Can you hear me now? Towards a model of age-related difficulty in noisy environments

Abstract

The National Institute on Deafness and Other Communication Disorders estimates that there are 40-million Americans who report having hearing difficulty. One of the chief complaints is the inability to hear in noisy environments. We currently do not understand how the brain separates background noise from attended sounds. The goal of this thesis is to observe how the brain detects sounds in noise and how these mechanisms are disrupted during normal aging. In the first chapter, we review the relevant literature surrounding what is known about how sound suppression occurs at the cortical level. We then review the literature surrounding what is known about cortical decline during healthy aging. In the second chapter, examine the differences in sound-encoding by recording from auditory cortex while animals passively listen to tones in noise. From this work, we see that old animals have deficits in temporal and spectral encoding. At the population level, we find that there are increased noise correlations, which we find limit the ability of cortex to encode the tone identity. In the third chapter, we examine the effect of behavior by recording from old and young animals while performing a tone detection task in noise. We confirm the temporal deficits and increased noise correlation seen in passive listening are still present. We additionally find that aging animals are unable to fully suppress their responses to the noisy background. This increased activity to the background leads the old animals to have more false alarms. In the fourth chapter, we examine how life-long auditory training can mitigate the problems found in chapters two and three. We train animals on a tone detection task for a period of 6 months and then see how their passive responses to noise differ from an animal that has not been trained. In the final chapter, we take the results from the three chapters in order to form a working model of how healthy aging affects the auditory cortex and leads to disruptions in encoding tones in noise. We then suggest future work that could be done to further test the predictions of this model.