Search This Blog

Most of the time I spend thinking about the brain, I think about the basal ganglia

Aug 3, 2017

Most of the time I spend thinking about the brain, I think about the basal ganglia.

The basal ganglia is mostly involved in choosing which action to do at the moment  -- e.g., right now should I write a blog post or make coffee or take a walk or call my friend on the phone?  Research on the basal ganglia suggests that it essentially 'assigns' a value to each action.

How does the basal ganglia know how much value to assign to a particular action?  It remembers how rewarding the action was last time I carried it out, and the time before that.  It basically keeps a running average of the reward I received for each particular action in the past.

But wait.  Doesn't the value of a particular action (how much I want to do it) depend upon the situation I find myself in, my mood, where I am, who I'm with, and so on?   That's exactly where the basal ganglia comes in.  It doesn't simply assign a value to an action per se.  Instead, the value it assigns varies according to the situation I'm in.  The basal ganglia remembers the situation I was in (at least some aspects of it) when I carried out the action in the past. And it keeps track of how rewarding the action was in the context of situation 1, situation 2, and so on. In order to tell me if a particular action is likely to be rewarding right now, it takes into account my current situation, looks for actions I've carried out under similar situations in the past, and tells me which ones led to the greatest reward (positive feelings).  The answer the basal ganglia gives me doesn't come in words. It comes in terms of how much motivation I feel when I consider choosing that particular action.

How does the basal ganglia keep track of values for each of the different situations I find myself in?  How does it know what the current situation is?  Areas of the the basal ganglia are mostly comprised of input neurons and output neurons.  The 'situation' is defined by the inputs. An area of my basal ganglia, right now, is receiving input from visual areas of the cortex telling it that I'm at a table in my in-laws' garden in Spain.  It also receives input about the temperature of the sun on my skin (a gentle warmth), my mood (relaxed) and signals from my body (my legs are tired from walking a lot this afternoon). The cerebral cortex is in charge of sensing these components of my current 'situation', and it sends input to inform the basal ganglia about it.

The outputs of the basal ganglia correspond to the particular actions I choose (e.g., writing this blog post).

The value of an action under a particular situation is represented as the strength of the connections between the inputs and outputs.

Dopamine -- which is released in large quantities when I perceive my experience to be better-than-expected - strengthens the input-output connections of the basal ganglia -- i.e., increases the value assigned to a given situation/action pair.

The details get technical.  Probably more technical than you're in the mood to hear about, or I'm in the mood to write about.  Besides, it would require me to sketch a diagram.

Like many things we do, I'm entering into behavior that doesn't resemble past actions I've taken.  My basal ganglia doesn't give me much of an indication of how much reward to expect from writing a blog post with details about how the basal ganglia works.  That means I have to rely on other parts of my brain that imagine outcomes of new behaviors.  The hippocampus, best known for its role in memory,  is also involved in imagining scenarios that never occurred.  As I consult with my hippocampus and related areas, my basal ganglia steps in and tells me to bring this post to a close.   There's fresh coffee on, and in my current situation (neuroscientists like to use the term 'state'), in my current state, serving myself coffee is almost guaranteed to be a high-value action.

No comments:

Post a Comment