Sensitivity to stimulus polarity in speech-evoked frequency-following responses

Abstract

It has been suggested that frequency-following responses recorded to speech sounds presented in opposite polarities can be added to emphasize responses related to the periodicity envelope (e.g., at the fundamental frequency, f0) or subtracted to emphasize responses related to the stimulus fine-structure (e.g., harmonics near formant peaks) because inverting stimulus polarity has little effect on the stimulus envelope. This hypothesis was tested by comparing frequency-following responses to several tokens of two vowels (/a/ and /i/) presented twice in one polarity and once in the opposite polarity, from 9 normal-hearing subjects. At harmonics near formant peaks, most subjects displayed frequency-following responses that followed stimulus polarity. At f0, subjects displayed responses that were insensitive to stimulus polarity. However, response amplitude varied across polarities for some subjects and some vowel tokens. Stable responses were obtained when stimuli were presented in the same polarity, so the polarity-sensitive amplitude changes were not likely related to temporal encoding difficulties or background electrophysiological noise. Future studies involving manipulations of speech stimulus polarity should consider response amplitude changes carefully. © 2013 Acoustical Society of America.