Noise induced hearing loss powerpoint presentation download,my ear is sensitive to noise thyroid,tinnitus glue ear adults remedy - Step 3

People with sensorineural hearing loss are often constrained by a reduced acoustic dynamic range associated with loudness recruitment; however, the neural correlates of loudness and recruitment are still not well understood. Sensorineural hearing loss is generally accompanied by loudness recruitment, in which the perceptual loudness of stimuli grows more rapidly than normal across a narrowed dynamic range of intensities (reviewed by Moore 1995). An additional complication that may have caused confusion is the existence of so-called component 2 (C2) responses at high sound levels (Liberman and Kiang 1984; Ruggero et al.
These considerations make it difficult to adequately evaluate neural correlates of loudness growth in damaged ears. Before physiological recording, animals were anesthetized with xylazine (2.0 mg im) followed by ketamine (120–150 mg im).
Isolated fibers were characterized initially by an automated tuning-curve algorithm, which tracked the minimum level required for a 50-ms tone burst to elicit at least one more spike than a subsequent 50-ms period of silence (Kiang et al. The stimuli for which rate-level functions were collected included: BF tones, 1- and 2-kHz tones, broadband noise, and a speech token (“besh”). Power spectra for the vowel (top) and fricative (bottom) portions of the speech token (“besh”). A closed acoustic system similar to that designed by Sokolich (1977) was used to deliver stimuli to the cat's ear.
Stimulus-driven rate at each level was based on the number of spikes that occurred within a window from 10 to 210 ms after the onset of the 200-ms stimulus. The slopes of rate-level functions were characterized by fitting each rate-level function with a simple one- or 2-line model as shown in Fig. The growth of auditory-nerve (AN) activity with sound level was compared in normal-hearing cats and in cats with a noise-induced hearing loss to test the hypothesis that AN-fiber rate-level functions are steeper in impaired ears. Loudness is small near the elevated threshold, but grows to near normal at intensities around 100 dB SPL. 1997) and auditory-nerve (AN) fiber discharge rate (B: after Sachs and Abbas 1974) for normal ears (solid) and in the presence of substantial outer hair-cell damage (dashed). This issue has potential significance for understanding sensorineural hearing loss and for hearing aid design. All animal care and use procedures were approved by the Johns Hopkins Animal Care and Use Committee (protocol number CA99M255).
1965); a craniotomy was made in the posterior fossa and the cerebellum was retracted medially.
For the speech token, rate was computed separately for the steady-state vowel portion (90–170 ms after stimulus onset) and for the fricative portion (230–290 ms). Stimuli included best-frequency and fixed-frequency tones, broadband noise, and a brief speech token. The standard model for loudness recruitment suggests that steepened loudness growth reflects a steepening of the basilar-membrane (BM) input–output function resulting from loss of the normal compression produced by outer hair cells (OHCs; Fig.
A: for tones at best frequency, BM velocity in normal ears grows linearly with sound level at low sound levels, but then shows compression over a broad range of levels as the cochlear-amplifier gain is reduced (solid line). The goal of the present study was to provide further information about intensity coding in impaired ears by comparing AN fiber rate-level functions between normal-hearing and acoustically traumatized cats.
AN-fiber recordings were made by inserting a 10- to 30-M? glass micropipette filled with 3 M NaCl into the AN under visual control. One stimulus presentation occurred every 1,000 ms and consisted of the 200- or 300-ms stimulus followed by silence. Rate-level functions were averaged across all repetitions of the stimulus and were smoothed with a 5-point (5-dB) triangular window.


The 2-line fit was motivated by the sloping-saturation rate-level function shape that is observed for some normal AN fibers (example A in Fig. With outer hair cell (OHC) loss, the cochlear amplifier is gone and BM velocity grows linearly with sound level (dashed line).
1983), where mixed lesions of both inner hair cells (IHCs) and OHCs are expected (Liberman and Dodds 1984a,b), did not show a steepening of rate-level functions. Component 1 (C1) responses dominate AN fibers at low and moderate sound levels and a transition to C2 is usually observed near 80–100 dB SPL.
Cats were initially anesthetized with xylazine [2.0 mg intramuscularly (im)] and ketamine (110 mg im).
Recordings were made in an electrically shielded, double-walled soundproofed room, and computer-controlled stimuli were presented by calibrated acoustic drivers attached to hollow ear bars. The syllable “besh” was synthesized using a Klatt synthesizer (Klatt 1980), as described in Franck (1994) and in Miller et al.
The speakers were connected to hollow ear bars, and the acoustic system was calibrated before each experiment by a probe tube placed near the tympanic membrane.
SR was estimated from the interval between 200 ms after the stimulus offset and the end of the silent period for the lowest 20 stimulus levels for the 2-kHz tone. 1) Fibers with rate-level functions that were similar across all stimuli typically had broad tuning, consistent with outer-hair-cell (OHC) damage. The results show diverse changes in rate-level functions, consistent with a mixture of lesions. Atropine (0.1 mg im) was given to control mucus secretions and eye ointment was used to protect the eyes from drying. Experiments generally lasted from 24 to 48 h, and were terminated by a lethal dose of barbiturate (Euthasol). It was rarely possible to collect more than 2–3 repetitions for each stimulus, and some fibers were lost before collecting data for all of the stimuli in the protocol. The frequency response of this system was relatively flat (±6 dB) up to at least 5–7 kHz for both speakers, which was sufficient for the purposes of this study.
In the few cases in which a systematic trend was observed in these data, another stimulus was used for which the SR estimates were consistent across the lowest 20 levels. 2) Fibers with a wide dynamic range and shallow slope above threshold often retained sharp tuning, consistent with primarily inner-hair-cell (IHC) damage. However, the means by which this change in BM response is conveyed to the central auditory system is not clear. 1 predicts that a lesion with damage only to IHCs could result in shallower rate-level functions, if the lesion produced threshold shifts without a loss of BM compression. Acoustic trauma can reduce or eliminate C1 with little evident effect on C2 (Liberman and Kiang 1984).
Additional doses of ketamine were administered as needed to maintain an anesthetized state throughout the noise exposure. The bulla was vented with a 40-cm length of polyethylene tubing to equalize the middle-ear pressure (Guinan and Peake 1967). Two least-square fits were made over the range from 10 to 90% of the maximum BF-tone driven rate (rate minus SR). 3) Fibers with very steep rate-level functions for all stimuli had thresholds above approximately 80 dB SPL and very broad tuning, consistent with severe IHC and OHC damage. Presumably, a steeper BM slope should produce steeper auditory-nerve (AN) fiber rate versus level functions; however, the schematic examples in Fig.


In addition, previous studies mainly used tones at the best frequency (BF) of the neuron under study. Because the remaining C2 response shows steep response growth, it may serve as a confounding variable in previous experiments. The animal's rectal temperature was maintained near 38.5°C with a feedback-controlled heating pad. The state of the cochlea was monitored by tracking thresholds as a function of BF over time and looking for abrupt increases above the minimum thresholds collected early in the experiment. The fits were done across all possible “elbow” points such that each line segment covered at least a 10-dB range and the fit with the least error was accepted. Impaired rate-level slopes were on average shallower than normal for tones, and were steeper in only limited conditions.
B: AN-fiber discharge rates for 2 theoretical fibers, a low-threshold high spontaneous-rate fiber (light solid line) and a high-threshold low spontaneous-rate fiber (heavy solid line).
For natural stimuli, such as speech, cochlear suppression in normal ears can lead to rate-level slopes that are less than those for BF tones (Schalk and Sachs 1980). One-line fits were used when there was no evidence of sloping saturation (example B in Fig. There was less variation in rate-level slopes across stimuli in impaired fibers, presumably attributable to the lack of suppression-induced reductions in slopes for complex stimuli relative to BF-tone slopes.
Low-threshold (high spontaneous rate) fibers normally saturate before compression sets in, so the slopes of their rate-level functions should not be changed between normal and impaired ears (light solid and dashed lines in Fig. The low threshold fiber is saturated at the sound level at which the BM becomes compressive (vertical dotted line), so its discharge rate does not reflect the compression. In this case, damage to OHCs, with or without IHC damage, should lead to steepening of rate-level functions because of the loss of suppression. By contrast, the high-threshold fiber has a knee in its rate-level function at the threshold for BM compression. However, the response properties of the fibers collected after the increase were similar to those of other impaired fibers with similar thresholds collected before the increase or from other experiments, and thus these data were typically included in the impaired population.
In some impaired cases, a steep C2 response occurred at very high levels (above 80 dB SPL). In the impaired ear (dashed lines), the low-threshold fiber's rate-level function is shifted without a change of slope, but the high-threshold fiber's is steepened at suprathreshold levels. Animals were allowed to recover for ?30 days because there is little to no temporary threshold shift in cat after 1 mo (Miller et al.
Only high-threshold (low spontaneous rate) fibers should show steeper rate-level functions with cochlear damage (heavy solid and dashed lines in Fig. Slopes were recorded separately for the low-level portion of one- and 2-line fits, the high-level portion of 2-line fits, and C2 responses. 1B), and then the steepening should occur only some number of dB above threshold, above the onset of BM compression. Slopes were also characterized by fitting a regression line to the smoothed rate-level function over a series of 11-dB ranges.
The 11-dB range was shifted across the range of levels presented in 1-dB steps, giving an estimate of slope versus level.



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