Cortical processing of speech sounds in adverse conditions

The brain has an amazing capacity to accurately represent information about myriad complex stimuli from its surrounding environment. Problems with maintaining accurate representations of stimuli have harmful effects and are reflected behaviorally. Speech is one such complex stimulus. Normal hearing humans are adept at understanding speech in adverse situations. However hearing and learning impaired populations have great difficulty processing speech sounds in noise. Examining neural responses to speech sounds may provide a better insight into how the auditory system represents speech sounds. Previous studies with EEG and fMRI techniques have revealed much about how the brain responds to speech sounds. However, these techniques lack the spectral and temporal precision necessary to study the underlying neural representation of individual speech sounds. In this dissertation, microelectrode recordings of speech responses in rat primary auditory cortex provide high spatial and temporal precision. Previous studies have shown that animals have similar behavioral and neural responses to speech sounds as humans. Primary auditory cortex (A1) responses in rats are correlated with their behavioral speech discrimination ability in quiet situations. Our study shows that rats can discriminate between speech sounds in noisy conditions and that this ability can be explained by the A1 spatiotemporal patterns observed in noisy conditions. Maintaining temporal resolution of neural responses is necessary to accurately predict behavior in noise. A number of previous studies suggest that speech processing disorders arise from underlying neural deficits in temporal processing. Given the importance of temporal information for the representation of speech stimuli, we tested whether changes in temporal response properties could affect neural responses to speech sounds. Vagus nerve stimulation (VNS) was paired with tone train stimuli to change the temporal response properties of A1 neurons. Results from this study indicate that increased temporal following capacity of neurons also increased neural discrimination ability of rapidly presented speech sounds.