Hi, I'm Michael Corayer and this is Psych Exam Review. In this video we're going to talk about split-brain patients. Now these are patients
who have had their corpus callosum severed. So you might recall that the corpus callosum is this
large band of nerve fibers. It connects the cerebral cortex of the left hemisphere
to the cerebral cortex of the right hemisphere.
So why would we ever want to sever this connection? Well for most of these patients this procedure is done because they have seizures.
So they have seizures which are these bursts of neurons firing, uncontrollable bursts of neurons firing and this spreads
so the neurons firing causes other neurons to fire and then that causes other neurons to fire.
This ends up spreading back and forth between the two hemispheres.
So the way to stop this is to sever the connection, the corpus callosum, between these two hemispheres
and this helps to reduce the spreading of this neuron firing.
The good news is that this procedure is very effective. If you have a patient who has very frequent seizures
very disruptive, severe, really affecting their ability to live a normal life
then they might undergo this procedure and this will hopefully stop the seizures from occurring. Now there are some side effects of
severing the corpus callosum. But the other good news is that the side effects are actually fairly small.
If you met somebody who had undergone this procedure and had a split brain
you probably wouldn't know. So there wouldn't be really obvious effects on their behavior that would stand out to you. But if you put them in
certain situations and tested this you could see that they have these two separated hemispheres of their brain.
unlike a person which an intact corpus callosum where the two hemispheres can communicate with one another freely.
So how do we go about finding these differences in behavior that occur? It turns out there are some specializations of each
hemisphere. So the left hemisphere has certain task that it's good at that the right hemisphere doesn't do
and vice versa. The right hemisphere has some tasks that it's specialized for that the left hemisphere doesn't do.
This is work that was done originally by Roger Sperry studying split brain patients and for this he was awarded the Nobel prize in 1981
and in fact that year it was a split Nobel prize because he shared it with
David Hubel and Torsten Wiesel who we're going to learn about in a future video
because they did work on visual processing that's also relevant for psychology.
Ok, so Sperry did this initial work on these hemispheric specializations
certain tasks that are specialized in either hemisphere. So what are these specializations and how do split brain patients reveal them?
So the first specialization, the most important for these split brain studies, is the idea that language for most people
occurs mostly in the left hemisphere and speech production
occurs mostly because of activity in the left hemisphere. So this area in the left frontal lobe called Broca's area coordinates the movements
for speech as well as other language processing areas are mostly in the left hemisphere.
OK so what this means is that if we send information to the left hemisphere we can talk about it.
But if we send to the right hemisphere and we can't get it over to the left hemisphere
via the corpus callosum, then we can't talk about it. So how could we demonstrate this with a split brain patient?
Well, let's say we gave them an object to hold.
So if I were to place this object in their right hand, remember the idea of contralateral control, this is going to get sent from the right
hand to the left hemisphere. So if I ask the person what are you holding, they'll be able to tell me, if I blindfold them, place it in their hand
then they say "it's a key", or "I think it's a key". They're grasping it with their right hand, the left hemisphere
tries to figure out what they're grasping, then sends it to the speech production area, and they can tell me it's a key.
On the other hand, literally, I can place it in the left hand, they feel it,
the right hemisphere is processing this information saying "I think it's a key"
the problem is the person can't say this because the right hemisphere doesn't control
language production. So the person knows what it is but they can't tell me. What is it?
"I'm drawing a blank, I can't
can't seem to say it".
So how do we know the right hemisphere even knows it? Maybe they really don't know. Well, if I put a bunch of objects in front of them
and I say, ok, using your left hand point the one that you were holding and they can immediately
point to the key. So this shows that the right hemisphere does know it, it's just not able to talk about it.
OK so that's one way we can reveal this specialization in the brain.
Now we'll get a little bit more complicated, we're going to look at how this works with visual information.
Not just touch sensation, which as I said before is purely contralateral, left side of the body goes to the right hemisphere.
Vision is a little bit more complicated but not too bad. So I want to go over this and if you have any questions
about this feel free to ask in the comments. But I'll do my best to simplify this.
OK so let's say we're looking down the top of somebody's head.
Here's their left eye, here's their right eye.
We've got their left hemisphere here, we're looking down so it's going to overlap the eye there.
But we'll do our best, this won't be the most anatomically correct
brain diagram you've ever seen. But hopefully it will get the point across. Here's the left eye, here's the right eye, here's the left hemisphere,
here's the right hemisphere.
The important thing about the eyes is that
the process information by visual field.
This means they're not contralateral the way the rest of the body is. It's not the case that the left eye goes to the right hemisphere or the
right eye goes to the left hemisphere. That's not true. It's a common mistake that students make, so banish that from your mind.
What we'll see is it's divided by visual field. So if I focus on the center of the screen here
everything over to the right is red and let's say on the left side I have blue here.
What's gonna happen is that this blue side, the left side of the screen
is gonna go
the light is going to travel straight across to this part of the retina on this eye.
And same thing on this eye here.
And the right side of the screen is going to travel across over here.
Now it goes to the opposite side of the eye because the light is just traveling straight through the hole in the eye, the pupil, and just
hitting that opposite side. But the more important thing is what happens after this.
What happens is this information, the important point is really that the eye is getting two sides, a left visual field and a right visual field.
What happens then is this information comes out
here, comes out the optic nerve
and in this case it's the right visual field and it's already in the left hemisphere so it's going to actually stay in the left hemisphere.
Alright so it's gonna go initially to an area called the optic chiasm then it goes to the thalamus, then the thalamus sends it out to the
appropriate area of the cortex, which in this case is the occipital lobe, to the primary visual cortex.
Now what's interesting is what happens
over here on the other side. This right visual field needs to get over to the left hemisphere. So in this case it comes out the optic nerve
and when it gets to the optic chiasm it actually crosses over
to the other hemisphere and joins this information which is also about the right visual field.
And then gets sent out to the occipital lobe for processing.
And the opposite would be true over here. So this side, the left visual field is going to go down here out the optic nerve
it's going to get to the optic chiasm but it's already in the right hemisphere so it stays over here
goes to the thalamus, then goes out to the occipital lobe for processing. And over here, this left visual field,
is going to come out here and when it gets to the optic chiasm it's going to cross over
to the other side
to the right hemisphere,
then go to the thalamus, then go out to the occipital lobe for processing.
So what this means is that everything in the right visual field goes to the left hemisphere and
everything in the left visual field goes to the right hemisphere.
So this is the right hemisphere over here, only seeing blue information
and this is the left hemisphere which is getting the right visual field
which is all red. Now if you think about this it makes sense that we would split by visual field instead of splitting by eye.
If you were to lose an eye, if we think in evolutionary terms about how this is helpful, if you lose an eye it would be
a real shame if that meant that that whole side of the brain was no longer getting any visual information. You'd be
wasting all that brain space just because you lost an eye. In this situation we see that even if you lost and eye, information is still gonna
get sent to both of the occipital lobes. So if we knocked out the right eye here, the left eye would still send some of its information to the
right and some to the left based on visual field.
OK let me just add a few labels in here.
So this point here would be the optic chiasm, that's where the information crosses over
to get to the appropriate hemisphere
and then here is the
this is where it's
going into the thalamus after the optic chiasm, then from there it ends up in the occipital lobe
in the primary visual
cortex. This is an area
called V1.
The primary visual cortex. Ok, so let's get back to our split brain patients.
What happens when we split the corpus callosum, we sever the corpus callosum, we split the brain, we don't cut the optic chiasm.
So this process still occurs, this transfer from visual field over to the opposite hemisphere still occurs. So what does this have to do with
split brain tasks? That means we can do this just like we did with the hand version.
Except now I put the information on the screen
and I have them focus on the center, that's important, they have focus on the center. If they move their eyes around then they can
send everything to both hemispheres. That's how they're normally going to do it in everyday life, that's why you aren't going to notice. But if
we focus their vision, we flash these on the screen briefly so they don't have time to move their eyes around, see what's going on
everywhere then anything to the right goes to the left hemisphere, anything to the left goes to the right hemisphere. If I flash these on the
screen, this screwdriver is going to my left hemisphere over here
so when you ask me "what did you see on the screen?" if I have a split brain, I'm going to say "I saw a screwdriver"
because that's all the left hemisphere saw, the right visual field.
If however, you ask me to draw with my left hand what I saw,
well the left side of the screen went to my right hemisphere which is going to tell my left hand to draw a key.
Now the really interesting thing is when I ask the person. So they draw the key, then they look at it, and I say
"why did you draw a key?".
Now I'm asking them to explain, using speech, so the left hemisphere saw a screwdriver
then sees that the person drew a key.
Why did this happen? It turns out the left hemisphere will make up an explanation.
It's something that the left hemisphere seems to be good at is coming up with logical explanations for behavior.
So the person will look at the key and make up some story with the left hemisphere. Well, I saw screwdriver,
but, last time I needed a screwdriver I couldn't find one, and so I ended up using a key to turn a screw.
They'll make up some plausible explanation to why did they draw key when they saw a screwdriver.
It's also the case that there's some things that the right hemisphere does better so one of these is face recognition. This, for most people,
is going to predominantly happen in the right hemisphere. This means if we show things that are arranged into a face
this certain detection of the pattern that makes up a face, rather than the individual parts
is happening in the right hemisphere and in one of the videos that I've posted in the description
you can see Michael Gazzaniga working with a split brain patient and he gives him this task where he shows these famous paintings
by Archimboldo that perhaps you've seen before, of faces made out of objects, made out of fruit, flowers,
books, things like that. If he shows it to the right visual field, goes to the left hemisphere,
the left hemispheres processes, sees the objects. What did you see? I saw books, flowers.
But if he sends that same painting
to the left visual field so it goes to the right hemisphere and the right hemisphere has this facial recognition area, it can see this pattern
so the right hemisphere says I saw
a face. Point with the left hand, what did you see? Right hemisphere saw a face. That stood out to the right hemisphere
because it's specialized to recognize faces. So that's another specialization that can be revealed through these split bring patients.
The key ideas here are to remember are, first of all, that it's split by visual field, not by eye,
keep that in mind, students often get tripped up by that. And the second point is
it doesn't really matter in daily life. We have these clever experiments revealing these differences between left hemisphere and right
hemisphere but don't take this too far. There's a lot of books being sold you know talking about "left brainers" and "right brainers" and it's
to me, all very silly because
most of us have our corpus callosum intact and everything is happening in all areas of our brain
the hemispheres are sharing information with each other.
Yes, it's true certain tasks are specialized to the left and right hemisphere but in terms of how that's going to affect your behavior it doesn't
really matter. So you don't sit there and say wow I looked at a bunch of faces so my right hemisphere is really tired.
Or I've been producing a lot of speech so my left hemisphere is exhausted.
It doesn't really matter that way. I mean if you want to get good at a certain task you just practice doing it. You don't need to think about
whether it's related to brain activity in the right or left hemisphere, I don't really see the point of that. But I guess it helps to sell books so...
That's the other thing to keep in mind, don't go too far in drawing conclusions from this. And even for people with split brains it's true that
both halves of their brain are very active all the time and coordinate in doing a number of activities and they're moving
their eyes around and sending information to both hemispheres so it really doesn't matter
too much about which hemisphere processes which information.
OK so I hope you found this helpful. Check out the videos that I linked in the description, I think you'll find them interesting, you can see
some actual split brain patients doing some of these studies. If you found this helpful, please like the video and subscribe to the channel for
more. Thanks for watching!
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