Auditory Brainstem Response

The auditory brainstem response (ABR) is a far-field auditory evoked potential that provides an objective, non-invasive measure of hearing sensitivity and neural conduction through the ascending auditory pathway. ConductMaze generates precisely calibrated acoustic stimuli — broadband clicks for rapid threshold screening or frequency-specific tone pips (typically 4, 8, 16, 24, and 32 kHz) for audiometric profiling — delivered through a calibrated speaker mounted inside a sound-attenuating chamber. Subdermal needle electrodes placed at the vertex (active), ipsilateral mastoid (reference), and contralateral mastoid or hindlimb (ground) record the far-field voltage generated by synchronous neural firing in successive auditory relay nuclei. The signal is amplified (×100,000), bandpass filtered (100-3000 Hz), and averaged across 256-1024 stimulus repetitions to extract the microvolt-level ABR waveform from background EEG noise.

The ABR waveform consists of five to seven vertex-positive peaks occurring within the first 8 milliseconds after stimulus onset. In mice, Wave I reflects the compound action potential of the auditory nerve (spiral ganglion neurons), Wave II arises from the cochlear nucleus, Wave III from the superior olivary complex, Wave IV from the lateral lemniscus, and Wave V from the inferior colliculus. ABR threshold — defined as the lowest stimulus intensity that produces a reliably identifiable Wave I or Wave IV — is the primary measure of hearing sensitivity. Inter-peak latencies (I-III, III-V, I-V) quantify central conduction time and detect retrocochlear pathology such as demyelination or auditory neuropathy. The ABR is the standard endpoint for ototoxicity studies (cisplatin, aminoglycosides, noise-induced hearing loss), age-related hearing loss (C57BL/6J presbycusis), and gene therapy for hereditary deafness.

ConductMaze synchronizes stimulus generation and electrode acquisition with microsecond precision using TTL-triggered hardware timing, eliminating software jitter artifacts. The system performs automated threshold search using a descending-ascending intensity bracketing algorithm (typically 90 dB SPL to 10 dB SPL in 5-10 dB steps), flags waveforms with excessive artifact rejection rates, and computes wave peak latencies and amplitudes using template matching. All waveforms are stored as individual traces for offline re-averaging or manual peak picking, and batch audiogram generation enables rapid group-level visualization of frequency-specific threshold shifts.