Age-related loss of activity of auditory-nerve fibers.
1. Characteristic frequencies (CF), spontaneous rates (SR), and thresholds were recorded from single fibers in the auditory nerves of gerbils aged for 36 mo in a quiet vivarium. The data from the quiet-aged animals were compared with similar data obtained previously from young controls. Fibers were classified as "low-SR" if their spontaneous rates were < or = 18 spikes/s and "high SR" for higher rates. 2. For CFs > 6 kHz, the percentage of low-SR fibers contacted declined from 57% of the population in young gerbils to 29% in the aged gerbils. This population change is statistically significant (P < 0.01). At CFs < 6 kHz, the population demographics did not change significantly with age, with the low-SR fibers comprising 30 and 39% of the population, respectively, for the young and aged animals. 3. To further test the hypothesis that low-SR fibers with CFs > 6 kHz become less active with age, additional experiments were conducted to examine the recovery of the compound action potential (CAP) response from prior high-level stimuli. Previous work has shown that the CAP recovery curve has two segments: a fast segment associated with the high-SR fibers and a slow segment associated with the low-SR fibers. The curves obtained from quiet aged gerbils show a faster recovery than young controls for probe tones at 8 and 16 kHz, but not at 2 and 4 kHz. Thus these results agree with our single-fiber data indicating that there is a loss of low-SR activity for CFs > 6 kHz in the aged animals. 4. Low-SR fibers typically have larger dynamic ranges than those of high-SR fibers, are better able to preserve information concerning stimulus timing and amplitude modulation, and their responses are more robust in the presence of masking noise. Moreover, low-SR fibers are likely inputs to the crossed-olivocochlear reflex, a reflex that serves an antimasking role in the detection of sounds in a binaural noise field. If true for humans, the loss of the low-SR system could explain many of the hearing deficits often seen in older individuals; e.g., decreased ability to understand speech in noise, changes in masking level differences, and decreased ability to localize sound sources using binaural cues.
Department of Otolaryngology and Communicative Sciences, Medical University of South Carolina, Charleston 29425-2242, USA.
R A
RA
Schmiedt
J H
JH
Mills
F A
FA
Boettcher
D000161:Acoustic Stimulation; D000375:Aging / Q000473:pathology / Q000502:physiology*; D000818:Animals; D005072:Evoked Potentials, Auditory / Q000502:physiology*; D005849:Gerbillinae; D016014:Linear Models; D009410:Nerve Degeneration / Q000502:physiology*; D009412:Nerve Fibers / Q000502:physiology* / Q000648:ultrastructure; D009474:Neurons / Q000502:physiology*; D000159:Vestibulocochlear Nerve / Q000502:physiology* / Q000648:ultrastructure