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CHAPTER 10 Maintaining the Internal Environment 191
One role of the amygdala is in evaluating the emotional valence of situations to
determine if they’re scary (fearful), happy, or sad. The amygdala helps the brain
recognize potential threats and contributes to autonomic responses through
output to the hypothalamus. The amygdala is also responsible for reward- or
punishment-based learning.
The amygdala is located anterior to the hippocampus in the temporal lobe. The
amygdala is involved in processing memories that have high emotional salience,
such as fear, and it projects back to the prefrontal cortex. The role of the amygdala
in fear memory has been studied in animal models using fear conditioning. Through
its inputs to the hippocampus, the amygdala also contributes emotional content
to declarative memories (see Chapter15 for the role of the amygdala in memory).
The amygdala encodes long-term emotional memories through connections with
the neocortex, primarily, the orbitofrontal cortex. The orbitofrontal cortex is
involved in emotional responses to contexts, such as being nervous when betting
a large sum of money. Lesions to the anterior temporal lobe that include the
amygdala produce hypo-emotionality, loss of fear, and hypersexuality in experi-
ments on nonhuman primates.
Other brain areas are involved in emotion, motivation, and reward processing.
These include the ventromedial prefrontal cortex, anterior cingulate cortex, and
anterior insula. These areas contribute to the recognition of relevant sensory
input, motivation for action, and generation of appropriate emotional responses.
Other subcortical regions involved in emotion, motivation, and reward include the
basal ganglia and cerebellum.
Assigning with the insula (insular cortex)
The insula (insular cortex) is a portion of the mesocortex located in the deep s-
sure that separates the temporal from the parietal and frontal lobes. Above the
insula is a structure called the operculum, which means “lid.” It’s highly intercon-
nected with the limbic system and is important in assigning emotional signi-
cance to sensory input. It receives inputs from sensory areas of the thalamus and
projects to the amygdala, orbitofrontal cortex, and ventral striatum of the basal
ganglia. It also projects directly to areas of the motor cortex.
The executive control of the insula includes coordinating the regulation of some
aspects of homeostasis, the conscious perception and awareness of pain, and
some aspects of motor control. Awareness of the amount of pain, and even imag-
ining pain, activates the insula, particularly on the right side. It’s also involved in
the high-level control of the autonomic system.
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192 PART 2 Controlling Muscles, Movement, and Survival
In motor control, the insula operates as an executive control center that regulates
heart rate and blood pressure to levels appropriate for the level of the body’s exer-
tion and emotional state. It also appears to have a role in speech articulation and
some poorly understood aspects of body awareness, emotions, and social aware-
ness via its connections to the limbic system.
Motivating with the basal ganglia
Chapter8 describes the important role of the basal ganglia in action selection in
the control of voluntary movement. The basal ganglia also function in processing
emotions, motivation, and reward.
Rewards are things you desire, such as objects, stimuli, or activities that have
positive value. They produce a conscious experience of pleasure through excite-
ment, satisfaction, relaxation, awe, or physical responses to touch, eating, or
sexual behavior. Rewards can induce an animal to switch from one behavior to
another. Motivation is the desire to seek a goal by initiating, guiding, and main-
taining goal- or reward-oriented behavior.
The striatum is the main input region for the basal ganglia that receives inputs
from many regions of the cerebral cortex. The dorsal striatum functions in motor
control and procedural memory. The ventral striatum includes an area called the
nucleus accumbens. Besides cortical inputs to the nucleus accumbens there are
inputs from the limbic system and dopaminergic inputs from the ventral tegmen-
tal area (VTA) in the midbrain called the mesolimbic system.
In humans, one proposed function of dopamine is to produce subjective pleasure
or the anticipation of pleasure. When rewarding stimuli are detected,the dopami-
nergicmesolimbic systemis activated, which causes the release of dopamine to
several areas, including the nucleus accumbens. Through the action of dopamine,
activity in the nucleus accumbens stimulates motivation and behaviors to obtain
a reward or goal.
The output from the nucleus accumbens pathway is to the thalamus, which relays
output to the cerebral cortex. For the nucleus accumbens pathway, the prefrontal
cortex, basal ganglia, and thalamus form a cortico-striatal-thalamo-cortical loop
that is an important emotional processing pathway in the brain.
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CHAPTER 10 Maintaining the Internal Environment 193
Coordinating with the cerebellum again
The cerebellum may also be involved in the subjective or feeling aspect of emo-
tion. It may contribute to emotional expressions that are important in social com-
munication. It may contribute to cognitive processes that determine the emotional
signicance of events and may modulate some emotions.
Together the basal ganglia and cerebellum may contribute to a range of functions
including recognition of emotional stimuli, including voices or faces. Both may
function in reward processing, contribute to autonomic responses, participate in
inferring the emotional state of others (theory of mind), and contribute to your
awareness of subjective feelings.
Integrating with the prefrontal cortex
Some more complex emotions are associated with social interactions and include
pride, empathy, embarrassment, shame, and guilt. These emotions are similar to
the primary emotions of sadness, pleasure and fear because they produce physi-
ological changes and behaviors that you experience as distinct conscious feelings.
Social emotions may depend especially on the prefrontal cortex, which has a sig-
nicant role in emotional control. We discuss these topics further in Chapter11
when we describe the neocortex in more detail.
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CHAPTER 11 Topping It O: The Neocortex 195
Chapter11
Topping It O: The
Neocortex
T
he neocortex is at the summit of the hierarchy of brain processing. It’s the
neural circuit area that is most enlarged in mammals, and it allows you to
set goals, change your goals when you need to, and store memories that
inform future decisions. Thanks to the development of the neocortex, we humans
evolved into the intelligent (well, mostly) and adaptable creatures we are today. In
this chapter, we tell you all about this important part of the brain.
You’re probably aware that your brain has two sides, or hemispheres— one left
and one right. The left side of the brain generally receives inputs from and
controls the right side of the body, while the right side of the brain interacts with
the left side. Here, you meet your two hemispheres and we tell you more about
what they do. We also discuss the dierences that exist between the male and
female brain.
IN THIS CHAPTER
»
Getting inside your head to the
neocortex
»
Seeing how your brain evolved
»
Making choices with working
memory
»
Acting appropriately and acting
without thinking
»
Meeting the brain’s left and right
hemispheres
»
Looking at dierences between the
male and female brain
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196 PART 2 Controlling Muscles, Movement, and Survival
Looking Inside the Skull: The Neocortex
The neocortex evolved in mammals. In some lineages, including whales, dolphins,
elephants, and primates, the neocortex expanded dramatically during evolution.
The neocortex is the region of the brain that’s involved in voluntary motor
control, perception, and a number of complex behaviors exhibited by mammals.
In the following sections we look at the anatomical and functions regions of the
neocortex and describe the organization and circuitry of the neurons.
Examining the lobes
Looking at a human brain from above or from the side, virtually all you see is neo-
cortex, with the cerebellum sticking out from the back of the brainstem below the
occipital lobe. Neurobiologists divide the neocortex into four anatomical lobes (see
Figure11-1):
»
Frontal: The frontal lobe extends from the most anterior part of the brain
behind the forehead to a groove near the middle of the brain called the
central sulcus. The anterior area is the prefrontal cortex concerned with plans,
FIGURE11-1:
Lobes of the
neocortex.
© John Wiley & Sons, Inc.
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CHAPTER 11 Topping It O: The Neocortex 197
ideas, and contingencies. It projects to many areas, including the motor cortex
that controls voluntary movement.
»
Parietal: The parietal lobe begins at the central sulcus and extends posteriorly
to the occipital lobe and to its ventral border with the temporal lobe. The
parietal lobe contains the somatosensory cortex and an association area
that processes somatosensory, visual, and auditory information. It provides
sensory feedback for planning and movement and is essential for your
awareness of space.
»
Temporal: The temporal lobe has three main divisions:
•
The superior temporal gyrus processes auditory information that then
projects to the association area in the parietal lobe.
•
The inferior temporal gyrus processes visual information along the ventral
stream that allows you to identify objects.
•
The medial temporal lobe has a high-level visual area called the fusiform
face area that processes faces and other complex visual stimuli. It also
contains older cortical regions called allocortex involved in memory,
olfaction, and emotion.
»
Occipital: The occipital lobe is dedicated to visual processing. It contains the
primary visual cortex V1, and higher-order visual areas such as V2 and V3.
Visual information is projected out of the occipital lobe along the dorsal
stream to the parietal lobe and the ventral stream to the temporal lobe.
Most cortical brain areas extend down into the sulci (grooves), which comprise a
large percentage of the total neocortical area (about 2,400 square centimeters).
Functional areas may extend over dierent gyri (ridges or folds) and sulci.
Illustrations of the brain typically show sections that can be in one of three planes,
as you can see in Figure11-2:
»
A horizontal section is parallel to the ground.
»
A sagittal section is from front to back, typically along the midline of the brain,
dividing it into left and right halves.
»
A coronal section is in a plane roughly parallel to the forehead.
Each of these section planes show the neocortex. It consists of a thin 2- to
4-millimeter gray area on the surface, with a larger white area below. The gray
matter is where the neurons, dendrites and synapses are located. The white
matter contains axon tracts myelinated by oligodendrocytes. Both gray and white
matter also contain astrocytes and blood capillaries.
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198 PART 2 Controlling Muscles, Movement, and Survival
Noticing uniform structure and circuits
One of the most important organizational principles of the neocortex is that it has
a standard, six-layered structure. Figure11-3 shows a region of the neocortex
consisting of a sulcus between two gyri (left). The gray matter is a thin sheet of
neurons sitting on top of the white matter.
FIGURE11-2:
Three ways to
section the brain.
© John Wiley & Sons, Inc.
BIG BRAINS
The larger the brain, the more white matter it has. Why? Suppose that in the brain,
every neuron were directly connected to every other neuron. If the number of neurons
were to grow, the number of axons would have to increase as the square of the number
of neurons. Connecting every one of the 86 billion neurons in the human brain to every
other neuron would require a white matter volume the size of a house!
The brain solves this problem by having any given neuron connected to a few thousand
other neurons (on average), with the densest connections to nearby neurons, so that
most axonal lengths are short. A nite number of major axon projection tracts send
information long distances in the brain. Information is highly compressed in these pro-
jections. For example, about one million visual axons from the thalamus drive about
200 million cortical neurons in V1.
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CHAPTER 11 Topping It O: The Neocortex 199
Dierent types of axon tracts form the white matter:
»
Projection tracts have axons leaving the neocortex and projecting to subcorti-
cal structures or the spinal cord.
»
Association tracts have axons projecting from one cortical area to a dierent
cortical area on the same side of the brain. They can be long or short range.
»
Commissural tracts have axons projecting from one cortical area to the same
area on the opposite side of the brain.
»
Ascending tracts have axons projecting from the spinal cord and brainstem to
the brain.
The right side of Figure11-3 is an illustration of the gray matter, which comprises
six layers of neuronal cell bodies, dendrites, synapses, and short unmyelinated
axons. The fundamental output neuron in the neocortex is the pyramidal neuron,
a type of excitatory glutamatergic neuron. The other neurons (not shown) include
excitatory glutamatergic stellate neurons and inhibitory GABAergic interneurons
(see Chapter6).
The types of neurons in the six layers and the inputs and outputs from neurons in
these layers are very similar across the neocortex. This is amazing given that the
neocortex processes inputs from very dierent senses and produces movements
all using similar neuronal circuits.
The organization of the neocortex involves cortical minicolumns. A cortical
minicolumn is a vertical column through the cortical thickness with a diameter of
30 to 40 micrometers, comprising about 100 neurons from layers 1 through 6.
Neurons within a cortical minicolumn are highly interconnected. Cortical mini-
columnsare considered the basic unit of cortical organization with each column
FIGURE11-3:
Neocortex gray
matter and
layers.
© John Wiley & Sons, Inc.
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200 PART 2 Controlling Muscles, Movement, and Survival
functioning as a computational unit.The human neocortex is composed of about
200 million minicolumns in parallel vertical arrays.
Communicating through connections
Each layer of the neocortex has a characteristic distribution of neurons and con-
nections with other cortical and subcortical regions. The layers also contain glial
cells and GABAergic inhibitory interneurons. The layers and connection in neo-
cortex are as follows:
»
Layer 1: Called the molecular layer, it consists mainly of apical dendrites from
pyramidal neurons in the cortical column and horizontally oriented axons. It
contains a few GABAergic neurons. It receives inputs from other cortical layers
and the thalamus.
»
Layers 2 and 3: They’re called the external granular layer and the external
pyramidal layer, respectively. Layers 2 and 3 are often grouped together and
called Layer 2/3 because they’re functionally similar. Layer 2/3 contains
stellate neurons and small- to medium-size pyramidal neurons. Pyramidal
neurons in Layer 2/3 project axons to other cortical areas. They also project to
Layer 5. Layer 2/3 is the target of axons coming from other cortical areas and
from Layer 4 neurons.
»
Layer 4: Known as the internal granular layer, it contains stellate and pyramidal
neurons. Layer 4 is the main target of incoming axons from the thalamus. Layer
4 axons project to neurons in Layer 2/3 and Layer 6 within the same column.
»
Layer 5: Known as the internal pyramidal layer, it contains large pyramidal
neurons. Their axons extend to subcortical areas including the basal ganglia,
brainstem, and spinal cord. Layer 5 neurons also project to other cortical
areas. Layer 5 neurons receive inputs from Layer 2/3.
»
Layer 6: Referred to as the multiform layer, it contains large pyramidal neurons,
small spindle-like pyramidal, and multiform neurons. Layer 6 neurons receive
input from and project to Layer 4. Layer 6 neurons send axons to the thalamus
for reciprocal connections between cortex and thalamus.
Figure11-4 shows the typical inputs, outputs, and intracortical connections within
the neocortical layers. Most layers also receive inputs from brainstem modulatory
neurons. Intracortical circuits also exhibit mutual excitation that must be bal-
anced by inhibitory pathways that aren’t shown. Though similar in neuronal types
and connections, the number and density of neurons and size of the layers vary
somewhat between dierent functional regions of neocortex. For example, layer 4
is larger in primary sensory cortex that receives massive inputs from the thala-
mus. Layer 5 is larger in primary motor cortex, the origin of the corticospinal and
corticobulbar tracts.
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