Auditory Brainstem Responses to Forward and Reversed Speech in Normal Hearing Individuals

Abstract

Differences in the coding of forward and reversed speech has indicated that the human auditory system is sensitive to different types of speech sounds. Infants as well as adults are reported to respond differently to forward and reversed speech. Functional magnetic resonance imaging (FMRI) have revealed that listening to forward speech activates large regions of the temporal lobe, whereas reverse speech evokes significantly diminished and nonlocalised brain responses. The objective of the present study was to assess the differences, if any, in the brainstem responses to forward and reversed speech stimuli. 50 normal hearing adults participated for the study. A synthesized 40msec short stimulus /da/ syllable was used as the stimulus for both forward and reversed conditions. The syllable was reversed with the help of Adobe Audition software. Auditory brainstem responses were recorded for the forward and reversed /da/ stimulus. Results revealed that the amplitude of wave V was larger for reversed speech as compared to the forward speech. On the other hand, the amplitude of the frequency following responses, fundamental frequency and the formant frequency were smaller in the reversed speech condition as compared to the forward speech condition. The findings of the present study suggest that differential processing of forward and reversed speech occurs at the brainstem level as well even for a short duration stimulus. The better response to forward speech could be due to the universal temporal and phonological properties of human speech which is familiar to the brainstem and hence is processed efficiently. These findings suggests that Speech evoked ABR may throw light to understand the encoding of complex acoustic stimulus at the brainstem level.