulation. This delay was mitigated by securing an additional six months of funding from the institute, allowing the experimental component of O2 to be completed during 2026.
Summary:
Overall, the project successfully established key experimental and modeling frameworks for optimizing oCI coding strategies, including opsin characterization, IC recording pipelines, and acoustic response prediction models. These achievements lay the foundation for ongoing and future optical stimulation experiments aimed at realizing the full potential of optical cochlear implants.
