Envelope following responses elicited by English sentences

It would be clinically valuable if an electrophysiological validation of hearing aid effectiveness in conveying speech information could be performed when a device is first provided to the individual after electroacoustic verification. This study evaluated envelope following responses (EFRs) elicited by English vowels in a steady state context and in natural sentences. It was the purpose of this study to determine whether EFRs could be detected rapidly enough to be clinically useful.

EFRs were elicited using 5 vowels spanning the English vowel space, /i/, /ε/, /æ/, /(Equation is included in full-text article.)/, and /u/. These were presented either as concatenated steady state vowels (total duration 10.04 seconds) or in three 5-word sentences (total duration 11.77 seconds), where each vowel appeared once per sentence. Single-channel electroencephalogram was recorded from vertex (Cz) to the nape of the neck for 190 and 160 repetitions of the steady state vowels and sentences, respectively. The stimuli were presented at 70 dBA SPL. The fundamental frequency (f0) track from the stimuli was used with a Fourier analyzer to estimate the EFRs to each vowel. Noise amplitudes were also calculated at neighboring frequencies. Fifteen normal-hearing subjects who were 20 to 34 years of age participated in the experiment.

In the analysis of steady state vowels, the mean response amplitude of /i/ was statistically the largest at 173 nV. The other 4 steady state vowels did not differ in mean response amplitude, which varied between 73 and 106 nV. In the analysis of vowels from the 3 sentences, the largest response amplitudes tended to be for /u/. Mean amplitudes for /u/ were 164, 111, and 140 nV for the words "booed," "food," and "Sue," respectively. The vowel /u/ produced statistically larger responses than /i/, /ε/, and /(Equation is included in full-text article.)/ when grouped across words, whereas other vowels did not differ. Mean response amplitudes for the other vowel categories in the sentences varied between 82 and 105 nV. All subjects showed significant EFRs in response to the words "Bee's" and "booed," but only 9 subjects showed significant EFRs for "pet," "bed," and "Bob."

The authors were readily able to detect significant EFRs elicited by vowels in a steady state context and from 3 natural sentences. These results are promising as an early step in developing a clinical tool for validating that vowel stimuli are at least partially encoded at the level of the auditory brainstem. Future research will require evaluation of the technique with aided listeners, where the natural sentences are expected to be treated as typical speech by hearing aid signal-processing algorithms.