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Publication Summary and Abstract

Humphries, M. D., Gurney, K. & Prescott, T. J. (2007), Is there a brainstem substrate for action selection?, Philosophical Transactions of the Royal Society B: Biological Sciences, 362(1485):1627-1639. [Code]

The search for the neural substrate of vertebrate action selection has focused on structures in the fore- and mid- brain, and particularly on the group of sub-cortical nuclei known as the basal ganglia. Yet, the behavioural repertoire of decerebrate and neonatal animals suggests the existence of a relatively self-contained neural substrate for action selection in the brainstem. We propose that the medial reticular formation (mRF) is this substrate’s main component and review evidence showing that the mRF’s inputs, outputs, and intrinsic organisation are consistent with the requirements of an action selection system. The internal architecture of the mRF is composed of inter-connected neuron clusters. We present an anatomical model which suggests the mRF’s intrinsic circuitry constitutes a small-world network, and extend this result to show that it may have evolved to reduce axonal wiring. Potential configurations of action representation within the internal circuitry of the mRF are then assessed by computational modelling. We present new results demonstrating that each cluster’s output is most likely to represent activation of a component action; thus, co-activation of a set of these clusters would lead to the co-ordinated behavioural response observed in the animal. Finally, we consider the potential integration of the basal ganglia and mRF substrates for selection and suggest they may collectively form a layered/hierarchical control system.
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