COCHLEAR NERVE ACTIVITY AFTER INTENSE SOUND EXPOSURE IN NEONATAL CHICKS

1. Single neuron behavior in the cochlear nerve of neonatal (3-day-old ) chicks was examined after exposure to a 120-dB SPL pure tone (0.9 kH z) for 48 h. Exposed animals were tested after 0 days or 12 days of re covery. Nonexposed chicks, age-matched to the exposed animals, formed two control groups. 2. Spectral response plots were obtained from each cell. These plots described the neuron discharge rates in response to 1,767 tone burst stimuli, each with a unique frequency-intensity comb ination. The tone bursts were presented at frequencies between 0.1 and 4.5 kHz and for intensities between 0 and 100 dB SPL. From these plot s the characteristic frequency (CF), CF threshold, and sharpness of tu ning (Q(10 dB)) were derived for each cell. Frequency response-area fu nctions at selected stimulus levels and rate-intensity functions at th e CF were also constructed from the spectral response plots. In additi on, spontaneous activity was determined. Data were obtained from 903 c ells. 3. Neuron activity in the control cells revealed no differences between CF thresholds, Q(10 dB), or spontaneous activity in the two ag e groups. However, age differences at all frequencies were noted in th e rate-intensity functions. 4. A frequency-dependent loss in CF thresh old was observed in the 0-day recovered cells. The threshold shift (re lative to age-matched control cells) was 55-65 dB between 0.8 and 1.5 kHz, but only 10-15 dB between 0.1-0.4 kHz and 2.5-3.5 kHz. The expose d cells showed no loss in frequency selectivity (Q(10 dB)) at < 0.5 kH z, whereas above this frequency an increasing deterioration in tuning was noted. Spontaneous activity in the 0-day cells was suppressed acro ss the entire range of CFs. The rate-intensity function of exposed cel ls had a steeper growth rate than that of control cells. 5. At 12 days of recovery, CF threshold, Q(10dB), and spontaneous activity all reco vered to the levels exhibited by age-matched control cells. However, t he rate-intensity function for cells with CFs between 0.8 and 1.0 kHz showed abnormal growth and higher discharge rates at saturation than t he control cells. Outside of this frequency range the rate-intensity f unctions of control and exposed cells were similar to each other. 6. R ecovery of function in the sound-damaged chick ear is accompanied by a lmost complete repair of the basilar papilla. The tectorial membrane, however, retains a major defect and only the lower layer of this membr ane regenerates. An important observation in this presentation was the abnormal rate-intensity functions (in the 12-day recovered cells) rep orted for frequencies served by that region of the sensory epithelium where the tectorial membrane defect was found. This observation may be related to sustained structural damage to the short hair cell region of the papilla and/or alterations in the efferent control of papilla f unction mediated by the short hair cells.