Importance of Vowel Harmonic Phase and Fundamental Frequency for Envelope Following Responses

The envelope following response (EFR) is a scalp-recorded potential that is phase-locked to envelope periodicities in auditory stimuli such as vowels. Vowel-evoked EFRs are influenced by stimulus characteristics; the most well-studied parameter is the fundamental frequency (f0). Many studies use lower f0 stimuli because they generally elicit larger response amplitudes. Comparatively, the influence of other stimulus characteristics, such as harmonic phase and amplitude spectra, is still poorly understood. The present study emphasizes the investigation of the potential influence of harmonic phase spectra using vowel stimuli. It is hypothesized that the alignment of beating envelopes, which are presumed to be generated by adjacent pairs of vowel harmonics, is a factor contributing to EFR amplitude.

Three experiments investigated the effects of varying stimulus parameters on EFRs using vowel tokens derived from 1 male talker with a lower f0 (107.9 Hz) and 1 female talker with a higher f0 (211.6 Hz). A total of 92 adults with normal hearing participated. A single channel was used to record EFRs between the vertex and nape with monaural stimuli. Experiment 1 progressively reduced f0 from the higher f0 to match that of the lower f0 to investigate whether control of f0 alone was sufficient to elicit equivalent amplitude EFRs. Experiment 2 mostly manipulated the phases of stimulus vowel harmonics while matching f0 to study their influence on EFR amplitude. Experiment 3 compared EFR amplitude for vowels with harmonic phases designed with a cochlear model to maximize or minimize EFR amplitude through alignment or opposition of beating envelopes presumed to be generated by adjacent pairs of vowel harmonics.

In experiment 1, EFR amplitudes increased as f0 of the female talker was reduced. However, even when f0 was close to the male f0, the EFR amplitude remained lower than that obtained with the male f0. Experiment 2 showed that EFR amplitude changes as harmonic phase spectra are varied while using matched f0 tracks. Furthermore, results suggested that synthesized vowels can elicit EFRs with amplitudes equivalent to a natural utterance by matching f0 and harmonic amplitude and phase spectra. Experiment 3 showed significantly smaller EFR amplitudes for vowels with alternating beating envelopes that encouraged destructive interference. With a lower f0, the alignment of vowel beating envelopes significantly increased EFR amplitude.

The results of experiment 1 support that, in addition to f0, other stimulus factors affect EFR amplitude. Experiment 2 supports that harmonic phase spectra are an important stimulus characteristic for EFR amplitude and that EFRs elicited by natural vowels can be simulated adequately by modeling f0 and harmonic amplitude and phase spectra. Results of experiment 3 support that alignment of stimulus beating envelopes, presumed to be generated in the cochlea between adjacent pairs of harmonics, can lead to larger scalp-recorded EFRs. Designing vowel stimuli with low f0 and harmonic phases optimized to encourage envelope alignment could improve EFR amplitudes and reduce the recording time required to detect them in some individuals.