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CHAPTER 11 Topping It O: The Neocortex 201
Given its common circuit types, the neocortex produces common neuronal com-
putations on its inputs. A given area of cortex seems to be able to process a set
number of inputs. This implies that when more sensory cells are in an area of
skin, or retina, or anywhere else for higher resolution, the brain uses more cortical
area that contains more local minicolumns to process the larger number of inputs.
More complex computations are done in a hierarchy of cortical areas. A low level
area processes the input from the thalamus, extracts some information, and sends
its output to a higher level area. This higher area does the same, sending its output
to some even higher, more selective area, building up a hierarchy of selectivity.
Using a single circuit type for all sensory and motor neural computation in
neocortex is one of the most remarkable traits of mammals. Many neurobiologists
believe that it’s the key to mammals’ success and the ultimate generator of human
intelligence and consciousness.
Getting to the Brain You Have Today
The protoreptilian brain corresponds to the hindbrain region of the mammalian
brain; it’s thought to have arisen rst about 550 million years ago (MYA) in ver-
tebrate evolution. It contains brainstem areas that sustain survival. The basal
ganglia evolved soon after this. The paleomammalian brain arose around 225 MYA
with the evolution of the limbic system in the rst mammals. It’s involved in
emotion and motivation for feeding and reproduction. The neomammalian
FIGURE11-4:
The neocortex:
Inputs and
outputs.
© John Wiley & Sons, Inc,.
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202 PART 2 Controlling Muscles, Movement, and Survival
complex arose as mammals evolved, around 200 MYA; this system is thought to
permit abstract planning and, ultimately in humans, language.
Here are two major ideas about how the neocortex relates to the rest of the brain:
»
The neocortex sits at the apex of a controller hierarchy. It sets the big-picture
goals and procedures and allows lower levels to take care of the details.
»
The neocortex functions like a set of subroutines in a computer program. The
main procedure in subcortical areas is actually the sequence controller that
calls subroutines (particular computational areas of neocortex) for doing
detailed calculations at high resolution.
These ideas appear to be quite distinct, so which is right? We argue that both are
correct and that the neocortex and subcortical areas interact to control behavior.
You decide to take a trip. Decisions involve your neocortex about various ways to
travel, such as by car or by plane. The decision also involves goals about the pur-
pose of the trip and your current state of aairs (money in your bank account,
vacation time, and so forth). Dealing with conicting goals and states involves the
anterior cingulate cortex (which is mesocortex, not neocortex) and the thalamus.
These subcortical brain structures use memory and information from the neocortex
to decide when and where to travel and how to get there. Figure11-5 illustrates this.
This is a hierarchical system because the details of how to carry out tasks in various
ways reside in the neocortex. If you were monitoring brain activity while a person
was taking a plane from point A to point B, you’d say that areas of the frontal lobe
neocortex were controlling the procedure. But also true is that the method of trans-
port you use (a plane) and the alternatives you take within that method (choosing
the window or aisle seat) are decided by lower-level brain systems with input from
FIGURE11-5:
Neocortex:
controller or
subroutine?
© John Wiley & Sons, Inc.
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CHAPTER 11 Topping It O: The Neocortex 203
the neocortex. In that sense the relationship between subcortical and cortical levels
is more like the former calling subroutines in the latter.
The next sections discuss how the neocortex develops. This information is useful
to understand the formation of minicolumns, the basic computational unit in the
neocortex, and the evolution of the neocortex.
Looking at how the neocortex develops
During embryonic development of the neocortex, stem cells in the neural tube pro-
liferate and then give rise to progenitor cells called radial glial cells (RGCs). RGCs also
divide and generate cells that migrate outward to form the layers of the neocortex.
After arriving at their layer, the RGC-derived cells dierentiate into glutamatergic
neurons or glial cells. Researchers propose that the direct neuronal descendants of
RGCs in vertical columns become interconnected and provide the basis of cortical
minicolumns. Chapter 3 provides more information about development of the
neocortex.
Enlarging the frontal lobes for
complex behavior
Mammals increased their brains’ abilities to do complex computations by devel-
oping and expanding the neocortex and adding more units, the cortical minicol-
umns. In mammalian evolution, this cortical expansion led to higher resolution
and more complex perceptions. In human evolution it led to increased executive
functions and language.
One aspect of the success of the neocortex is its uniform organization. The same
neural circuit type is used to increase resolution in everything from processing
visual detail to sequencing muscle movements.
Neocortical resolution is enhanced in two ways:
»
The area devoted to a task increases the resolution at which a limited
number of computations are performed within in a particular area. For
example, in the visual system, animals with larger numbers of retinal
ganglion cells have a primary visual cortex with more area— the area of
visual cortex per ganglion cells is about the same. Attributes such as orienta-
tion, depth perception, and color are extracted within an area.
»
Resolution is increased in the kinds of perceptions that can be produced by increas-
ing the number of distinct cortical areas in a hierarchy. Each area does some set of
feature extractions. These extracted features are fed into the next area, which
starts with those features as a basis and extracts even more complex features.
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204 PART 2 Controlling Muscles, Movement, and Survival
Setting and accomplishing goals
During primate evolution the size of the neocortex increased— specically, the
frontal and prefrontal cortex. Figure11-6 shows the relative sizes of the neocortex
in the cat, rhesus monkey, chimpanzee, and human. The prefrontal cortex in each
is in a darker shade. Prefrontal cortex increased as a percent and in absolute area,
from nonprimates to primates to monkeys to apes to humans.
The frontal lobe constitutes about 40 percent of the neocortex. It contains the pre-
frontal cortex, motor cortex, frontal eye elds, and Broca’s area. The prefrontal
cortex is the seat of abstract planning, decision-making, reasoning, and working
memory. It allows you to keep complex ideas in mind. The prefrontal cortex moves
you from having actions that are largely reactive and dependent on a stimulus to
having plans that are more exible and driven by your goals.
As Chapter8 describes, the motor cortex contains the supplementary and premo-
tor cortices that organize instructions for groups of muscles to accomplish spe-
cic tasks. In the primary motor cortex, neurons send their axons to the brainstem
or spinal cord and synapse on alpha motor neurons (and interneurons) that con-
trol muscle contraction. The frontal eye elds control eye movements and are
involved in visual attention. Broca’s area is a language area that we describe in the
section “Specializing for language” later in this chapter.
The prefrontal cortex can be functionally divided into two broad regions, lateral
and ventromedial. The lateral prefrontal cortex includes the dorsolateral and ven-
trolateral prefrontal cortex. Another name for the ventromedial prefrontal cortex
is the orbitofrontal cortex.
FIGURE11-6:
Cross-species
comparison of
the prefrontal
cortex.
© John Wiley & Sons, Inc.
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CHAPTER 11 Topping It O: The Neocortex 205
The prefrontal cortex is responsible for executive functions, with two main types:
»
Organizational executive functions: Working memory, planning, and
problem-solving are examples.
»
Regulatory abilities: Self-control, emotional regulation, and moral reasoning
are examples.
Making Decisions: The Lateral
Prefrontal Cortex
We as humans don’t always live in the present. We think about the future and we
think about the past. This ability allows people to make plans with a complex
hierarchy of goals and subgoals and modify these plans as we carry them out if
things don’t go as expected.
The dorsolateral prefrontal cortex enables the brain to represent information that
isn’t in your current environment, as working memory. Working memory is what
you purposely keep in your mind to complete a task. It’s necessary to remember
what you’re doing, and what needs to be done next, by preventing you from being
distracted by irrelevant stimuli and thoughts. This ability to use a representation
of your goal to stay focused — as we examine in the following sections —
underlies all higher executive functions.
Keeping it all in mind
The function of the prefrontal cortex is executive because it controls the ow of
information in other brain structures. For example, suppose a friend asks you for
directions to a local restaurant. Neurons in the prefrontal cortex must maintain an
auditory image of the spoken request, translate this auditory image into an image
of the restaurant, and then, while maintaining that image, begin constructing
images of the streets and turns between here and there. All the time you’re giving
directions, you must maintain an image of the goal of the directions, and where
you are in the sequence of steps.
Changing your plans
The ability to follow complex abstract goals involves making a series of decisions
about what actions are currently appropriate for that goal, and which are not. For
example, if you’re driving in slow trac to an appointment and running late, one
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206 PART 2 Controlling Muscles, Movement, and Survival
response is to drive faster. Your frontal lobe causes you to weigh the usually minor
problem of being late against the possibility of a speeding ticket or, worse, car
accident. People with damage to their frontal lobes have particular trouble chang-
ing strategies according to circumstances.
Axons in the frontal lobes are the last parts of the brain to be fully myelinated
(wrapped with a glial process, such as from an oligodendrocyte), usually not until
early adulthood. Because the prefrontal connections are still developing, teens
tend to be more impulsive. It’s also one reason why the teenage brain is particu-
larly susceptible to concussions.
Dialing that number: Working memory
The concept of a working memory is derived from the concept of short-term
memory. An example of short-term memory is holding a security code number in
mind just long enough to type it into your phone or computer.
A model for dierent memory stores was published by Richard Atkinson and
Richard Shirin in 1968. It proposes that sensory information is processed and
becomes stored in sensory memory for a few seconds. Sensory memory that
attracts your attention can be transferred to short-term memory over several
minutes. If you use and manipulate that short-term memory, it’s called working
memory. Short-term memory that is rehearsed, relearned, or is very important
can be transferred into a long-term memory store.
In 1974 Alan Baddeley and Graham Hitch published a working memory model. It
involves a central executive that relies on three short-term memory subsystems
that maintain information in visual and auditory systems and retrieved from
long-term memory. Chapter15 describes the features of these memory models.
Most neurobiologists agree that the lateral prefrontal cortex is where working
memory is instantiated. It’s been proposed that working memory involves rever-
berating circuits of excitatory glutamatergic neuronal activity in the neocortex
that maintain a representation of what is to be remembered. Mechanisms pro-
posed mechanisms for working memory include short-term neuronal plasticity
and neural oscillations.
Studies have identied neuronal activities recorded in the lateral prefrontal cortex
that have the properties necessary for representing working memory. In a repre-
sentative experiment, a monkey was presented with an image for less than a sec-
ond. He was trained to wait until a go signal, and then he could do something
based on the stimulus to receive a reward. Neurons recorded in the lateral pre-
frontal cortex didn’t re during the stimulus; they only began ring when the
stimulus was removed and continued ring the entire time until the go signal
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CHAPTER 11 Topping It O: The Neocortex 207
(this was a variable interval). These neurons then stopped ring at the go signal
when the monkey reached for the reward.
Neurons in the lateral prefrontal cortex are connected with virtually the entire rest
of the brain. Although researchers don’t know the connectivity of the neurons in
the monkey test, the following scenario is possible: If the stimulus is a green
square in the upper-left corner of a card, active neurons in visual cortex that code
these properties would be set up in a reverberant circuit by the lateral prefrontal
neurons. In this way, the prefrontal neurons controlled the existence of the mon-
key’s mental image of the green square in the upper left, while he waited through
the interval until he could press the right key to get the reward.
Recalling long-term memory
The hippocampus is required for the encoding of long-term declarative and spa-
tial memories. Long-term memories aren’t stored in the hippocampus; instead,
they’re stored in neocortical circuits, most likely in those cortical areas that were
originally active in processing the stimulus that a person is remembering.
The process of creating long-term memory storage involves reciprocal activation
between the hippocampus and cerebral cortex that occurs particularly during
sleep. Presumably, the areas of the sensory system that were originally activated
by the sensory input are the ones altered, but we do not, in fact, know about this
in any detail. (Refer to Chapter 15 for more about memory, the hippocampus,
mechanisms, and the role of sleep.)
Doing the Right Thing
Have you ever caught yourself remembering something funny, and then realized
that you were actually laughing out loud? You probably have— everyone does—
but wouldn’t it be awkward if that happened during a business meeting or in the
middle of a religious service? Humans all experience various urges. Most of these
urges are okay in some circumstances, but highly inappropriate in other circum-
stances. The next sections look at how you know which behaviors are appropriate
and how you suppress those that aren’t.
Responding with the orbitofrontal cortex
Virtually all mammals are born fearing certain things, such as snakes, very large
birds, spiders, and heights. Many of these fears are probably present in nonmam-
malian vertebrates as well and may be built into portions of the limbic system.
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Other fears, however, aren’t innate and humans need to learn them, such as fear
of exposed electric wires or a certain expression on your boss’s face. Knowing to
avoid the dangers of complex human society requires:
»
The ability to accurately represent the circumstances that present a threat
»
The ability to remember the salient aspects of threat circumstances so that
the memory will guide future behavior
In the previous section, we describe how working memory, mediated by the dor-
solateral prefrontal cortex, maintains current and past sensory input and goals,
which allows you to carry out your plans. The orbitofrontal cortex also holds a
kind of working memory. However, the orbitofrontal cortex is particularly con-
cerned with behavior that has a learned or social dimension with negative costs
(embarrassment or shame, for example). It also interacts with consciousness
indirectly by giving you something you may experience as an intuitive, gut feeling
that something is about to go wrong.
Damage to the lateral prefrontal cortex results in the inability to alter plans or
strategies according to circumstances. In contrast, damage to the orbitofrontal
cortex results in the inability to inhibit inappropriate behavior. For example,
patients with orbitofrontal damage typically engage in risky behavior such as
gambling or unsound investments. Some tend to use excessive and inappropriate
profanity and otherwise act in a way that makes them social outcasts. Patients
with severe orbitofrontal damage may be unable to experience empathy for the
pain of others, and in extreme cases, can be sociopaths.
Orbitofrontal damage also takes away a person’s ability to have a gut feeling, which
we humans all get when experiencing the urge to do something we know is inap-
propriate given the circumstances. The orbitofrontal cortex appears to be involved
in representing the memory of situations that may have negative outcomes, partic-
ularly social situations. The representation function of the orbitofrontal cortex
depends on its interaction with its specic memory device, the amygdala.
Getting that bad feeling: the amygdala
and cortical connections
The lateral prefrontal cortex holds the context of a given situation in working
memory, so the hippocampus can encode a context-sensitive memory of the most
important parts of an experience. Just anterior to the hippocampus is another
limbic system component, the amygdala. The amygdala works with the orbito-
frontal cortex to learn and recall situations that warn us of negative consequences
(refer to Chapter10).
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CHAPTER 11 Topping It O: The Neocortex 209
Patients with damaged amygdalae can have normal fear responses to intrinsically
threatening scenarios such as snakes or heights. But they don’t learn or experi-
ence fear of things typical people learn to fear, such as risky gambling bets and
socially inappropriate behavior. Patients with orbitofrontal damage may lack fear
responses to things they rationally know are dangerous.
Going with your gut
The frontal lobe mediates approach-withdrawal behavior. You can think of many
situations as things to either avoid or seek out because things to avoid have nega-
tive consequences, while things to seek out have rewards.
A left-right brain asymmetry seems to exist with approach-withdrawal behavior.
The left prefrontal cortex appears to be approach oriented, so damage to this area
tends to cause depression and withdrawal. The right prefrontal cortex appears to
be withdrawal oriented, so damage to it can cause, at least in some cases, manic
behavior.
The contrast seems to be stronger for the orbitofrontal than the prefrontal cortex.
The orbitofrontal cortex communicates its assessment of approach-withdrawal
judgments unconsciously, through gut feelings. This shows that emotions don’t
just stem from some evolutionarily old part of the brain that is inferior to the
rational portion. In fact, emotions can be the result of complexly represented, very
real, learned experiences. Even if an emotion isn’t communicated by a rational,
verbal thought representation, gut feelings themselves can be based on complex,
learned representations of similar situations.
Gut feelings can tell you when your behavior in a social group is unacceptable,
even when no one tells you. Although you aren’t aware of it, your brain is con-
stantly processing facial expressions, tones of voice, and body language of those
around you. The orbitofrontal-amygdala system provides an intelligence that
can’t be put into words for situations involving risks and rewards. This intelli-
gence is highly complex and certainly not a throwback to a more primitive brain.
Seeing Both Sides: The Left and
Right Hemispheres
When you look at a human brain, you can see an obvious mirror-symmetry
between the left and right halves. As we mention throughout this book, the left
half of the brain tends to deal with the right side of the body and vice versa.
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If you look carefully at brains, although the major gyri and sulci have a consistent
location, the locations and sizes of some minor gyri and sulci can show variation.
Two brains almost never have totally identical patterns of gyri and sulci.
Figure 11-7 shows a view from the side of the left half of the brain (a) and a
mid-sagittal section of the interior of the right half of the brain (b). The numbers
correspond to regions dened by the German anatomist Korbinian Brodmann in
the early 1900s.
Although the two brain hemispheres are nearly mirror-symmetrical in how they
look, functional asymmetries do exist between the left and right hemispheres.
Virtually all right-handed people (greater than 95 percent) and the majority of
left-handed people (about 60 percent) have most of their language function on
the left side of the brain. The right hemisphere is much more competent at han-
dling spatial skills. Since the French physician Pierre Paul Broca found language
localized to the left hemisphere in the 1800s, neurobiologists have wondered
about the origin and signicance of left–right functional brain asymmetries.
Here we describe the two well-characterized areas of neocortex that have essen-
tial functions in language.
Specializing for language
Two areas of the brain known to be left-specialized for language are Broca’s area
(around areas 44 and 45in Figure11-7) and Wernicke’s area (near the area marked
39in Figure 11-7). Broca’s area is in the frontal lobe, near the premotor cortex
area that controls the speech apparatus. As its location implies, it’s particularly
important for producing speech.
FIGURE11-7:
Brodmann areas
of the neocortex.
© John Wiley & Sons, Inc.
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