Sensitivity of envelope following responses to vowel polarity

Envelope following responses (EFRs) elicited by stimuli of opposite polarities are often averaged due to their insensitivity to polarity when elicited by amplitude modulated tones. A recent report illustrates that individuals exhibit varying degrees of polarity-sensitive differences in EFR amplitude when elicited by vowel stimuli (Aiken and Purcell, 2013). The aims of the current study were to evaluate the incidence and degree of polarity-sensitive differences in EFRs recorded in a large group of individuals, and to examine potential factors influencing the polarity-sensitive nature of EFRs. In Experiment I of the present study, we evaluated the incidence and degree of polarity-sensitive differences in EFR amplitude in a group of 39 participants. EFRs were elicited by opposite polarities of the vowel /ε/ in a natural /hVd/ context presented at 80 dB SPL. Nearly 30% of the participants with detectable responses (n = 24) showed a difference of greater than ∼39 nV in EFR response amplitude between the two polarities, that was unexplained by variations in noise estimates. In Experiment II, we evaluated the effect of vowel, frequency of harmonics and presence of the first harmonic (h1) on the polarity sensitivity of EFRs in 20 participants with normal hearing. For vowels /u/, /a/ and /i/, EFRs were elicited by two simultaneously presented carriers representing the first formant (resolved harmonics), and the second and higher formants (unresolved harmonics). Individual but simultaneous EFRs were elicited by the formant carriers by separating the fundamental frequency in the two carriers by 8 Hz. Vowels were presented as part of a naturally produced, but modified sequence /susaʃi/, at an overall level of 65 dB SPL. To evaluate the effect of h1 on polarity sensitivity of EFRs, EFRs were elicited by the same vowels without h1 in an identical sequence. A repeated measures analysis of variance indicated a significant effect of polarity on EFR amplitudes for the vowel /u/ and a near-significant effect for /i/, when h1 was present. EFRs elicited by unresolved harmonics and resolved harmonics without h1 demonstrated no significant differences in amplitude due to polarity. The results suggest that h1 contributes to the polarity sensitivity of EFRs elicited by low frequency F1 carriers. However, it is unlikely that this is only due to the influence of a polarity-sensitive frequency-following response to the fine structure at h1. Removing h1 by filtering also decreased the asymmetry of the vowel envelope, especially for those with low first formant frequencies. A measure called the envelope asymmetry index was computed to evaluate the relationship between stimulus envelope asymmetry above and below the baseline, and polarity-sensitive differences in EFR amplitude. A significant positive correlation between envelope asymmetry index and absolute amplitude differences in EFR due to polarity suggests that one of the causes contributing to the polarity sensitivity of EFRs could be the asymmetry in stimulus envelope. This stimulus characteristic, however, explains only a fraction of the variability observed and there may be other factors that contribute to individual differences in polarity sensitivity of the EFR to naturally produced vowel stimuli.