D4 Auditory Evoked Potentials (ABR / ASSR)

Topic

The auditory evoked potential equipment places surface electrodes on the scalp—one at the crown of the head and another over the mastoid bone behind the ear—and delivers a brief sound stimulus to the ear via an earphone: either a broadband click or a short-duration tone. The surface electrodes do not pick up the sound stimulus directly; instead, they record the electrical activity that the stimulus generates in the auditory nerve and brainstem as the signal propagates from the ear to the brain.

Because a broadband click is such an abrupt sound stimulus, it causes a large number of auditory nerve fibers to fire almost simultaneously, rather than firing at different times as they would in response to a continuous sound. This synchronization produced by the broadband click is precisely what allows surface electrodes to detect auditory nerve activity from outside the skull: when many auditory nerve fibers fire at once, their individual electrical signals summate into a combined signal strong enough to reach the scalp, whereas the activity from a continuous sound—lacking fiber synchronization—cancels out before it can be recorded at the surface.

This synchronized auditory nerve activity does not reach the scalp in a single leap; it passes sequentially through the various relay stations of the central auditory pathway on its way to the brainstem. Each relay station the synchronized activity traverses adds an additional peak to the signal ultimately recorded by the surface electrodes. Among these successive peaks, Wave V serves as the primary marker, as it is typically the most consistent and easily identifiable peak across different individuals. Wave V latency—the time elapsing between the sound stimulus and the appearance of that peak—depends on how long it takes for synchronized activity to traverse all the relay points of the central auditory pathway to reach the site where Wave V is generated; the slower this conduction—whether because the sound stimulus reaches the auditory nerve with less intensity or because a relay point in the central auditory pathway conducts with a delay—the longer the Wave V latency becomes. Unlike a broadband click, which simultaneously synchronizes fibers across many different frequencies—thereby making it impossible to determine at which specific frequency conduction fails—an amplitude-modulated continuous tone (a tone whose intensity repeatedly rises and falls at a fixed rate) causes the central auditory pathway to synchronize its activity with that rising-and-falling rhythm, rather than producing a single peak like that generated by a broadband click. This sustained, repetitive synchronization of the central auditory pathway with the rhythm of the amplitude-modulated continuous tone is what is recorded as the auditory steady-state response; since the amplitude-modulated continuous tone can be set to a specific frequency, the auditory steady-state response allows for the estimation of the hearing threshold at that specific frequency. By presenting multiple amplitude-modulated continuous tones simultaneously—each at a different frequency and with a different modulation rate—the auditory steady-state response estimates hearing thresholds at several specific frequencies at the same time. Since the auditory evoked potential equipment used in this model does not require the individual to provide a voluntary response, neither the Wave V latency recorded via broadband click nor the frequency-specific auditory threshold estimated via the auditory steady-state response depends on the individual's cooperation: a longer-than-expected Wave V latency indicates that synchronized activity takes longer than normal to traverse the auditory nerve and central auditory pathway, whereas an elevated auditory threshold at a specific frequency—determined using the auditory steady-state response—indicates that the specific frequency requires greater-than-usual intensity to generate any detectable synchronization within the central auditory pathway.

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