The Centre Surround model and Disorders of the Basal Ganglia

Hello! after a break that was far too long I'm back to the blogging (those first two posts obviously took it out of me!). Hopefully I can pick up a bit of momentum this time round and manage a few more in quick succession. For this post I want to follow on from the first I did about the Basal Ganglia, in particular let's talk about how the centre surround model I introduced in the first post has helped clinical scientists in understanding what causes lie behind symptoms they observe in patients with a range of high profile disorders of the Basal Ganglia. I'll be taking my first post as a start point and building on that because I want to avoid any returning readers (wishful thinking?) from having to read the same things twice (plus I'm lazy and don't want to type the same thing multiple times) so now would be a good time to go back now and have a look here.

Action selection and the Basal Ganglia

What is action selection and how could the brain do it?

For my first full post I thought that rather then pick out some recent paper that’s made a big splash or something prominent in the media I would just talk about a topic in neuroscience I find interesting, it isn’t even related to what I study for my PhD, just something I find interesting and hopefully you will to. I’m going to talk about the action selection: what it is and how it's carried out in the brain. To put it simply as we engage with the world around us any number of potential actions are possible and action selection is choosing the next action. If we elaborate a bit, while any number of actions is available not all choices are made equal. Depending on your current needs and goals, certain courses of action are more or less appropriate. So, action selection refers not only to the initiation of one course of action but also to the decision processes that lead to the initiation of (hopefully) the most appropriate course of action and the inhibition of others.

One way this could be achieved in the brain is to have a dedicated action selection module. To carry out the task of action selection successfully this module would require certain fundamental features. First, it would need access to neural representations of competing action choices i.e. receive input from a wide range of brain regions involved directly in motor control. Second, its output would need to have a direct influence on the expression of the winning movement, and inhibition of losing movements. Finally, to be able to make a decision on the most appropriate course of action the module would also need access to information about goals, motivations, context, internal drives (e.g. hunger, thirst) etc. in a form that would have direct influence over movement choice.

The basal ganglia and its anatomy

A candidate for one of the brains action selection modules is the basal ganglia. Which is? The basal ganglia is a group of interconnected nuclei* nestled deep below the bumpy outer casing of the cerebral cortex, more or less at the core of the brain. The four nuclei to keep in mind when it comes to action selection are the striatum, subthalamic nuclei (STN), globus pallidus (in particular the internal portion or GPi) and substantia nigra (a particular subregion called the substantia nigra pars reticulata or SNr is most important here).