Masking Differentially Affects Envelope-following Responses in Young and Aged Animals

Area of Science:

• Auditory Neuroscience
• Gerontology
• Speech Processing

Background:

• Age-related hearing loss involves threshold shifts and reduced auditory brainstem response (ABR) wave I amplitudes, indicative of cochlear synaptopathy/neuropathy.
• Older individuals often struggle with speech envelope processing, particularly in noisy environments, complicating the isolation of specific auditory deficits.
• Distinguishing the impacts of elevated hearing thresholds, synaptopathy, and noise on neural representations of sound is challenging.

Purpose of the Study:

• To compare the effects of notched, low-pass, and high-pass noise on envelope-following responses (EFRs) in aging.
• To investigate how peripheral (wave I ABR amplitude) and central (EFR amplitude) matching influence noise effects on EFRs in young and aged subjects.
• To assess the neural synchronization capabilities to modulated noise (NAM) in young versus aged animals.

Main Methods:

• EFRs to sinusoidally amplitude-modulated (SAM) tones were measured in young (60-80 dB SPL) and aged (85 dB SPL) animals under various noise conditions (notched, low-pass, high-pass).
• Sound levels were adjusted to peripherally match wave I ABR amplitudes or centrally match EFR amplitudes between age groups.
• Responses to SAM tones in modulated noise (NAM) were analyzed to assess neural synchronization.

Main Results:

• Low-level notched noise reduced EFRs in young animals up to 2-octave notch widths; high-pass noise also reduced EFRs, with greater effects at lower levels in young animals.
• Low-pass noise did not affect EFRs in either age group. High-pass noise appeared to impact young animals more than aged ones.
• Aged animals showed reduced ability to synchronize to NAM and maintain dual amplitude modulation (AM) representations compared to young animals, indicating age-related deficits in neural coding.

Conclusions:

• EFR amplitudes are significantly affected by aging and the presence of competing sounds.
• Competing sounds likely reduce or alter the pool of responsive neurons, impacting auditory processing in aged individuals.
• Aging compromises the neural mechanisms for processing complex auditory signals, especially in the presence of noise.