Rate/level functions of auditory-nerve fibers in young and quiet-aged gerbils.

Steady-state rate/level functions of single auditory-nerve fibers to characteristic frequency (CF) tone bursts were measured in quiet-aged (35-37 months) and young control (4-7 months) gerbils. Rate/level functions of aged gerbils are different from those of young controls in that the thresholds are shifted to higher sound levels, but otherwise the shapes of the aged and young rate/level functions are similar. Specifically, there is little difference in the slope of the dynamic range portion of the rate/level functions when comparing aged gerbils to young controls. This is in contrast to whole-nerve input/output (I/O) functions of aged gerbils, which exhibit slopes that are less steep than those of the young controls (Hellstrom and Schmiedt, 1990b). Thus, it is likely that the deterioration of the CAP I/O function in aged animals is not due to a deterioration of rate/level functions in single units, but rather to other factors such as spiral ganglion cell degeneration or a loss of synchrony.