Introduction

Auditory brainstem responses (ABR) are series of surface recorded potentials that reflect the activity of the eighth nerve and the various nuclei in the brainstem. The presence of a normal ABR signifies normal neural synchrony and processing of sound at the level of the brainstem [1, 2]. The ABR is sensitive to several characteristics of the stimulus for instance type, duration, intensity, frequency, bandwidth, and rate. Recently, speech elicited ABR have been shown to be an objective measurement of speech processing in the brainstem [1,2,3,4].

It is well known that background noise impairs the listening abilities of individuals with normal hearing [5,6,7] and individuals with hearing impairments [6, 7]. Both physiological and psychophysical experiments show degraded auditory performance in the presence of background noise. Background noise typically adversely affects: (i) Detection tasks e.g., elevates auditory thresholds for tones and speech [8, 9], and (ii) Speech intelligibility tasks [10]. The importance of signal-to-noise ratio (SNR) is well documented in behavioral speech perception experiments [5,6,7], 11], and psychophysical measurements [12,13,14].

Communication in difficult listening environments depends on how the auditory system can extract signals of interest from other competing information. Listening in background noise, in particular, presents a difficult challenge that often leads to communication breakdowns [6, 7]. Several factors contribute to the ability to hear a signal in the presence of noise including, but not limited to, reduced audibility, as well as how signals in noise are encoded throughout the brainstem and central auditory systems. Although the effect of signal-to-noise ratio (SNR) has been demonstrated in several human psychophysical and speech perception studies and animal physiological studies, the relation between SNR and absolute tone level as measured by human brainstem electrophysiology is less clear. One approach to studying how signal level and SNR are processed in the human brainstem is to use ABR [15]. ABR as a measure of brainstem function can provide valuable information about how large populations of neurons, recorded at the scalp, are encoding signals in noise [15].

The measurement of the ability to process auditory input in background noise is very important from both scientific and clinical perspectives. For instance, in the clinical population, it would help in the diagnosis and management of the auditory disorder, and in scientific research, it would advance our knowledge on the processing of signals in the auditory system. Studies on ABR related to the encoding of signals in ipsilateral noise are very limited. Only one study by Burkard and Hecox [16] investigated the interaction between click stimulus parameters and ipsilateral SNR. There are dozens of behavioral measures of auditory ability in noise that are frequently used in the clinic, for example, speech-in-noise, tone-in-noise, hearing-in-noise test, etc. The behavioral measures can be used in adults and older children but cannot be administered in difficult-to-test populations, for instance, infants, toddlers, children, and adults with severe or multiple disabilities.

Currently, there are no objective tests that can assess auditory ability in noise. ABR appears to be a test of choice for such purposes as it can be reliably recorded in newborns and other difficult-to-test populations. Also, ABR can be evoked by a wide range of stimuli such as click, tone burst, and speech. Another motivation for this study was to identify if the human auditory brainstem is responsible for differential processing of sound in the presence of noise between children and adults. It is well known that adults and children significantly differ in speech-in-noise tasks even when the language variable is controlled. However, it is also unknown if and how the human auditory brainstem in adult and children differ to process sounds in background noise. The lack of studies in this area and the motivation for objective tests for measuring SNR laid the foundation for current research. The purpose of this study was to determine the effect of signal-to-noise ratio (SNR) on brainstem activity to further increase our understanding of how the human central auditory system encodes signals in noise. We systematically investigated the effect of various SNRs on the latency and amplitude of ABR to a range of stimuli. We also compared the latency and amplitude of ABR recorded in various ipsilateral SNRs in children and adults.

Method

Participants

Participants were divided into two groups in the present study. One group was normal hearing children (n = 10, males) age ranged from 7 to 12 years whereas, the other group was adults (n = 10, males) age ranged from 18 to 40 years. Both the groups had normal hearing threshold (< 20 dB HL) for the octave frequencies of 250 to 8000 Hz, normal middle ear function and no history of ontological and neurological impairments. Consent was taken from the parents of children. Whereas adult participants signed the consent form prior to the participation.

Instrumentation

Diagnostic Audiometer OB 922 was used for estimating the pure-tone thresholds. Middle ear functioning was assessed by using GSI tympstar middle ear analyzer. ABR measurements were carried out by Intelligent Hearing System (IHS) with Smart EP Version 3.94 USbez.

Procedure

Participants reclined on a cot inside a double-walled sound isolating room. Surface electrodes recorded electrical activity from test ear mastoid (inverting electrode), forehead (non- inverting electrode) and non-test ear mastoid (ground electrode) using disc electrodes. Inter-electrode impedance was ≤ 5 kΩ. ABR was recorded with click, tone burst and speech stimuli. A speech token /da/ of 40 ms duration, a 1000 Hz tone burst and click was used as stimuli. ABR to all the three stimuli were measured in four conditions; Quiet, + 10 dB SNR, 0 dB SNR and − 10 dB SNR. The broadband noise was presented in ipsilateral condition and the level was varied to achieve a given SNR, while the stimulus level was fixed at 60 dB HL for all stimuli. The testing was done by using ER-3A insert earphones. The protocol used to record ABR is presented in Table 1. It is important to note that because ABR waveform and peak differ for the three types of stimuli used in this study that is the reason only wave V was considered for simple and direct comparison. Also, a few pilots recording suggested wave V as the only reliable peak in the presence of ipsilateral noise. All the participants were asked to respond initially before actual testing if they can detect the test signal in various SNR conditions.

Table 1 Acquisition parameters for ABR recording
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Results

The condition of -10 SNR was not considered for statistical analysis as ABR was not present for any participants for all the studied parameters at − 10 SNR condition. Separate statistical analysis was done on the two parameters of ABR under investigation; Latency and Amplitude of wave V. The mean and standard deviation of the latency of wave V for both the groups (adults and children) in three conditions (Quiet, + 10 dB SNR and dB SNR) obtained for clicks, /da/ and 1000 Hz tone bursts stimuli are shown in Figs. 1, 2 and 3, respectively. Mixed ANOVA was done to check the effect of different stimuli (Click, speech stimuli /da/ and 1000 Hz tone-burst) and different testing conditions (Quiet, + 10 dB SNR and 0 dB SNR) on the latency of wave V of ABR in children and adults. Overall, adults and children show significantly different latency of wave V peak for three stimuli and at different conditions [F(1,18) = 8.175 p < 0.001]. Latency of wave V was significantly different [F(2,36) = 352.398, p < 0.001] for clicks, /da/ and 1000 Hz tone bursts stimuli in both adult and children at different conditions. Latency of wave V in quiet, + 10 dB SNR and 0 dB SNR was significantly different [F(2,36) = 232.182 p < 0.001] in both adults and children for clicks, /da/ and 1000 Hz tone bursts stimuli.

Fig. 1
figure 1

Graph showing Mean latency with Standard Deviation of wave V for click stimulus in the three conditions (Quiet, + 10 dB SNR and 0 dB SNR) for adults and children

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Fig. 2
figure 2

Graph showing Mean latency with Standard Deviation of wave V for /da/ stimulus in three conditions (Quiet, + 10 dB SNR and 0 dB SNR) for adults and children

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Fig. 3
figure 3

Graph showing Mean latency with Standard Deviation of wave V for 1000 Hz tone burst stimulus in the three conditions (Quiet, + 10 dB SNR and 0 dB SNR) for adults and children

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Figures 4, 5 and 6 show the mean and standard deviation of the amplitude of wave V for both the groups in three conditions (Quiet, + 10 dB SNR and dB SNR) obtained for clicks, /da/ and 1000 Hz tone bursts stimuli, respectively. Mixed ANOVA was done to check the effect of different stimuli (Click, speech stimuli /da/ and 1000 Hz tone-burst) and different testing conditions (Quiet, + 10 dB SNR and 0 dB SNR) on the amplitude of wave V of ABR in children and adults. Overall, adults and children show significantly different amplitude of wave V peak for three stimuli and at different conditions [F (1,18) = 11.709, p < 0.001]. Amplitude of wave V was significantly different [F (2, 36) = 44.447, p < 0.001] for clicks, /da/ and 1000 Hz tone bursts stimuli in both adult and children at different conditions. Amplitude of wave V in quiet, + 10 dB SNR and 0 dB SNR was significantly different [F (2, 36) = 1226.503, p < 0.001] in both adults and children for clicks, /da/ and 1000 Hz tone bursts stimuli.

Fig. 4
figure 4

Graph showing Mean amplitude with Standard Deviation of wave V for click stimulus in the three conditions (Quiet, + 10 dB SNR and 0 dB SNR) for adults and children

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Fig. 5
figure 5

Graph showing Mean amplitude with Standard Deviation of wave V for /da/ stimulus in the three conditions (Quiet, + 10 dB SNR and 0 dB SNR) for adults and children

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Fig. 6
figure 6

Graph showing Mean amplitude with Standard Deviation of wave V for 1000 Hz tone burst the three conditions (Quiet, + 10 dB SNR and 0 dB SNR) for adults and children

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Discussion

Importance of SNR Measurement

It is evident from the literature that signal to noise ratio (SNR) affects various psychophysical and physiological audiological tests [5,6,7, 13, 14]. The competing noise has an inhibitory effect on the outer hair-cells functioning, which reduces the amplitude of otoacoustic emissions [17]. Varying SNR results in different auditory thresholds for tone and speech stimuli [8, 9] also, speech intelligibility scores vary with the changing SNR [10].

Several audiological tests are performed by varying SNR and hold clinical significance. Speech in Noise tests which utilize different SNR’s are psychophysical and theses tests are used to assess auditory processing disorders [18]. The suppression effects that are evident by decrease in the amplitude of otoacoustic emissions when an additional stimulus (tone) is added are used to assess the function of the efferent auditory system [19]. So, further, the measure of the effect of different SNR on ABR can be utilized as a clinical tool to assess the sound-in-noise processing ability objectively.

Effect of SNR on ABR for Three Types of Stimuli

The result revealed that there is a significant effect of SNR on the latency and amplitude of ABR wave V for the three stimulus conditions in both groups. The latency of wave V was significantly different for quiet and different noise conditions for all three stimuli. The latency of wave V increased as the SNR decreased (i.e. at + 10, and 0 dB SNR) for both groups. Conversely, ABR wave V was absent at − 10 dB SNR, for both groups. Similarly, the amplitude of ABR wave V was also significantly different for quiet and different noise conditions for all three stimuli. Unlike latency, amplitude decreased with the decreasing SNR in both groups. The results of latency and amplitude variations of the ABR wave V for different SNR conditions correlate with the results of Bucard and Hicox [16] in adults with click and 1000 Hz stimuli. The variation in latency and amplitude of the wave V for 1000 Hz tone-burst was larger than click and speech stimuli. This could be because single frequency tone burst excites a smaller area on the basilar membrane whereas, click and speech stimulus excites a larger area on the basilar membrane in the cochlea. Thus, the masking effect on tone-burst was more than that on clicks and /da/ stimuli.

Effect of SNR on ABR in Children and Adults

Overall results showed that the SNR affects children and adults differently in terms of increase in latency and decrease in amplitude of the ABR wave V. Further, the Independent t-test results revealed that the difference in both latency and amplitude occurs only at 1000 Hz tone-burst, but not for click and speech stimuli in both the groups at different SNR. The result for click stimuli is concurrent with the findings of Burkard & Sims [20] where no difference was observed between children and adults with changing SNR for click stimuli. This may be attributable to the fact that noise has more adverse effect on the auditory function of children than adults. This correlates with the results of Stuart [21] and Hall et al. [22] on differential functioning of children and adults for speech reception threshold and spondee recognition, respectively. The adverse effect of noise may also account for the differential performance in children and adults for tone-burst because single frequency tone burst excites a smaller area on the basilar membrane whereas, click and speech stimulus excites a larger area on the basilar membrane in the cochlea hence, more. The present findings also suggest that the difference in speech-in-noise perception between children and adults can at least be partly due to the sound-in-noise processing at the level of the auditory brainstem.

Relationship Between Behavioral Response and ABR to Stimuli in Different Condition

For behavioral response participants in both the groups were able to detect all the stimulus in quiet, at + 10 SNR and 0 SNR but not at − 10 dB SNR. The difficulty to detect stimuli was increased as the SNR was decreased. Similarly, for ABR when the SNR decreased the latency of wave V increased and the amplitude decreased. Conversely, at − 10 dB SNR, the wave V was absent for all the stimuli in both groups.

Clinical Implications

Measurement of ABR in ipsilateral SNR can be performed in infants and difficult-to-test populations. The concept of ipsilateral SNR measurement can be used at least in two ways; (i) the ipsilateral SNR level can be used as a reliable measure by varying it to find the minimum SNR needed to record ABR reliably, and (ii) the ABR can be recorded in several pre-determined SNR level to measure latency/amplitude- SNR functions. These measures are particularly important as conventional measures of SNR such as speech-in-noise cannot be used in this population. The information on SNR estimated from ABR measurement may be used for hearing rehabilitation purposes and for monitoring the benefits of auditory training and management.

Conclusion

Based on the results of the study few conclusions can be drawn. SNR affects the latency and amplitude of the wave V peak differentially for the different stimuli. A difference in the performance of children and adults was also observed. SNR measurements using ABR provide an objective index of brainstem ability to process sound in the presence of background noise. This measure is important and can be used to assess the sound-in-noise processing ability in the difficult-to-test population such as infants and children with communication disorders where conventional measures using signal-to-noise tests cannot be administered. Future studies may be directed towards finding how ABR can be used for estimating SNR depending on various test parameters and the characteristics in the clinical population.