A faint click reaches the ear, and within milliseconds, a cascade of electrical signals races through the brainstem. This silent conversation within the brain holds the key to understanding hearing disorders in humans.
The brainstem evoked response audiometry (BERA), commonly known as the auditory brainstem response (ABR), is an electrophysiological test that captures the brain's electrical activity in response to sound. In mice, this technique has become an indispensable tool for neuroscientists and geneticists. It allows researchers to unravel the mysteries of how we process sound and what happens when this complex system fails. By studying the tiny brainstems of mice, scientists are making giant leaps in diagnosing and treating human hearing and neurological disorders 5 7 .
At its core, BERA is an objective electrophysiological measurement of the brainstem's response to auditory stimuli. Unlike a behavioral test where a patient indicates when they hear a sound, BERA passively records the nervous system's own electrical signals, making it perfect for subjects who can't communicate what they hear, from infants to laboratory mice 5 7 .
When a sound is presented to the ear, it triggers a precise sequence of neural activity along the auditory pathway. This journey begins in the inner ear, travels along the auditory nerve, and proceeds through several relay stations in the brainstem. Each of these stations generates a distinct, measurable electrical peak, labeled with Roman numerals as Waves I through VII 2 5 .
By analyzing the latency (the time it takes for a wave to appear) and the amplitude (the size) of these waves, researchers can pinpoint not just if an animal can hear, but also where in the auditory pathway a problem might exist.
You might wonder why so much hearing research is conducted on mice. The answer lies in the remarkable conservation of biological processes across mammals. The fundamental architecture of the mouse and human auditory systems is very similar, making mice excellent models for understanding human hearing 2 .
However, researchers must navigate a significant challenge: the most commonly used transgenic mice, the C57BL/6 strain, begin to lose their hearing due to a specific genetic mutation when they are just a few weeks old. This progressive hearing loss can severely confound research results 4 .
To solve this, scientists have developed innovative strategies. One effective approach is using a different mouse strain called B6.CAST-Cdh23Ahl+/Kjn. These mice have been bred to maintain low-threshold hearing well into adulthood. By crossing this strain with other transgenic lines, researchers can create mice with the desired genetic traits without the complication of early hearing loss, leading to more reliable and interpretable data 4 .
Similar auditory system architecture to humans makes mice ideal for hearing research.
The following section details a standard protocol for acquiring and analyzing ABR data in mice, as used by leading research institutions and phenotyping consortia worldwide 2 8 .
Mice are housed and cared for under strict ethical guidelines. Before testing, the acoustic system must be meticulously calibrated. A microphone is placed inside the sound-attenuating chamber to mimic a mouse's ear, and the speaker's output for both clicks and tone bursts is verified using an oscilloscope to ensure accurate sound pressure levels and frequencies 3 .
The mouse is anesthetized using an intraperitoneal injection of a ketamine and xylazine mix. Once deeply sedated, it is placed on a heating blanket inside a sound-attenuating chamber to maintain body temperature and prevent external noise interference 8 .
Subdermal stainless steel needle electrodes are inserted at key locations. The active electrode is placed at the vertex (top) of the skull, the reference electrode is placed ventro-lateral to the pinna (ear) being measured, and the ground electrode is positioned at the hip. Proper placement and low impedance are critical for obtaining a clear signal 3 8 .
Acoustic stimuli—clicks or tone bursts of varying frequencies (e.g., 4, 8, 16, 32 kHz)—are presented through a free-field speaker. For each sound level and frequency, the stimulus is repeated hundreds of times (e.g., 300 times at a rate of 20 per second). The corresponding brainwave activity is recorded, and these responses are averaged by a computer to filter out background brain activity (EEG) and isolate the tiny auditory evoked potentials, which are only a few microvolts in amplitude 2 3 8 .
The software generates a waveform for each stimulus condition. Researchers then perform:
In a typical experiment, a normal-hearing mouse will show clear, well-defined waves at low sound levels (e.g., 20 dB SPL). As the sound level decreases, the waves become smaller and their latencies increase, until they disappear at the hearing threshold 8 .
The power of this method is evident when comparing different mouse models. For example, a study might reveal that mice with a specific gene mutation (e.g., Cav3.2 -/-) exhibit significantly elevated hearing thresholds compared to their control littermates. This means they require a much louder sound to generate a detectable ABR, indicating hearing loss 3 .
Furthermore, latency analysis can show if the defect is primarily in the inner ear (affecting Wave I latency) or involves the brainstem pathways (affecting the I-V interpeak latency). This level of detail is crucial for understanding the specific biological mechanism affected by the gene mutation 2 .
| Wave | Anatomical Origin | Typical Latency Range (ms) |
|---|---|---|
| I | Auditory Nerve | 1.5 - 2.0 |
| III | Cochlear Nucleus | 3.0 - 4.0 |
| V | Inferior Colliculus | 4.5 - 6.0 |
| I-V | Central Conduction Time | 4.0 - 5.5 |
Table based on standard values from human and animal studies 7
| Method | What it Assesses | Best For | High-Throughput? |
|---|---|---|---|
| ABR/BERA | Entire auditory pathway (nerve to brainstem) | Detecting a wide spectrum of hearing loss pathologies | Yes |
| Acoustic Startle | Simple motor reflex to loud sound | Quick detection of severe hearing loss only | Yes, but limited |
| Otoacoustic Emissions (OAEs) | Function of outer hair cells in the cochlea | Ruling out cochlear (inner ear) damage | Yes |
| Electrocochleography (ECoG) | Summating potential of cochlear hair cells | Diagnosing Meniere's disease and endolymphatic hydrops | No |
Information adapted from a comparison of functional assessment methods 8
Carrying out a successful BERA experiment requires a suite of specialized equipment and reagents. Here are some of the key components.
| Item | Category | Function |
|---|---|---|
| Ketamine/Xylazine Mix | Anesthetic | Renders the mouse unconscious and immobile for accurate recording. |
| Atipamezole | Anesthetic Reversal | Antisedan used to safely reverse the effects of the anesthetic after the procedure. |
| Sound-Attenuating Chamber | Equipment | A lined, sealed cubicle that blocks external noise to prevent contamination of the neural signals. |
| Multifield Speaker | Equipment | Precisely delivers calibrated click and tone-burst stimuli. |
| Subdermal Needle Electrodes | Equipment | Stainless steel electrodes that capture the microvolt-level electrical signals from the scalp. |
| Bio-Amp & Digital Signal Processor | Equipment | Amplifies the tiny neural signals thousands of times and processes them for analysis. |
| ABR Analysis Software | Software | Custom software for stimulus generation, data acquisition, and automated peak detection. |
Items compiled from detailed protocol descriptions 2 8
Ketamine/Xylazine mix for sedation and Atipamezole for reversal.
Sound-attenuating chamber to block external noise interference.
Subdermal needle electrodes for precise signal capture.
The applications of mouse BERA research are vast and directly impactful on human health. It serves as a cornerstone in:
By screening thousands of mutant mice, international consortia like the International Mouse Phenotyping Consortium (IMPC) are discovering new genes critical for hearing, shedding light on the genetic causes of human deafness 8 .
From a tiny click in a silent chamber to a detailed graph of neural activity, brainstem evoked response audiometry in mice is a powerful window into the brain. This elegant technique continues to decode the complexities of hearing, offering hope for millions affected by hearing and neurological disorders. As researchers refine these methods and develop better mouse models, each recorded waveform brings us one step closer to silencing the burden of deafness.