Electrophysiological methods

This methodology relies on the principle of measuring neurological responses to auditory stimuli. For calibration measurements and comparisons to behavioural methods trained animals can be used. They provide for a behaviourally established auditory threshold and electrophysiological measurement.

Electrodes are usually placed on the skin (for instance in dolphins) or subdermally (many species) to record voltage differences that are elicited when playing sounds of different frequency and intensity. These usually very small electrical currents are called auditory evoked potentials (AEP). Of these, only the fast potentials (less than ten milliseconds after the sound is received - FAEPs) are used to measure auditory thresholds. Other activities like muscle contractions also result in nerve responses and therefore potentially mask the auditory stimulus and its response potenials. Hence, AEPs are strongly influenced by any activity and the animal usually has to be sedated for these measurements. The process of signal transmission between the cochlea and the brain is called the auditory brainstem response (ABR). It is usually referred to as the first 0-12 ms of the AEP signal and therefore very much alike the FAEPs. The terms AEP and ABR are often used synonymously for any method that relies on eliciting nerve potentials by playing back sound and analysing the FAEPs.

It is necessary to average multiple evoked potentials to see a clear averaged response, that then contains multiple waves, which in turn are associated to different brain regions. Usually at least 100 to 2000 repetitions are needed to get a clear waveform of the averaged response. When those averaged responses fade out into the electrophysiological background noise, it is assumed that the animal does not hear the sound any more. AEP measurements have been used for many studies conducted on animals, that are either not trainable or that can only be accessed in the wild. They sometimes represent the best available knowldedge about auditory thresholds, but comparisons between electrophysiological methods and behavioural methods to estimate hearing thresholds show considerable differences with thresholds determined on the basis of AEPs usually being higher.

AEP measurements therefore might not give true and absolute representations of hearing thresholds. For many species (including fishes) AEP experiments do not represent the hearing bandwidth or sensitivity accurately but are rather a situation-specific representation of electrophysiological nerve potentials. There is however also the indication that some studies found comparable hearing thresholds based on both methods.

Electrophysiological methods have their benefits whenever fast assessement of hearing is necessary. A striking example for this is the newborn screening in humans, where hearing sensitivity is actually not measured. It is rather assumed that when a clear response is detected at well audible sound pressure levels, that then hearing is not compromised at the tested frequency.

In general it can be assumed that a behavioral audiogram is therefore to be preferred to assess best hearing in an animal species, but that electrophysiological measurements allow scientis to determine hearing abilities in non-cooperative animals or age classes.