Auditory brainstem response findings in hypothyroid and hyperthyroid disease

Objective: Retrocochlear sensorineural hearing loss, associated with hypothyroidism, has been reported. This study investigated that claim. Methods: Audiometric and auditory brainstem response (ABR) measures of patients with hyper- and hypothyroidism. Results: The hyperthyroid group (14 patients) gave no audiometric or ABR results that differed significantly from normal controls. The audiometric findings for the hypothyroid group (21 patients) showed that 36% of this group had a four frequency average threshold greater than 25 dB. The statistically significant findings for ABR were a reduction in the amplitude of waves III and V and an abnormal increase in the I-V interval, appearing to confirm a retrocochlear disorder. However, as these patients have a low body temperature, the data were analysed by temperature and the results showed that the ABR abnormalities could be explained by body temperature. Conclusions: Hyperthyroid patients did not differ significantly from the controls. The four frequency average threshold and ABR results in hypothyroid patients did differ from the controls. However there is no clear evidence that the retrocochlear involvement is a direct result of the hypothyroidism. The abnormalities in the ABR can be explained by the low body temperature seen in these patients. Significance: This corrects the impression that hypothyroid patients have retrocochlear problems (indicated by ABR); a conclusion confirmed by data from 7 patients after thyroxin treatment when body temperatures. were close to normal values and ABR I-V intervals within or near normal limits

Volterra Slice otoacoustic emissions recorded using maximum length sequences from patients with sensorineural hearing loss

When normally hearing ears are stimulated with maximum length sequences (MLS) of clicks, a family of non-linear temporal interaction components of otoacoustic emissions (OAEs) can be derived, which have been named Volterra Slice OAEs (VS OAEs). This study investigates the sensitivity of VS OAEs to sensorineural hearing impairment in adults, compared to that of the widely used derived non-linear click evoked OAE (DNL CEOAE). VS OAEs and DNL CEOAEs were obtained from 24 normally hearing and 24 hearing impaired ears using a custom-built MLS system and a Otodynamics 'IL088' OAE Analyzer, respectively. The results show that, based on waveform reproducibility, VS OAEs are as successful as DNL CEOAEs at separating normal from impaired ears at the audiometric frequencies of 1 and 2 kHz, where a strong correlation is found between the amplitudes of the two OAE types. At 4 kHz however, VS OAEs are a significantly better indicator of hearing loss than DNL CEOAEs. This difference at 4 kHz appears to be due to the lack of stimulus artefact contamination of VS OAEs in the early, high frequency portion of the response. The findings suggest that VS OAEs may provide a better diagnostic and monitoring tool for hearing loss at high frequencies than the conventional DNL CEOAE

Effect of subject task on contralateral suppression of click evoked otoacoustic emissions

Contralateral suppression of click evoked otoacoustic emissions (CEOAEs) is widely used as a non-invasive measure of the activity of the (uncrossed) medial olivocochlear bundle (MOCB). There is evidence that the uMOCB receives descending input from the cortex, potentially mediating top-down control during higher order processing. This study investigated whether the contralateral suppression measure is affected by top-down influences during different tasks performed by the participants during recording. Suppression of CEOAEs evoked at 50 and 60 dB SPL was measured under four different task conditions: (1) no task; (2) passive visual (watching a silent subtitled DVD); (3) active visual (responding to visually presented sums); (4) active auditory (detecting tone pips embedded in the evoking click train). The most significant effect of task was found on the recording noise, with both the passive visual and the active auditory task producing significantly lower noise levels than the no task condition. In the passive visual task, this was associated with a reduced inter-subject variability, which enhanced the effect size relative to the no task condition. A main effect of subject task was also found on the change in CEOAE I/O slope due to contralateral noise. This effect reflected a significantly smaller suppression during the active auditory task compared to the no task condition, leading to a reduced effect size. No significant difference in suppression strength between the no task condition and the two non-auditory tasks was observed, suggesting that the main effect of task reflects a specific effect of auditory attention. The data suggest that MOCB activity is inhibited due to top-down influences when selective attention is focussed on the ipsilateral ear

Linear and nonlinear changes in the auditory brainstem response of aging humans

Objective: This experiment was designed to characterize the changes in linear and nonlinear temporal interactions in the aging auditory brainstem of humans using maximum length sequence (MLS) stimulation. Methods: The MLS technique uses a quasi-random sequence of clicks and silences to determine the linear (linear averaging of single responses) and nonlinear (interactions between pairs or triplets of responses) temporal interactions in the auditory brainstem response (ABR). A group of 30 normal hearing females aged between I I and 61 years were tested. They were divided in three groups: young ( 41 years). Results: The linear component showed a decrease in wave 5 amplitude with age which could also be related to increased thresholds for higher frequencies. The nonlinear component of the MLS-ABR showed significant differences between middle and old groups for wave I and 5 latencies and inter-wave interval (1-5). Conclusions: The results suggest that the linear and nonlinear components of the MLS-ABR could be more sensitive to changes occurring in the auditory system before any functionally significant changes in hearing status. Significance: The nonlinear components of the auditory brainstem could be used as an investigative tool to assess early changes in the aging auditory brainstem in young middle-aged women

An investigation into the relationship between input-output nonlinearities and rate-induced nonlinearities of click-evoked otoacoustic emissions recorded using maximum length sequences.

The maximum length sequence (MLS) technique allows otoacoustic emissions (OAEs) to be recorded using clicks presented at very high presentation rates. It has previously been found that increasing the click presentation rate leads to increasing suppression (termed "rate-suppression") of the MLS evoked OAE (Hine, J.E., Thornton, A.R.D., 1997. Transient evoked otoacoustic emissions recorded using maximum length sequences as a function of stimulus rate and level. Ear Hear. 18, 121-128). It has been suggested that the source of rate-suppression arises from the same nonlinear processes that give rise to the well-known nonlinear growth of OAEs. Based on this assumption, a simple model of rate-suppression (Kapadia, S., Lutman, M.E., 2001. Static input-output nonlinearity as the source of nonlinear effects in maximum length sequence click-evoked OAEs. Br. J. Audiol. 35, 103-112) predicts that both input-output (I/O) nonlinearity and rate-suppression can be unified by characterising the stimulus in terms of its acoustic power which, at high rates, is proportional to the click presentation rate. The objective of this study was to test this simple model by recording MLS OAEs from a group of normally hearing adults over a range of stimulus rates from 40 to 5000 clicks/s, and of stimulus levels from 45 to 70 dB peSPL. The results are broadly in agreement with the predictions from the model, though there appears to be some tendency for the model to slightly overestimate the degree of rate-suppression for a given degree of I/O nonlinearity. It is also suggested that the model may break down more significantly in the presence of spontaneous OAEs

Properties of volterra slices of otoacoustic emissions in normal-hearing humans obtained by using maximum length sequences of clicks

Nonlinear temporal interaction components of otoacoustic emissions (OAEs) may be investigated by presenting a stream of clicks in maximum length sequences. This yields responses, termed here Volterra slices, which are related to the Volterra kernels of the system. The aim of this study was to obtain normative data on Volterra slices over a range of click rates and stimulus levels. OAEs were recorded in 12 normally hearing adult ears at six rates and four click levels. In addition to the first order kernel, six slices from the Volterra slices of orders 2–5 were extracted from the recordings. It was found that higher order kernel slices could be reliably measured in all 12 ears tested and that they have properties that differ from those of the conventional OAEs. These findings may facilitate the study of cochlear function in both normal and pathological ears

Selective attention increases the temporal precision of the auditory N-100 event-related potential

Selective attention increases the amplitude of the averaged N-100 event-related potential (ERP). This increase may result from more neurons responding to the stimulus or from the same number of neurons better synchronised with the stimulus, or both. We investigated the synchronization mechanism using a new response latency jitter measurement algorithm that performed well for all the signal-to-noise ratios obtained in the experiment. We found that the significantly increased N100 amplitude is accounted for by a significantly decreased latency jitter variance for the attended stimuli

Nonlinear properties of otoacoustic emissions in normal and impaired hearing

Click-evoked otoacoustic emissions (CEOAEs) exhibit nonlinearities in amplitude and time domains. The first objective of this study was to investigate whether there is any correlation between the temporal and amplitude nonlinearities of CEOAEs in normals. Additionally there is evidence that pathology affects the normal cochlear nonlinearity. The second objective was to investigate whether pathology affects the temporal nonlinear components.Conventional and maximum length sequence (MLS) CEOAEs were recorded in normal subjects and in patients with mild hearing loss. The slope of the input–output (I/O) function of the conventional CEOAE measured the amplitude nonlinearity. Two measures of temporal nonlinearity were the magnitude of the suppression that occurs with increase in stimulus rate and the amplitudes of the second and third order temporal interaction components (Volterra slices).The amplitude nonlinearity is well correlated with both the magnitude of the rate suppression and the amplitudes of the Volterra slices. The ‘linear’ CEOAE amplitude showed no differences between the normal and patient groups but the differences in the Volterra slices were substantial. This suggests that the first sign of damage to the cochlea is that the system becomes more linear. Hence the Volterra slices may provide a sensitive measure of cochlear damage