Electrophysiological responses measured to sine-vocoded speech

Abstract

Objectives The goal of the current study was to investigate the effects of altering spectral and temporal cues on fundamental frequency (F0) magnitude by measuring the Frequency Following Response (FFR) on listeners with normal hearing. Spectral and temporal cues were altered by varying the number of channels and lowpass cutoff frequencies, respectively. Methods Six adults with normal hearing were recruited as participants in this study. FFRs were recorded in response to the steady state English back vowel /u/ for twelve different sine-vocoded stimuli with varying numbers of channels (1, 2, 4, 8, 16, and 32), while the lowpass cut off frequency was fixed at either 50 or 500 Hz. A Fast Fourier Transform (FFT) was used to compute the magnitude spectra over the duration of the recording. Peak FFT magnitudes were measured at the F0 from the FFR spectra. A two-way analyses of variance (ANOVA) on repeated measures was completed on the data collected to determine the effects of vocoded stimuli on the neural representation in the brainstem of the F0 spectral peaks. Results Significant differences in F0 magnitudes were seen between the two different lowpass cutoff frequencies, such that stronger responses were obtained using a 500 Hz as compared to a 50 Hz lowpass cutoff frequency. No significant differences were seen between the different numbers of channels. Furthermore, there was no significant interaction between channel number and cutoff frequency. Conclusions The results of this study provided evidence as to the feasibility of using the FFR to study brainstem representation to vocoded stimuli and showed consistencies with previous behavioral and electrophysiologic studies. More research is needed to look further at the effects of varying the lowpass cutoff frequency, effects of higher order processing on F0 magnitude, and the effects of variable placements of electrode arrays in the use of cochlear implants.