Key Points:
1- NB CE-Chirp ABR thresholds were found closer to PTA thresholds than the TB ABR thresholds.
2- NB CE-Chirp ABR significantly shortens the test time.
3- Chirp stimulus is more available for hearing screening.
4- NB CE-Chirp ABR can be used in the prediction of behavioral hearing thresholds in adults whose behavioral hearing thresholds are difficult to determine.
5- Chirp stimulus provides larger and more detectable wave V amplitudes according to Tone Burst.
1. Introduction 
The auditory brainstem response (ABR) is an objective method to evaluate the neurologic functioning of the auditory pathway, auditory brainstem neurologic integrity, and an estimate of hearing thresholds in individuals (1). ABR waveforms include a series of positives and negatives waves occurring during the first 10 ms following an acoustical stimulation. They reflect the synchronous activity of the auditory system, and phrases with roman numerals (I–V) (2). (ABR) is used in children and adults for universal newborn hearing screening, evaluation of hearing functions, diagnosis of peripheral and central nervous system diseases, and also intraoperative monitoring ( 3, 4). The ABR evaluation represents a standardized and comprehensive method for determining the potential risk of drug-induced hearing dysfunction (5). Also, ABR studies have reported abnormal (prolonged) latencies for waves III and/or V with autism spectrum disorders (6). Stimulus variants used in ABR according to frequency bands (350-10,000 Hz) are click stimulus (all frequency bands), Tone-Burst (TB) stimulus (narrow frequency band) and chirp stimulus ( 1, 7). Chirp stimulus was developed to compensate for the delay of the cochlear wave traveling in ABR by stimulated different neural units along with the cochlear partition. (8). Different models of chirp stimulus have been identified in the literature to obtain efficiently ABR (9, 10).
Tone-Burst (TB) stimulus type is widely used to obtain frequency-specific responses (11). However, the TB stimuli causes cochlear wave travelling delay in a specific frequency range (12). It is also difficult to identify the wave V at low stimulus levels (<60 dB nHL) with Tone-Burst stimuli (13). Although the design of CE-Chirp stimulation is an important development in the auditory electrophysiological field, CE-Chirp stimulus is insufficient in the estimation of frequency-specific thresholds (14). The Narrow Band (NB) CE-Chirp stimulus has been developed for compensating for some deficiencies of the Tone-Burst stimulus (9, 15). NB CE-Chirp stimulus has designed for including four central frequencies as 0.5, 1, 2, 4 kHz. The stimulated area of the NB CE-Chirp stimulus is slightly wider than the TB stimulus and allows synchronized firing of different neural units along the base to the apical end of the cochlea. (3). Ferm et al. (16) compared NB Chirp and TB ABR and NB CE-Chirp ABR wave V amplitudes were found to be larger than TB ABR wave V amplitudes. Rodrigues at al.(12) detected that NB CE-Chirp ABR wave V amplitudes were greater than TB ABR wave V amplitudes at all levels except 500 Hz 80 dB nHL.
Studies about the behavioral hearing thresholds (BHT) estimation by electrophysiological methods are up to date in the literature(17). For many years, TB ABR has been used to estimate behavioral hearing thresholds (18). The elimination of the delay in the cochlear traveling wave with the chirp stimulus and the synchronous stimulation of the afferent nerves are superior to the traditional click stimulus. For this reason, studies about the prediction of behavioral hearing thresholds with chirp stimulus are increasing in the recent literature (19). BHT is used clinically to detect hearing loss and to prescribe the amplification of suitable hearing aids (20). In determining hearing thresholds, PTA is used preferential, and Tone Burst ABR is commonly used in children with hearing loss and neurological disease (21, 22). Also, repeated measurements showed that hearing thresholds may be detected false and not consistent with previous thresholds in 50% of newborns with hearing loss, therefore the accurate estimation of thresholds becomes essential (22).
In addition, there may be adults who cannot fully adapt to subjective hearing tests (PTA). This can happen due to poor understanding of the test procedure and low motivation. Even though there is no pathology in the hearing system, people can act as if they have a hearing loss to get personal or financial profit. This situation is called “nonorganic hearing loss”, “pseudohypacusis” or “functional hearing loss” (23). It is difficult to accurately determine behavioral hearing thresholds in adults with nonorganic hearing loss or and intellectual deficit.
In this study, we aimed to determine which Tone Burst ABR and NB CE-Chirp ABR thresholds are closer to frequency-specific behavioral hearing thresholds in normal hearing adults. Thus, it can be determined which ABR stimulus is more effective in estimating hearing thresholds in adults whose behavioral hearing thresholds are difficult to determine
Accurate estimation of frequency-specific hearing thresholds can also allow proper hearing aid adjustment in adults with hearing loss and intellectual deficit.
2. Materials and methods 
2.1Study Design: This study designed prospectively in the audiology department of a tertiary university hospital between June 2018 and October 2018. The study included 24 adults (17 males, 7 females) aged 20-48 years with normal hearing. Participants were tested in a sound-treated room, pure-tone audiometer Interacoustics model AC40 (calibrated as per ANSI S3.6, 1996) (Interacoustics AS, Assens, Denmark). Pure-tone audiometry for both air conduction (for the frequency range 250–8000 Hz) and bone conduction (for the frequency range: 500–4000 Hz) were tested using headphones TDH 39 (Telephonics Co.Farmingdale, NY, USA) and bone vibrator RadioEar B-71 (RadioEar Co. Middelfart, Denmark). Only those participants whose hearing sensitivity was ≤ 20 dB HL at each frequency (250-8000) without any otological, psychological or neurological dysfunction were selected for the study. ABR recordings were taken using the Eclipse Ep 25 ABR system.
The assessment was done with two stimuli namely, NB CE-Chirp and Tone Burst at four frequencies 500 Hz, 1000 Hz, 2000 Hz and 4000 Hz. The stimuli were calibrated to ISO 389-6 (2007) for the click and to manufacturer’s data for the chirps. Positive (active) electrode was placed on the top of the forehead (Fz), ground electrode was placed at the bottom up the forehead (Fpz), reference electrodes were placed on the right (M2) and left (Ml) mastoids, The measured potentials were recorded with impedance below 5kΩ.
The four (0.5, 1, 2, 4 kHz) Tone Bursts and NB CE-Chirps® were presented with rarefaction polarity through insert ER-3A (Etymotic Research) earphones at a rate of 44.1/s. Measurements were carried out using High Pass Filter (HPF): 75 Hz and Low Pass Filter (LPF): 1500 Hz. Tone Bursts and Narrow Band CE-Chirps were presented monaurally.
ABRs records obtained for 0.5, 1, 2, 4 kHz and 60, 40, and 20 dB nHL intensity levels for both Narrow Band (NB) CE-Chirp and Tone Burst (TB) stimuli. Behavioral hearing thresholds (BHT) were identified at 0.5, 1, 2, 4 kHz. The ABR threshold was taken as the lowest of levels containing a clear response. Tests were started from 60 dB nHL level, stimuli were decreased until no response was obtained. Initially 10 dB nHL decreases were used to estimate BHT, and then 5 dB nHL changes were made to determine a clear wave V response. Each measurement was carried out using an average of 2000 sweep to clarify the waveforms.
The absolute peak amplitude and absolute peak latencies were recorded for wave V in all four frequencies (0.5, 1, 2, 4 kHz) for TB and NB CE-Chirp stimulus. ABR wave V amplitudes and absolute latencies were determined by visual inspection. Duration of TB and NB CE Chirp ABR tests for each ear was recorded. The study was prepared with the reporting guideline of the STROBE checklist.
2.2. Participants: Inclusion criteria were: adults with no history neurological or otological dysfunction and audiometric thresholds ≤ 20 dB HL for the 250-8000 Hz frequencies. Patients with hearing loss, neurological, psychological were excluded from the study. Also, patients with previous ear surgery, chronic otitis media, Meniere’s disease, subjective tinnitus, tympanosclerosis were excluded, Patients with chronic systemic disease (diabetes mellitus, autoimmune diseases), chronic systemic drug use were not included in the study.
2.3. Statistical Analysis: Results were analyzed with the SPSS 22.0 package program. Threshold values were compared with the Paired t-test and test times were compared with the Wilcoxon test and the results with p significance value less than 0.05 were considered as significant. Intraclass correlation coefficient (ICC) was used to determine consistency of thresholds identify by PTA, TB, and NB CE-Chirp stimulus. Sample calculation and power analysis were assessed with the G Power 3.1 package program. By using the result of similar research of Mourtzouchos and et al. (24), the effect size (d) 1.21 specified as α=0.05 and β=0.80 and the sample size was calculated as 20.
2.4. Ethics Committee Approval: Verbal and written consent was obtained after all participants were informed. This study was conducted in accordance with the Declaration of Helsinki. Ethics committee approval was obtained.