Spontaneous rates, thresholds and tuning of auditory-nerve fibers in the gerbil: comparisons to cat data

Characteristics of 245 auditory nerve fibers in eleven Mongolian gerbils are described in terms of spontaneous rates, thresholds, and tuning curves. The animals were reared in a low-noise environment and had similar hearing thresholds across frequency. Tuning curves were obtained with an algorithm developed to characterize the tuning of auditory fibers in cat, thereby allowing direct comparisons to published data from cat. The results demonstrate that basic similarities exist between gerbil and cat data, although some minor differences are also apparent. Tuning curve bandwidths, as measured 10 and 40 dB above the thresholds at the characteristic frequency (CF), follow trends found in cat data. Like cat, auditory nerve fibers in the gerbil have a range of spontaneous rates. In individual gerbils, fibers associated with low spontaneous rates have higher thresholds than do fibers of similar CF with high rates. Five of the eleven gerbils showed profiles of spontaneous rate across frequency reminiscent of those obtained from quiet-raised young cats. The profiles of the remaining gerbils tended to be compressed to a smaller range of spontaneous rates for characteristic frequencies above about five kHz, much like older cats with unknown noise histories. The cause of the spontaneous compression is not obvious. The correspondence between cat and gerbil with regard to spontaneous rate and CF threshold implies the presence of fundamental mechanisms that are common to mammalian auditory systems.