Elsevier

Neural Networks

Volume 109, January 2019, Pages 113-136
Neural Networks

Roles for globus pallidus externa revealed in a computational model of action selection in the basal ganglia

https://doi.org/10.1016/j.neunet.2018.10.003Get rights and content

Abstract

The basal ganglia are considered vital to action selection - a hypothesis supported by several biologically plausible computational models. Of the several subnuclei of the basal ganglia, the globus pallidus externa (GPe) has been thought of largely as a relay nucleus, and its intrinsic connectivity has not been incorporated in significant detail, in any model thus far. Here, we incorporate newly revealed subgroups of neurons within the GPe into an existing computational model of the basal ganglia, and investigate their role in action selection. Three main results ensued. First, using previously used metrics for selection, the new extended connectivity improved the action selection performance of the model. Second, low frequency theta oscillations were observed in the subpopulation of the GPe (the TA or ‘arkypallidal’ neurons) which project exclusively to the striatum. These oscillations were suppressed by increased dopamine activity — revealing a possible link with symptoms of Parkinson’s disease. Third, a new phenomenon was observed in which the usual monotonic relationship between input to the basal ganglia and its output within an action ‘channel’ was, under some circumstances, reversed. Thus, at high levels of input, further increase of this input to the channel could cause an increase of the corresponding output rather than the more usually observed decrease. Moreover, this phenomenon was associated with the prevention of multiple channel selection, thereby assisting in optimal action selection. Examination of the mechanistic origin of our results showed the so-called ‘prototypical’ GPe neurons to be the principal subpopulation influencing action selection. They control the striatum via the arkypallidal neurons and are also able to regulate the output nuclei directly. Taken together, our results highlight the role of the GPe as a major control hub of the basal ganglia, and provide a mechanistic account for its control function.

Introduction

The basal ganglia are an evolutionarily conserved group of subcortical nuclei, which have long been implicated in action selection Frank, 2005, Frank et al., 2004, Grillner and Robertson, 2016, Hikosaka et al., 2000, Lindahl et al., 2013, Redgrave et al., 1999, Schroll et al., 2012, Stephenson-Jones et al., 2011. Several computational models have been developed, examining their role in action selection Berthet et al., 2016, Frank et al., 2004, Gurney et al., 2001a, Gurney et al., 2001b, Hikosaka et al., 2000, Kamali Sarvestani et al., 2011, Mink, 1996, Schroll et al., 2012. They propose the basal ganglia as a ‘selection machine’ resolving conflicts between competing behaviours for common and restricted motor resources Frank, 2005, Redgrave et al., 1999, Schroll and Hamker, 2013. This notion is backed by studies showing that the stimulation of the striatum, the main input nucleus, can either trigger actions or inhibit them Freeze et al., 2013, Kravitz et al., 2010. Furthermore, loss of dopamine neurons in the substancia nigra pars compacta (SNc), result in a reduced ability to select motor responses (Wylie et al., 2009) in pathological conditions like Parkinson’s disease. In furtherance of the selection hypothesis, the basal ganglia are also implicated in learning of stimulus–response associations (Alexander, DeLong, & Strick, 1986) as well as in establishing stimulus–response–outcome associations (Redgrave & Gurney, 2006).

Existing models have dealt with a variety of aspects of basal ganglia function and anatomical context. Thus, many discuss the role of reinforcement learning Brown et al., 2004, Frank, 2006, Gurney et al., 2015, Redgrave and Gurney, 2006, Schroll et al., 2012 and have also incorporated the thalamo-cortical loops Beiser and Houk, 1998, Chersi et al., 2013, Frank et al., 2004, Humphries and Gurney, 2002, van Albada and Robinson, 2009. These models also cover a range of levels of biological description — from abstract system-level to detailed multi-compartmental neuronal models, as well as simulations of ensembles of neurons. Addressing computations at the level of the subnuclei of the basal ganglia, there have been several models of the striatal microcircuitry Damodaran et al., 2015, Humphries, Lepora et al., 2009, Humphries, Wood, and Gurney, 2009, the subthalamic nuclei (STN, Frank 2006), as well as examinations of the oscillations associated within the STN–GPe network Blenkinsop et al., 2017, Corbit et al., 2016.

Most models are based on the classical architecture of connectivity of the basal ganglia (Fig. 1 (A)), focusing on the direct pathway – the striatal D1 projections to the output nuclei, globus pallidus interna and substantia nigra pars reticulata (GPi/SNr), and the indirect pathway – the striatal D2 projections to the GPe, and the GPe projections directly to GPi/SNr and the STN–GPe/GPi loop. The GPe has been considered as homologous in structure and function in most of these models. However, recent studies have revealed a new subpopulation of GPe neurons, the arkypallidal cells (Mallet et al., 2012) that are active in anti-phase to their more common counterparts, the prototypical GPe neurons (Mallet et al. 2012, see also Methods). These two classes are also referred to as the TA and TI neurons respectively (Mallet et al., 2012). The arkypallidal cells provide a major input to the striatum (Mallet et al., 2012).

We aimed to incorporate the arkypallidal neurons into a well-tested model architecture of the basal ganglia Gurney et al., 2001a, Gurney et al., 2001b. The architecture has been validated at several levels of description: at the systems level using rate coded neural populations constrained by anatomical and physiological data (see Blenkinsop et al., 2017, Gurney et al., 2004, Humphries and Gurney, 2002); spiking neuron models challenged with physiological data Chersi et al., 2013, Humphries et al., 2006, Stewart et al., 2012; and at the behavioural level in embodied (robotic) models (Prescott, Montes González, Gurney, Humphries, & Redgrave, 2006). Most recently, it has been used to link a raft of neurobehavioural phenomena to neuronal mechanisms observed in vitro (Gurney et al., 2015). Thus, this model architecture offers a strong platform to try to understand the role and function of arkypallidal neurons and their afferent and efferent pathways in action selection. Furthermore, we also included another scheme of organisation in the GPe in terms of neuronal subpopulations — the outer and inner GPe neurons (Sadek, Magill, & Bolam, 2007). We built on the original model and used the methodologies developed therein to assess them, on extended architectures of connectivity of the GPe. The arkypallidal neurons have been accommodated in a few computational models Bahuguna et al., 2017, Bogacz et al., 2016, Lindahl and Hellgren Kotaleski, 2016, Moolchand et al., 2017 and their function in supporting optimal action selection (Bogacz et al., 2016) as well as in network dynamics underlying basal ganglia movement disorders have been investigated Bahuguna et al., 2017, Lindahl and Hellgren Kotaleski, 2016. However, their role in action selection and their influence on other basal ganglia subnuclei, needs additional investigation. Further, the outer and inner neuron dichotomy has not been included in any model so far (to our knowledge), and their role in action selection remains unknown. Our work addresses these lacunas and reveals important functions for different neuronal subpopulations within the GPe, and unites these two prevalent schemes of organisation within the GPe (GPe TI/TA and GPe outer/inner, Mallet et al. 2012 and Sadek et al. 2007) and furthermore, places the GPe in perspective as an important control centre of the basal ganglia.

Section snippets

Anatomy of the basal ganglia

The classical anatomy of the basal ganglia Bolam et al., 2000, Calabresi et al., 2014, Redgrave et al., 1999 is shown in Fig. 1 (A). It consists of the following principal nuclei: the striatum, the globus pallidus ((GPe) and internal (GPi) divisions in primates), the STN and the substantia nigra (SNr and SNc). The primary input nuclei are the striatum and the STN. The output nuclei are the GPi and the SNr. The input nuclei receive afferent signals from most of the cerebral cortex and the

Results

Recall from the methods that we make use of step-wise and combined models, investigating single and multiple pathways respectively, and that their deployment is carried out in three modelling phases. This approach is reflected here in reporting the Results.

Discussion

We have investigated the newly discovered intrinsic connectivity of GPe in considerable detail. Quantitative evaluation of selection performance in this model has revealed several new functions of GPe that may be understood within the selection framework. The prototypical neurons have been shown to be the principal subpopulation influencing action selection. The arkypallidal neurons are used by both the prototypical neurons and the STN, to modulate the activity of the striatum. These

Concluding remarks

The simulations have thrown light on the importance of the GPe in the basal ganglia, and its crucial and myriad role in action selection. It seems to be a ‘control centre’ of the basal ganglia with considerable influence on the functioning of other basal ganglia nuclei. The results show the GPe controlling the striatum, the GPi/SNr and as shown also in previous models, the STN (Gurney et al., 2001a). In particular, the prototypical GPe TI (outer/inner) neurons, seem to be the ‘controllers’,

Acknowledgements

We acknowledge the following grant sponsors: European Horizon 2020 Framework Programme (Grant 720270 (Human Brain Project SGA1), Grant 785907 (Human Brain Project SGA2)), the Swedish Research Council and the Swedish e-Science Research Center . We are also grateful for comments on the manuscript by Dr. Brita Robertson and Associate professor Dr. Peter Wallén.

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