ENCODING COMPLEX SPEECH SOUNDS IN THE AUDITORY MIDBRAIN OF CATS

Sensorineural hearing loss (SNHL) in humans affects the processing of complex stimuli like speech, particularly in noisy environments. There are few to no studies exploring the effects of SNHL on responses to complex stimuli in the auditory midbrain. We investigated the degree to which patterns of neural activity evoked in the inferior colliculus (IC) by speech sounds in various levels of background noise allows discrimination between these sounds in normal hearing, and deaf cats. Neural responses were recorded simultaneously from 32 sites across the cochleotopic axis of the IC in anaesthetised cats. Speech sounds were presented at 20, 40, 60 and 80dB Sound Pressure Level in quiet and increasing levels of additive noise (signal-to-noise ratios (SNRs): -20,-15,-10,-5,0,+5,+10,+15,+20dB). The speech sounds were consonant-vowel-consonant (CVC) tokens used previously to investigate spectral and temporal resolution in implant users (bad, bed, bid, bod, bud, bead, board, bird, bard, bared, booed). Neurograms were created, and neural discrimination (d’) was assessed using Euclidean distance in n-dimensional space, resulting in a function reflecting differentiation of speech sound encoding across various SNRs and noise levels. d’ was computed from the hit-rate and false-alarm rate for the CVC classifications, and the neural discrimination threshold was defined as the lowest SNR at which d’ crossed 1. Our results showed that d’ in quiet was dependent on the audibility of the stimuli, defined as the level of the stimulus above the individual animals pure-tone average (1, 2, 4kHz). There was no dependence of discrimination threshold on audibility.