A Multi-Channel EEG Mini-Cap for Recording Auditory Brainstem Responses in Chinchillas
According to the World Health Organization, disabling hearing loss affects nearly 466 million people worldwide. Sensorineural hearing loss (SNHL), which is characterized as damage to the inner ear (e.g., cochlear hair cells) and/or to the neural pathways connecting the inner ear and brain, accounts for 90\% of all disabling hearing loss. One important clinical measure of SNHL is an auditory evoked potential called the auditory brainstem response (ABR). The ABR is a non-invasive measure of synchronous neural activity across the peripheral auditory pathway (auditory nerve to the midbrain), comprised of a series of multiple waves occurring within the first 10 milliseconds after stimulus onset. In humans, oftentimes ABRs are recorded using a high-density EEG electrode cap (e.g., with 32 channels). In our lab, a long-term goal is to establish and characterize reliable and efficient non-invasive measures of hearing loss in our pre-clinical chinchilla models of SNHL that can be directly related to human clinical measures. Thus, bridging the gap between chinchilla and human data collection by using analogous measures is imperative. \par
For this project, a 32-channel EEG electrode mini-cap for recording ABRs in chinchillas was studied. Firstly, the feasibility of this new method to record ABRs demonstrated. Secondly, the sources of bias and variability relevant to the mini cap were investigated. In this investigation, the ability of the mini cap to produce highly reliable, repeatable, reproducible, and valid ABRs was verified. Finally, the benefits of this new method, in comparison to our current approach using three subdermal electrodes, were characterized. It was found that ABR responses were comparable across channels both in magnitude and morphology when referenced to a tiptrode in the ipsilateral ear canal. Consequently, averaging across several channels led to a reduction in overall noise and the need for fewer repetitions (in comparison to the subdermal method) to obtain reliable response. Other methodological benefits of the mini cap included closer alignment with human ABR data collection, more efficient data collection, and capability for more in-depth data analyses, like source localization (e.g., in cortical responses). Future work will include collecting ABRs using the EEG mini-cap before and after noise exposure, as well as exploring the potential to leverage different channels to isolate brainstem and midbrain contributions to evoked responses from simultaneous recordings.