Monkeys were implanted with permanent recording electrodes having tips placed in the auditory nerve-cochlear nucleus region. AP input-output relationships and VDLs were obtained from computer-averaged evoked responses to pure-tone pulses and selected monosyllabic words before and after five consecutive daily 15-min exposures to 115 dB SPL of steady broad-band noise. Threshold shifts and sustained reduction in AP amplitudes occurred at higher frequencies. Response patterns evoked by speech appeared coded in the form of amplitude modulation, and words could be identified by the summed waveform associated with each stimulus. Response patterns for some words were altered by noise exposure. Results suggest that noise intensity-duration combinations currently accepted as nonhazardous to hearing (on the basis of TTS recovery) may produce persistent changes in hearing function for suprathreshold sounds, and that speech reception may be altered by noise stimulation followed by only minimal changes in pure-tone VDLs.
Guinea pigs were fitted with permanent recording macroelectrodes having tips located in the auditory-nerve-cochlear-nucleus region. After recording detection levels and input-output functions for clicks or pure-tone bursts under waking conditions using computer-averaging techniques, the animals were exposed to 115 dB SPL of broad-band noise for 1 h. Postnoise measurements were made within the hour following exposure and for at least four weeks thereafter. VDLs shifted for clicks and midfrequency tonal stimuli, although corresponding suprathreshold AP responses showed sustained reduction of peak-to-peak amplitudes. The altered form of the AP input-output functions is characteristic of nerve fiber depopulation conditions and suggests that damage occurred to the ear in spite of the fact that VDLs returned to prenoise levels.
Squirrel monkeys were fitted permanently with rigid depth electrodes having tips localized in the auditory nerve region and with screw-type electrodes fixed on the surface of the parietal brain area (“vertex”). Summed records of pure-tone evoked potentials were recorded from both electrode sites using either a digital or an analog signal averager. Compared with cortical AERs, nerve evoked potentials showed greater amplitudes, greater resistance to habituation, less test-retest variability, and were characterized by less ambiguous waveforms at near-threshold levels. Acuity functions based on nerve AERs generally show greater sensitivity (lower detection levels) than those based on brain cortex responses.
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