Selective Attention: How does Neural Response Modulation in Auditory Cortex Enable Auditory Scene Analysis?

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Listening to conversation in a crowded room is one of the greatest challenges that the auditory system faces, and the most common cause of complaint for many of the 10 million people in the UK who suffer hearing loss. For example, to fully appreciate the piece of gossip that your friend is telling you in a restaurant, you must be able to separate his voice from the voices of other people, the clatter of glasses, and the music playing in the background. Your brain is able to 'select' the voice of your friend over all these other sounds - perhaps on the basis of where he is standing, or the pitch of his voice. Normal hearing listeners achieve this feat effortlessly, although engineers have yet to create a machine that can successfully match such signal separation in noisy backgrounds. In this proposal we try to understand how the neural machinery of the brain is able to extract sounds of interest while ignoring others. Our work focuses on a brain area called the auditory cortex; an area that is thought to be necessary for listening in complex situations like the one described above. Our goal is to understand how the responses of neurons in auditory cortex represent multiple competing sounds, and how the neural responses can be shaped in order to best represent sounds according to the listener's current demands.In this proposal we train animals in a series of listening tasks that will enable us to impose different demands on auditory cortex. Animals will listen for a target word amongst a series of non-target words. In some cases they will do this in silence, in others in the presence of background noise. In further variations they will listen to two streams of speech, each from a different talker, and from different locations, and be asked to selectively attend to one talker over the other (equivalent to trying to listen to your friend while ignoring the loud man behind her). We will record from neurons in auditory cortex while animals perform these tasks in order to understand how the different task requirements change the way in which sounds evoke neural activity. Auditory cortex is made of multiple, hierarchically organised areas that are thought to perform different functions. We will determine whether areas early in this hierarchy are affected by attention differently from those in higher areas.In the second part of this project we will use a technique called optogenetics to selectively silence neural activity in particular regions of auditory cortex. We will test the hypothesis that different areas of Auditory Cortex facilitate different sorts of attention - for example separating sounds according to their location in space, as opposed to the pitch or timbre or a particular talker's voice. Finally we will determine whether feedback from higher auditory areas to primary auditory areas is essential for active listening. This work would represent a fundamental advance in our knowledge of the role of these 'feedback' projections and the role that they play in active listening.Our work has the potential to enable the development of more sophisticated, biologically inspired, signal processing devices for hearing aids and cochlear implants - both of which perform poorly in many real-world listening conditions. Listeners whose hearing is assessed via an audiogram as normal can still struggle with listening in noisy situations - this problem is particularly acute in aged listeners. Problems in processing complex sounds underlie Central Auditory Processing Disorder, and disorders of attention are thought to underpin a variety of developmental disorders including autism, attention related hyperactivity disorder, as well as dementia and other neuropsychiatric conditions. Understanding the neural mechanisms in the healthy brain responsible for engaging attention to select sources in a sound scene will lay the foundation for understanding, and potentially treating, conditions in which these mechanisms are impaired.