Brain Matters: Frontal Lobes (4 of 5)


Uploaded by ltoddrose on 31.07.2011

Transcript:
To begin this chapter lets take a step back and consider what we know so far. Recall from
the previous chapter that we grouped the brain into four broad areas: the brain stem, the
cerebellum, the diencephalon, and the cerebrum. Recall also that the cerebrum we differentiated
into three broad areas, the cortex, white matter tracks and subcortical structures including
the basil ganglia, the amygdala, and the hippocampus. In this chapter, we’ll focus on the cortex.
Specifically, we’ll talk about a way of grouping the cortex into four broad lobes.
when talking about the cortex it’s important to point out that you can divide it any number
of ways depending on your purpose. Here we’re going to differentiate between four areas
of the cortex we call lobes: the frontal lobes, the parietal lobes, the temporal lobes, and
the occipital lobes. These lobes all do different things, although they’re closely related.
Each lobe has rich connections with several brain structures all the way down from the
brain stem, the cerebellum, the diencephalon, and other cortical areas. But it’s this
last piece, it’s connections with its cortical areas, that are perhaps most relevant to this
discussion. With each lobe that we discuss we’ll make a distinction between two broad
areas: the primary area, which is a dedicated area that processes very specific kinds of
signals, either motor or sensory, and in contrast to that, broader association areas, that take
that basic signal and turn it into a more complex, meaningful experience. With that
in mind let’s begin by discussing the frontal lobes.
The frontal lobes consist of the front one third of the cerebral cortex. Now given its
size, it’s perhaps not surprising then that the frontal lobes are involved in a wide range
of functions including planning, organization, and emotional regulation to name but a few.
The frontal lobe can be differentiated between its primary area, in this case a primary motor
area, and several association areas. Lets talk about each of them beginning with the
primary motor cortex.
The primary motor cortex is the most posterior part of the frontal lobe, and it’s a thin
strip that runs parallel to the central sulcus. Interestingly enough, the primary motor cortex
consists of a direct mapping for every muscle that it conrols in the body. We call this
the motor homunculus. Now that mapping is slightly disfigured. You might think that
the relative size of the cortex area dedicated to a certain muscle group would correspond
to say the size of the actual body part that it controls. If that were true you would expect
a larger area of the cortex dedicated to processing something like your torso. Instead, the motor
homunculus is slightly disfigured in that the size of a given area of the cortex is
in proportion to the precision of the movements that that area controls. So for example you
have much larger cortical areas responsible for the movement of lips and your mouth than
you do say of your torso. That’s really no that surprising, you just need more neurons,
so more cortical area, to do the kind of complex things that your mouth has to do for say speech,
language, things like that.
Lying just anterior to the primary motor cortex are the motor association areas. The motor
association areas consist of a few components including the supplementary motor area and
the premotor cortex, but here we’ll go ahead and talk about the motor association area
as one functional unit. That unit has rich reciprocal connections with several areas
of the cortex including the parietal lobes and the prefrontal cortex. Now the major role
of the motor association area is to plan and refine complex movements, but not execute
them, remember that’s the job of the primary motor cortex. Let’s think about an example
of the importance of the motor association areas. Let’s say you’re sitting at the
breakfast table and you reach out and you grab a cup of coffee off the table. That seems
like a pretty simple movement, and in reality it is especially compared to the complex movement
that you’re going to coordinate throughout the rest of the day. But if you think about
it, even that basic movement requires incredible amounts of coordination of muscles. Not only
the sequence and timing of the muscle execution, but in terms of differential force and velocity
of each muscle just to reach out and grab a coffee cup. So, in even the simplest of
actions the motor association areas play a vital role.
Lying anterior to the motor association areas is another very important association area
for the frontal lobe called the prefrontal cortex. It includes deep reciprocal connections
with everything from the brain stem, the cerebellum, subcortical structures, and other cortical
areas. In fact, it’s perhaps the most widely connected area of the brain. Now it’s possible
to further differentiate the prefrontal cortex into additional subdivisions however for our
purposes we’ll continue to talk about the prefrontal cortex as one functional whole.
Given how richly connected the prefrontal cortex is to so many areas of the brain it’s
not surprising that it tends to play a vital role in a wide range of very important functions.
Notably, it’s crucial for things like executive functions, such as planning, organization,
goal directed behavior as well as emotional regulation, basic reasoning and thought, and
delayed gratification – that is, the ability to override a gut response for the possibility
of better outcome later on. Perhaps the best way to think about the role of the prefrontal
cortex is to consider the most famous case of prefrontal cortex damage in history: this
is the story of Phineas Gage. Let me paint the picture. It’s September 13, 1848 and
Phineas Gage is a 25 year old foreman for a railroad company that’s making their way
through Kavendish, Vermont. Now Phineas is widely regarded by his employers as a model
employee. One of Phineas’s responsibilities as the head foreman was to oversee the destruction
of large boulders that would often lie in the way of the planned track that to lay down.
How do you destroy a large boulder? Well, as Phineas could have told you it involves
drilling a hole deep down to the center of that boulder, and packing that hole with a
combination of gunpowder, a fuse, and sand. And how do you pack it? You use a tamping
iron. A tamping iron is something about like a javelin, it weighs about 13 pounds, it’s
about three and half feet long, and about one and a quarter inch in diameter. So, Phineas
did this quite often but on this day Phineas forgot one important thing in that mixture,
he didn’t include sand. So when he took the tamping iron and started packing down
gunpowder and a fuse, it ignited on him, and unfortunately it turned his tamping iron into
a projectile. The tamping iron was immediately shot straight through his prefrontal cotex
and out the top of his head. It was said that they found the tamping iton about eighty feet
from the site of the accident. So what happened to Phineas Gage? I wouldn’t
blame you if you assumed he died. Well that would be kind of a boring story and not tell
us much about the prefrontal cortex, so of course he didn’t die immediately. In fact,
not only did he not die, when everyone came running to see what the problem was, Phineas
could talk to them. He was clearly a little dazed and confused, but he was conscious and
able to articulate what had just happened. He was even able to walk on his own volition
to the carriage that was to take him into town to see a doctor. Now when he gets to
town the doctor says, “you know I didn’t believe him until Phineas started to feel
nauseous, and as he bent over to vomit, the pressure of vomiting pushed out a teacup full
of Phineas’s brain on to the floor. Recall I mentioned that consistency of the brain
is something like tapioca pudding and jello, and it lacks a lot of internal structure.
So if you ever find yourself with a hole in your head my advice is don’t vomit or you’re
going to loose some cortex. Okay, so now the doctor believes that something’s
wrong with Phineas and he treats him. After about a month, Phineas seems fine, the doctor
releases him and Phineas goes home. It’s not until Phineas tries to resume the things
he did in his past life that the role of the prefrontal cortex becomes obvious. So what
where they? First, when Phineas went to get his job back at the railroad it became immediately
evident to his employers that, as they said, Phineas wasn’t Phineas anymore. Instead
of this young, incredibly mature, well-liked, respectful employee, what stood before them
now was a very aggressive, belligerent, vulgar individual who had no respect for authority
and would no sooner create a plan as abandon it for another plan. That is, he couldn’t
do any kind of goal directed behavior and lacked some basic executive functions, so
he couldn’t be a foreman anymore. In fact, they couldn’t even employ him on the railroad
at all. So Phineas does some odds and ends jobs including at some point putting himself
on display at the Barnum’s Museum in New York. At some point, we even find Phineas
several years later driving stage coach in Chile. So, he hand an eventful life.
Now the story of Phineas Gage is important for two reasons. First, and the most obvious,
is that judging by what happened to him it seems pretty clear that the prefrontal cortex
is important for several of these sort of executive functions we talked about. Clearly
Phineas Gage wasn’t as responsible, didn’t have as much emotional regulation, didin’t
have the kind of goal directed behavior that he once had. But I think just as important
is what Phineas still could do. I mean think about it. Yes, he did lack the kind of overall
maturity that he displayed before the accident but it wasn’t as if he had no executive
functions anymore. I mean, in the 19th century, simply making your way from New Hampshire
to New York City to be on display, heck even knowing that you could put yourself on display
and that people would pay money to see your misfortune requires a little bit of complex
thinking. But more important, I don’t think you could be a stagecoach driver in Chile
without a little bit of executive functioning. So what does Phineas Gage’s story specifically,
and the prefrontal cortex in general perhaps tell us about the organization of the brain
as a whole? Perhaps the most important thing that this teaches us is that it’s incredibly
difficult for us to talk about brain structures and functions in a way that effectively deals
with complexity inherent in the brain. In fact, one of the problems is, our language
is somewhat impoverished in the way we’re able to communicate about the brain, so we
end up talking about structures and functions as if there is this one-to-one mapping. So
we’ll say “the prefrontal cortex does delayed gratification.” Well that’s not
quite right. It’s true that the prefrontal cortex is necessary for delayed gratification,
but in reality, it exists as part of a network of brain structures that together give rise
to this complex thing called delayed gratification. A good example of this is thinking about starting
your car in the morning. If before you started your car, you opened the hood and you removed
a spark plug, what would happen? Well, clearly your car wouldn’t start. And it would be
fair for you to then say “well, clearly the spark plug is important for starting the
car, because if I remove it, it doesn’t start.” That’s fine. The problem comes
when we make a broader leap and say “well clearly this shows that the spark plug is
the area for starting your car.” That’s not quite right. In reality, the spark plug
is part of a broader network of engine structures that all contribute to this idea of car starting.
So in that way, the spark plug is necessary but not sufficient by itself. Likewise, the
prefrontal cortex, just like other brain areas, plays a vital role in several high-level functions
including inhibitory control for example. But alone they are not sufficient to give
rise to that complex function.
Okay, to summarize. In this chapter we discusses a way of grouping the cortex into four lobes:
the frontal lobes, the parietal lobes, the temporal lobes, and the occipital lobes. And
we spent a lot of time going into greater detail on one of those lobes, the frontal
lobes, where we differentiated between the primary motor area, motor association areas,
and the prefrontal cortex, which was involved in higher order thinking. We also put particular
emphasis on the fact that all these parts are a part of a broader network of brain structures
and that any one of them alone does not give rise to a specific function. In that way,
any given brain area, not just in the prefrontal cortex but anywhere, is maybe necessary but
not sufficient for a given function. In the next chapter we’ll dive deeper in
the other cortical lobes: the parietal lobes, the occipital lobes, and the temporal lobes.