All animals must continuously sequence and co-ordinate behaviors appropriate to both their context and current internal state if they are to survive. It is natural to wonder what parts of the nervous system – the neural substrate – evolved to carry out this action selection process. For simpler animals, like C. elegans, a small, fixed, behavioural repertoire is handled by specialist neurons that direct motor responses to specific stimuli. By contrast, vertebrate behavourial repertoires are both extensible and flexible. The evolution of this broader scope for behaviour seems related to the evolution of a central nervous system,
The elaboration of the vertebrate nervous system has led to multiple, partially segregated neural systems that lie interposed between primary sensory and motor circuits. Each of the elaborated sensory, homeostatic, memory, planning, and emotion neural systems could in principle guide behaviour; yet each is essentially competing for access to a single final common motor pathway formed by the motor neurons of the spinal cord and cranial nerve nuclei. We will discuss the proposal that the vertebrate brain has co-evolved (or co-opted) specialised and centralised neural systems for action selection to handle both the competition between systems accessing the final motor pathway and the open-ended nature of a flexible, extensible behavioural repertoire (Prescott et al, 1999; Redgrave et al, 1999).
Two key structures have been identified, the basal ganglia of the fore- and mid-brain (Redgrave et al, 1999), and the medial reticular formation of the brainstem (Humphries et al, 2007). Both are intimately involved in motor control. The basal ganglia are key to learning and expressing voluntary behaviour, the medial reticular formation seems to co-ordinate simple motor actions. Moreover, in keeping with the hypothesis of evolved specialised action selection structures, they have been identified in all mammal species, and homologous structures exist in all other vertebrates. Computational modelling of both structures has been crucial in understanding how they may represent and resolve action selection competitions (Gurney et al, 2001; Humphries et al, 2006; Humphries et al, 2007). It will also be crucial to understanding how these systems work together.
Gurney, K., Prescott, T. J. & Redgrave, P. (2001a) A computational model of action selection in the basal ganglia Biological Cybernetics, 85, 401-423.
Humphries, M. D., Stewart, R. D. & Gurney, K. N. (2006) A physiologically plausible model of action selection and oscillatory activity in the basal ganglia Journal of Neuroscience, 26, 12921-12942.
Humphries, M. D., Gurney, K. & Prescott, T. J. (2007) Is there a brainstem substrate for action selection? Philosophical Transactions of the Royal Society B, 362, 1627-1639.
Prescott, T. J., Redgrave, P. & Gurney, K. (1999) Layered control architectures in robots and vertebrates Adaptive Behavior, 7, 99-127
Redgrave, P., Prescott, T. J. & Gurney, K. (1999) The basal ganglia: A vertebrate solution to the selection problem? Neuroscience, 89, 1009-1023