Threshold-duration functions of chinchilla auditory nerve fibers.

The purpose of the present study was to measure the change in threshold as a function of stimulus duration in single auditory nerve fibers. Thresholds were measured at each neuron's characteristic frequency (CF) for eight stimulus durations ranging from 8 to 1024 ms. Using an adaptive, two-interval, forced-choice threshold-tracking procedure with a 2-down, 1-up rule, thresholds were estimated based on a decision criterion of one spike or greater difference between tone and no-tone intervals. The results showed that mean thresholds decreased with increasing stimulus duration by approximately 14.6 dB over the range of durations tested. Analysis of group and individual data showed that thresholds decreased by approximately 6-7 dB per decade of duration. The slope of threshold improvement decreased systematically with increasing CF, consistent with previous physiological and psychophysical data.

Effects of selective inner hair cell loss on auditory nerve fiber threshold, tuning and spontaneous and driven discharge rate.

Current theories assume that the outer hair cells (OHC) are responsible for the sharp tuning and exquisite sensitivity of the ear whereas inner hair cells (IHC) are mainly responsible for transmitting acoustic information to the central nervous system. To further evaluate this model, we used a single (38 mg/kg) or double dose (38 mg/kg, 2 times) of carboplatin to produce a moderate (20-28%) or severe (60-95%) IHC loss while sparing a large proportion of the OHCs. The surviving OHCs were functionally intact as indicated by normal cochlear microphonic (CM) potentials and distortion product otoacoustic emissions (DPOAE). Single-unit responses were recorded from auditory nerve fibers to determine the effects of the moderate or severe IHC loss on the output of the surviving IHCs. Most neurons that responded to sound in the single-dose group had normal or near-normal thresholds and normal tuning. Relatively few neurons in the double-dose group responded to sound because of the severe IHC loss. The neurons that did respond to sound had narrow tuning curves. Some neurons in the double-dose group also had thresholds that were within the normal range, but most had thresholds that were elevated a mild-to-moderate degree. These results indicate that intact IHCs can retain relatively normal sensitivity and tuning despite massive IHC loss in surrounding regions of the cochlea. However, the spontaneous and driven discharge rates of neurons in the carboplatin-treated animals were significantly lower than normal. These changes could conceivably be due to sublethal damage to surviving IHCs or to postsynaptic dysfunction in the auditory nerve.

Effect of high-frequency interrupted noise exposures on evoked-potential thresholds, distortion-product otoacoustic emissions, and outer hair cell loss.

The effect of high-frequency interrupted noise exposures on evoked potential (EP) thresholds, distortion-product otoacoustic emissions (DPOAEs), and status of the outer hair cells was studied with the aim of understanding the correspondence among the three measures. Animal subjects were exposed to an octave band noise centered at 4 kHz at 85 dB SPL for 6 hr/day for 10 days. EP and DPOAE recordings were made before the exposure and on days 1, 2, 4, 6, 8, and 10 of exposure. A final set of measurements were made 5 days after the last exposure, following which the animals were sacrificed and their cochleas were examined using scanning electron microscopy. Both EPs and DPOAEs showed a worsening of auditory function after the first exposure and then showed a progressive recovery toward baseline. However, there was no consistent relationship between changes in EP thresholds and changes in DPOAEs nor were there any systematic changes in outer hair cells that corresponded with the changes in DPOAEs. Furthermore, EP thresholds often revealed considerable deficits in function while DPOAEs were normal.

Elevation of auditory thresholds by spontaneous cochlear oscillations.

The inner ear sometimes acts as a robust sound generator, continuously broadcasting sounds (spontaneous otoacoustic emissions) which can be intense enough to be heard by other individuals standing nearby. Paradoxically, most individuals are unaware of the sounds generated within their ears. Two hypotheses could explain this paradox: (1) the spontaneous emissions may not be transmitted to the central nervous system; or (2) the spontaneous emission produces a continuous, high rate of neural activity, which, like the natural pattern of spontaneous activity, is ignored by the central nervous system. Here we demonstrate that high-intensity spontaneous otoacoustic emission can vigorously activate auditory nerve fibres in mammals (Chinchilla laniger). This 'internal biological noise' creates a 'line busy' signal that significantly degrades a neuron's ability to respond to sound and results in a hearing loss completely different from that caused by damage to sensory cells.

Changes in distortion product otoacoustic emissions and outer hair cells following interrupted noise exposures.

Changes in distortion product otoacoustic emissions (DPOAEs) were examined during and after interrupted noise exposures and compared to the condition of the outer hair cells (OHCs) and inner hair cells (IHCs) as assessed by scanning electron microscopy (SEM). Binaural, adult chinchillas were exposed to a 95 dB SPL, octave band noise centered at 0.5 kHz for 15 days using a 3 h on/9 h off schedule. DPOAEs were measured before, during and after the exposures. DPOAE amplitudes decreased significantly during the first few days of the interrupted noise exposures and then began to recover. At most frequencies, the emission amplitudes recovered completely to pre-exposure baseline values by five days after the last exposure. The results of the present study indicate that the changes in DPOAE amplitude paralleled the recovery in the amplitude and threshold of the compound action potentials as reported previously (Boettcher et al., 1992). Although the DPOAEs completely recovered, considerable OHC loss and stereocilia disarray was evident even four weeks after exposure.

Neural correlates of temporal integration in the cochlear nucleus of the chinchilla.

Single unit thresholds were measured as a function of stimulus duration for Primary-like and Chopper units in the anteroventral cochlear nucleus (AVCN) of the chinchilla to examine the neural correlates of temporal integration. Thresholds were measured with a two-alternative, forced-choice (2AFC) adaptive tracking procedure. The time constants and the slopes of the threshold-duration functions were estimated by fitting the threshold data with an exponential function and a power law function. The results showed that Primary-like units exhibited greater threshold improvement and a longer time constant than Chopper units. Units with low characteristic frequencies (CF) showed a larger decrease in threshold with increasing duration and a longer time constant than mid-CF or high-CF units. Units with low spontaneous rates (SR) showed a smaller threshold decrease with increasing duration and a shorter time constant than mid-SR or high-SR units. The single unit time constants and the rate of threshold improvement are similar to those measured psychophysically in the chinchilla.

The spectral shaping of neural discharges by refractory effects.

It has previously been shown that post-stimulus time (PST) histograms have autoregressive properties, which implies that the neural firings have spectral components that are determined by these properties. The expected power spectral density of neural discharges is derived when the process is firing at a constant rate (similar to tonal stimulation at CF with no phase locking). Although the unconditional intensity of the process is not time varying, the spectrum exhibits prominent spectral peaks. The effect of histogram bin size, stimulus intensity, and refractory effects are examined with respect to spectral shape and it is shown that stimulus intensity determines the magnitude of spectral peaks while refractory duration determines the peak locations. The effectiveness of predicting the spectrum is demonstrated with eight-nerve data and point process simulations.

Discharge patterns of cochlear ganglion neurons in the chicken.

Physiological recordings were made of the compound action potential from the round window and single neurons in the cochlear ganglion of normal adult chickens (Gallus domesticus). The compound action potential threshold to tone bursts decreased from approximately 42 dB at 0.25 kHz to 30 dB between 1 and 2 kHz and then increased to 51 dB at 4 kHz. Most of the cochlear ganglion cells had characteristic frequencies below 2 kHz and the thresholds of most neurons were roughly 30-35 dB lower than the compound action potential thresholds. At any given characteristic frequency, thresholds varied by as much as 60 dB and units with the highest thresholds tended to have the lowest spontaneous rates. Spontaneous discharge rates ranged from 0 to 200 spikes/s with a mean rage of 86 spikes/s. Interspike interval histograms of spontaneous activity often contained regular peaks with the time interval between peaks approximately equal to 1/(characteristic frequency). Tuning curves were sharply tuned and V-shaped with approximately equal slopes to the curves above and below characteristic frequency. Q10dB and Q30dB values for the tuning curves increased with characteristic frequency. Post stimulus time histograms showed sustained firing during the stimulus and were characterized by a slight-to-moderate peak at stimulus onset. Most units showed vigorous phase-locking to tones at characteristic frequency although the degree of phase-locking declined sharply with increasing characteristic frequency. Discharge rate-level functions at characteristic frequency had a mean dynamic range of 42 dB and a mean saturation firing rate of 327 spikes/s. In general, the firing patterns of cochlear ganglion neurons are similar in most respects to those reported in other avians, but differ in several important respects from those seen in mammals.

Evoked potential tone-on-tone masking patterns in the chinchilla.

Tone-on-tone masking patterns were measured in the chinchilla using the auditory evoked response from the inferior colliculus. The masker was a 2 kHz continuous tone presented at one of five levels from 20 to 70 dB SPL. The evoked response was elicited with tone bursts presented over a wide range of frequencies. Masking was defined as the difference between evoked response thresholds measured in the presence of the masker versus quiet. At low masker levels, the masking profile was symmetrical and centered around 2 kHz. However, as masker level increased, masking spread toward the high frequencies. Furthermore, at masker levels above 50 dB SPL, a low-threshold notch occurred above the masker. The masking profiles obtained with the evoked response are in good agreement with those obtained psychophysically.