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CHAPTER 16 The Frontal Lobes and Executive Motor Control 311
territories. This requires complex spatial memory that includes somatosensory,
visual, olfactory, and audio sensory processing. The complex associations between
what’s perceived in the environment, experience, and learned contingency-
specic behaviors depend on processing in the frontal lobes.
The following sections explain how the frontal lobes use planning and past
experiences to override your more primitive goals and control behavior.
Originating abstract plans
The frontal lobes are connected extensively with the limbic system. One way to
think about limbic system control of behavior is to think about the limbic system
as a device for switching states. Most animals have a nite number of goal states,
such as seeking water, food, a mate, or shelter. The limbic system receives input
from the body’s homeostatic mechanisms, and, with limited sensory input depen-
dency, selects the highest priority from a set of evolutionarily programmed
behaviors, such as seeking a water hole or a mate.
Role of frontal lobes
The frontal lobes add the ability to represent reality in the context of past experi-
ence and current situation and goals. Even if an animal is extremely thirsty, let-
ting another member of the herd drink rst may be a better idea, in case the
crocodile it remembers has quietly returned. The frontal lobes are overriding the
highest-priority state selected by the limbic system based on something that isn’t
detected now but is remembered for this situation.
Frontal lobes allow abstract decisions and planning not closely linked to current
body state or observable environmental cues. A thirsty collection of animals may
drink from a water hole until satised and then leave. A frontal lobe competent
troupe may avoid the water hole at a particular time of day during which they
remember that lions show up — allowing the other animals water access at a
very bad time.
Complexity of contingency planning
This kind of abstract contingency planning doesn’t require consciousness, but it
does require complex learning, memory, and nuanced representations of reality.
Vocalizations that refer to internal representations of reality, rather than to what
is actually there, can be the rst underpinnings of language. For example, a mon-
key gives a call that signals the presence of a lion because the monkey remembers
that lions show up at sunset, not because the monkey is actually looking at a
lion now.
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312 PART 3 Understanding Perception and Cognition
Converting plans to body control
A problem with abstract plans is that one can carry them out in many ways. Com-
plex plans have multiple steps, each of which is contingent on the availability of
certain resources, on having the prior steps completed, and on the ability to begin
the next step in the sequence.
At one level, complex processing in the frontal lobes is necessary to execute a
sequence of subgoals necessary to accomplish an overall goal. A higher level of
cortical processing can veto the action sequence if any of the subgoals can’t be
accomplished. So don’t even start going to the water hole if the wind is blowing in
such a direction that you won’t be able to smell the lions.
Commanding with the prefrontal cortex
The prefrontal cortex, located in the anterior-most half of the frontal lobes, is
considered the command center for goal-directed behavior. Highly interconnected
with the rest of the brain, the prefrontal cortex has key roles in executive func-
tions and emotional control. Its basic executive functions include attention, work-
ing memory, and cognitive control – the intentional selection of thoughts,
emotions, and behaviors based on current task demands and social context.
Higher-order executive functions include planning, reasoning, decision-making,
and problem-solving.
The prefrontal cortex provides coordination and execution of motor actions through
its involvement in goal setting, decision-making, motivation, and cognitive con-
trol. Four main anatomical regions comprise the prefrontal cortex that are well
connected with each other, with several overlapping or complementary functions.
Both dorsal and ventral regions of the prefrontal cortex have been implicated in
goal-directed motor control, though the exact circuits and outputs remain elusive.
The activation of high-level goals in the prefrontal cortex appears to activate all
the neural circuit subgoal chains that can satisfy the goal. Athletic coaches know
that imagining hitting a tennis forehand activates the premotor sequence for
doing so and can improve a player’s skill, even though the actual output to the
muscles is suppressed. A similar thing occurs in movement dreams in REM sleep
where motor sequences are activated all the way to the spinal level, but the nal
output is suppressed so you don’t actually move (except for cases where thrashing
and sleepwalking occur).
Executing a goal requires that abstract plans be translated into specic motor
sequences and then to muscle contractions. Neural activity starts with represen-
tations of high-level goals in the prefrontal cortex and moves to activate the sup-
plementary motor area or premotor cortex in the motor cortex, which then drives
motor neuron outputs in the primary motor cortex.
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CHAPTER 16 The Frontal Lobes and Executive Motor Control 313
Initiating Action in the Basal Ganglia
The section “Comparing Reexes versus Conscious or Goal-Directed Behavior”
earlier in this chapter discusses the frontal lobes overriding the limbic system and
basal ganglia in control of behavior (refer also to Chapter8). The basal ganglia
remain involved in executing the action, however. This fact is dramatically obvi-
ous from observations of patients with Parkinson’s disease, which is associated
with loss of dopaminergic input from the substantia nigra, a component of the
basal ganglia.
Patients with Parkinson’s can certainly formulate abstract motor plans, but they
have trouble initiating and executing them. This suggests that the instantiation of
the motor sequence conceived in the prefrontal cortex still depends on subcortical
structures to initiate, implement, and provide feedback control to deal with errors.
The following sections delve into the ins and outs of the basal ganglia.
Preparing for action
The basal ganglia receive inputs from prefrontal, motor, and sensory association
areas of the neocortex and the thalamus for voluntary motor control. The function
of the basal ganglia is to select, among the simultaneously active frontal lobe
motor programs, the one to execute at a particular time. The overall output of the
basal ganglia inhibits all the motor sequences except the one selected, via its
release from inhibition. The basal ganglia operate in a loop through the thalamus
with the frontal lobes, such that the particular frontal motor sequence that the
basal ganglia selects is partly controlled by the frontal lobes.
One hypothesis is that the basal ganglia operate in a winner-takes-all selection
among active frontal lobe motor program sequences. This selection may involve a
threshold eect by which one program sequence in the frontal lobes that is slightly
more active than the others is selected and reinforced through the thalamus. This
selection process is important in procedural learning and learning routine behav-
iors in particular situations.
Patterning and oscillating
Many neural oscillatory rhythms (brainwaves) occur in the brain that are evident in
electroencephalograph (EEG) recordings. Dierent frequency waves (bands) have
been given dierent names, such as
»
Theta (3 to 10 Hz)
»
Beta (15 to 30 Hz)
»
Gamma (30 to 80 Hz)
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314 PART 3 Understanding Perception and Cognition
These waves are important in the normal functioning of the basal ganglia. In the
subthalamic nucleus, a signicant upregulation in the amplitude of beta waves is
associated with Parkinson’s disease and is thought to contribute to bradykinesia
and rigidity. Oscillations in the gamma band that facilitate movement (prokinetic)
are decreased in Parkinson’s disease. Medications or treatments in patients with
Parkinson’s disease that increase dopamine reduce the beta oscillations and
increase oscillations in the gamma band.
Also, between the basal ganglia and the cerebellum is an important neural loop
(see Chapter8). This loop regulates the selection and initiation of action via pat-
tern classication operations in the striatum (the input layer of the basal ganglia).
Medium spiny neurons in the striatum disinhibit one particular thalamo-cortical
loop activity pattern that increases at the expense of others. The loop with the
cerebellum then amplies and renes this pattern as it continues during the
action sequence.
Learning with the basal ganglia
The striatum is the major input region of the basal ganglia. In addition to its roles
in action selection and initiation, it also represents a major site of learning and
memory for goal-directed actions and habit formation. These are types of implicit/
nondeclarative memory called procedural memory (see Chapter15), which involve
encoding and storage of cognitive, motor, and perceptual habits and skills. Proce-
dural memory also involves the cerebellum, which we describe in the section
“Sequencing in the cerebellum and strengthening synaptic pathways” later in this
chapter.
The dorsal striatum is composed of two subregions:
»
Cortical inputs from associative and prefrontal cortexproject to the dorsome-
dial striatum. This region is involved in reward-guided instrumental learning,
which supports goal-directed behavior.
»
Cortical inputs from sensorimotor and premotor cortices project to the
dorsolateral striatum. This region is involved in learning phases associated
with habit formation and performance.
The majority of neurons in the striatum are GABAergic inhibitory neurons, called
medium spiny neurons. They receive cortical input from somatosensory, prefrontal
and motor cortices, and thalamic neurons. The great majority of these cortical
inputs are glutamatergic. The striatum also receives inputs from dopaminergic
neurons in the substantia nigra and from nearby cholinergic neurons within the
striatum.
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CHAPTER 16 The Frontal Lobes and Executive Motor Control 315
Long-term synaptic plasticity at the major inputs, the cortico-striatal synapses,
has been proposed as a mechanism that enables the basal ganglia circuits to func-
tion in the acquisition and maintenance of motor skills and habits. Both long-
term potentiation and long-term depression have been observed in striatal
synapses. Both types of synaptic plasticity are proposed to be involved in encoding
procedural memory. In many features, the striatum is analogous to the hippo-
campus, which binds cortical and subcortical inputs and encodes declarative
memory using long-term synaptic plasticity.
Coordinating through the Motor Cortex
The supplementary motor area and premotor cortex lie functionally between the
abstract representation of goals and plans in the prefrontal cortex and the control
of muscles or muscle groups in the primary motor cortex. The supplementary
motor area and premotor cortex organize goals into muscle movement sequences.
But they do this dierently. Well-known, internally generated (driven) movement
involves the supplementary motor area. Sensory-guided (externally cued)
movement— such as manipulating a novel object— involves the premotor cortex.
The following sections expand on the connections and roles of the premotor cor-
tex and supplementary motor area in controlling voluntary movements.
Feeding back to guide movement
The premotor cortex is involved in more complex movements than mediated by
the primary motor cortex, which might by itself control simple actions like
squeezing a ball with the hand. The location of the premotor cortex places it near
the primary motor cortex areas that control the body’s trunk muscles.
The premotor cortex receives sensory feedback from the parietal lobe (posterior
parietal cortex— a major sensory association area) through the thalamus. The
premotor cortex projects not only to primary motor cortex, but also directly to the
spinal cord, the striatum, and back to the motor thalamus. The premotor cortex is
important for guiding your movement using sensory feedback. Complex move-
ments, such as climbing, are associated with the premotor cortex. (If, say, you’re
climbing a ladder, you need to move all four limbs in a coordinated way while
being aware of the space around your body.) Because of the direct projections from
premotor cortex to the spinal cord, the hierarchy between the premotor cortex and
the primary motor cortex isn’t absolute, and the premotor cortex can control
movement bypassing the primary motor cortex.
Neurons in the premotor cortex are active when you’re learning motor sequences.
Neurons in the dorsal premotor cortex become active in response to a stimulus cue
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316 PART 3 Understanding Perception and Cognition
to act and may remain active during any delay before the action occurs. Some evi-
dence exists that entire action sequences are represented in the premotor cortex
as behavioral repertoires, such that dierent parts of the movement repertoire are
represented in dierent premotor cortex subregions. The subregions compute
spatial trajectory and control the movements of joints and muscles.
Using the supplementary motor area
The supplementary motor area is medial and anterior to the primary motor cortex
leg representation. The supplementary motor area controls movements and
movement sequences that are generated internally, rather than triggered by sen-
sory events. It may also be involved in stabilizing the body, particularly in coordi-
nating both sides of the body together. Like the premotor cortex, neurons in the
supplementary motor area also project directly to the spinal cord and may directly
control movement.
The supplementary motor area receives strong inputs from most prefrontal cortex
but much less input from the posterior parietal cortex. This area is strongly con-
nected to the thalamus and the basal ganglia. It’s important for complex locomo-
tion such as climbing, leaping, and other complex behavioral repertoires,
particularly those that have been learned and internalized.
The supplementary eye eld (SEF) is located anteriorly in the supplementary
motor area. This area and the frontal eye eld (FEF) anterior to that in the frontal
cortex are involved in generating saccadic eye movements. Some evidence sug-
gests that FEF represents saccades with respect to the target location on the ret-
ina, whereas saccades may be represented in the SEF with respect to their location
relative to the head. The pattern of connections between these two areas suggests
a mapping between them that converts the head-oriented saccade representation
in SEF to a retina-oriented saccade representation in FEF.
Learning motor sequences
Motor sequence learning is the process by which a sequence of motor actions is
produced with improved accuracy and precision. Early behaviorists believed that
motor sequence learning comprised a reex chain in which the completion of each
movement in the chain triggered the next. However, this can’t be the whole story
because movements can occur even without sensory feedback, and some move-
ment sequences occur too quickly to depend on sensory feedback.
Learned motor sequences are eciently represented in hierarchical plan organi-
zations that combine lower-level sequences into larger units, called chunking.
Motor sequence learning can occur with or without conscious awareness of the
learning process.
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CHAPTER 16 The Frontal Lobes and Executive Motor Control 317
Practicing mentally
Motor sequence learning can be explicit or implicit:
»
Explicit sequence learning means being aware (conscious) that you’re practicing
a skill, like hitting a tennis ball.
»
Implicit learning means that, during practice, you get better at something
without being aware (not conscious) of exactly how that’s occurring.
In a typical sequence when you’re acquiring a skill, you’re more attentive in the
initial phase (explicit learning), but after repeated practice, the skill becomes
almost automatic as you improve (implicit learning).
Even if it’s raining outside, you can still practice your tennis serve— in your
mind! Practice can be eective even if the activity is only imagined. This imagined
activity is called motor imagery, and it has been shown to be eective in sports
training and to rehabilitate motor decits associated with some disorders such as
multiple sclerosis and motor decits from strokes.
Motor imagery activates early and late stages of motor control, including the sup-
plementary motor area, primary motor cortex, posterior parietal cortex, basal
ganglia, and cerebellum. Activation of the primary motor cortex during motor
imagery is, however, at a less intense level than during actual action execution.
This is sucient to increase muscular activity and is specic to the muscles asso-
ciated with the imagined action.
Sequencing in the cerebellum and
synaptic pathways
When a neural code or representation for a learned skill is created in the brain, it’s
called a procedural memory. Refer to the section “Learning with the basal gan-
glia” in this chapter for more about the basal ganglia. The cerebellum also plays
an essential role in motor and procedural learning, particularly where ne adjust-
ments have to be made to the action sequence.
The cerebellum has several major cell types in a stereotypical, repeated circuit
(see Figure16-1). Purkinje cells in outer layers of the cerebellum inhibit deep cer-
ebellar nuclei. The thin dendritic arbors of aligned Purkinje cells form stacks
through which parallel bers from the deeper layers pass. The parallel bers (the
axons of the granule cells) make excitatory (glutamatergic) but weak synapses
onto Purkinje cell dendritic spines. Climbing bers, which originate from the
inferior olivary nucleus in the medulla, have a more potent excitatory input to the
Purkinje cell that is nearer to dendrites and cell soma.
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Motor learning in the cerebellum is mediated primarily by LTD of parallel ber
synapses onto Purkinje cells. Climbing bers provide the teaching signal that
induces synaptic modication in parallel ber–Purkinje cell synapses. Climbing
ber activity represents an error signal.
Purkinje cells have two dierent types of action potentials:
»
Simple spikes: They occur spontaneously and when Purkinje cells are
activated by the parallel bers.
»
Complex spikes: They exhibit an initial large-amplitude potential followed by
a burst of smaller-amplitude spikes induced by climbing ber activation.
Complex spikes can generate calcium-mediated action potentials in the
Purkinje cell dendrites. This can shift Purkinje cell activity from a quiet state to
a spontaneously active state. The complex spike, generated by the climbing
ber, serves as an error signal during learning— complex spike activity
increases when a mismatch occurs between an intended movement and the
movement actually executed.
Basket and stellate cells in the cerebellar molecular layer (refer to Figure16-1)
make GABAergic inhibitory inputs to the Purkinje cell. Stellate cells synapse onto
Purkinje cell dendrites, but basket cells contact the Purkinje cell axon initial seg-
ment. Basket cells mediate feedforward inhibition (see Chapter8) onto Purkinje
cells as part of the process of sequence tuning.
FIGURE16-1:
The organization
and connections
of major cell
types of the
cerebellum.
© John Wiley & Sons, Inc.
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CHAPTER 16 The Frontal Lobes and Executive Motor Control 319
Whereas feedback inhibition, such as in spinal cord reexes, tends to maintain a
limb in a particular position, feedforward inhibition is used to program actions
such as slowing down a limb before it reaches the target, so that if you reach to
grab an apple hanging from a tree limb, your hand doesn’t go past the apple, but
stops in the right place. This occurs prior to any sensory feedback coming from
contact with the apple.
Activation of presynaptic NMDA receptors in stellate cells increases their GABA
release and changes the composition of their glutamate receptor subunits. These
long-lasting changes are one of the associative learning cellular mechanisms in
the cerebellum.
Mirroring Others: Mirror Neurons
In primates, mirror neurons have been recorded in the premotor cortex, the sup-
plementary motor area, the primary somatosensory cortex, and the inferior pari-
etal cortex. These neurons re when an animal performs a specic act and,
interestingly, when the animal observes the same action performed by another
animal or a human. The neurons are named mirror neurons because their activity
reects the behavior of another animal doing what would activate the neuron in
the observer animal.
Mirror neurons were discovered by the Italian researcher Dr. Giocomo Rizzolatti
and colleagues in the 1980s while recording from neurons in the ventral premotor
cortex of the macaque monkey that were involved in the control of hand and
mouth actions during eating. Some of the neurons responded when the monkey
saw a person pick up food, as well as when the monkey picked it up itself.
In humans, neuronal activity consistent with that of mirror neurons has been
observed in the motor cortex, somatosensory cortex, and inferior parietal cortex.
The following discusses potential functions of mirror neurons.
Dening mirroring behaviors
Mirror neurons appear to help us do many things:
»
Learn new skills by imitating others. If mirror neurons simulate observed
actions, their activity may be like a kind of mental practice for an
action sequence.
»
Understand the actions and intentions of others. In social situations,
mirror neurons have also been suggested to help an animal understand
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320 PART 3 Understanding Perception and Cognition
others. This is consistent with suggestions that autism may involve some
problem with the mirror neuron system. It has also been suggested that the
human Broca’s region is homologous to the area of the monkey ventral
premotor cortex where mirror neurons have been recorded. A signicant
number of neurons in the macaque monkey inferior parietal cortex also have
mirror properties, responding to direct hand gestures and observed ones.
»
Empathize. Some evidence exists for the involvement of mirror neurons in
empathy. The anterior cingulate cortex, anterior insula, and inferior frontal
cortex are active when people experience emotions such as disgust, pain, and
happiness, as well as when subjects see another person experiencing these
emotions. People who score higher on self-report questionnaires for empathy
show stronger activation of mirror neurons for emotions and hand gestures.
A more controversial idea is that mirror neurons may provide the neurological
basis of self-awareness in humans by representing actions at an abstract level.
Imitating others through mirroring
If mirror neurons mediate automatic imitation and motor mimicry, then the
mirror neuron system may contribute to cognitive functions such as social aware-
ness and other social behavior according to having what’s called a theory of mind.
Automatic imitation is where a person observing a body movement deliberately
performs a similar one. This can mediate empathizing with the person we’re
observing and understanding his or her intentions.
Some researchers argue that human language evolved from a mirror neuron sys-
tem for understanding gestures. Human brain areas homologous to those where
mirror neurons have been found in monkeys are located in the inferior frontal
cortex, close to Broca’s area, a main language region of the brain. When people
gesture to each other, as in the game of charades, fMRI activity increases in brain
areas of human observers, consistent with being driven by mirror neurons.
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