Seminars in Hearing Research at Purdue (SHRP)

Date: Thursday, November 04, 2021

Time: 10:30 – 11:20 am

Location: LYLE 1150

Speaker: Jonatan Märcher-Rørsted, PhD candidate, Technical University of Denmark (DTU)

Title:  Interactions between peripheral and central measures of temporal coding in a chinchilla model of noise-induced cochlear synaptopathy 

Abstract:

Steady-state electrophysiological responses phase-locked to the carrier or modulation frequencies of an auditory stimulus are reduced with age in humans. Age-related reductions in frequency following responses (FFRs) have been attributed to a decline in temporal processing in the central auditory system. Yet, age-related cochlear synaptopathy may reduce synchronized activity in the auditory nerve, which may also contribute to reduced FFR responses. Here, we investigate the effect of noise-induced cochlear synaptopathy on peripheral and brainstem temporal coding (i.e., the FFR) in a chinchilla model of temporary threshold shift (TTS) by simultaneously recording electrocochleography (ECochG) and (brainstem) FFR responses. 

We collected simultaneous electroencephalography (EEG) and ECochG responses from 10 anesthetized chinchillas. Four of the chinchillas were exposed to two hours of 100 dB SPL octave-band noise (centered at 1 kHz), producing a significant TTS measured one day post exposure. This exposure has been shown previously to create a broad region of significant (up to 50%) cochlear synaptopathy in chinchillas, with minimal permanent threshold shift. Electrophysiological responses in exposed chinchillas were measured at least two weeks post exposure. Two other animals were treated as controls. Additionally, we collected data from an older (n=4, 4-5 years of age) group of behavioral chinchillas, which were treated as an ”urban aged” group. FFRs to the carrier frequency of 10-ms tone bursts at low (516 Hz), mid (1032 Hz) and high (4064 Hz) frequencies, presented at levels ranging from 40 to 80 dB SPL, were recorded to examine potential level- and frequency-dependent effects. Additionally, responses to tonal frequency sweeps from 0.2-1.2 kHz at 80 dB SPL were collected. Evoked responses to clicks from 0-80 dB SPL were also recorded to quantify level-dependent latencies of different sources in the auditory pathway.  

Reduced ECochG responses to the carrier of low (516 Hz) frequency tones were observed in exposed animals. In peripheral neurophonic ECochG responses, we observed more pronounced reductions at higher levels. Brainstem FFR responses to lower-level low-frequency tones (516Hz at 60 dB SPL) showed more pronounced reductions compared to the peripheral neurophonic response in this condition, suggesting a level-dependent interaction between peripheral and central responses to low-frequency tonal stimuli. Reductions of the phase-independent second harmonic of the tonal carrier (two times the fundamental) were also observed in both peripheral and central measures, consistent with a neural origin of the reduced response. Aged chinchillas followed a similar trend as the noise exposed group in the ECochG responses, but showed further reduction of the FFR measured with a vertical EEG montage. This suggests that aging and noise induced cochlear synaptopathy could be disentangled measuring both ECochG and EEG responses simultaneously. 

Disentangling peripheral and central responses is crucial for our understanding of the underlying generation mechanisms of the FFR, and its connection to peripheral neural degeneration. These results suggest level- and frequency-dependent interactions between peripheral and central generators in both normal, noise-exposed TTS and aged animals, which may guide further advancements in diagnostics for peripheral neural degeneration. 

Zoom Info:

https://purdue-edu.zoom.us/j/93108158900?pwd=RDdTQ0Z4UE9Rb0JUenhjMG1SMkp2QT09

Meeting ID: 931 0815 8900

Passcode: 11501150

---

The working schedule for this academic is here:

https://purdue.edu/TPAN/hearing/shrp_schedule

The titles and abstracts of the talks will be added here:

https://purdue.edu/TPAN/hearing/shrp_abstracts