Relationships between ECAP amplitude growth functions and perceptual growth in cochlear implant users: Implications for individualized cochlear implant fitting.

OBJECTIVE: This study examined if the amplitude growth function (AGF) of the electrically evoked compound action potential (ECAP) can objectively predict the perceptual loudness growth function (LGF) in cochlear implant (CI) users. METHODS: ECAP AGFs were measured in 16 CI users across all active electrodes using the MED-EL AutoART fine-grain procedure. Perceptual LGFs were obtained with psychophysical loudness scaling at stimulation rates between 150 and 1200 pulses per second (pps). A third experiment collected ECAP and LGF data simultaneously at 80 pps. Both physiological and perceptual functions were normalized to their dynamic ranges, and a power-law transformation was fitted to model the transformation between ECAP amplitude and perceived loudness. The effects of stimulation rate and electrode position on the model parameters were examined statistically, and the model's predictive accuracy was assessed using a goodness-of-fit (R²) analysis. RESULTS: Loudness growth functions showed a systematic rate dependency: shifting from compressive at 80 pps to expansive at 1200 pps. Electrode-specific effects were less pronounced. The power-law exponent increased significantly with stimulation rate (p < 10⁻¹⁵), indicating steeper loudness growth at higher rates. The transformation model was remarkably accurate predicting perceptual loudness: 66% of fits yielded R²>0.8 and 45% >0.9, demonstrating strong correspondence between physiological and perceptual growth functions. CONCLUSIONS: A strong, systematic relationship exists between ECAP AGFs and perceptual loudness growth across stimulation rates in CI users. Modeling loudness growth from ECAP data provides a viable path toward objective, physiology-based individualization of compression functions in future CI fittings.