Representations of fricatives in sub-cortical model responses: comparisons with human consonant perception

Abstract

Fricatives are obstruent phonemes made by constricting airflow in the vocal tract, producing turbulence across the constriction site or downstream from the constriction. Fricatives exhibit significant intra/inter-subject and contextual variability. Yet fricatives are perceived with high accuracy. The current study investigated modeled neural responses to fricatives in the auditory nerve (AN) and inferior colliculus (IC), with the hypothesis that response profiles across populations of neurons provide robust correlates to perception. Stimuli were 270 intervocalic fricatives (10 speakers x 9 fricatives x 3 utterances). Computational model response profiles had 50 characteristic frequencies, log-spaced from 125 to 8 or 20 kHz. Confusion matrices generated by KNN subspace classifiers used the average rates for the 50 characteristic frequencies as the features and compared them with published behavioral data. The modeled AN and IC neural responses provided better predictions of perceptual accuracy than the stimulus spectra, with IC showing better accuracy than AN. Perceptual fricative accuracy was explained by modeled neural response profiles, whereas confusions were only partially explained. Extended frequencies improved accuracy based on the model IC, corroborating the importance of extended high frequencies in speech perception. Future electrophysiological studies from IC cells and neural response simulations of hearing-impaired ears could guide the development of processing strategies.