Frequency following responses measured to noise-vocoded speech in adults with normal hearing sensitivity

Abstract

Objectives The goal of the current study was to investigate the effect of noise-vocoders on the brainstem neural representation of fundamental frequency (F0) to better understand the way in which cochlear implant (CI) users process speech. In order to examine brainstem neural representation of noise-vocoded stimuli when spectral and temporal cues are varied, we presented noise-vocoded speech at low-pass cutoff (LPC) frequencies of 500 and 50 Hz, and varied the channel number from 1-32. Methods Six normal hearing adults were recruited as participants in the study. Frequency Following Response (FFRs) were recorded for each participant using a synthetic steady state English back vowel /u/ (F0= 120 Hz; F1= 360 Hz; F2= 970 Hz; duration: 250 millisecond) presented in noise-vocoded conditions. The effect of varying spectral cues were studied by varying the number of channels (1, 2, 4, 8, 16, 32). The effect of varying temporal cues was studied by changing the LPC frequency from 50 to 500 Hz. A Fast Fourier Transform (FFT) analysis was performed on FFR waveforms reflecting brainstem neural representation of the F0. Non-Parametric Wilcoxon Signed-Ranked tests for each channel (1, 2, 4, 8, 16, and 32) were conducted on the FFR data to evaluate the effects of varying the temporal envelope cutoff frequency (50 Hz and 500 Hz) on the F0 peak magnitudes in the FFR envelope spectra. Results There was a significant difference of the LPC frequency on the F0 magnitude, such that 500 Hz LPC frequency produced a stronger mean F0 than the 50 Hz LPC frequency. The mean F0 magnitudes were higher and increased as the channel number increased. Conclusions Examination of brainstem neural representation of F0 using the FFR is an important step toward understanding how CI users’ processes speech. Further research using vocoders, in individuals with normal hearing is necessary before any assumptions can be made about how CI users hear and process speech.