Cochlear Nerve and Cochlear Nucleus

Mapping the human subcortical auditory system using histology, postmortem MRI and in vivo MRI at 7T

Studying the human subcortical auditory system non-invasively is challenging due to its small, densely packed structures deep within the brain. Additionally, the elaborate three-dimensional (3-D) structure of the system can be difficult to understand based on currently available 2-D schematics and animal models. Wfe addressed these issues using a combination of histological data, post mortem magnetic resonance imaging (MRI), and in vivo MRI at 7 Tesla. We created anatomical atlases based on state-of-the-art human histology (BigBrain) and postmortem MRI (50 µm). We measured functional MRI (fMRI) responses to natural sounds and demonstrate that the functional localization of subcortical structures is reliable within individual participants who were scanned in two different experiments. Further, a group functional atlas derived from the functional data locates these structures with a median distance below 2 mm. Using diffusion MRI tractography, we revealed structural connectivity maps of the human subcortical auditory pathway both in vivo (1050 µm isotropic resolution) and post mortem (200 µm isotropic resolution). This work captures current MRI capabilities for investigating the human subcortical auditory system, describes challenges that remain, and contributes novel, openly available data, atlases, and tools for researching the human auditory system.

Engaging and disengaging recurrent inhibition coincides with sensing and unsensing of a sensory stimulus

Even simple sensory stimuli evoke neural responses that are dynamic and complex. Are the temporally patterned neural activities important for controlling the behavioral output? Here, we investigated this issue. Our results reveal that in the insect antennal lobe, due to circuit interactions, distinct neural ensembles are activated during and immediately following the termination of every odorant. Such non-overlapping response patterns are not observed even when the stimulus intensity or identities were changed. In addition, we find that ON and OFF ensemble neural activities differ in their ability to recruit recurrent inhibition, entrain field-potential oscillations and more importantly in their relevance to behaviour (initiate versus reset conditioned responses). Notably, we find that a strikingly similar strategy is also used for encoding sound onsets and offsets in the marmoset auditory cortex. In sum, our results suggest a general approach where recurrent inhibition is associated with stimulus ‘recognition' and ‘derecognition'.

Sensory stimuli evoke temporally dynamic responses. Here the authors report that responses to odour onset and offset are orthogonally represented in the locust antennal lobe, differentially entrain oscillations, and propose a model in which they are necessary for initiation and termination of behaviour.

Electrically-evoked frequency-following response (EFFR) in the auditory brainstem of guinea pigs

It is still a difficult clinical issue to decide whether a patient is a suitable candidate for a cochlear implant and to plan postoperative rehabilitation, especially for some special cases, such as auditory neuropathy. A partial solution to these problems is to preoperatively evaluate the functional integrity of the auditory neural pathways. For evaluating the strength of phase-locking of auditory neurons, which was not reflected in previous methods using electrically evoked auditory brainstem response (EABR), a new method for recording phase-locking related auditory responses to electrical stimulation, called the electrically evoked frequency-following response (EFFR), was developed and evaluated using guinea pigs. The main objective was to assess feasibility of the method by testing whether the recorded signals reflected auditory neural responses or artifacts. The results showed the following: 1) the recorded signals were evoked by neuron responses rather than by artifact; 2) responses evoked by periodic signals were significantly higher than those evoked by the white noise; 3) the latency of the responses fell in the expected range; 4) the responses decreased significantly after death of the guinea pigs; and 5) the responses decreased significantly when the animal was replaced by an electrical resistance. All of these results suggest the method was valid. Recording obtained using complex tones with a missing fundamental component and using pure tones with various frequencies were consistent with those obtained using acoustic stimulation in previous studies.

Psychophysical assessment of the level-dependent representation of high-frequency spectral notches in the peripheral auditory system

To discriminate between broadband noises with and without a high-frequency spectral notch is more difficult at 70-80 dB sound pressure level than at lower or higher levels [Alves-Pinto, A. and Lopez-Poveda, E. A. (2005). "Detection of high-frequency spectral notches as a function of level," J. Acoust. Soc. Am. 118, 2458-2469]. One possible explanation is that the notch is less clearly represented internally at 70-80 dB SPL than at any other level. To test this hypothesis, forward-masking patterns were measured for flat-spectrum and notched noise maskers for masker levels of 50, 70, 80, and 90 dB SPL. Masking patterns were measured in two conditions: (1) fixing the masker-probe time interval at 2 ms and (2) varying the interval to achieve similar masked thresholds for different masker levels. The depth of the spectral notch remained approximately constant in the fixed-interval masking patterns and gradually decreased with increasing masker level in the variable-interval masking patterns. This difference probably reflects the effects of peripheral compression. These results are inconsistent with the nonmonotonic level-dependent performance in spectral discrimination. Assuming that a forward-masking pattern is a reasonable psychoacoustical correlate of the auditory-nerve rate-profile representation of the stimulus spectrum, these results undermine the common view that high-frequency spectral notches must be encoded in the rate-profile of auditory-nerve fibers.

Psychophysical estimates of level-dependent best-frequency shifts in the apical region of the human basilar membrane

It is now undisputed that the best frequency (BF) of basal basilar-membrane (BM) sites shifts downwards as the stimulus level increases. The direction of the shift for apical sites is, by contrast, less well established. Auditory nerve studies suggest that the BF shifts in opposite directions for apical and basal BM sites with increasing stimulus level. This study attempts to determine if this is the case in humans. Psychophysical tuning curves (PTCs) were measured using forward masking for probe frequencies of 125, 250, 500, and 6000 Hz. The level of a masker tone required to just mask a fixed low-level probe tone was measured for different masker-probe time intervals. The duration of the intervals was adjusted as necessary to obtain PTCs for the widest possible range of masker levels. The BF was identified from function fits to the measured PTCs and it almost always decreased with increasing level. This result is inconsistent with most auditory-nerve observations obtained from other mammals. Several explanations are discussed, including that it may be erroneous to assume that low-frequency PTCs reflect the tuning of apical BM sites exclusively and that the inherent frequency response of the inner hair cell may account for the discrepancy.

Cochlear traveling-wave amplification, suppression, and beamforming probed using noninvasive calibration of intracochlear distortion sources

Originally developed to estimate the power gain of the cochlear amplifier, so-called "Allen-Fahey" and related experiments have proved invaluable for probing the mechanisms of wave generation and propagation within the cochlea. The experimental protocol requires simultaneous measurement of intracochlear distortion products (DPs) and ear-canal otoacoustic emissions (DPOAEs) under tightly controlled conditions. To calibrate the intracochlear response to the DP, Allen-Fahey experiments traditionally employ invasive procedures such as recording from auditory-nerve fibers or measuring basilar-membrane velocity. This paper describes an alternative method that allows the intracochlear distortion source to be calibrated noninvasively. In addition to the standard pair of primary tones used to generate the principal DP the noninvasive method employs a third, fixed tone to create a secondary DPOAE whose amplitude and phase provide a sensitive assay of the intracochlear value of the principal DP near its characteristic place. The method is used to perform noninvasive Allen-Fahey experiments in cat and shown to yield results in quantitative agreement with the original, auditory-nerve-based paradigm performed in the same animal. Data obtained using a suppression-compensated variation of the noninvasive method demonstrate that neither traveling-wave amplification nor two-tone suppression constitutes the controlling influence in DPOAE generation at close frequency ratios. Rather, the dominant factor governing the emission magnitude appears to be the variable directionality of the waves radiated by the distortion-source region, which acts as a distortion beamformer tuned by the primary frequency ratio.

Detection of high-frequency spectral notches as a function of level

High-frequency spectral notches are important cues for sound localization. Our ability to detect them must depend on their representation as auditory nerve (AN) rate profiles. Because of the low threshold and the narrow dynamic range of most AN fibers, these rate profiles deteriorate at high levels. The system may compensate by using onset rate profiles whose dynamic range is wider, or by using low-spontaneous-rate fibers, whose threshold is higher. To test these hypotheses, the threshold notch depth necessary to discriminate between a flat spectrum broadband noise and a similar noise with a spectral notch centered at 8 kHz was measured at levels from 32 to 100 dB SPL. The importance of the onset rate-profile representation of the notch was estimated by varying the stimulus duration and its rise time. For a large proportion of listeners, threshold notch depth varied nonmonotonically with level, increasing for levels up to 70-80 dB SPL and decreasing thereafter. The nonmonotonic aspect of the function was independent of notch bandwidth and stimulus duration. Thresholds were independent of stimulus rise time but increased for the shorter noise bursts. Results are discussed in terms of the ability of the AN to convey spectral notch information at different levels.

The inferior colliculus of the rat: A quantitative analysis of monaural frequency response areas

Frequency response areas (FRAs) were measured for 237 single units in the inferior colliculus (IC) of urethane-anesthetized pigmented rats using monaural pure-tone stimulation. Based on qualitative criteria [J Neurosci 21 (2001) 7303], FRAs were classified as V-shaped in 69% of neurons, non-V-shaped in 29%, and unclassifiable in the remaining 2%. Non-V-shaped FRAs were heterogeneous, comprising a number of subtypes including narrow, closed, low- and high-tilt, multipeaked, U-shaped, mosaic and inhibitory. To complement this subjective classification, we applied quantitative measures used by others (e.g. [J Neurophysiol 84 (2000) 1012]), including the inverse slope of the upper and lower FRA borders, Q-values, and other measures of bandwidth. The results suggest that FRAs in the rat IC are best described as forming a continuous distribution among subtypes, rather than clustering into discrete categories. Moreover, there is a broad range of frequency tuning characteristics and FRA types across the entire frequency spectrum. Within this general pattern, however, there are some frequency-specific differences in FRA type distribution. The relative proportion of V-shaped FRAs was greatest at the high and low ends of the auditory range, with the highest proportion of non-V-shaped FRAs in the mid-range from 6 to 12 kHz. For most neurons with multipeaked FRAs, the peak frequencies were not harmonically related. Frequency tuning in the pigmented rat IC is generally similar to that in other species. Comparison of Q values across auditory nuclei shows little evidence that FRAs are sharpened at levels above the auditory nerve. Rather, there is a broad range of frequency tuning properties at each level.

Neural adaptation to pulsatile acoustical stimulation in the cochlear nucleus of the rat

This study, carried out in adult Long-Evans rats, was designed to investigate the adaptive properties of the cochlear nucleus to pulsatile acoustical stimuli. To achieve this purpose, near-field evoked potentials were picked up from the ventral cochlear nucleus in awake animals. Individual auditory thresholds were measured and responses to 250 ms trains of repetitive clicks with pulse rates ranging from 100 to 2000 pulses per second were collected. The amplitude of the first negative (N(1)) component of the evoked potentials to consecutive individual pulses in the train was measured by using a subtraction method. As expected, a rapid amplitude decrement of the responses in the train was obtained and a three phase adaptation was described. The decrease of individual N(1) component amplitude was fitted for each rate of stimulation with exponential decrease equations and time constants were calculated. Such an analysis allowed us to characterize three distinct adaptive processes which were discussed. The results were comparable to those obtained in previous studies in the auditory nerve and suggest that the adaptation recorded in the ventral cochlear nucleus by using near-field evoked potentials reflects the adaptive properties of auditory nerve fibers.

Modelling convergent input onto interaural-delay-sensitive inferior colliculus neurones

Convergent input from cells in the medial superior olive (MSO) and lateral superior olive (LSO) onto a single inferior colliculus (IC) cell explains many findings that are not compatible with a simple coincidence detector mechanism. Here this explanation is tested using a physiologically accurate computer model of the binaural pathway in which the input to the IC cell is either from two MSO cells or a MSO and a LSO cell. Auditory nerve (AN) spike trains are formed by a stochastic hair cell model following a basilar membrane simulation using a gammatone filter. In subsequent cells input spikes cause post-synaptic potentials (PSPs) which are summed causing the cell to fire when the sum crosses a threshold. The individual cells are matched to the physiology by varying the number of inputs, the magnitude and duration of the PSPs and the firing threshold. Non-linear best-phase-versus-frequency functions arise if the two IC inputs have different best frequencies and different characteristic delays. One input can be selectively suppressed by turning on an additional tone at the worst phase of that input. Non-zero characteristic phases arise if the characteristic frequencies of the AN fibres feeding into a single superior olive cell are mismatched.

Discharge rate of the auditory nerve during noise revealed by electrocochlear stimulation

The purpose of this study was to evaluate the average discharge rate of all fibres in the whole auditory nerve (R(wn)) when a broad-band noise with steady-state effects is applied to the ear. We assessed the R(wn) parameter by detecting the state of refractoriness of the nerve during noise stimulation using an electric stimulus (ES) as a probe. The technique, applied in awake pre-implanted guinea pigs (Charlet de Sauvage et al., 1994), made it possible to obtain electro-acoustic responses (EARs), from which an estimate of the R(wn) parameter could be deduced. Negative current pulses of 100 micros duration, each followed by an identical pulse of positive polarity for charge balance, were applied between round window and indifferent vertex electrodes at intervals of 160 ms. The 120 ms wide-band noise masker started 92 ms before every other negative ES. The signal on the stimulating electrodes was averaged over a 5.12 ms window in synchrony with the negative pulse. EARs were obtained by alternately subtracting recordings during noise from those during silence. The R(wn) parameter was determined by comparing experimental and computed EAR patterns. For this purpose, a model of unit response incorporating changes in amplitude and conduction velocity during the relative refractory period was designed. The recovery function of the firing probability in response to ES was evaluated. Fibres were classified in different categories according to their background discharge rates. The probability of response of single fibres to ES in each category was calculated on the basis of their interval histograms during silence and noise. Individual spikes were combined accordingly to obtain the computed EAR waveform. R(wn) was determined by adjusting the EAR amplitude of the model in relation to that of the experimental EAR. R(wn) generally increases in a linear fashion with respect to noise intensity expressed in dB, thus following the increase in loudness perception estimated by Weber's law. At the highest noise levels, R(wn) tends to saturate. The estimated saturation rate was found to be about 380 spikes/s.

A micromechanical model of the cochlea with radial movement of the tectorial membrane

The function of the tectorial membrane in the cochlear micromechanics is uncertain. In modeling approaches some models have assumed it to be a resonator that participates in the sharp tuning mechanisms of the cochlea with its mass coupled to the ciliary stiffness of outer hair cells, being driven by the shear force between the reticular lamina and itself. This paper presents a different type of micromechanical model which assumes that the tectorial membrane is driven by a lymphatic fluid flow that can be shown to have a substantial radial component. It also assumes that the reticular lamina is relatively stiff and thereby restrains the top end of outer hair cells that exert a force to the basilar membrane via Deiters cells. When combined with a three-dimensional block model, it can simulate the sharp tuning mechanisms of the cochlea well.

A frequency-dependent saturation evident in rate-intensity functions of the chinchilla auditory nerve

The shape of rate-intensity functions recorded from individual neurons of the auditory nerve using stimulus frequencies at and below the characteristic frequency have been both well-characterized and modeled by other researchers. However, previous studies of rate-intensity functions using stimulus frequencies above the characteristic frequency have primarily focused on the slopes of the rising phases of the functions. Hence, they did not determine whether rate-intensity functions recorded using stimulus frequencies above the characteristic frequency saturate, and, if so, at what firing rates the saturation occurs. In this study, rate-intensity functions have been obtained from neurons of the eighth nerve of the chinchilla in response to gated, sinusoidal stimuli in order to investigate saturation firing rates for frequencies above the characteristic frequency. For each neuron, rate-intensity functions were obtained for stimulus intensities up to 90 dB SPL at the characteristic frequency and at as many frequencies above the characteristic frequency as time would allow. These data clearly reveal that, for frequencies above the characteristic frequency, saturation occurs at a rate that decreases monotonically as the frequency of stimulation is increased. In addition, an empirical equation is given which summarizes the dependence of saturation on stimulus frequency for the data of this study.

Spatial resolution of cochlear implants: the electrical field and excitation of auditory afferents

This paper investigates the spatial resolution of electrical intracochlear stimulation in order to enable further refinement of cochlear implants. For this purpose electrical potential distributions around a conventional human intracochlear electrode (NUCLEUS-22) were measured in a tank, in cat cadaver cochleae and in living cat cochleae. Potential gradients were calculated where of importance. The values were compared to spatial tuning curves from cat primary auditory afferents in electrical mono-, bi-, and various tripolar stimulation modes. Finally, a lumped element model was developed to elucidate the single fiber data. Tank potential measurements show the principal features of the different stimulation modes but are not sufficient to explain all the features of experimental data from single fibers. Intracochlear potential measurements indicate an increase in spatial resolution in an apical direction. The single fiber data also confirm that a tripolar stimulus configuration provides significantly better spatial resolution than any other stimulation mode presently in use.

Auditory processing of complex sounds: an overview

The past 30 years has seen a remarkable development in our understanding of how the auditory system - particularly the peripheral system - processes complex sounds. Perhaps the most significant has been our understanding of the mechanisms underlying auditory frequency selectivity and their importance for normal and impaired auditory processing. Physiologically vulnerable cochlear filtering can account for many aspects of our normal and impaired psychophysical frequency selectivity with important consequences for the perception of complex sounds. For normal hearing, remarkable mechanisms in the organ of Corti, involving enhancement of mechanical tuning (in mammals probably by feedback of electro-mechanically generated energy from the hair cells), produce exquisite tuning, reflected in the tuning properties of cochlear nerve fibres. Recent comparisons of physiological (cochlear nerve) and psychophysical frequency selectivity in the same species indicate that the ear’s overall frequency selectivity can be accounted for by this cochlear filtering, at least in band width terms. Because this cochlear filtering is physiologically vulnerable, it deteriorates in deleterious conditions of the cochlea - hypoxia, disease, drugs, noise overexposure, mechanical disturbance - and is reflected in impaired psychophysical frequency selectivity. This is a fundamental feature of sensorineural hearing loss of cochlear origin, and is of diagnostic value. This cochlear filtering, particularly as reflected in the temporal patterns of cochlear fibres to complex sounds, is remarkably robust over a wide range of stimulus levels. Furthermore, cochlear filtering properties are a prime determinant of the ‘place’ and ‘time’ coding of frequency at the cochlear nerve level, both of which appear to be involved in pitch perception. The problem of how the place and time coding of complex sounds is effected over the ear’s remarkably wide dynamic range is briefly addressed. In the auditory brainstem, particularly the dorsal cochlear nucleus, are inhibitory mechanisms responsible for enhancing the spectral and temporal contrasts in complex sounds. These mechanisms are now being dissected neuropharmacologically. At the cortical level, mechanisms are evident that are capable of abstracting biologically relevant features of complex sounds. Fundamental studies of how the auditory system encodes and processes complex sounds are vital to promising recent applications in the diagnosis and rehabilitation of the hearing impaired.

Hearing in the frog: a neurophysiological study of the auditory response in the midbrain

The evoked response in the torus semicircularis of Rana temporaria and R. esculenta to acoustic stimulation has been investigated. The amplitude of the potential is maximal when the recording electrode is placed 1 mm below the tectal surface. We show that responses are evoked by tones over the frequency range 50 to 3500 Hz, but that the upper frequency limit can be extended beyond 5000 Hz by mechanical vibration of the middle ear. The projection from lower centres to the torus semicircularis is organized tonotopically; fibres responding to high frequencies project more rostrally than those responding to lower frequencies. Moreover, maximum response is evoked from each region in the torus semicircularis by incident sound from a particular direction. Best responses are obtained in the anterior region of the torus semicircularis for high frequencies presented rostrally and in the posterior region by lower frequencies presented at 90° to the rostrocaudal axis. In this way, a map of auditory space is represented in the midbrain.

Comparing the level of the acoustic distortion product 2f1-f2 with behavioural threshold audiograms from normal-hearing and hearing-impaired ears

The level of the acoustic distortion product, 2f1-f2, was measured across frequency from two different groups of adults: (1) a group of 57 drawn from the university population who considered that they had normal hearing and (2) a group of 26 patients referred from the ENT Department of the County Hospital. These patients complained of reduced 'quality' of hearing, but no organic condition had been diagnosed to explain it. Behavioural audiograms were obtained for both groups. In both groups of listeners the distortion frequency sweep is related to the respective subjective threshold audiogram. In subjects from whom distortion can be recorded across a wide frequency range, acoustic distortion product level and auditory sensitivity were significantly correlated in one-third of the subjects. In hearing-impaired patients acoustic distortion is usually detectable across frequency regions of normal hearing, but falls below the noise floor if hearing threshold levels are above 15-20 dB.

Diversity of characteristic frequency rate-intensity functions in guinea pig auditory nerve fibres

Rate-intensity functions at characteristic frequency (CF) were recorded from single fibres in the auditory nerve of anaesthetised guinea pigs. Within the same animal, CF rate-intensity functions, although probably forming a continuum, could be conveniently divided into three groups; (1) Saturating; reach maximum discharge rate within 30 dB of threshold, (2) Sloping-saturation; initially rapid growth in discharge rate leading to a slower growth in discharge rate but not saturating and (3) Straight; approximately constant increase in firing rate per decibel increase in sound pressure up to the maximum sound pressures used. Thresholds for individual fibres were plotted relative to compound action potential thresholds at the appropriate frequency. Fibres with straight CF rate-intensity functions had the highest thresholds. Fibres of the saturating CF sloping-saturation CF rate-intensity type had thresholds intermediate between saturating and straight. There was a close relationship between the type of CF rate-intensity function exhibited by a fibre and its spontaneous discharge rate. Fibres with saturating CF rate-intensity functions generally had high spontaneous discharge rates (greater than 18/s), whereas those with straight CF rate-intensity functions generally had low spontaneous discharge rates (less than 0.5/s). The majority of fibres with sloping-saturation CF rate-intensity functions had spontaneous rates between 0.5/s and 18/s. There was a negative correlation (r = -0.59) between the logarithm of the spontaneous discharge rate and relative threshold at CF with the lowest spontaneous rate fibres having the highest thresholds and vice-versa. This diversity of CF rate-intensity functions has functional implications for both frequency and intensity coding at high sound pressures in the mammalian auditory system.

Mechanical properties of the frog ear: vibration measurements under free- and closed-field acoustic conditions

The acoustically induced motion of the eardrum of the frog was measured by an incoherent optical technique. When free-field sound stimulation was used, the eardrum vibration had a band-pass characteristic with maximum amplitude at 1-2.5 kHz. However, when the sound was presented in a closed-field acoustic coupler the response was low-pass (cut-off frequency about 2.5 kHz). We demonstrate that the motion is the result of the mechanical properties of the eardrum and the sound pressure acting upon it. The net pressure is due to a combination of sound incident directly on the front of the drum and of sound conducted to the rear via internal (resonant) pathways. The frog ear therefore acts as a pressure-gradient receiver at low frequency and a pressure receiver at high frequency. A model is proposed and analysed in terms of its electrical analogue. This model accounts for both our own experimental observations and those of previous studies.

Comparison between AP and SP parameters in trans- and extratympanic electrocochleography

Results of the comparison of action potential (AP) and summating potential (SP) parameters in trans- (TT) and extratympanic (ET) methods in 2 normal-hearing ears and 20 hearing-impaired ears with various etiologies and audiogram shapes are reported. In normal and impaired ears the TT/ET ratio of N1 amplitude was intensity dependent and was greater at higher intensities than at lower intensities. N1 latency was identical in both methods. The AP waveform was almost identical in both methods in ears except in a noise-induced hearing loss: N1/N2 ratio was greater in the TT method than in the ET method. In the TT method +SP at high frequency tone bursts and -SP at low frequency tone burst were recorded in Menière's disease and progressive sensorineural hearing loss, while in the ET method only -SP was recorded at a tone burst of each frequency. Origins of N1 and N2 and a clinical value of -SP and +SP are discussed.

The frequency selectivity of the normal and pathological human cochlea

Human AP tuning curves (tone on tone simultaneous and forward masking curves) were measured during transtympanic electrocochleography. For subjects with near normal thresholds, average Q10 dB values (simultaneous masking) were 2.3 at 2 kHz, 3.6 at 4 kHz, and 4.7 at 8 kHz. Patients with threshold elevations of more than 40 dB, resulting from sensorineural hearing loss of cochlear origin, had tuning curves less sharply tuned by a factor of 2-3, with Q10 dB values of 1-2 at 2 kHz and at 4 kHz, and 1-2.3 at 8 kHz. AP tuning curves and single fibre tuning curves (frequency threshold curves) were measured in normal guinea pigs; cochlear fibre tuning is sharper than AP tuning (simultaneous masking) by a factor of 1.8 (2-20 kHz). Assuming that this factor can be applied to the human cochlea, estimates of normal human cochlear fibre Q10 dB values are 4.2 at 2 kHz, 6.5 at 4 kHz, and 8.5 at 8 kHz.

Slow cortical responses and the diagnosis of central hearing loss in infants and young children

To evaluate the usefulness of slow cortical responses (ERA) for threshold estimation in infants and young children, 83 children were investigated with combinations of pure-tone audiometry, electrocochleography (ECochG) and ERA. The deviations between ECochG/ and ERA thresholds were correlated to brain function in order to diagnose central hearing losses. By comparing corresponding values of 2 kHz pure-tone and ERA thresholds, 32% (10/31) errors were found, mainly below 35 dB HL. In a group of patients with no sign of brain disorder, an overall error rate of 37% (20/53) was found. Below 35 dB HL, 72% (11/15) errors were found. In a group of patients with brain dysfunction, the overall rate was 70% (21/30), below 35 dB HL it was 84% (21/25). In the range below 35 dB HL, no significant difference (p greater than 0.05) in errors was found between the groups with and without brain disorders. It is concluded that ERA is unreliable for the estimation of moderate hearing losses and cannot per se detect a central hearing dysfunction. Elevated ERA thresholds may indicate a central hearing loss, but to establish this topical diagnosis, ECochG and neuropsychological examinations are necessary.

Click polarity inversion effects upon the human brainstem auditory evoked potential

Parameters of the brainstem auditory evoked potentials (BAEPs) to high-intensity clicks of initial rarefaction (R) and condensation (C) phases differed. The amplitudes of Waves I, II and IV were greater with R clicks, while that of Wave V was greater with C clicks. The peak-latencies of Waves I and VI were shorter with R clicks and those of the remaining components tend to shorten with C clicks. At low stimulus intensities the preserved BAEP components (Waves III, V and VI) did not change noticeably with click phase inversion.

Olivocochlear bundle activity recorded in awake cats

Multi-unit activity was recorded in awake cats from the crossed olivocochlear bundle (COCB). Miniature stainless stell concentric electrodes were chronically implanted onto the floor of the fourth ventricle of six animals. There was no activity at the electrode tip in anesthetized animals, while in awake cats a great deal of unit activity could be seen. A correlation between COCB activity and ongoing behavioral activity such as scratching, grooming, yawning, or orientation could not be established. It was found, however, that the multi-unit responses in the COCB statistically increased their firing rate during acoustic stimulation, and a 500-Hz tone was found to be most effective. The electrode locations were histologically confirmed. The present results are similar to other data that describe unit activity in the olivocochlear bundle of decerebrate cats. The capacity to record from this fiber tract in awake animals, however, provides a new tool for studying the peripheral efferent pathway of the auditory system.

Estimation of the critical bandwidth from loudness summation data

An automatic method for critical band estimation from loudness summation data is presented. A mathematical model, based on a power function, is fitted to the data and the critical bandwidth is defined at the intersection of the asymptotes. The model is designed for clinical use, involving the treatment of the data from single test persons; it represents an operational solution to a difficult task. The model is able to describe data from normals and patients with a sensorineural hearing loss. Variability of the critical band estimates, intrasubject as well as intersubject, is larger than for visually obtained estimates. However, visual estimation is difficult, subjective, and probably heavily biased. The model produces estimates which in logarithmic form have a Normal distribution at medium loudness level, while visual estimation gives rise to irregular distributions at all levels. A normal range for model estimates from sets of data obtained at medium loudness level is defined by mean and standard deviation.

Clinical critical band estimation

A modified version of the method of loudness summation, developed for clinical critical band estimation, is presented. The stimuli are noise bands centred around 1 kHz. The standard procedure includes 1) determination of the "sensitivity curve": the loudness difference between broad band noise and narrow band noise as a function of the level. For clinical use this function is necessary for evaluation of the level with the most rapid growth of loudness with bandwidth, i.e. the optimum level for sharp determination of the critical bandwidth. 2) Determination of the critical band function, i.e. the difference in sound pressure level required for equal loudness of the test noise band and the reference as a function of the bandwidth. This determination is performed at a level with maximum growth of loudness, evaluated by the sensitivity curve. 3) Critical band estimation by a mathematical model, described in another work (Bonding et al., 1978) from the data obtained. The method is analysed regarding feasibility and reproducibility.

The sharpening of cochlear frequency selectivity in the normal and abnormal cochlea

In the normal (anaesthetized) animal cochlea, the frequency threshold curves for single primary fibres are up to an order of magnitude sharper than the analogous function derived from various reported measurements of the basilar membrane amplitude of vibration. This enhanced neural frequency selectivity is found in the same species and under conditions similar to those in which the mechanical measurements are taken. The sharpening process (at least near threshold) appears to be linear and is not dependent upon lateral inhibitory mechanisms. The variability of the neural frequency selectivity and its vulnerability to metabolic, chemical and pathological influences suggests the hypothesis that the sharpening is due to some form of "second filter" subsequent to the relatively broadly tuned basilar membrane. All fibres recorded from in the cochlear nerve in the normal cochlea show this enhanced frequency selectivity; in contrast, in pathological cochleas, all fibres, or a substantial proportion, have high-threshold, broadly tuned characteristics, approximating to those of the basilar membrane. The frequency selectivity of normal cochlear fibres is adequate to account for the analogous psychophysical measures of hearing. It is proposed that loss of this normal frequency selectivity occurs in deafness of cochlear origin, accounting for widening of the critical band. A new hypothesis for recruitment is proposed on this basis.