Temporal and speech processing deficits in auditory neuropathy.

Auditory neuropathy affects the normal synchronous activity in the auditory nerve, without affecting the amplification function in the inner ear. Patients with auditory neuropathy often complain that they can hear sounds, but cannot understand speech. Here we report psychophysical tests indicating that these patients' poor speech recognition is due to a severe impairment in their temporal processing abilities. We also simulate this temporal processing impairment in normally hearing listeners and produce similar speech recognition deficits. This study demonstrates the importance of neural synchrony for auditory perceptions including speech recognition in humans. The results should contribute to better diagnosis and treatment of auditory neuropathy.

Auditory threshold sensitivity of the human neonate as measured by the auditory brainstem response.

The absolute auditory sensitivity of the human newborn infant was investigated using auditory brainstem response thresholds (ABR). ABRs were elicited with clicks and tone-bursts of 0.5, 1.5, 4.0 and 8.0 kHz, embedded in notched noise, in healthy, full-term human neonates and young adults with known, normal-hearing sensitivity. Stimuli were calibrated using a probe microphone positioned near the tympanic membrane in the ear canal of each subject to control for differences in resonance characteristics of infant and adult ear canals. ABR thresholds were also characterized relative to group psychophysical thresholds (nHL) and relative to individual psychophysical threshold or sensation level (SL) for the adult subjects. Infant ABR thresholds measured in p.e. SPL for all stimuli are elevated by to 3-25 dB relative to adult thresholds. Threshold elevation is greatest for the high-frequency stimuli. Result are consistent with neural immaturity for high-frequency stimuli in the auditory system of human neonates.

Induced temporary threshold shift in guinea pigs.

The etiology of an incidentally discovered temporary threshold shift observed in an experimental animal (Harley guinea pig) is discussed with its potential implications for auditory research.

Gender distinctions and lateral asymmetry in the low-level auditory brainstem response of the human neonate.

Threshold measures of auditory brainstem response (ABR) were generated in 72 full-term newborn infants in response to clicks and tone burst stimuli between 500 and 8000 Hz as detailed in a previous study. These results were further analyzed for differences in response related to ear (lateral asymmetry) and subject gender. Thresholds obtained in male infants were significantly lower than those of females (P = 0.0485). The greatest differences in threshold between male and female infants occurs in the right ear (7.45 dB) as opposed to the left ear (1.56 dB). Both male and female infants have significantly larger wave V amplitude elicited from the right ear than the left (P = 0.0002) using low-level stimuli. Also, as has been noted in adults, female infants have larger amplitude ABRs than males (P = 0.0018), but amplitude differences across gender are significant only in the right ear (ear by gender interaction P = 0.0278). Results of this study indicate that gender differences and lateral asymmetry in auditory function are not a result of gender bias for or unbalanced auditory trauma, but a biologically significant phenomenon that is present at birth. The argument is made that superior right ear performance may be part of cerebral laterality in auditory function.

Distortion product otoacoustic emission suppression tuning curves in human adults and neonates.

Distortion product otoacoustic emission (DPOAE) iso-suppression tuning curves (STC) were generated in 15 normal-hearing adults and 16 healthy term-born neonates for three f2 frequencies. The 2f1-f2 DPOAE was elicited using f2/f1 = 1.2, LI = 1.2, LI = 65 and L2 = 50 dB SPL. A suppressor tone was presented at frequencies ranging from 1 octave below to 1/4 octave above f2 and varied in level until DPOAE amplitude was reduced by 6 dB. The suppressor level required for 6 dB suppression was plotted as function of suppressor frequency to generate a DPOAE STC. Forward-masked psychoacoustic tuning curves (PTC) were obtained for three of the adult subjects. Results indicate that DPOAE STCs are stable and show minimal inter- and intra-subject variability. The tip of the STC is consistently centered around the f2 region and STCs are similar in shape, width (Q10) and slope to VIIIth-nerve TCs. PTCs and STCs measured in the same subject showed similar trends, although PTCs had narrower width and steeper slope. Neonatal STCs were recorded at 3000 and 6000 Hz only and were comparable in shape, width and slope to adult STCs. Results suggest: (1) suppression of the 2f1-f2 DPOAE may provide an indirect measure of cochlear frequency resolution in humans and (2) cochlear tuning, and associated active processes in the cochlea, are mature by term birth for at least mid- and high-frequencies. These results provide significant impetus for continued study of DPOAE suppression as a means of evaluating cochlear frequency resolution in humans.

Auditory neuropathy in childhood.

OBJECTIVES: Auditory neuropathy is a recently described disorder in which patients demonstrate hearing loss for pure tones, impaired word discrimination out of proportion to pure tone loss, absent or abnormal auditory brainstem responses, and normal outer hair cell function as measured by otoacoustic emissions and cochlear microphonics. We have identified eight pediatric patients having hearing deficits that are most likely due to a neuropathy of the eighth nerve. In this study, the results of audiologic testing performed with these eight children are described. STUDY DESIGN: Retrospective review of audiologic findings in eight children with auditory neuropathy. METHODS: Each subject was tested with pure tone and speech audiologic testing, auditory brainstem response, and click-evoked otoacoustic emissions. Results of these tests were tabulated and summarized. RESULTS: Pure tone audiologic testing revealed five children with upsloping sensorineural hearing loss, two with high frequency loss, and one with a mild, flat configuration. Six children demonstrated poor word discrimination scores, and the other two had fair to good word discrimination. All eight subjects had normal distortion product and transient otoacoustic emissions. All eight children demonstrated absent or marked abnormalities of brainstem auditory evoked potentials. These findings suggest that while cochlear outer hair cell function is normal, the lesion is located at the eighth nerve. CONCLUSIONS: Recent advances in otoacoustic emissions testing permit differentiation of neural deafness from sensory deafness. This paper describes the clinical presentation and audiologic findings in pediatric auditory neuropathy, as well as the recommended management of these patients. Otolaryngologists should be aware of this disorder and implications for its management, which differs from treatment of sensorineural hearing loss.

The case for early identification of hearing loss in children. Auditory system development, experimental auditory deprivation, and development of speech perception and hearing.

Human infants spend the first year of life learning about their environment through experience. Although it is not visible to observers, infants with hearing are learning to process speech and understand language and are quite linguistically sophisticated by 1 year of age. At this same time, the neurons in the auditory brain stem are maturing, and billions of major neural connections are being formed. During this time, the auditory brain stem and thalamus are just beginning to connect to the auditory cortex. When sensory input to the auditory nervous system is interrupted, especially during early development, the morphology and functional properties of neurons in the central auditory system can break down. In some instances, these deleterious effects of lack of sound input can be ameliorated by reintroduction of stimulation, but critical periods may exist for intervention. Hearing loss in newborn infants can go undetected until as late as 2 years of age without specialized testing. When hearing loss is detected in the newborn period, infants can benefit from amplification (hearing aids) and intervention to facilitate speech and language development. All evidence regarding neural development supports such early intervention for maximum development of communication ability and hearing in infants.

Cochlear implantation of auditory neuropathy.

Auditory neuropathy (AN) is a hearing disorder that presents with a grossly abnormal or absent neural response as measured by evoked potentials in the presence of normal outer hair cell function evidenced by present otoacoustic emissions or cochlear microphonics. Rehabilitation for patients with AN is challenging due to abnormal temporal encoding at the auditory nerve leading to severely impaired speech perception. Although patients with AN may demonstrate improvement in thresholds with amplification, temporal encoding dysfunction, and consequently speech perception degradation, is not alleviated by amplification. Another issue is the heterogeneity of the AN population in terms of audiologic and neurologic findings, in addition to uncertain etiology and pathophysiology. For children with prelingual onset of AN, development of auditory and oral communication skills is particularly compromised. All children with hearing loss in the severe-to-profound range who do not benefit from conventional amplification can be considered candidates for a cochlear implant (CI). This paper presents a case study of a child with AN who received a CI. Whereas no synchronous neural response auditory brainstem response could be elicited to acoustic stimuli, an electrically evoked auditory nerve action potential was evident following implantation, suggesting restoration to some degree of neural synchrony. Significant improvement in speech perception was found post-CI. Recommendation to implant all patients with AN would be premature, but these findings suggest that electrical stimulation in some cases of auditory neuropathy can be a viable option.

The varieties of auditory neuropathy.

Auditory neuropathy (AN) was initially described as impairment of auditory neural function, with preserved cochlear hair cell function. In this report, 67 patients with audiological and neurophysiological criteria for hearing loss due to auditory neuropathy are described. Reviewing this large body of patients, AN appears to consist of a number of varieties, with different etiologies and sites affected. All varieties share a relatively spared receptor function, and an impaired neural response, with diminished ability to follow fast temporal changes in the stimulus, but different varieties in this general scheme can be distinguished. Analyses of the clinical features indicate that auditory neuropathies vary in several measures including age of onset, presence of peripheral neuropathy, etiology, and behavioral and physiological measures of auditory function. The sites affected along the peripheral auditory pathway may include dysfunction of the outer hair cells, the synapse between hair cell and auditory nerve, and the auditory nerve fibers, with myelin as well as axonal impairments contributing to the disorder.

Electrode Configuration for Auditory Brainstem Response Audiometry.

A vertical recording montage (Cz to the seventh cervical vertebra or C7) has been shown to yield significantly lower auditory brainstem response (ABR) threshold when compared with a horizontal or anterior-posterior montage (Sininger & Don, 1989). The present study further examines the relationship between electrode placement and the amplitude of the ABR to low-level stimuli. Four electrode arrays were compared-the commonly-used clinical configuration of forehead to ipsilateral mastoid, the vertical array from the previous study (Cz to C7), and two additional montages employing linked mastoids as reference to either the vertex or forehead electrode. ABRs were recorded simultaneously in four channels in response to 8000 click stimuli at 10, 20, and 30 dB SL from 9 male and 9 female adult subjects with normal hearing. The effect of electrode channel was significant at <.0001, with the vertical channel revealing the largest wave V amplitudes at all stimulus levels. Female subjects, as expected, produced larger ABR amplitudes than male subjects in all recording conditions, but electrode-channel effects were independent of gender.

Filtering and spectral characteristics of averaged auditory brain-stem response and background noise in infants.

Filtering of electrode-recorded activity before averaging is used in evoked-potential measurements to reduce background noise under the assumption that unwanted spectral components will be suppressed without substantially altering neural activity. Desired filter characteristics depend on signal and noise spectra and filter choice can affect the validity and efficiency of ABR measurement. Spectral characteristics of the human auditory brain-stem response (ABR) change with subject age and state and with the level and spectrum of the stimulus. Brain-stem evoked potentials from infants comprise greater low-frequency spectral components than do adult ABRs. Physiologic noise can also change with age of the subject. This study compares the effects of 30- and 100-Hz high-pass filters on ABR amplitude and response Fsp in full-term infants. Stimuli used to elicit ABRs are moderate- and low-level clicks and 500-Hz tone bursts. Spectral characteristics of the averaged ABR and the no-stimulus background noise from full-term infants are also characterized. Results indicate that (1) energy in the infant ABR is concentrated below 100 Hz and (2) a high-pass recording filter of 30 Hz reveals a larger-amplitude ABR and enhances the overall signal-to-noise ratio as measured by Fsp as compared to a 100-Hz high pass.

Auditory brain stem response for objective measures of hearing.

The auditory brain stem response (ABR) is felt to be an objective technique for predicting hearing thresholds because a voluntary response is not required from the subject. However, determination of ABR threshold can be a subjective process. This article discusses a technique, termed Fsp, which adds objectivity to ABR threshold detection by creating a ratio of signal plus averaged background noise over an estimate of the averaged background noise for any given averaged ABR. Fsp values have an F distribution. Consequently, the confidence of true detection for a given ABR can be determined by comparing its calculated Fsp value to statistical tables. Using a technique such as Fsp not only adds objectivity to ABR threshold detection, but also optimizes test time by allowing the averaging process to stop as soon as the background noise has been reduced and the true neural potential can be judged to be present. The estimate of the background noise can be used as a weighting factor to reduce the influence of noisy segments during the averaging process as well. Using this technique, we have found ABR threshold to be within 5 or 6 dB of psychophysical threshold for like (click) stimuli and, in our pediatric clinic, ABR click thresholds are within 10 dB of puretone average for children with losses ranging from mild to profound.

Effect of click polarity on ABR threshold.

This study investigated the effects of click polarity on threshold detectability (threshold level in dB SL) of the auditory brain stem response (ABR). ABRs were obtained from 10 normally hearing adult subjects in response to rarefaction and condensation clicks presented from 0 to 10 dB SL in 2 dB steps. An objective response signal-to-noise estimator, known as Fsp, was the dependent variable. ABR detectability functions (Fsp by dB SL) were not significantly influenced by click polarity. Conclusions can only be drawn for normally hearing adults. For this population, click polarity does not affect threshold detection with the ABR.

The development of cochlear frequency resolution in the human auditory system.

OBJECTIVE: The objectives of this study were: 1) to evaluate the maturity of cochlear frequency resolution in human neonates, and 2) to further elucidate the differential time course for development of frequency resolution at the cochlear and auditory-neural levels of the auditory system. DESIGN: This paper describes a relatively new technique using distortion product otoacoustic emision (DPOAE) suppression to study cochlear tuning. DPOAE suppression tuning curves (STCs) were generated in 15 normal-hearing adults and 26 healthy, term-born neonates at 1500, 3000, and 6000 Hz. The 2f1-f2 DPOAE was measured in all subjects with primary tones of 65 and 50 dB SPL (L1 > L2) and a 1.22 f2/f1 frequency ratio. Initially, an unsuppressed DPOAE was recorded. After this, a suppressor tone was introduced, and its level varied until DPOAE amplitude was reduced by 6 dB. By plotting the suppressor level required to achieve criterion amplitude reduction by suppressor frequency (for many tones), a DPOAE STC was generated. DPOAE STC shape, width, slope, and tip characteristics were analyzed for both adults and neonates. RESULTS: General shape and appearance of DPOAE STCs were comparable for adults and neonates, as was STC tip frequency and level. Statistical analyses of tuning-curve width (Q) and slope (dB/octave) failed to show age effects, further confirming the similarity between adults and neonates. DPOAE STCs were stable, show minimal intra- and intersubject variability, and closely resemble and behave like physiologic measures of tuning from the VIIIth nerve. CONCLUSIONS: Results suggest that: 1) cochlear tuning and related active processes are basically mature by term birth in the human auditory system, 2) tuning immaturities reported in infants as old as 6 mo of age probably involve auditory-neural immaturities, and 3) suppression of the 2f1-f2 DPOAE seems, to provide an indirect measure of cochlear frequency resolution in humans.

Hearing threshold as measured by auditory brain stem response in human neonates.

OBJECTIVES: This article evaluates the concept of auditory threshold and discusses the limitations of assessing threshold in human neonates. The advantages and limitations of assessing neonatal threshold by means of auditory brain stem response (ABR) are discussed, and data from several studies of newborn ABR threshold are compared. The authors report data from their own study designed to compare adult and neonatal ABR threshold using tonal stimuli. EXPERIMENTAL DESIGN: Several studies are compared. Data from the authors are ABR thresholds for tone bursts of 0.5, 1.5, 4, and 8 kHz, determined from 2-channel recordings in full-term neonates and adults. Stimuli were calibrated in SPL by means of a probe microphone inserted into the ear canal along with the insert transducer of each subject. RESULTS: All studies find a degree of threshold elevation in neonates relative to adult threshold. Neonatal ABR thresholds from our laboratory for stimuli from 500 through 8000 Hz are elevated relative to adult thresholds by 5 to 25 dB. Threshold elevation in our data and in other studies has found that neonatal ABR thresholds to high-frequency stimuli show the largest elevation relative to adults and low-frequency stimuli the most mature. CONCLUSIONS: Thresholds of neonates, as measured by the ABR, are immature especially for high-frequency stimuli. Proper stimulus calibrations, which removes the influence of ear canal resonance, are important for comparisons of data across age groups. Developmental differences in the conductive mechanism and neural immaturity are the most harmonious explanations for elevation of neonatal ABR thresholds.

Effects of click rate and electrode orientation on threshold of the auditory brainstem response.

This study evaluated the effects of stimulus presentation rate and electrode orientation on ABR threshold. Six normal-hearing adults served as subjects. ABRs were recorded from three orthogonal electrode pairs in response to click stimuli at rates of 48 and 21/s. Psychophysical thresholds were determined for each of these stimuli, and ABRs were recorded with stimuli between 0 and 10 dB SL in 2-dB steps. A ratio of ABR signal strength to noise variance (Fsp) was the criterion used to define threshold. Specifically, an Fsp value of 2.25, equivalent to a 95% response confidence using 5 and 250 degrees of freedom, was the criterion used to determine response presence within a maximum of 10,000 sweeps per average. Results revealed that, for normal-hearing adult subjects, click rate up to 48/s does not influence the sensation level at which a detectable response can be found, and a nearly vertical electrode orientation is optimal for recording the response at low levels. The predicted difference between psychophysical and ABR thresholds with a vertical electrode orientation, using 10,000 sweeps per average, up to 48 clicks per second, and an Fsp criterion of 2.25 is 4.4 dB.

Asymmetric cochlear processing mimics hemispheric specialization.

Otoacoustic emissions or OAEs (reflections of cochlear energy produced during the processing of sound) were measured in response to two types of stimuli, rapid clicks and sustained tones, in each ear of neonates. OAEs were larger to tones when elicited in the left ear and to clicks when elicited in the right. This finding is similar to those of enhanced processing of tones in right auditory cortical areas and of rapid stimuli on the left, given strong crossed connections from ear to brain. These findings indicate that processing at the level of the ear may facilitate lateralization of auditory function in the brain.

Distortion product otoacoustic emission suppression in subjects with auditory neuropathy.

OBJECTIVE: The objective of this experiment was to address: 1) whether normal efferent system function is required for normal cochlear tuning as measured by distortion product otoacoustic emission (DPOAE) suppression in humans and 2) whether cochlear function, assessed by DPOAE suppression tuning, is normal in a small group of patients with auditory neuropathy. DESIGN: DPOAE suppression tuning curves (STCs) are similar to other physiologic measures of tuning. They are generated by evoking a DPOAE with two simultaneously presented pure tones and then suppressing the distortion product with a third tone of varying frequency and level. In this study, DPOAE STCs were generated with f2 frequencies of 1500, 3000, and 6000 Hz in 15 normal-hearing adults and four subjects with documented auditory neuropathy. Tuning curve width, slope and tip characteristics, as well as rate of suppression growth were measured in each group. Contralateral suppression of otoacoustic emissions (OAEs) was also recorded as an index of medial efferent function. RESULTS: Results show that the four subjects with auditory neuropathy lacked efferent suppression of OAEs. However, these four subjects showed normal estimates of cochlear tuning as measured by DPOAE suppression results. CONCLUSIONS: This finding suggests that normal efferent system function is not required at the time of test for normal DPOAE suppression tuning. It also suggests that cochlear function as evaluated by detailed measures of DPOAE suppression, is normal in these "typical" patients with auditory neuropathy.

Memory scanning speed in language-disordered children.

Twelve students from classrooms for children with severe language disorders and 12 age-matched controls were evaluated for short-term memory scanning speed using the Sternberg task. Sets of two, three, and four digits were presented via earphones as memory sets, followed by single-probe digits. Reaction time was measured for a verbal response of "yes" or "no" indicating whether the probe was a member of the initial set. Average per-item scanning speed for the normal children was 66 ms, which was significantly different from the 253-ms average scanning speed of the language-disordered group. Many similarities were noted in the performance of the two groups on the memory scanning task. No differences were found in estimates of encoding and response speed, in percentage of errors or in serial or numerical position effects across groups. Results indicate that children with developmental language disorders may demonstrate reduced processing speed or memory retrieval time, which could contribute to linguistic deficits.