A model of intrinsic processing in the basal ganglia
K. Gurney, P. Redgrave, and T. J. Prescott
REFERENCE: Gurney, K., Redgrave, P. and Prescott, T. J. (1998) A model of intrinsic processing in the basal ganglia. Society for Neuroscience Annual Meeting, 24, Los Angeles, 640.13.
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We have recently suggested (Redgrave et al 1997) that the basal ganglia (BG) play a crucial role in solving the action selection problem in vertebrates . We now present a quantitative model of intrinsic BG processing which shows how this might occur. While much is known of the anatomy and physiology of the basal ganglia, comparatively little is known of the computational operations conducted within its functional architecture. The prevailing model (Albin et al 1989) - framed in terms of ‘direct’ and ‘indirect’ pathways - has several shortcomings . First, it fails to emphasise the significance of several anatomically important pathways. Second, it remains to be developed as a full computational model so that its claims remain unsubstantiated. Third, It fails to accommodate recent clinical and animal data (Chesselet and Delfs 1996, Parent 1998). Given this state of affairs we believe the functional architecture of the BG is ready for review, an opinion shared by the authors of the current model (Albin et al 1995).
- To reinterpret the BG anatomy as a set of neural mechanisms for selection.
- To explain quantitatively the selection operations of BG.
- To understand the role of dopamine (DA) within the context of action selection.
BG connectivity is very rich, and a principled approach is required to tease apart the strands of functionality within it. Starting from the computational premise of action selection, we examined the neural network architectures available for selection  and looked for correspondences with BG anatomy . We then used the resulting architecture to build a computational model and subjected this to analysis of its equilibrium properties and general simulation.
- BG functional anatomy can be interpreted in terms of two subsystems or pathways: a selection pathway proper (output GPi/SNr) , and a control pathway (output GPe) which modulates the activity in the selection pathway . The architecture supports the basic selection process by disinhibition .
- One role of the control pathway is to ‘normalise’ activity levels in the selection pathway to ensure selection .
- Dopamine modulation of input to both pathways works synergistically to make selection more or less exclusive .
- The effect of DA depletion is to prevent selection - consistent with symptoms of PD. Unlike the prevailing model, the new model does not rely solely on decreased GPe activity to explain this.
- The effect of large DA levels is to allow promiscuous selection - consistent with symptoms of ADHD, Tourette’s syndrome or stimulus intoxication, for example.
- Several transient signal properties of the model are similar to those observed in recordings from animal BG .
- The ability of the model to display several Neurophysiological ‘signatures’ suggests it may have captured key features of the biology
The functional decomposition of the BG architecture we have suggested here is supported by systemically elegant features (e.g. signal normalisation, synergistic DA modulation) and the model’s consistency with both neurophsyiological and behavioral data. We believe it will provide the basis for a programme to discover how the BG works to select actions in a robust, adaptive and hierarchical way.