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.
Schmiedt
R A
RA
Department of Otolaryngology and Communicative Sciences, Medical University of South Carolina, Charleston 29425-2242, USA.
Mills
J H
JH
Boettcher
F A
FA
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