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Okay, so now in this module, we're going to talk about the basal ganglia.

And the basal ganglia is one of the oldest structures of the

central nervous system and it's, it's a deep, deep part of the telencephalon.

We'll look at it in a minute.

But, first I want to share with you why we have

the basal ganglia and why it's present in all of the vertebrates.

It's present in the earliest vertebrates.

This is a picture of a hag fish.

Now, a hag fish has a pretty limited behavioral repertoire.

It can kind of go that way or that way, but that's about it.

It doesn't have a wit.

It doesn't have fins that are moveable.

And, it doesn't even have eyes.

It has eye spots that just sense dark and light.

So, it has no complexity at all, and yet

it's still, it still has to decide, that big decision.

Do I go the right, or do I go to the left?

And, it can't go both ways at the same time, and so it uses this, this basal

ganglia, the core structures of the basal ganglia, we share with the hag fish.

And that, the core function of that str,

those structures, is to select which action to perform.

So, the hag fish has a limited repertoire.

We're a little more fancy.

We're a little more fancy for a few

reasons and let, let us enumerate those reasons.

Well, first of all, we've got limbs.

And on those limbs, we have digits.

And all of those limbs, and digits, can move.

And they, they move in complex ways.

In addition, the hag fish is living in the water, and

the challenge of living in the water, the challenge of moving is,

very reduced because the gravitational pull on the hag fish is very limited.

But we are terrestrial, and therefore we face the

challenge of gravity at every moment of the day.

And here is, one of our fellow vertebrates facing that challenge.

And succeeding.

So to, to, to make movements that allow you to oppose gravity, and stay upright.

That's, that's a complicated challenge.

And the basal ganglia has to

choose between various movements that oppose gravity.

In addition to having limbs and in addition to being on the,

on the ground and subject to gravity, we also have, we have faces.

So, not just limbs and digits, but faces and we move, and we have eyes and

those eyes move in very complicated ways and they have to be very carefully

the movements of eyes have to be very carefully controlled.

And, in fact, in more complicated vertebrates than the hag fish in, in

mammals such as us and the cat we have a lot

of basal ganglia circuitry that's dedicated to making sure that our eyes

move in one direction at a time and so our eyes are always yoked.

They look up or they look down but they can't look up and down at the same time.

In addition, for each body part that we have and we have all these

additional body parts that the hag fish

doesn't have, we can do multiple movements.

So, we can walk or skip or jump.

We can whisper or sing or yodel.

I can't, but some people can.

We can shout.

We can do all of that with our laryngeal muscles.

We can do all of those different movements with our legs.

And so, there's a lot of different choices even with the same body part.

But we can't do multiple movements with the same body part all the time.

We are limited by, what we call, the motor plant.

We got one motor plant.

It can do lots of things, but only one at a time.

In addition, mammals, and particularly primates, have a very different,

have a very complex social structure, so that a very similar stimulus, let's say

a person or a dog can illicit a very different

reaction depending on who that particular person is or who that particular dog is.

In addition, we, we are informed by our previous

experiences and this happens right from the get go.

This is a picture of, Oscar, who's cuddled up to a pit bull.

And, and he knows this pit bull.

He lives with it, he's always known, he's never not

known this pit bull and so he has these feelings.

But a stranger pit bull, he would not do the same thing with.

So, we are capable of very complicated choices with

what movements we make and what movements we don't.

And the ultimate selector is the basal ganglion.

Action Selection

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