A litter of four cats, born and raised in a soundproofed chamber, was studied in an attempt to determine which, if any, features of the auditory-nerve response from routinely available cats might be due to the chronic effects of noise exposure. Two features of routine-normal response were especially suspect in this regard: (1) a "notch" in the distribution of single-unit thresholds centered at characteristic frequencies (CF's) near 3 kHz and (2) a compression of the distribution of rates of spontaneous discharge for units with CF above 10 kHz. A third feature of response in routine animals was the presence of a small number (roughly 10%) of units with virtually no spontaneous discharge and very high thresholds, sometimes 80 dB less sensitive than high-spontaneous units of similar CF. In the data from chamber-raised animals, the high-spontaneous units showed exceptionally low thresholds at all CF regions, however, there were signs of the midfrequency notch in the threshold distribution of at least two of these animals. The compression of the spontaneous rate distribution was not seen in any of the three most sensitive animals. The data suggest that there is a significant amount of "normal pathology" in the high-CF units from routine animals. Low-spontaneous, high-threshold units were present in all four chamber-raised ears with the same characteristics as in routine animals (exceptionally narrow tuning curves and exceptionally low maximum discharge rates) and at roughly the same percentage of the unit sample. A class of units with medium spontaneous rates and intermediate thresholds could also be identified. The possible significance of a classification of auditory-nerve units according to spontaneous rate is discussed.
