Brainstem auditory evoked responses (BAER)
In the field of neuroanatomy, brainstem auditory evoked responses (BAERs), also known as brainstem auditory evoked potentials (BAEPs), represent the electrical response of the inner ear, auditory nerve, and brainstem structures to specific sounds characterized by distinct features and intensities. This electrical response reflects neuronal activity and is propagated to higher regions of the brain. BAERs are captured through scalp electrodes and typically exhibit a response latency of no more than ten milliseconds, accompanied by an amplitude of a few microvolts.
Measuring BAERs enables a functional assessment of the auditory sensorineural pathway, offering valuable insights into its characteristics. Under appropriate conditions, this measurement allows for evaluating the response amplitude and latency of each structure within the sensorineural pathway, spanning from the cochlea to the termination in the brainstem. The BAER waveform exhibits species-specific morphology and demonstrates typical characteristics, subject to specific conditions. Generally, it manifests as a sequence of electrical activity peaks (typically from four to seven) that emerge milliseconds after auditory stimulation, with each peak corresponding to the activation of a specific sensorineural structure.
BAER recording serves the following purposes:
Verification of an individual’s hearing status, including unilateral or bilateral hearing ability and the adequacy of ear thresholds. The auditory threshold represents the minimum sound pressure level (in dB) at which each ear of the subject begins to perceive stimulation and transmits information to the remaining Auditory Pathway, which subsequently becomes conscious in the cerebral cortex.
Detection of potential issues within the Auditory Pathway by studying the morphology of BAERs. It facilitates differentiation between pre-cochlear or non-sensory deafness and those originating from sensorineural locations. Moreover, within the latter category, it can identify whether an auditory problem is cochlear or extracochlear (brainstem) in nature. Consequently, BAER recording serves as a valuable diagnostic tool for assessing the presence and nature of auditory pathway problems.
BAER recording procedure
At Ototox, we have extensive experience in recording BAER in various species, both rodents and non-rodents. The BAER measurements are performed under sedation or anesthesia, with the specific protocol tailored to each species.
Before proceeding with the BAER recording, a comprehensive examination of each ear is conducted while the animal is under sedation. This examination, which includes both an external inspection and an otoscopic examination of the ear canal, aims to check for any ear effusion, cerumen, otitis, or abnormalities in the ear structure. It should be noted that the feasibility of the otoscopic examination is dependent on the size of the animal; for example, it may not be possible in smaller animals such as mice but can be performed in larger animals like dogs. The auditory functional assessment is typically performed in the right ear unless any condition or abnormality is identified during the examination, which may hinder an accurate evaluation.
For BAER recording, we utilize specialized equipment consisting of a “TDT RZ6 MULTI-I/O PROCESSOR” connected to a “MEDUSA-Z4 AMPLIFIER” and a calibrated “TD MF1 MULTI-FIELD MAGNETIC SPEAKER.” These components allow us to deliver sounds in a controlled far-field setting, with the loudspeaker placed approximately 10 cm from the entrance of the ear canal.
During the BAER recording, a series of sound stimuli are presented, including clicks and gated tone bursts at frequencies ranging from 4 to up 32 kHz (typically 4, 8, 16, 24, and 32 kHz). Each set of sounds is played at least 500 times, spanning intensities ranging from 10 to 90 dB SPL. The sound-induced midbrain electrical responses are captured using three small subcutaneous needle electrodes (13 x 0.9 mm): one placed at the inion, another positioned below the ear canal being assessed, caudal to the mandibular bone, and a third electrode serving as the ground, placed at the nasion. The recorded waves are measured and averaged as necessary, with a minimum of 500 responses (N ≥ 500). Typically, the recording of BAER following clicks takes around ten minutes, while BAER following tone bursts is recorded for approximately 35 minutes.
In cases where the responses in the assessed ear (usually the right ear) are inconsistent, the left ear is also evaluated to rule out any monoaural interference resulting from ear pathologies.
Once the BAER recording is completed, the following parameters are provided:
- Hearing thresholds for click sounds and tone bursts at selected frequencies (e.g., 4, 8, 16, 24, and 32 kHz), measured in dB SPL.
- Peak latencies of evoked potential peaks I, II, III, IV, and V, measured in milliseconds.
- Interpeak latencies (IPL) including IPL I-III, IPL I-V, and IPL III-V, measured in milliseconds.
- Peak amplitudes of evoked potential peaks I, II, and IV, measured in microvolts.
Additional parameters can be provided upon request, such as those required for the study of specific synaptopathies.
Statistical tests are conducted on the obtained parameters as deemed appropriate for analysis and interpretation.