Allocentric and egocentric representations of sound space in auditory cortex

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We can recognise and localise the many sounds that make up our everyday environment. For example when sitting outside a café, we can hear the rumble of a plane overhead, the pop music coming from the shop across the street, the chink of cutlery from the table behind us and the voice of the friend to our right who we're having coffee with. However, the cochlear, which encodes the frequency composition of sounds, does not provide information about where a sound comes from. Instead sound location must be computed in the brain by comparing the signals arriving at the two ears. Auditory space thus must be constructed from measurements relative to the head - sounds from our left will hit our left ear sooner than the right ear and will be louder in left than right ear because of the shadow cast by our head. Nonetheless, we can intuitively describe sound location in several ways: relative to ourselves (the toddler is giggling on my left) or relative to the world (the clock is chiming on the fireplace). Indeed as we move around the world, our perception of sound sources in the world remains stable - if we turn our heads towards a ringing phone we don't perceive the phone rotating around us; rather we are aware that we are turning towards it. This stability is remarkable because the sounds arriving at the ear are very different before and after we move - if I turn my head 180 degrees, a sound that originally arrived first at my left ear may arrive at my right ear first after turning. To achieve this, the brain must distinguish changes in sounds caused by our own movement and compensate to make the world stable. Perceptual stability and our intuitive experience of sound location in the world, suggests that the brain can represent both head-centered and world-centered space. We recently developed new methods to determine whether spatial sensitivity in the brain is head-centered (egocentric) or world-centered (allocentric). In previous studies of spatial processing, subjects are held static at the centre of a ring of speakers while neural activity is recorded. Because the subject never moves, the sound location relative to the head and the world are always the same and so it's impossible to tell which space a neuron represents. In our study however, subjects freely explored an environment while turning and moving their heads so that sound location relative to the head and the world could differ and we could determine if neural coding was head or world centered. For the first time we proved what most scientists had assumed, that many neurons in auditory cortex represent sound location relative to the head and are thus egocentric. But surprisingly, we also discovered a smaller population of allocentric neurons that could represent sound location in the world regardless of head position / direction. In this project we seek to better understand how egocentric and allocentric neurons are organised in the brain and how they contribute to sound perception. We will use a grid of speakers to map in unprecedented detail how space is represented by egocentric and allocentric neurons. We will also ask how animals experience sound location by training animals to perform either a head-centered or world-centered sound localization task. We will then compare neurons when animals are actively performing tasks or passively listening to understand how attention changes spatial processing. Finally, we will investigate how experience influences how the brain represents sound source location by comparing neurons in animals trained in each task or with no training. The ability to localise a sound in space is crucial for survival and communication: when listening to one voice in a noisy background our use of spatial cues significantly improves our ability to pick the target voice out from the background. Understanding how the brain represents auditory space may influence the design of hearing aids and inspire improvements in speech recognition.