Responses from AVCN units in the cat before and after inducement of an acute noise trauma.
Acoustically evoked responses of single units in the anteroventral part of the cochlear nucleus (AVCN) in the cat were studied together with compound eighth-nerve action potentials (AP). Halfway through the experiments the cats were exposed for half an hour to pink noise at 105 dB SPL, producing an average threshold shift of 30 dB (maximum 50 dB) in the 2-6 kHz region. The effect of noise exposure was studied in two ways. On the one hand we compared the results for one population of units measured before the noise exposure with those found for another population measured afterwards. On the other hand we compared the results before and after the noise exposure for one unit that could be kept under observation during the noise exposure. After the noise exposure spontaneous activity and phase-locking of the responses of the units to the stimulus waveform were not significantly different from the pre-exposure findings. Response latency tended to increase. Therefore, the decrease of latency found for APs must be due to a shift to higher frequencies of the population of units contributing to the AP. The sharply tuned tip segments of tuning curves shift to higher levels whereas the low-frequency tails remain at about the same level. Q10 decreases by at most 50%, which was also found for AP tuning curves. Response spectra for clicks and noise (reverse correlation function) did not show a significant decrease of frequency selectivity. Units with CF less than 3 kHz may show a shift of CF to a lower frequency by 10-20%. After inducement of the noise trauma the sharply tuned tip segment of a tuning curve may not be found and CF may be assigned to a local minimum in the low-frequency tail of the tuning curve.