The effect of coffee on contralateral suppression of transient evoked otoacoustic emissions

Introduction

Coffee is a non-alcoholic beverage which is widely consumed by humans across the world.^1–3 It contains a variety of bioactive chemicals that have anti-oxidant, anti-inflammatory, and anti-cancer properties.¹ It also contains caffeine which is a stimulating agent. Caffeine is also found in various other food items such as tea, cocoa beans, chocolate, energy drinks, among others.^1,2 The amount of caffeine in any food product is determined by the serving size, product type, and preparation method.⁴ Caffeine improves perception, increases the ability to remain awake for longer periods, and reduces fatigue.⁵ The stimulatory effect of caffeine is due to the blocking of adenosine receptors, consequently regulating the neurotransmitter levels and activities in the central nervous system.³

Otoacoustic emissions (OAEs) are very small amplitude sounds produced by the cochlea as a by-product of motile function of the outer hair cells (OHCs) (i.e. amplifier function of OHCs).^6,7 The OAEs generated in the cochlea travel backwards through the middle ear to the external ear canal, and it can be recorded using a sensitive microphone from the external ear canal.^6–8 In the cochlea, the OAEs are produced spontaneously and also in response to an external acoustic stimuli, referred as spontaneous OAEs and evoked OAEs respectively.⁸ The OAEs are commonly elicited using clicks, tone-bursts, and pure-tones. The OAEs elicited using short-duration stimuli such as clicks and tone-bursts are known as transient evoked OAEs (TEOAEs).^6–8 The OAEs elicited using pure-tones are known as distortion product OAEs (DPOAEs) and stimulus frequency OAEs (SFOAEs).⁸ The OAEs are very useful for assessing the cochlea and efferent pathways of the auditory system.

The human auditory system comprises afferent and efferent auditory pathways. The efferent pathways have an inhibitory function in the auditory system. In the cochlea, the efferent fibres cause hyperpolarization of OHCs, subsequently reducing their motile function.⁹ The reduced motility of OHCs in the presence of efferent activity result in a reduction of the amplitude of OAEs.¹⁰ This reduction in the amplitude of OAE due to efferent activity is called suppression of OAE. In humans, the suppression of OAE can be measured by presenting noise to the test ear or non-test ear during the recording of OAEs. The suppression of OAE obtained by presenting noise to the non-test ear is known as the contralateral suppression of OAE.⁸

Several studies have investigated the effect of caffeine on the auditory system. Studies have been carried out to investigate the effect of caffeine on auditory evoked potentials,^11–20 speech perception,^21,22 and OAEs.²³ Few studies have investigated the effect of caffeine on the auditory brainstem response (ABR). Findings from these investigations have showed significantly shorter latency and larger amplitude for ABR peaks following caffeine ingestion.^11,12,16 Similarly, the latency of peaks of the auditory middle latency response and the peak P1 of the auditory late latency response was decreased after caffeine ingestion.¹² In general, findings of the above studies suggest a positive effect (improved neural transmission) of caffeine on the central auditory pathway. Very few studies have investigated the effect of caffeine on speech perception ability.^21,22 Altin et al.²¹ investigated the effect of caffeine on speech identification score in noise. Results showed a significant improvement for speech identification score in noise after caffeine ingestion. Taghavi et al.²² investigated the short-term effect of caffeine on the acceptable noise level (ANL) in individuals with normal hearing. The results showed a significant reduction in the ANL after caffeine intake, suggesting that caffeine increases tolerance to noise, improving speech perception in noise. Based on findings from the above investigations, caffeine could be assumed to improve the perception of speech in noise.

Studies investigating the effect of caffeine on the OAEs in humans are scarce. Various studies investigating the effect of caffeine on the cochlea have reported that, the caffeine causes hyperpolarization of OHCs in the cochlea which suppress the amplifier function of OHCs.^24–26 Therefore, caffeine could be assumed to have a negative effect on the amplitude of OAEs. Recently, Drepath et al.²³ reported no significant effect of coffee on the amplitude of DPOAE. In contrast, Bobbin²⁷ reported a negative effect of caffeine on the amplitude of DPOAE in animal study. Therefore, although Drepath et al. ²³ reported no effect of coffee on the amplitude of OAE in humans; similar investigations should be conducted before generalizing the results. Thus, the first objective of the present study was to investigate the effect of coffee on the amplitude of TEOAE. The second objective of the present study was to investigate the effect of coffee on the contralateral suppression of TEOAE, to understand the effect of caffeine on efferent activity in the auditory system. Studies investigating the effect of caffeine on the ABR have reported an improved transmission of neural activity in the auditory pathway. This improved transmission in the afferent pathways could have an influence on the activity in the efferent pathways. However, none of the studies have investigated the effect of caffeine on the efferent auditory activity. Further, studies investigating the effect of caffeine on speech perception have reported a positive effect of caffeine on the perception of speech in noise. The improvement in speech perception after caffeine ingestion could be a consequence of increased efferent activity in the auditory system. In this connection, several studies have reported a significant relationship between the magnitude of efferent activity and speech perception in noise.^28,29 Therefore, there is a need to understand the effect of caffeine on the efferent activity.

Method

Procedure

The data collection was carried out in two phases. In phase I, the TEOAE and contralateral suppression of TEOAE were recorded before and after consumption of coffee (coffee-first group) or milk (milk-first group). In phase II, the TEOAE and contralateral suppression of TEOAE were recorded before and after consumption of milk (coffee-first group) or coffee (milk-first group). Phase II of the study was carried out after a gap of one week. Further, participants were informed to restrain from consuming caffeinated substances such as coffee, tea, energy drinks, or chocolate. for at least 12 hours prior to data collection. The procedure followed for data collection is shown in Figure 1.

Figure 1. Schematic diagram showing the procedure followed for data collection. In each session TEOAEs were recorded in non-linear mode and in linear mode with and without delivering noise to the contralateral ear.

Recording of TEOAEs

The TEOAEs were recorded using the Otodynamics Echoport 292II otoacoustic emission analyzer. During the recording of TEOAEs, participants were made to sit comfortably on the reclining chair. They were instructed not to move throughout the duration of recording of TEOAEs. The OAE probe was inserted to the test ear and E-A-RTone 5A insert phone was inserted to the contralateral ear of participants. Initially, the TEOAE was recorded in non-linear mode. A total of 260 click-trains (1040 clicks) were presented at 80 dB SPL, and the responses were averaged. Following this, the TEOAEs were recorded in linear-mode. A total of four recordings were obtained with and without presenting noise to the contralateral ear of participants. In each recording, a total of 400 click-trains (1600 clicks) were presented at 60 dB SPL and the responses were averaged. The first recording of TEOAE was always obtained without presenting noise to the contralateral ear, and referred to as baseline TEOAE. The remaining three recordings were obtained by presenting white noise to the contralateral ear of participants at 60 dB SPL, 50 dB SPL, and 40 dB SPL. The order of noise level presented to the contralateral ear was randomized. All the recordings of TEOAE were obtained without disturbing the placement of OAE probe (i.e., single-fit condition). Further, the TEOAEs were recorded from both ears of the participants.

The TEOAEs were recorded in four sessions. The first two sessions were scheduled on day 1 and the remaining two sessions were scheduled after one week. In the first session, the TEOAEs were recorded in non-linear and linear modes and these recordings were referred to as ‘pre-drink measurements’. After completing the baseline measurements, coffee was given to participants in the ‘coffee-first’ group and milk was given to participants in the ‘milk-first’ group. After one hour, the second session of TEOAE recording was initiated. The procedure of recording TEOAEs in the second session was similar to the first session and referred as ‘post-drink measurements’. After one week, the pre-drink (session 3) and post-drink (session 4) measurements were repeated. After the third session, milk was given to participants in the ‘coffee-first’ group and coffee was given to participants in the ‘milk-first’ group.

Coffee preparation

One sachet of instant coffee powder (1.3 g – 70% coffee and 30% chicory) and two tablespoons of powdered milk were mixed in 150 mL of warm water and sugar was added to improve the flavour for each serving. This is a method of coffee preparation followed in India. The amount of caffeine ranged from 27 to 40 mg per cup of coffee. Milk was prepared similarly without adding the coffee powder.

Results

Transient evoked otoacoustic emissions

Figure 2 shows the mean global amplitude of TEOAE (recorded in non-linear mode at 80 dB SPL) for both ears before and after consumption of coffee and milk. The mean amplitudes were similar for both ears across the conditions (i.e., before and after consumption of coffee and milk). The Shapiro-Wilk test revealed that the amplitude of TEOAE of both ears across conditions was normally distributed. Thus, a repeated-measures ANOVA was carried out with ears (right and left), condition (before and after consumption), and drink (coffee and milk) as repeated measures. Results showed no significant effect of ear [F(1,42)=0.505, p=0.481], condition [F(1,42)=0.162, p=0.689], and drink [F(1,42)=0.644, p=0.427] on the amplitude of TEOAE. Further, no significant interaction was found between ears and condition [F(1,42)=2.016, p=0.163], drink and condition [F(1,42)=0.644, p=0.427], ears and drink [F(1,42)=0.09, p=0.765], and ears, drink, and conditions [F(1,42)=0.135, p=0.751]. Bayesian repeated measures-ANOVA showed moderate evidence in favour of the null hypothesis for the effect of condition [BF₁₀=0.127] and drink [BF₁₀= 0.156] on the amplitude of TEOAEs. Further, it showed anecdotal evidence in favour of the null hypothesis for the effect of ears [BF₁₀=0.366] on the amplitude of TEOAE, which suggests more data/sample need to be collected to draw a firm conclusion.³¹

Figure 2. Mean global amplitude of TEOAE (recorded in non-linear mode at 80 dB SPL) before and after consumption of coffee (unfilled triangles connected with solid line) and milk (filled triangles with dashed line) for right ear (red colour) and left ear (blue colour).

Figure 3 shows the mean global amplitude of TEOAE (recorded in linear mode at 60 dB SPL) for both ears before and after consumption of coffee and milk. The mean amplitude of TEOAE was larger in the right ear across the conditions (i.e., before and after consumption of coffee and milk). Further, the mean amplitudes of TEOAE before and after consumption of coffee or milk were similar for both ears. The Shapiro-Wilk test revealed that the amplitude of TEOAE of both ears across conditions was normally distributed. Repeated-measures ANOVA was carried out with ears (right and left), conditions (before and after consumption), and drink (coffee and milk) as repeated measures. Results showed no significant effect of ear [F(1,46)=2.815, p=0.1], condition [F(1,46)=0.604, p=0.441], and drink [F(1,46)=0.288, p=0.594] on the amplitude of TEOAE. Further, no significant interaction was found between ears and conditions [F(1,46)=1.267, p=0.266], drink and conditions [F(1,46)=0.752, p=0.39], ears and drink [F(1,46)=0.959, p=0.333], and ears, drink, and conditions [F(1,46)=0.146, p=0.704]. Bayesian repeated measures ANOVA showed extreme evidence in favour of the null hypothesis for the effect of conditions [BF₁₀=0.002] and drink [BF₁₀= 0.002] on the amplitude of TEOAEs. Further, it showed anecdotal evidence for the effect of ears [BF₁₀=1] on the amplitude of TEOAE which suggests more data need to be collected to draw a firm conclusion.³¹

Figure 3. Mean global amplitude of TEOAE (recorded in linear mode at 60 dB SPL) before and after consumption of coffee (unfilled triangles connected with solid line) and milk (filled triangles with dashed line) for right ear (red colour) and left ear (blue colour).

Contralateral suppression of transient evoked otoacoustic emissions

Figures 4 and 5 show the mean magnitude of contralateral suppression of TEOAE for both ears across the levels of noise before and after consumption of coffee and milk. Panel A and panel B of Figure 4 show the mean contralateral suppression of TEOAE across noise levels for baseline measurement. The mean contralateral suppression was slightly lower in the left ear compared to the right ear. Further, the mean contralateral suppression of TEOAE decreased with the reduction in the level of noise in the contralateral ear. Panels C and D of Figure 4 show the mean contralateral suppression of TEOAE before and after consumption of coffee and milk respectively in the two ears. The results for the same are depicted in Figure 5 for better visualization. The mean contralateral suppression before and after consumption of milk was similar at each level of noise in the two ears. In contrast, a slightly greater suppression was noted after consumption of coffee at each level of noise in the two ears, except for the right ear at 40 dB noise.

Figure 4. Mean magnitude of contralateral suppression of TEOAE before and after consumption of coffee and milk and across the levels on noise in the contralateral ear for both ears.

Panel A shows the mean contralateral suppression of TEOAE for both ears before consumption of coffee. Panel B shows the mean contralateral suppression of TEOAE for both ears before consumption of milk. Panel C shows the mean contralateral suppression of TEOAE for both ears before and after consumption of coffee. Panel D shows the mean contralateral suppression of TEOAE for both ears before and after consumption of milk.

Figure 5. Mean magnitude of contralateral suppression of TEOAE across levels of noise in the contralateral ear before and after consumption of coffee and milk in right ear (panel A) and left ear (panel B).

The Shapiro-Wilk test revealed that the magnitude of contralateral suppression of TEOAE across conditions and levels of noise for both ears were not normally distributed. To investigate the effect of condition (before and after consumption) and drink (coffee and milk) on the contralateral suppression of TEOAE, the Friedman test was carried out separately for each ear (right and left). Results showed no significant difference for the magnitude of contralateral suppression of TEOAE before and after consumption of coffee and milk at each level of noise for right ear [60 dB (χ²₍₃₎=4.021, p=0.259) and left ear [60 dB (χ²₍₃₎=3.952, p=0.267)]. Further, to investigate the effect of ear on the contralateral suppression of TEOAE, the data obtained before consumption of coffee were subjected to the Wilcoxon signed ranks test. It showed the contralateral suppression of TEOAE was not significantly different between ears (Z=1.208, p=0.227).

Discussion

Results of the present study showed no significant effect of coffee on the amplitude of TEOAE. This finding is consistent with results of Drepath et al.²³ which showed no effect of coffee on the amplitude of DPOAEs in humans. These findings suggests no effect of caffeine on the amplitude of OAEs in humans. But, in contrast to the findings of human studies, animal studies have reported an effect of caffeine on the amplitude of DPOAEs. Bobbin²⁷ reported a stimulus level-dependent effect of caffeine on the amplitude of DPOAE. The amplitude of DPOAE was found to be reduced when elicited with lower-intensity stimuli and the amplitude was increased when elicited with higher-intensity stimuli. In addition, Bobbin²⁷ also investigated the effect of caffeine on the compound action potentials (CAP), summating potential (SP), cochlear microphonics (CM) and latency of N1. The caffeine had a suppressive effect on the CAP, SP, and latency of N1. Bobbin²⁷ attributed the reduction in the amplitude of DPOAE at low intensity to diminished amplifier function of OHCs in the cochlea, which is a consequence of caffeine. In the cochlea, caffeine causes activation of Ca²⁺-dependent K⁺ channels in the OHCs, which leads to hyperpolarization of the OHCs and subsequently suppresses the amplifier function of OHCs.^{24–26,32,33} Recently, Castellano-Muñoz et al.³⁴ investigated the effect of caffeine on the electrical properties of OHCs and postsynaptic activity in auditory fibers. The results showed caffeine has no effect on the electrical properties of OHCs, but it has an effect on the postsynaptic activity in auditory fibers. Thus, findings of the above study suggest that functioning of the OHCs may not be influences by caffeine, and hence the amplitude of OAE could be similar before and after consumption of caffeine.

The present study also investigated the effect of coffee on the contralateral suppression of TEOAE. The contralateral suppression of TEOAE was measured by presenting white noise to the contralateral ear at 40, 50, and 60 dB SPL. Results showed an increase in the magnitude of contralateral suppression of TEOAE with an increase in the level of noise in the contralateral ear. These findings are consistent with results of several investigations.^35–38 The increase in contralateral suppression of TEOAE with noise level has been attributed to the strength of efferent activity. Further, results of the present study showed a slightly greater contralateral suppression after coffee consumption; however, the difference was not significant. As studies investigating the effect of caffeine on the contralateral suppression of TEOAE are not available in the literature, the results of the present study cannot be compared with other investigations. Further, although findings of the present study showed no significant effect of coffee or caffeine on the contralateral suppression of TEOAE, similar studies are essential before generalizing the findings.

Based on the findings of the present study, we understand that consuming coffee before an audiological evaluation has no significant effect on the amplitude of TEOAE and contralateral suppression of TEOAE. However, there are few limitations to the present study. In the literature, studies investigating the effect of caffeine on the ABR have shown a dose-dependent effect of caffeine on the peaks of ABR.¹⁶ A similar a dose dependent effect of caffeine could be present on the amplitude of TEOAE and contralateral suppression of TEOAE. However, in the present study a fixed amount of coffee was provided to participants, thus currently it is not understood whether increasing the dose of caffeine would have any effect on the amplitude of TEOAE and contralateral suppression of TEOAEs. Further, the amount of caffeine present in coffee is dependent on the type of coffee (i.e., brewed, instant, or decaffeinated).^39,40 In the present study instant coffee was given to participants, which contains lower amount of caffeine compared to brewed coffee. Therefore, if coffee has a dose-dependent effects of caffeine on the TEOAE and contralateral suppression of TEOAE, then findings of the present study cannot be generalized to all types of coffee.

To conclude, findings of the present study suggest no effect of coffee on the findings of TEOAE. The procedure used for recording the non-linear TEOAE in the present research was similar to the protocol used in clinics for routine evaluation. Thus, based on findings of the present study, we understand that consuming coffee before an audiological evaluation may not affect clinical measurement of TEOAEs and the inferences drawn from it don’t change with coffee consumption.

Data availability

Mendeley Data: The effect of coffee on TEOAE and contralateral suppression of TEOAE, https://doi.org/10.17632/p4pgd57zgd.1.⁴¹

This project contains the following underlying data:

Data are available under the terms of the Creative Commons Attribution 4.0 International license (CC-BY 4.0).