Stimulus polarity effects on individual components of speech-evoked frequency following response

In literatures of auditory frequency following response (FFR), it has been suggested that adding the frequency following responses evoked by opposite stimulus polarities can be used to emphasize neural activities related to the stimulus envelope, while subtracting the responses can enhance the activities related to fine structure such as the formants in speech stimuli. To assess the effectiveness of the additive and subtractive technique, our study focused on the effects of stimulus polarity on individual harmonic frequency components of FFR. Specifically, the effects of stimulus polarity on the amplitude and phase of individual component were analyzed by comparing FFRs to two stimulus polarities. In addition, individual component was also compared between addition FFR and subtraction FFR, to figure out the effects caused by the additive and subtractive technique. Results showed no significant effects of stimulus polarity on the amplitudes of individual components in FFR, as well as the phase of the component at the fundamental frequency. However, significant change on the phase of component at the first formant frequency was observed due to stimulus polarity inversion. According to amplitude analysis, the addition FFR did emphasized the F0 component, while the subtraction FFR enhanced F1 component. The present study supported the hypotheses that the additive and subtractive technique was effective in emphasizing the envelope-related responses and fine-structure-related responses, respectively. The proposed method of analyzing the amplitude and phase of individual component in FFR was useful for testing the additive and subtractive technique directly.