Envelope and spectral frequency-following responses to vowel sounds

Frequency-following responses (FFRs) were recorded to two naturally produced vowels (/a/ and /i/) in normal hearing subjects. A digitally implemented Fourier analyzer was used to measure response amplitude at the fundamental frequency and at 23 higher harmonics. Response components related to the stimulus envelope (“envelope FFR”) were distinguished from components related to the stimulus spectrum (“spectral FFR”) by adding or subtracting responses to opposite polarity stimuli. Significant envelope FFRs were detected at the fundamental frequency of both vowels, for all of the subjects. Significant spectral FFRs were detected at harmonics close to formant peaks, and at harmonics corresponding to cochlear intermodulation distortion products, but these were not significant in all subjects, and were not detected above 1500 Hz. These findings indicate that speech-evoked FFRs follow both the glottal pitch envelope as well as spectral stimulus components.