Frequency following response (FFR), also referred to as Frequency Following Potential (FFP), is an evoked response generated by continuous presentation of low-frequency tone stimuli. Unlike the Acoustic Brain Reflex (ABR), the FFR reflects sustained neural activity; integrated over a population of neural elements. It is phased 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 response characteristics and clinical utility.

History

In 1930, Weaver and Bray discovered a potential called the “Weaver-Bray Response”. They originally believed the potential originated from the cochlear hair cells, but later discovered that the response was from the auditory nerve. The original discovery of this potential may have been accidentally discovered back in 1930, however renewed interest in defining the FFR didn’t occur until the mid 1960s. While a 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 Jewitt and Williston’s work on click ABRs).

Stimulus parameters

The recording procedures for the scalp recorded FFR is essentially the same as the ABR. The primary differences are related to the stimulus and response parameters. The FFR can be evoked to tone bursts below 2000 Hz, complex tones, steady-state vowels, tonal sweeps, and consonant-vowel syllables. The duration of those stimuli are generally between 15-150 ms, with a rise time of 5 ms.. The presentation rate is often between 3.1 and 7.1/sec anywhere from 45-80 dBnHL. The polarity of the stimulus can either be fixed or alternating, however alternating polarity causes the extinction of the cochlear microphonic and a doubling of FFR frequency.

Clinical applicability

Due to the lack of specificity at low levels FFR has yet to make its way into the clinical setting. Only recently has FFR been evaluated for encoding complex sound and binaural processing. There may be uses for the information FFR can provide regarding steady state, time-variant, and speech signals for better understanding of individuals with hearing loss and its affects. FFR Distortion Products (FFR DPs) could supplement low frequency (< 1000 Hz) DPOAEs. FFRs have the potential to be used to evaluate the nature of neural representation of speech sounds processed by different strategies employed in cochlear implants, primarily identification and discrimination of speech. Also phase-locked neural activity reflected in the auditory steady state responses have been successfully used to predict auditory thresholds.

Research directions

Currently, there is renewed interest in using the FFR to evaluate: the role of neural phase locking in encoding of complex sounds in normally hearing and hearing impaired subjects, encoding of voice pitch, binaural hearing, and evaluating the characteristics of the neural version of cochlear nonlinearity.

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