Contribution of local- and circuit-based mechanisms to adaptive coding in the auditory midbrain

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The ability to code accurately the prevailing sensory information is critical to an animal's survival, and brain mechanisms have evolved that enable this information to be extracted from the environment in an optimal manner. As part of this ability, many sensory neurons appear able to adjust their neural tuning in 'real-time' to take account of the unfolding sensory environment. This so-called 'adaptive coding' often results in improved accuracy with which the ambient sensory environment is coded. How this is achieved in the brain remains to be determined, both in terms of cellular mechanisms and in terms of brain circuits that contribute to adaptive coding in any one brain centre. Our experiments will investigate three specific aspects of adaptive coding. First, they will determine mechanisms within neurons themselves that contribute to adaptive coding for the unfolding sound intensities present in a sound. Second, they will assess the contribution of cortical feedback on adaptive coding in lower brain centres. Although sensory information is thought to 'flow' from the peripheral hearing sensors in the ears towards higher brain centres such as the cortex, a substantial number of cortical feedback circuits are also known to exist. The function of these feddback circuits remains unclear, but preliminary evidence from my laboratory suggests that one role is to modify adaptive coding in the auditory midbrain, increasing the time taken by neurons to optimise their sensitivity to sound intensity when the prevailing sound enviroment suddenly changes. Finally, experiments will also examine how specific adaptive coding is for the stimulus feature currently being heard, assessing whether or not adapting a neuron's response to one stimulus parameter (e.g. sound intensity) also means that the responses are adapted to sound-source location, for example.