Donald H. Eldredge

Cochlear Responses to Acoustic Transients: An Interpretation of Whole‐Nerve Action Potentials

Intracochlear electrodes in the guinea pig are used to measure the relations among cochlear potentials in response to slow acoustic transients. The traveling wave of Bekesy is described in terms of cochlear‐microphonic (CM) voltage as functions of time and place along the cochlear partition. The results are consistent with previous observations in the ear and on models of the basilar membrane. Interpolations of wave velocity and wave amplitudes between places used for the measurements allow continuous representations of the traveling‐wave pattern of CM in either space or time. From these representations, it is clear that the duration of the stimulating phase of CM along the cochlear partition significantly exceeds the apparent duration of the whole‐nerve action‐potential (AP) response to these transients.Selective changes in the waveforms of the AP responses, as opposed to simple reductions in amplitude, are observed when the transients are accompanied by bands of noise and after local chemical or mechani...

Hazardous Exposure to Intermittent and Steady‐State Noise

The following document was prepared by NAS‐NRC CHABA Working Group 46. This group was asked to specify damage‐risk criteria for exposure to sound. The paper contains graphs of maximum sound‐pressure levels and durations of exposures that the Working Group believes would be tolerable and examples of the use of these graphs in addition to background information and a discussion of the rationale, assumptions, limitations, and general problems pertinent to the development and application of a damage‐risk criterion and related exposure contours.

A frequency‐position map for the chinchilla cochlea

Frequencies of tones are mapped on to distances along the organ of Corti by associating behaviorally measured threshold shifts with regions of hair-cell loss. The central tendency found for 95 frequency-position matches by four observers on 21 ears is approximated by a straight-line, log-linear relation between frequency and position. Only a small portion of the considerable variation of individual matches around this function could be explained by length of the organ of Corti. Other unidentified factors appear to be responsible for most of these variations.

Model for the Nonlinear Characteristics of Cochlear Potentials

In an effort to clarify certain ideas about the nonlinear behavior of the cochlea, simple nonlinear network was studied. The network was constructed to produce peak‐clipping distortion similar to that seen in the cochlear microphonic (CM) of the basal turn of the cochlea. The interference between pairs of tones and the growth of combination tones as well as other manifestations of nonlinearity were similar for the ear and for the network. The degree of similarity between the output of the network and CM from the ear is shown by instantaneous waveforms and by input‐output functions. An important finding of this paper is that the linearization effect of two‐tone interference in the ear is a property of nonlinear systems, which occurs in the network also. The results also support recent theories that summating potential (SP) is produced by asymmetric nonlinearities in the ear.

Temporary threshold shifts in chinchilla: electrophysiological correlates.

Carder and Miller [Trans. Amer. Acad. Ophthalmol. Otolaryngol. 75, 1346–1354 (1972)] exposed chinchillas for seven days to an octave band of noise centered at 500 Hz and at 95 dB SPL to produce temporary shifts of behavioral auditory thresholds (TTS) of about 48 dB which required four to seven days to recover to normal. In the present study physiological potentials were measured about 5, 24, and 48 h after exposures to the same noise for two or three days. Cochlear microphonic (CM) responses and dc endocochlear potentials were measured in each of the three cochlear turns. Input‐output functions for whole‐nerve action potential (AP) responses to clicks and visual detection levels (VDL) for early averaged evoked responses (AER) arising in the brain stem were also measured. Endocochlear potentials were unchanged. Shifts of sensitivity for CM in the second and third turns showed the closest numerical correspondence to behavioral TTS. Loss of sensitivity for AP was greater. Changes in VDL for the AER were cons...

Cochlear Potentials and Electron Microscopy Applied to the Study of Small Cochlear Lesions

Many investigators have attempted to define the relations among behavioral auditory thresholds, cochlear potentials and cochlear pathology after excessive exposure to noise. We have observed that some noise exposed chinchillas have fewer missing sensory cells yet lower cochlear potentials than nonexposed controls. By comparing the cochleae of animals with the above characteristics, we found that the noise exposed ear had many misshapen outer hair cells, possible loss of some of the peripherally arranged mitochondria in the same cells and an increase in vesicles, vacuoles and smooth endoplasmic reticulum in the afferent nerve fibers in the region of the organ of Corti from which the low cochlear potentials were recorded. It is clear from these findings that hair cell counts alone do not always give a true indication of the functional status of the cochlea. In describing the permanent anatomical effects of a particular noise exposure on the inner ear, the condition of the cells and nerve fibers which remain after injury must be determined in addition to the number of sensory cells and nerve fibers which were lost.

Species differences in cochlear fatigue related to acoustics of outer and middle ears of guinea pig and chinchilla

After guinea pigs were exposed to octave‐band noise, which in chinchillas produces behavioral threshold shifts of more than 50 dB, as well as significant shifts of cochlear microphonic potentials, only minimal changes in guinea pig cochlear microphonic responses were found. Differences in sensitivity to noise exposure between the two species could be accounted for by four factors. (1) At lower frequencies, the attenuation caused by the closed bulla is important up to frequencies which are higher for guinea pig than for chinchilla. (2) The resonance associated with sound‐pressure transformation from tragus to tympanic membrane is observed at frequencies about an octave higher in the guinea pig than in the chinchilla. (3) Cochlear microphonics with sound pressure measured at the drum and the bulla opened are less sensitive for guinea pig than for chinchilla. (4) Departure from linearity and/or onset of distortion of cochlear microphonics occurs at sound‐pressure levels which are different for guinea pig tha...

Remote Masking in Selected Frequency Regions

Masked audiograms for six different masking stimuli were obtained from five observers. The audiograms show that remote masking can be controlled with respect to frequency region by controlling the envelope of the masking sound. Regular variations in the envelope at, say, 500 cps produce remote masking in the 500‐cps region; while irregular, “random,” variations produce equal amounts of masking everywhere outside the frequency region corresponding to the masking band or tone. It is argued that the constant masking seen in remote masking is related to the fact that signal frequencies are differentially attenuated in their transmission from the tympanum to the cochlear partition, while the masking frequencies are not since they are actually generated at the cochlear partition itself.

Preliminary Observations on the Effects of Exposure to Noise for Seven Days on the Hearing and Inner Ear of the Chinchilla

Growth and decay of temporary threshold shifts (TTS), small permanent threshold shifts (PTS), and cochlear injury are described for two chincillas exposed to an octave band of thermal noise (0.3–0.6 kHz) at a sound‐pressure level (SPL re 0.0002 μbar) of 100 dB for seven days. TTS grew with the duration of the exposure for 24–48 h and remained constant thereafter. After termination of the exposure, TTS decayed slowly and reached near‐zero values in about five days. Three months after exposure there were small PTSs (less than 10 dB) for some test tones, reduced cochlear potentials, and clear losses of external hair cells in the second and third turns of the cochlea. Observations of TTS for identical exposures limited to a duration of 193 min are also described.

Factor Analysis of Cochlear Injuries and Changes in Electrophysiological Potentials Following Acoustic Trauma in the Guinea Pig

The changes in “threshold” and maximum responses for cochlear microphonic potentials and whole‐nerve action potentials were measured following acoustic trauma with 500‐cps tones. The cochlear‐microphonic responses were measured locally in the first turn of the cochlea for 500‐, 2000‐, and 8000‐cps test tones and locally in the third turn for a 500 cps test tone. The whole‐nerve action potentials were measured in response to 500‐ and 5000‐cps tone pips. Injury to the organ of Corti in the first and third turns was rated after examination of histological sections of the temporal bones. The physiological measurements, the injury ratings, and the sound pressures and durations of the traumatic exposures were examined together for correlations and analyzed for common factors. The results served primarily to confirm previous conclusions. For durations of exposure from 5 to 80 min, equal total acoustic energy in the exposure produced equal changes in the electrophysiological responses. The ratings of anatomical i...