The Envelope Following Response (EFR) can be used to estimate hearing thresholds using subdermal or dermal electrodes. A very good explanation was established by William Dolphin in 1993. He explains that scalp potentials follow (phase lock) the periodicity of sinusoidal signals up to 1 kHz. This is called the Frequency Following Response (FFR). It has furthermore been shown that if continous sinusoidal amplitude modulated sounds are used above 1 kHz (in humans) then a phase locking onto the envelope frequency (the modulation frequency) is observed. This results in a spectral component in the response to the used SAM tones that is not observed in continous sinusoidal signals without modulation. Since this phase locking results in significant energy at the modulation frequency it is called the "Envelope Following Response".
This allows for a rapid testing of different frequencies at the same time using multienvelope signals as explained by Dolphin (1996) and utilized for instance in the Evoked Response Study Tool (EVREST) framework (Finneran, 2009). Especially this abilitiy to screen different frequencies using the so called Modulation Rate Transfer Function (MRTF) has increased the knowledge on physiology of hearing wild populations of dolphins and seals. This function describes how intense the response to a given SAM tone stimulus is at the modulation rate (Dolphin et al. 1995). The different modulation rates elicit peaks in the spectra of the responses and allow for a similar evaluation as the responses to single pure tone sinusoidal signals in electrophysiological trials.