About Electromyographic Signals and the Automatic Detection of the Stapedius Reflex

Cochlear implants help regain the auditory sense in cases where the cochlea is not able to transform the sound signals into electric stimulation for the auditory cortex. By their very nature, cochlear implants require a calibration process, which is often based on the electrically evoked stapedius reflex threshold (ESRT), since it very well correlates with a person's most comfortable loudness level[PZS + 09]. During the ESRT-based calibration process, the implant stimulates the cochlea with increasing currents until the stapedius muscle contraction can be observed visually. The calibration process described above yields a first, rough implant adjustment, which requires an additional, extensive calibration procedure a few weeks after the implantation. The goal of this project is the development of an autonomous calibration scheme that is constantly monitoring and occasionally re-calibrating the implant. To this end, the system utilizes the electromyographic (EMG) signals that it directly derives at the stapedius muscle by means of specific electrodes. The system basically correlates the external (objective) loudness and the observable stapedius muscle activity, and correlates them with the expected, pre-defined stapedius activity. Too large a difference between the expected and observable stapedius activity, the system triggers an adaptation cycle of the implant's gain factors. The processing of the EMG signals of the stapedius muscle employs a series of three filters: (1) a 4th-order band-pass filter with corner frequencies of 30 and 500 Hz for removing steady components and high-frequency noise, (2) a rectifier, and (3) a 4th-order low-pass filter with a corner frequency of 2 Hz for extracting the EMG signal's envelope. In its subsequent processing stages, the system considers this envelope as the physical activity of the stapedius muscle[SRSV10]. This system has undergone a series of different tests. Since no solid EMG signal data 1