Jonatan Märcher-Rørsted1, Miguel Temboury Gutierrez1, Gerard Encina-Llamas1, Jens Hjortkjær1,2, Torsten Dau1
1Hearing Systems Section, Department of Health Technology, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark
2Danish Research Centre for Magnetic Resonance, Centre for Functional and Diagnostic Imaging and Research, Copenhagen University Hospital Hvidovre, DK-2650 Hvidovre, Denmark

Sound perception, in particular for speech and music, relies on accurate neural encoding of fast sound fluctuations. Neurons in the healthy auditory periphery and brainstem are able to phase-lock to these temporal fluctuations. Despite this being a fundamental aspect of the healthy hearing system, the limits of phase locking in humans remain unclear. Neuronal mass activity can be recorded in the form of electrical activity by electrodes placed on the scalp (i.e., electroencephalography, EEG). Phase-locked EEG responses to the carrier or the envelope of an auditory stimulus elicit a frequency following response (FFR). FFRs are reduced with increasing age. The reduction of FFR amplitude with age, particularly in listeners with clinically normal thresholds, has previously been associated with a decline in temporal processing due to desynchronization in the brainstem. However, motivated by outcomes from a recent modeling study, neural degeneration in the cochlea could account for such FFR reduction. If this would be supported experimentally, FFRs could be used as a biomarker of peripheral neural degeneration in humans. In the present study, FFRs in young and older clinically normal-hearing (NH) adults were recorded simultaneously using two electrode montages: a traditional vertical EEG montage mainly sensitive to central sources and through electrocochleography using tympanic membrane electrodes and ear canal electrodes to capture mainly peripheral sources. FFRs were recorded using tone-bursts at two frequencies (516 and 1086 Hz) and two durations (10 and 250 ms, respectively). Auditory brainstem responses (ABR) with the same electrode montages were also recorded. First results indicate that it may be possible to disentangle peripheral sources from central sources in both the FFRs and ABRs using this recording technique. If this is further validated, this technique may clarify the peripheral vs. central contributions to the reduction of FFR amplitudes in older participants.