Behavioral and Cortical Responses to Frequency Changes in Vowel-Like Stimuli in Listeners With Normal and Impaired Hearing

OBJECTIVES: This study proposes a new method for detecting cortical responses to formant frequency changes using the auditory change complex (ACC). These can be used as a proxy for discrimination performance between spectral characteristics of vowels. DESIGN: We generated two sets of vowel-like stimuli, each of which contained four sinusoids with carrier frequencies chosen to correspond to the first three formants of the vowels/u/ and/i/, respectively. During EEG recordings, the second formant (F2) frequency changed in different step sizes to evoke an ACC response. We collected responses for both a hearing-impaired group (10 participants) and a control group with normal hearing (9 participants) and furthermore compared these to a behavioral measure of formant frequency discrimination. RESULTS: The size of the change in the frequency of the F2 formant, as well as the presence of hearing impairment, significantly affected the ACC response. Larger frequency changes produced larger ACC response amplitudes, specifically for the/u/ stimuli with more lower-frequency content. Responses tended to be lower overall for the hearing-impaired participants. Moreover, the ACC magnitude was correlated with behavioral formant discrimination thresholds. CONCLUSIONS: Taken altogether, these results lend support to the use of an ACC paradigm to assess frequency discrimination abilities objectively with speech-like stimuli. This study shows reliable responses as well as correlation with behavioral discrimination, which highlight the paradigm's potential suitability for clinical purposes.