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

Abstract

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.

Keywords:

• Auditory brainstem response (ABR)
• auditory evoked potentials
• electrophysiology
• EEG

Notes

Acknowledgments

This work was supported by a grant from the University of Memphis Research Investment Fund (UMRIF) awarded to G.M.B. The authors declare no other competing conflicts or other financial interests.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes

1 Steeper growth for tones compared to speech was also confirmed via linear regression analysis. We computed slopes for each listener’s tone and speech I/O functions between 25 and 65 dB SPL (i.e. the linear growth segment) using standard least-square regression (MALTAB fitlm function). Across listeners, F0 I/O slopes were steeper for tones than speech (t₁₂ = 3.14, p = .0084). I/O slopes for FFR RMS amplitudes were similar across stimulus conditions (t₁₂ = 1.72, p = .11).

2 Rectification components are also expected in the speech and tone stimuli (Figures 1 and 2). However, those stimuli were complex so they contained stimulus energy at the F0 and its harmonics. In those cases, rectifier components (which are harmonically related to F0), would overlap with FFRs to the actual stimulus harmonics rendering them inseparable. Nonlinear components become apparent when using simple stimuli (puretones, chirps), since FFRs appearing at frequencies other than the stimulus F0 must reflect a nonlinear (biological) response (see also Figure 7 of Hoormann et al. 1992).

3 Single trial FFR amplitudes of Figure 4 are much larger (by several µV) than grand averaged data shown in Figure 2. This apparent discrepancy reflects the fact that any single trial FFR (Figure 4) contains both evoked potential (EP) and background EEG noise (BN) (Elberling and Don 2007). In contrast, the averaged FFR (averaged over 2000 trials; Figure 2) reflects largely the deterministic EP signal (∼0.5µV), since the residual BN has been reduced by a factor of √N.

4 N = 12 are plotted since one subject showed high-frequency artifacts in their EEG recording.