Synaptopathy in the Aging Cochlea: Characterizing Early-Neural Deficits in Auditory Temporal Envelope Processing

Area of Science:

• Auditory Neuroscience
• Gerontology
• Neurobiology

Background:

• Aging is associated with difficulties in processing complex sounds, like speech in noise, even without significant hearing threshold loss.
• Evidence suggests a progressive loss of synapses between inner hair cells and auditory nerve fibers occurs with normal aging.
• The specific role of this cochlear synaptopathy in age-related temporal processing deficits remains unclear.

Purpose of the Study:

• To investigate age-related changes in encoding stimulus envelopes using population envelope following responses.
• To examine the impact of cochlear synaptopathy on temporal processing at different stages of the auditory pathway.
• To correlate neural coding deficits with the degree of cochlear synaptopathy in aging mice.

Main Methods:

• Utilized an age-graded series of CBA/CaJ mice (male and female).
• Measured population envelope following responses and other hair cell/neural-based indicators of auditory processing.
• Employed the neurotoxin ouabain to selectively assess pre- and post-synaptic function in the cochlea.

Main Results:

• Deficits in neural coding of auditory stimuli emerge at the earliest neural stages of processing in aging mice.
• These early neural deficits are significantly correlated with the extent of cochlear synaptopathy.
• Age-related changes were more pronounced in suprathreshold sound processing and preceded changes in neural thresholds.

Conclusions:

• Progressive cochlear synaptopathy is a key factor contributing to age-related deficits in temporal coding.
• These early neural processing impairments affect suprathreshold sound perception, impacting real-world communication.
• The noninvasive methods used may be adaptable for clinical detection of cochlear synaptopathy in aging individuals.