Response properties of the human frequency-following response (FFR) to speech and non-speech sounds: level dependence, adaptation and phase-locking limits.

OBJECTIVE: The frequency-following response (FFR) is a neurophonic potential used to assess auditory neural encoding at subcortical stages. Despite the FFR's empirical and clinical utility, basic response properties of this evoked potential remain undefined.

DESIGN: We measured FFRs to speech and nonspeech (pure tone, chirp sweeps) stimuli to quantify three key properties of this potential: level-dependence (I/O functions), adaptation and the upper limit of neural phase-locking.

STUDY SAMPLE: n = 13 normal-hearing listeners.

RESULTS: I/O functions showed FFR amplitude increased with increasing stimulus presentation level between 25 and 80 dB SPL; FFR growth was steeper for tones than speech when measured at the same frequency. FFR latency decreased 4-5 ms with decreasing presentation level from 25 and 80 dB SPL but responses were ∼2 ms earlier for speech than tones. FFR amplitudes showed a 50% reduction over 6 min of recording with the strongest adaptation in the first 60 s (250 trials). Estimates of neural synchronisation revealed FFRs contained measurable phase-locking up to ∼1200-1300 Hz, slightly higher than the single neuron limit reported in animal models.

CONCLUSIONS: Findings detail fundamental response properties that will be important for using FFRs in clinical and empirical applications.