**4. Frequency-specific ABR**

This section gives a brief overview of frequency-specific ABR techniques that are now commonly used to establish hearing thresholds in audiological assessment following the newborn hearing screening and discusses why these techniques may be considered appropriate.

Traditionally, 100 μs click stimuli are used to evoke ABR responses (**Figure 10a**, top left). There are a number of advantages of using this stimulus: (1) it generally results in well-formed and detailed responses, (2) it helps in determining auditory neuropathy, and (3) it generates relatively large responses and therefore responses can be obtained in a brief amount of time [32]. Various studies describe a good correlation between click-evoked ABR thresholds and behavioral thresholds in the 2–4 kHz range, e.g. [31, 32], with correlations as high as 0.94. However, other studies report issues with the use of click stimuli for threshold estimates and report a much poorer correlation [33, 34]. The click-evoked ABR may seriously over-or underestimate sensory hearing loss, depending on hearing loss configuration. Though click ABR thresholds correlate well with the 2–4 kHz region on a population level, this does not necessarily result in accurate threshold estimates for individual patients. Stapells & Oates attribute these issues to the broadband spectrum of clicks and conclude that the click-ABR threshold probably represents the "best" hearing in a wide frequency range [33].

Over the years, several methods for obtaining frequency-specific ABR thresholds have been explored, for example, involving ipsilateral masking of frequency regions or derived response methods with filtered clicks. Hall gives a review of these various approaches [35]. The most common clinical approach for recording frequencyspecific ABRs is more straightforward and involves brief tone stimuli, or tone-bursts. A tone-burst stimulus is a transient stimulus of typically 5 tone cycles within a Blackman window (**Figure 10a**), or a 2 cycles rise-time-1 cycle plateau-2 cycles falltime envelope [35]. This stimulus configuration gives an acceptable trade-off between

#### **Figure 10.**

*Waveform of ABR stimuli as recorded with an interacoustics eclipse loopback test (a) 100 μs click, 4, 2, 1, 0.5 kHz Blackman window tone-burst stimuli (b) broadband LS-CE chirp, 4, 2, 1, 0.5 kHz NB-CE chirp.*

the short stimulus onset needed to evoke an auditory response, and the bandwidth needed to obtain frequency specificity. Several studies describe high correlations (0.85–0.95) between pure tone audiometry thresholds and tone-burst ABR thresholds in adults [36, 37] and in infants [34, 38] and the authors conclude that tone-burst ABR is a clinically feasible and accurate method of estimating the pure tone audiogram when appropriate correction factors are applied.

Larger and clearer ABR responses can be evoked by using chirp stimuli, mathematically designed to compensate for frequency-dependent traveling wave delays in the cochlea and to generate synchronous stimulation across a wide frequency region. These level-specific (LS) chirp stimuli generate larger amplitude responses than clicks or tone-bursts, thus increasing the signal-to-noise ratio and reducing test time [39]. Elberling and Don derived narrow-band (LS NB-CE) chirps from these broadband chirps with approximately one-octave bandwidth (**Figure 10b**) [40]. These LS NB-CE chirps facilitate frequency-specific ABR.

Ferm et al. found significantly larger ABR responses with LS NB-CE Chirp stimuli compared to tone-bursts and anticipated a vast reduction in test time for achieving a similar SNR [41, 42]. They also established correction factors, compensating for the offset between ABR threshold (dB nHL) and estimated hearing level (dB eHL), as well as threshold confidence intervals for these stimuli. These correction factors are currently in use in the British Newborn Hearing Screening Program (Guidelines for the early audiological assessment and management of babies referred from the Newborn Hearing Screening Programme. British Society of Audiology, 2014).

### **5. Binaural auditory brainstem responses**

An important feature of the auditory system is the ability to determine the location of sound sources relative to the head. Information from two ears can be used to estimate the location of a sound source in the horizontal plane using ITD and ILD. Using these binaural cues, normal hearing individuals can localize with high accuracy and precision [43]. Auditory localization allows humans to quickly detect and orient towards relevant sounds in the environment. This can be important, for example, when trying to safely navigate through traffic by bike or when walking, or when trying to focus on a single conversation in a noisy environment.

Measuring auditory localization accuracy and precision in a clinical setting requires specialized setups with a large number of speakers and, ideally, eye- or head-tracking. Although objective measures of hearing ability are often applied in the clinic, objective measures of auditory localization are not frequently used or wellknown. An interesting objective measure of auditory localization can be found in the Binaural Interaction Component (BIC) of the ABR since the later peaks (IV and V) originate from binaural nuclei SOC and LL (see Section 2.2).

The amount of binaural interaction between the ears can be used as an objective measure of binaural hearing. ILDs and ITDs can be presented via headphones by introducing level and time differences between the left and right channels of a stereo sound. The BIC can be obtained by subtracting the ABR to a stereo sound from the sum of the monaural left and right ABRs [44, 45]. In normal-hearing listeners, the binaural ABR and the monaural sum are not the same, resulting in a different waveform: the BIC (see **Figure 11**). The most prominent peak in the BIC is the first negative peak, often called DN1 (sometimes called beta). The amplitude and latency of the DN1 systematically vary with ILD and ITD in humans and animals [46–48].

*Short-Latency Evoked Potentials of the Human Auditory System DOI: http://dx.doi.org/10.5772/intechopen.102039*

#### **Figure 11.**

*The BIC is calculated by subtracting the binaural ABR from the sum of the monaural ABRs. Figure obtained from Laumen et al. [45].*

The largest amount of interaction (the largest DN1 amplitude) is typically observed at an ILD of 0 dB and/or an ITD of 0 μs.

The DN1 amplitude, and thus the amount of binaural interaction, gradually decreases with increasing ILD or ITD. The most likely sources of the DN1 are the MSO and LSO in the SOC [45].

Given that the BIC is a difference waveform and the fact that ABR peaks are typically of low amplitude, measuring the BIC requires a high signal-to-noise ratio in the binaural and monaural ABRs. Additionally, to obtain the DN1 amplitude for multiple ILDs requires a quite extensive testing and may be less practical in the clinic where less time may be available for measurements [48]. Some studies also report that the BIC is absent for some participants with normal localization skills (e.g. [49, 50]), making it difficult to rely on for individual diagnostic purposes in some cases. However, the BIC can be used to study the processing of binaural cues in the brainstem in various populations at a group level. For example, a study of children at risk for central auditory processing disorders (CAPD) showed that their BIC amplitude was reduced relative to normal hearing children [51]. Interestingly, the children in the CAPD group showed normal ABR thresholds, suggesting that binaural interaction can be specifically affected in certain conditions. That the presence of the BIC has some diagnostic value can be seen in the results of a study in which the presence of the BIC was used to detect children at risk for CAPD. The investigators could distinguish between children at risk for CAPD and those not at risk with a 76% sensitivity and specificity [52].

To conclude, the BIC of the ABR provides important information regarding binaural processing in the brainstem. Although some studies suggest that it may not be the best objective measure for diagnosing binaural hearing disorders at an individual level, it does provide a unique window into binaural cue interactions early in the auditory processing pathway.

### **6. Conclusions**

Sources of the ABR are the auditory nerve and brainstem auditory nuclei. Clinical application of ABRs includes identification of the site of lesion in retrocochlear

hearing loss, establishing functional integrity of the auditory nerve and objective audiometry. To help interpretation and establish reliability, separate subaverages may be obtained for ipsi- and contralateral registrations, and for test-retest reliability. Hearing threshold estimation of infants who are referred for audiological assessment after hearing screening relies on accurate estimation of hearing thresholds. Frequency-specific ABR using tone-burst or narrow band chirp stimuli is a clinically feasible method for this. Whenever possible, obtained thresholds should be confirmed with behavioral testing. The binaural interaction component of the ABR provides important information regarding binaural processing in the brainstem. Although some studies suggest that it may not be the best objective measure for diagnosing binaural hearing disorders at an individual level, it does provide a unique window into binaural cue interactions early in the auditory processing pathway.
