Frequency following response

The frequency following response (FFR), also referred to as frequency following potential (FFP) or envelope following response (EFR), is an evoked potential generated by periodic or nearly-periodic auditory stimuli. Part of the auditory brainstem response (ABR), the FFR reflects sustained neural activity integrated over a population of neural elements: "the brainstem response...can be divided into transient and sustained portions, namely the onset response and the frequency-following response (FFR)". It is often phase-locked to the individual cycles of the stimulus waveform and/or the envelope of the periodic stimuli. It has not been well studied with respect to its clinical utility, although it can be used as part of a test battery for helping to diagnose auditory neuropathy. This may be in conjunction with, or as a replacement for, otoacoustic emissions.

In 1930, Wever and Bray discovered a potential called the "Wever-Bray effect". They originally believed that the potential originated from the cochlear nerve, but it was later discovered that the response is non-neural and is cochlear in origin, specifically from the outer hair cells. This phenomenon came to be known as the cochlear microphonic (CM). The FFR may have been accidentally discovered back in 1930; however, renewed interest in defining the FFR did not occur until the mid-1960s. While several researchers raced to publish the first detailed account of the FFR, the term "FFR" was originally coined by Worden and Marsh in 1968, to describe the CM-like neural components recorded directly from several brainstem nuclei (research based on Jewett and Williston’s work on click ABR's).

The recording procedures for the scalp-recorded FFR are essentially the same as the ABR. A montage of three electrodes is typically utilized: An active electrode, located either at the top of the head or top of the forehead, a reference electrode, located on an earlobe, mastoid, or high vertebra, and a ground electrode, located either on the other earlobe or in the middle of the forehead. The FFR can be evoked to sinusoids, complex tones, steady-state vowels, tonal sweeps, or consonant-vowel syllables. The duration of those stimuli is generally between 15-150 milliseconds, with a rise time of 5 milliseconds.

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