Temporal nonlinearity revealed by transient evoked otoacoustic emissions recorded to trains of multiple clicks

A bispectral approach to analyze nonlinear cochlear active mechanisms in transient evoked otoacoustic emissions

A new approach to study nonlinearity in cochlear active mechanisms, as evaluated in transient evoked otoacoustic emissions (TEOAEs), is presented. TEOAEs are signals generated in the cochlea by a mix of linear and nonlinear mechanisms. This new approach was designed to complement the traditional TEOAE analysis performed by currently available systems used in objective hearing screening and assessment. Nonlinearity of TEOAEs was studied by means of the bispectrum, which is able to find out quadratic frequency couplings (QFCs) that occur when a frequency is not only generated by an independent cochlear source, but it is the result of the interaction among a number of cochlear sources. To fit with the technical constraints of currently available TEOAE systems, the bispectrum was estimated by the third-order scaled polyperiodogram. The proposed method was characterized with synthesized TEOAEs as a function of the main TEOAE parameters and then used to analyze TEOAEs recorded in normal hearing adults and full-term neonates. Results revealed the presence of QFCs in both adult and neonatal TEOAEs, with peculiar patterns and significantly different frequency content in the two groups: adults had QFCs mainly around 2 kHz and neonates had QFCs mainly in the range 3.5-4 kHz.

Temporal suppression of the click-evoked otoacoustic emission level-curve

The click-evoked otoacoustic emission (CEOAE) level-curve grows linearly for clicks below 40-60 dB and saturates for higher inputs. This study investigates dynamic (i.e., time-dependent) features of the CEOAE level-curve by presenting a suppressor-click less than 8 ms before the test-click. An alteration of the CEOAE level-curve, designated here as temporal suppression, was observed within this time period, and was shown to depend on the levels and the temporal separation of the two clicks. Temporal suppression occurred for all four subjects tested, and resulted in a vertical offset from the unsuppressed level-curve for test-click levels greater than 50 dB peak-equivalent level (peSPL). Temporal suppression was greatest for suppressors presented 1-4 ms before the test click, and the magnitude and time scale of the effect were subject dependent. Temporal suppression was furthermore observed for the short- (i.e., 6-18 ms) and long-latency (i.e., 24-36 ms) regions of the CEOAE, indicating that temporal suppression similarly affects synchronized spontaneous otoacoustic emissions (SSOAEs) and purely evoked CEOAE components. Overall, this study demonstrates that temporal suppression of the CEOAE level-curve reflects a dynamic process in human cochlear processing that works on a time scale of 0-10 ms.

Temporal suppression and augmentation of click-evoked otoacoustic emissions

This study investigates temporal suppression of click-evoked otoacoustic emissions (CEOAEs), occurring when a suppressor-click is presented close in time to a test-click (e.g. 0-8ms). Various temporal suppression methods for examining temporal changes in cochlear compression were evaluated and measured here for seven subjects, both for short- and long-latency CEOAEs. Long-latency CEOAEs (duration >20ms) typically indicate the presence of synchronised spontaneous otoacoustic emissions (SSOAEs). Temporal suppression can only be linked to changes in CEOAE-compression if the suppressor-click affects the CEOAE magnitude. Phase changes induced by the suppressor-click were shown to bias suppression in two ways: (i) when a specific asymmetric measurement method was used and (ii) when synchronisation between the CEOAE and the click-stimuli was incomplete. When such biases were eliminated, temporal suppression and augmentation (the opposite effect) were observed and shown to be subject-dependent. This indicates that the nonlinearity underlying temporal suppression can work in a more (i.e., suppressed) or less (i.e., augmented) compressive state, depending on the inter-click interval and the subject under test. Temporal suppression was shown to be comparable for CEOAEs and SSOAEs, indicating similar underlying cochlear nonlinear mechanisms. This study contributes to a better understanding of the temporal properties of cochlear dynamics.

Temporal suppression of long-latency click-evoked otoacoustic emissions

A comprehensive set of results from double click suppression experiments on otoacoustic emissions (OAEs) have been presented by Hine and Thornton [1] and Kapadia and Lutman [2]. They found that suppression of a click-evoked otoacoustic emission (CEOAE) varied with the timing and level of a suppressor-click presented close in time to the test-click. Maximal suppression was found when the suppressor-click led the test-click by 2-4 ms. The double click suppression experiment set out by Hine and Thornton was repeated here and the analysis extended to the 'long-latency' CEOAE (duration > 20 ms) whereas previous studies only focused on the 'short-latency' CEOAE (duration < 20 ms). The hypothesis was that suppression would continue over the long-latency CEOAE since this region is probably dominated by spontaneous OAEs (SOAEs) synchronising with the click stimulus. The results for two exemplary subjects showed that the nonlinear suppression effect remained on the long-latency CEOAE, indicating that both SOAEs and CEOAEs originate from the same cochlear nonlinearities, as earlier suggested by Kemp and Chum [3]. The apparent similar origin of both types of emissions implies that the same temporal effects influence their responses.

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.

Otoacoustic emissions

PURPOSE OF REVIEW

Otoacoustic emissions offer the practitioner a number of beneficial features as a noninvasive and objective measure of the ear's ability to process acoustic stimuli.

RECENT FINDINGS

Since their discovery, a number of clinical applications of otoacoustic emissions have been established, including their utility in the differential diagnosis of sensorineural hearing loss, in the screening of cochlear function in infants and other difficult-to-test patients, and in the monitoring of outer hair cell healthiness in patients who are exposed to potentially damaging agents or who have progressive hearing ailments.

SUMMARY

Clinical applications of OAEs have developed very rapidly and OAEs have become the standard of care, at least, for pediatric patients.

Properties of Volterra slices of otoacoustic emissions in normal-hearing humans obtained 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.