Kristin Levine

I look up from my word processor and make eye contact with my German
Shepherd. Her head rises, ears straighten, and I know that she is
anticipating an action from me. When I direct my gaze to my seven year old
daughter, she asks if I'm done writing and ready to go shopping for the
present that she needs to bring to the birthday party next week. In my
dog's look is simple hopefulness, in my daughter's a fuller array of
emotions and thoughts: hopefulness, purposefulness, anticipation of the
more distant future, expectations for my fulfillment of my parental role,
etc. etc.

If I had been in the middle of a thought and returned to my work without
speaking, the dog would lay her head down and return to her nap. My
daughter would react with a far more complex set of affective and thought

What is it in the human brain that sets it apart from the lower life forms,
making its reactions and capabilities so much more complex? How do these
differences develop and what are the relationships between neural
maturation, emotion and thought?

The purpose of this paper is to briefly review the literature in an attempt
to answer these questions.

Evolution of the Human Brain
The human brain, as we know it, has remained essentially unchanged for
approximately forty thousand years (Gazzaniga, 1988), but how did it get
this way? Although theories of the origin of life itself vary, the
evolution of increasingly complex cellular organisms has progressed since
shortly after the Earth was formed 4.6 billion years ago (Joseph, 1993).


Many scientists believe that life on this planet began with single-celled
organisms that contained a single strand of DNA within the protoplasm of
their cells (Hyman, 1942). If one subscribes to the theory of evolution,
these prokaryotes were the basis for the eventual development of
multi-celled organisms including birds, plants, cats, dogs, and even
humans. The first single-celled prokaryotes reproduced by dividing and
producing identical copies of themselves and their DNA. Despite their
cellular simplicity and lack of consciousness, these cells were capable of
sensing their surroundings and attending to those features of the
environment that were necessary for their survival. Even beyond this, these
single-celled organisms were capable of communicating and cooperating in
some fashion, and formed single-celled nations which benefited the
individuals and the social group. This primitive ability to communicate and
cooperate is thought to be due to the presence of DNA (Joseph, 1993).

The first multi-celled organisms contained double strands of DNA, and
therefore possessed many times the memory, intelligence, planning skills,
and capacity to communicate in comparison to the prokaryotes (Joseph,
1988). Since the time of these early uni-celled and multi-celled creatures,
DNA has served as the intellectual and memory center of all cells. It
wasn't until one billion years ago that multicellular creatures engaged in
sex, thereby indulging in more complex modes of communication (Joseph, 1993).

Multicellular creatures ranging from the simple early organisms described
above to humans, exhibit the capacity to combine and interchange DNA from
one cell to another. This makes possible the creation of a third organism
which, through the recombined DNA of its parents, carries some of the
genetic plans and memories of its predecessors (Watson, 1979).

Even in the case of modern-day humans, all the tissues of our bodies are
derived from a single sexually fertilized ovum cell. This primal cell
divides through the process of meiosis, as its daughter cells divide after
it, to form a complex multicellular human being. Based on the DNA
instructions within each nucleus of each daughter cell, cells are sent to
specific locations within the developing body and once there, form specific
connections which enable them to carry out certain functions.

The Neuron
The next important step in evolution occurred around 700 million years ago,
with the cellular metamorphosis that resulted in the creation of the neuron
(Joseph, 1988). Rather than merely dividing in order to pass on complex
memories and life plans through DNA, neurons are capable of creating
memories and plans and communicating this information to other neurons.
Neurons eventually developed dendrites and axons which are highly
specialized for reception versus transmission of electrical and chemical
messages, making them capable of communicating more specific and
differentiated information.


Over the course of evolution, the number of these secreting and
transmitting nerve cells increased in higher life forms. The resulting
organisms could now control, coordinate and direct their behaviors in a
much more sophisticated manner because different areas of the body could
communicate together almost simultaneously through what has been called the
"nerve net" (Joseph, 1988). As the interconnections of the nerve net
increased in size and complexity, true "brains" developed.

Long after the development of differentiated axonal and dendritic fibers,
myelin sheaths began to form insulation around the axon fiber for more
efficient transmission of information from one neuron to another.


The earliest nervous systems were nerve nets that were quite indiscriminate
in their responses. Activation of a single neuron excited the entire
network and the organism responded as a whole. The primitive network
evolved into the primitive nerve cord (such as that seen in flatworms), and
soon after, the specialized structure of "the head" followed (Glezer,
Jacobs, and Morgane, 1988). This differentiation of one end of the nerve
cord continued as enlargement and concentration of internal communication
and internal exchange mechanisms (Mahoney, 1991).

Cell bodies migrated inward and gathered together over time, eventually
forming collections of nuclei and the various lobes of the brain. The first
two ancient lobes were the result of ganglia of like-minded cells forming
to serve two vital functions. The olfactory lobe developed to analyze
olfactory, pheromonal or chemical information, and the optic lobe developed
to analyze visual input. The expansion and axonal-dendritic
interconnections of these first two lobes has, over time, led to the
formation of the modern brain (Joseph, 1993).

>From the olfactory system developed the limbic system, which is concerned
with basic survival needs: feeding, fighting, fleeing, and fornicating
(Barr, 1979). As a growing number of nerve cells accumulated for the
purpose of analyzing olfactory information, they began to form layers which
became the first layers of cortex. This olfactory-limbic cortical tissue
eventually gave rise to the first motor cortex and to the two cerebral
hemispheres that completely encase the remainder of the human brain. Much
of the human brain has evolved from this ancient olfactory-limbic system
(Maclean, 1990), a fact with much relevance for understanding human behavior.

According to the theory of evolution, about a half billion years ago, many
different types of vertebrates swam the ocean and plants proliferated
wildly over the earth's land surface. This was followed by a large variety
of insects, amphibians and eventually the first dinosaurs. Over time, the
brain also evolved in response to the effects of the continually changing
environment (Joseph, 1993; Mahoney, 1991). Up till the arrival of the true
mammals, around 100 million years ago, the sharks, reptomammals, dinosaurs,
and birds possessed only two layered cortical motor centers and limbic
system tissue. With the true mammals the neocortex developed outward, layer
by layer, to enshroud the old brain. This "new brain" consisted of six to
seven new layers of cortex which formed the cerebral hemispheres. Given the
intellectual superiority that their neocortex provided them, mammals
rapidly evolved, multiplied, and soon dominated a planet that had
previously been ruled by less intelligent, although more physically
powerful life forms. Over time neocortex continued to accumulate, and gyri
were formed so that expanding cortical connections could fit within the
confines of the bony skull. A wide variety of mammals now exist, with
varying amounts of cerebral cortex adapted to meet specific environmental
demands. The limbic cortex (old cortex) is not terribly dissimilar in
shape, location and size among mammals. The neocortical mantle that covers
it, however, expands progressively as one ascends from primitive mammals to
primates and then humans (Joseph, 1990). The limbic system and "reptilian
brain" of more primitive life forms have not been replaced, but merely
expanded upon (Joseph, 1992) . It is our highly developed neocortex, with
increased gyri to accommodate the expansion of the frontal and other lobes
of the cerebrum, that makes our brains uniquely "human." Even our fellow
primates who possess considerable neocortex do not have the brain area that
is considered essential for the production of complex spoken language, the
angular gyrus (Joseph, 1993). So I can communicate with my dog, often
unconsciously, because we possess much of the same limbic brain tissue. The
older communication system that we share continues to work well for both of
us; she can recognize my moods and anticipate my simple actions from my
gestures, sounds, touch, and smells. My daughter and I can communicate
through these older systems also, but we can utilize the additional complex
associative, anticipatory, planning and language systems made possible by
more extensive neocortex as well. It has not always been this way, however.
During her infancy I could not communicate with my daughter nearly as well
as I could with my dog. Nor did my infant daughter exhibit the graded
affective response, level of understanding or control, or judgment
demonstrated by the Shepherd. How does an individual's brain develop and
what effect does that have on emotion and thought?

Developmental Processes  [[[[[this was one HUGE paragraph, so I've
abitrarily divided it up for easier reading...Sheri]]]]

Cowan (1979) divided the neuronal development process into six basic
stages. First, the cell is generated; second, it migrates from its birth
citeto its terminal location; third, cells within specific brain regions
aggregate; fourth, axons and dendrites grow and cells differentiate; fifth,
synaptic connections form; and finally, cells, axons, and dendrites are
eliminated. This last process continues throughout an individual's life.
Rutter and Rutter (1993) describe the process in four overlapping phases.
First, the main structure of the brain forms, followed by proliferation of
brain cells. Next, cells migrate to their final destination and
simultaneously, synaptic connections increase to further elaborate the
neuronal network. Finally, as in Cowan's description, extensive cell death
results in loss of about half of the neurons. This loss is thought to serve
a fine-tuning function, and to be associated with increasing specialization
of function in different parts of the brain. Along with the development and
differentiation of this complex neuronal network, the myelinization of
axonal fibers, and development of neurotransmitters occur at different
rates in different portions of the brain. As various fibers become
myelinated, the functions that they subserve are performed more efficiently
(Milner, 1967; Rutter and Rutter, 1993). Obviously, the process of brain
development is very complex.

As Rutter and Rutter (1993) have noted, the precise migration of neurons
and formation of billions of synapses can not be controlled genetically.
Sensory input is thought to play a "driving" role in organizing neuronal
development, and lack of relevant experience can have a lasting effect on
brain development. Because of the role of sensory input in normal brain
development, the effects of environment (nurture) versus maturation
(nature) are nearly impossible to separate out. The developmental processes
described here will assume optimal environmental exposure, and address
changes that occur in the brain over the course of development.

Prenatal Brain Development

Rutter and Rutter (1993) note that unlike most organs of the body, the
brain experiences its growth spurt during the prenatal period and first few
years of life. Moore (1982) describes how the fetal brain quadruples in
size by the end of the first trimester, and almost triples again by the end
of the third trimester. The human nervous system begins as a neural groove
which closes into a neural tube by the forth week of prenatal life (Milner,
1967). Milner describes how four flexures develop as the tube lengthens and
demarcate five subsections. Each of these subsections acts as a discrete
focus of growth for a major nerve center. The lowest differentiates into
the spinal cord, the oldest phylogenetically, and lowest functional level
of the mammalian neuraxis. The remaining sections differentiate into the
medula oblongata, the pons and cerebellum, the mid-brain, the diencephalic
centers, and the top segment of the tube becomes the cerebral hemispheres,
which develop last. The brain attains all of its general structural
features by the fourth month, and the order of the structure's emergence
parallels the order of their phylogenetic appearance (Milner, 1967;
Persinger, 1987; Joseph, 1993; Gazzaniga, 1988).

 As we saw in the evolution of the brain, the lowest centers differentiate
first, and the highest centers last. This is consistent even within the
cerebral cortex, as the phylogentically oldest limbic lobe and hippocampus
begin to differentiate before the neocortex. As described by Joseph (1982),
prior to the development of cerebral cortex, primitive neuronal cell bodies
(neuroblasts) migrate outward to form an outer cortical zone called the
primordial cortex. At about the third month, this zone receives massive
migrations of cells from the inner regions of the brain. The six concentric
neocortical layers are not formed simultaneously, so that their functions
develop at differential rates. The first layers to develop are the deepest
layers, which consist of cortico-spinal tract (pyramidal) cells and
subserve motor function. During this period the motor regions of the
frontal lobes and the deep layers of the temporal lobes (including limbic
areas) begin to form (Milner, 1967).

The second wave of migration of neuroblasts into the primordial cortex
results in formation of layers 2, 3, and 4. These layers receive specific
sensory fibers directly from the thalamus or association fibers from other
cortical regions. As such, they have predominantly receptive (sensory)
functions. The final migratory blast forms the most superficial layer.
Joseph notes that these last three layers do not differentiate completely
until middle childhood. As described by Milner, the motor (ventral) roots
of the spinal cord begin to show myelin between the fourth and fifth
months, and myelination of the sensory pathways in the cord begins a month
later. Myelination of the "old cortex" begins at forty weeks, just before
birth, and when the infant is born the neocortex remains largely
undifferentiated and nonfunctioning.

Postnatal Neural Development
After birth, the brain continues its rapid development. It more than
doubles in weight in the first year of life and reaches 90% of its adult
size by age five years (Moore, 1982). Much of the complex process of brain
development after birth is not well understood, or more intricate than this
discussion warrants. Therefore this account will present general trends in
neuronal maturation that are believed to affect the development of emotion
and thought. In general, the progression of central nervous system
development continues as seen in the prenatal period. Reflexes and feedback
loops (servomechanisms) become progressively more complex. Inhibitory
centers tend to predominate over excitatory centers, damping and modifying
excitatory impulses to increase the complexity and specificity of
responses. Growth, development and maturation begin in the cord and end in
the neocortex.

A hierarchy of control develops with higher level (later developing)
centers exerting an inhibitory effect on lower level centers and increasing
the complexity of function (Moore, 1982; Joseph, 1993). At birth, spinal
level structures are primarily myelinated and brain stem structures
responsible for maintaining life functions in homeostasis are not yet fully
functional. During the first month, physiological functions (breathing, EEG
regularity, body temperature maintenance, etc.) stabilize and cortical
functioning begins (Milner, 1967). The onset of aerobic respiration at
birth is thought to be the trigger for the spread of electrical conduction
from subcortical to cortical cells, and once started growth in the
neocortical lobes is rapid.

According to Turner's (1950) study of gross structural characteristics of
brain growth, the most rapid increases in cortical surface area in the
first two years of life occurs in the parietal and frontal lobes. These
lobes are associated with the sensory, motor, language and speech functions
that undergo rapid behavioral change during early childhood. More moderate
growth occurs in the occipital and temporal lobes, which are associated
with visual perception and experiencing of sound, as well as functions of
the limbic system. Between the second and sixth year, rapid growth is noted
in the temporal and frontal lobes and after age six little or no growth
occurs in any of the lobes except the frontal lobes. Growth in the frontal
lobe continues at a moderate, even pace until age 10, and then continues
more slowly until age 20. In general, as seen in evolution, brain
development progresses upward and outward.

>From the central life-sustaining functions of the spinal cord and brainstem
("reptilian brain") which are present at birth, develops increasingly
complex memory storage and ability of various portions of the brain to
communicate with each other. The brain develops as a series of four
separate brains, each with its own memory, motor and other functions
(Mahoney, 1991). Each brain elaborates on the preceding level and adds
increasing degrees of organization and self-preservation capacity to the
vegetative functions of the hindbrain, midbrain, and spinal cord. The first
"brain" described by Maclean (1990) is this "reptilian brain." This part of
the brain is responsible for primitive levels of genetically transmitted
knowing that result in repetitive and ritualistic migratory,
territoriality, aggression and courtship behaviors. Maclean describes an
important achievement of the reptilian brain as "homing", or the tendency
to return to a recognized frame of reference after reaching out for a mate
or food, etc. Mahoney (1991) relates this to the development of human
"reality," which is our creation of an orderly and temporally stable world.
The second "brain" to develop is the limbic system, or "paleomammalian
brain". This level integrates and refines life-relevant behavior patterns
(feeding, aggression, and reproduction) and is best known for its role in
emotional intensity and motivational complexity (Mahoney, 1991). The limbic
system coordinates homeostatic life support, purposive action, memory,
learning, and emotionality. As such, it involves its own primitive form of
reflective intelligence and self-regulatory control. The third, or
"neomammalian" brain, also known as the "neocortex", accounts for 85% of
the entire adult human brain. The frontal area, which is associated with
higher level mental organization, intentionality, and self- awareness, is
over six times as large as that of non-human primates of similar size
(Mahoney, 1991). Mahoney cautions against thinking that, because it
develops later, the rational intellectual functions of the neocortex enable
it to override or control the passions of the limbic brain. Although under
inhibitory control of the neocortex, parts of the limbic system with their
primitive survival functions, can override neocortical control as will be
discussed later (Joseph, 1992; Joseph, 1993; Mahoney, 1991). The fourth
human brain is seen in differentiation of the neocortex into two separate
and independently functioning "higher brains" or cerebral hemispheres. In
his original description of "the triune brain," MacLean denied the need to
describe this fourth level of independent brain functioning, however the
majority of modern neuroscientists have disagreed (Mahoney, 1991).
Differentiation of these four brain systems and concomitant changes in
emotion and thought occur primarily during early childhood, but continue
into adolescence and even adulthood.

As pointed out by Mahoney (1991), the term "emotion" is derived from the
Latin "e movere" which means, literally, "to move." Emotionality is
basically protective in function, and is closely related to movement and
action. It either promotes survival of the individual through fight or
flight responses or survival of the species through reproductive or social
cooperative responses (Joseph, 1992).

The Limbic System
Although the right hemisphere is involved in emotional expression, the
subcortical limbic structures are thought to be the major sites for
elicitation of emotional arousal (Joseph, 1993). The limbic system is
described as the background of emotional tone (Moore, 1982), and is
involved in: monitoring, mediation and expression of emotional,
motivational, sexual and social behavior. Fight or flight, attraction or
avoidance, arousal or calming, hunger, thirst, satiation, fear, sadness,
affection, happiness, and the control of aggression are all responses
mediated by the limbic system (Joseph, 1992). The limbic structures receive
projections from all sensory receptors which enables the individual to
judge the appropriate response to sensory input. In sufficient intensity
any sensation (pressure, heat, sound, smell, movement, touch, etc.) will
result in emotional characteristics leading to approach or avoidance. The
limbic structures of primary importance for consideration of the
development of emotion are the hypothalamus, amygdala, hippocampus, and the
septal nuclei. These limbic nuclei functionally mature at different rates.
Corresponding behaviors and capacities appear, overlay previously developed
capacities, become differentiated, and become suppressed or eliminated as
further neuronal development and myelination occur (Joseph, 1992).

The hypothalamus emerges and differentiates before all other limbic nuclei,
and according to Joseph (1992), constitutes the most primitive and purely
biological aspect of the psyche. It reacts in an on/off manner to maintain
pleasurable or avoid noxious conditions. The hypothalamus is largely
concerned with monitoring the internal environment and maintaining
homeostasis in body tissues. Emotions elicited by the hypothalamus are
largely undirected, unconnected with events in the external environment,
and consist of feelings such as aversion, rage, hunger, thirst, pleasure
and unpleasure. The hypothalamus is functional at birth, however because of
its lack of connections with higher order nuclei, has no way to mobilize
the infant for effective action. Newborns first experience or express the
most powerful emotionality in response to bodily needs, tactile sensations
and loss of body support (Joseph, 1992). The earliest emotional responses
consist of screaming, crying, rage-like vocalizations, acceptance and
acquiescence, and are all mediated by the hypothalamus. Rutter and Rutter
(1993) discuss how it used to be thought that newborns exhibited only
undifferentiated emotions. Specific emotions such as fear, anger, and
happiness were thought to emerge gradually as a result of learning and
maturation. More recent research reportedly demonstrates that a range of
different discrete emotions are present in early infancy, although they
undergo further differentiation with maturity and experience (Mahoney,
1991). With maturation of higher order limbic nuclei, the infant becomes
more aware of external reality, begins to differentiate and associate
externally occurring events, and forms memories. This results in the
differentiation of more complex emotional responses such as surprise, fear,
or anxiety. The context in which various emotions are elicited also changes
over the course of development (Rutter and Rutter, 1993).


One of the nuclei most involved in more differentiated control of emotion
is the amygdala. During the course of evolution, the hypothalamus initially
controlled and expressed raw and reflexive emotionality in response to
monitoring of internal homeostasis and basic needs. The development of the
amygdala enabled the organism to monitor and test the external emotional
features of the environment and to act on them (Joseph, 1992; Joseph,
1993). In the infant, as in phylogenesis, when the amygdala becomes
functional it hierarchically takes over control of emotion from the
hypothalamus. At birth, the hypothalamus signals pleasure or displeasure in
response to the infant's internal needs, but because of its functional
isolation, has no way to get these needs met. Over the course of the first
few months of life, the amygdala and then the hippocampus develop. These
two limbic nuclei enable the infant to monitor the external world, while
registering and remembering events and objects (including people)
associated with pleasure, or tension reduction. The amygdala is
interconnected with various neocortical and subcortical regions, so it is
capable of monitoring and abstracting information from the environment that
is of motivational significance to the infant (Joseph, 1992). The amygdala
assigns emotional or motivational meaning to that which the infant
experiences. The ability to distinguish and express subtle socio-emotional
nuances including friendliness, fear, distrust, anger develops during the
first several months with maturation of the amygdala. Because of the
polymodal nature of amygdaloid neurons, this structure is involved in
attention, learning, and memory as well as emotional and motivational

The hippocampus is also associated with learning and memory, and
complements and interacts with the amygdala in regard to attention and the
generation of emotional imagery. The left amygdala/hippocampus is thought
to be involved in attending to and processing verbal information (Joseph,
1992). The right is involved in learning and memory of motivational,
tactile, olfactory, facial, nonverbal, visual-spatial, environmental and
emotional information. So, with the maturation of the amygdala and
hippocampus over the first few months of life, the infant is able to orient
and selectively attend to the external environment based on
hypothalamically monitored needs. He or she is increasingly able to
differentiate what occurs externally, to determine what is satisfying, and
to remember this information. Once these capacities are developed, further
associations, memories, differentiations and more specific and complex
emotional responses develop as the infant interacts with the environment.
These emotional responses also determine the behavior of the infant, and
play a key role in the way the infant organizes his or her experiences
(Mahoney, 1991). Izard (1978, p.391) describes emotions as "the principle
organizing factors in consciousness."

Septal nuclei
The septal nuclei, or septum, is interconnected with all regions of the
hippocampus, as well as projecting heavily throughout the hypothalamus and
connecting with the amygdala (Joseph, 1992). It appears to function in an
inhibitory manner, dampening and quieting arousal and limbic system
functioning. As such, it reduces extremes of emotionality and maintains the
individual in a state of quiet readiness to respond. In contrast to the
amygdala which promotes social behavior, the septum counters socializing
tendencies (Joseph, 1992). With maturation of the septum, the infant
develops an increasing capacity for controlling emotional responses based
on information from past or anticipated future experiences. More specific
emotional reactions are seen in response to various individuals in the
infant's environment, with pleasure or comfort associated with familiar
caretakers and fear or anxiety seen in response to strangers.

Neocortical Development
Although the limbic system is considered the seat of emotion and emotional
control, neocortical areas are also important in the development of
emotional response and regulation. As described previously, the frontal
lobes are the last part of the brain to finish developing, and continue to
change until adulthood. The frontal lobes allow the predominance of two
important behaviors that are relevant to the development of emotional
control: the ability to inhibit and the ability to anticipate (Persinger,
1987). The ability to inhibit enables us to control the impulses that arise
from the lower level limbic lobe- impulses that would lead us to eat,
express aggression, and have sex in a manner that would not be compatible
with living within a society. Along with the septal nuclei of the limbic
system and the amygdala, the frontal lobes contribute to regulation and
modification of emotional response. As previously noted, the most rapid
growth in the frontal lobe is seen during the first six years of life, when
social interactions result in understanding of social rules and
consequences that are crucial for developing optimal control over one's
impulses. This frontal lobe development may also be a contributor to the
increased emotional control seen in the so-called "latency aged" child, and
to the gradual increase in control that develops into early adulthood. The
differential rates of development of the cerebral hemispheres is another
factor to be considered in the development of emotion, although it will be
covered in more detail in conjunction with the development of thought. As
the functions of the hemispheres become differentiated, right hemispheric
activity is associated with greater emotionality than the left (Mahoney,
1991). This is thought to be due to the greater abundance of reciprocal
interconnections between the right hemisphere and the limbic system
(Joseph, 1982). Joseph argues that the left cortex develops before the
right, although the right may actually start earlier, but develop more
slowly and over a more extended period of time. Regardless, the relevant
implication here is that emotional regulation and specificity (associated
with the right hemisphere and it's connection to the limbic system) develop
more slowly and over a greater number of years than the capacity for motor
and verbal functions associated with the left hemisphere. Even the frontal
lobes appears to be split in terms of emotional representation. There is
some evidence that positive emotions are more commonly associated with
activity in the left, and negative emotions more often related to activity
in the right frontal regions (Buck, 1986). Emotion and thought are
virtually inseparable, and develop in an interdependent manner (Mahoney,
1991). With the development of cognitive ability, increased memory, and
growing associations, children develop more complex emotional responses.

For example, older children and adolescents experience emotions such as
guilt, envy, and embarrassment, that are not within an infant's emotional
repertoire (Rutter and Rutter, 1993). The more complex emotional response
known as "guilt" does not develop until shortly after the child's second
birthday, when a child is capable of appreciating standards and the
expectations of others that these be met. It is likely that the capacity
for development of this kind of complex emotional response is made possible
through maturation of the limbic system as previously described. The
complexity and specificity of the response, however, is dependent upon
development of the frontal lobes, right hemisphere, and upon cognitive
developmental processes which continue into late childhood, adolescence,
and possibly even adulthood.

Thinking is described by Joseph (1982, p. 4) as "a means of organizing,
interpreting, and explaining impulses that arise in the non-linguistic
portions of the nervous system so that the language-dependent regions may
achieve understanding." He also considers thought to be a form of language
which exists as an organized hierarchy of symbols, labels and associations
through which ideas, impulses, plans, objects in the environment, and
desires can be understood and possibly acted upon or prevented. Linguistic
thinking is therefore a process by which one accesses and organizes
information that is possessed within the brain so as to explain it to
oneself in language form. Thinking also occurs in feelings, images, musical
ideas and mixtures of associations which may be visual, verbal or both.
These associations may be coupled, through connections to the limbic
system, with an emotional tone that directs the entire process (Joseph,
1993). Because of their role in the communication between parts of the
brain, the following neural changes are relevant to a discussion of the
development of thought. The development of the "forth brain", as previously
discussed, involves the differentiation of two independently functioning
cerebral hemispheres, each with its own specialized functions. The
increasing maturation of intra-cortical and subcortical structures and
pathways corresponds with the development and internalization of language,
and the myelination of the corpus callosum results in increasing
information transfer between the two hemispheres.

Asymmetry of Cerebral Function
As pointed out by Mahoney (1991), it is now widely accepted that one of the
cerebral hemispheres (usually the left) specializes in higher order
symbolic processes such as language, mathematics and analytic logic. The
other hemisphere (most often the right) is adapted for dealing with space-
time relationships such as rhythm, form and synthetic operations. Joseph
(1982, p. 5) calls this lateralization the "hallmark of the human brain."
Although there is considerable overlap of functional representation and
expression, these two independent mental systems coexist side by side, each
capable of acting on information independently and without interference
from the other. They use different strategies for analyzing and expressing
information, and can transfer information across the corpus callosum for
further analysis. The left hemisphere uses predominantly verbal-analytic
strategies and the right uses primarily visual-spatial and
sensory-affective associational strategies. Although this specialization
results in increased range and speed of information analysis, Joseph (1982)
points out the potential for miscommunication and distortion that exists
because of the different modes of coding, processing, and storing
information. Transfer of information, even in adulthood when the corpus
callosum is fully mature, is sometimes inefficient or incomplete. The
development of two kinds of processing abilities in the two hemispheres
results in different modes of thought and different language or expressive
systems, linked by the slowly myelinating corpus callosum.

Development and Internalization of Language
Stern (1985) discusses the ability of six to seven month old infants to
recall memories for affective as well as motor experiences. He proposes
that infants can recall affective experiences before the development of
linguistic encoding vehicles, through other vehicles. This is consistent
with the neurological development of the limbic system, particularly the
maturation of the amygdala and hippocampus in the first few months of
postnatal life. As previously discussed, these nuclei enable the infant to
monitor the external, as well as internal environment, to form associations
between need states and events, objects, or people that bring pleasure or
displeasure, and to remember these experiences. These early memories are
thought to be stored in the form of images, feelings, and associations that
are not tied to language, or higher level thought, through limbic
structures and eventually through interconnections with the right
hemisphere (Joseph, 1982). As previously discussed, these emotional or
affective memories may drive the infant's behavior and form the basis for
further self-organization. Language is originally limbically based, and
this limbic language "heralds the founding drive from which all purposeful
and intellectual activities develop" (Joseph, 1982, p. 18). Limbic speech
is basically concerned with expression of moods, impulses, feelings,
desires, etc. and may be expressed in the form of crying, babbling, or
later, calling out "mama." This form of communication is primarily
emotional, automatic, and yet is symbolic since it serves as a command
and/or accompaniment to action. Initially these limbically induced motoric
responses do not signify the specific desire, state, etc. An infant's early
cry indicates discomfort and the caretaker figures out the specifics. This
limbic speech is social, however, and provides the context for
vocalization-experience pairings and the construction of schemas.
Maturation of the left hemisphere and its sequential, analytical, and motor
functions, together with external stimulating activities that enable the
infant to interact and develop associations, result in the development of
denotative speech. Denotative speech is concerned with naming and labeling,
stating fact or belief, and statements of assertion. This form of speech is
closely related to the eventual expression of thoughts, although thinking,
as defined by Joseph (1982; 1993) does not occur until much later.
Egocentric speech develops at approximately three years of age, and
consists of the child's self-directed verbal explanation of his/her own
actions to him/herself (Piaget & Inhelder, 1963). Initially, this
commentary occurs after the action is performed, and with progressing age,
the child explains the action during its performance and finally, before it
occurs. Shortly after the child develops the ability to access this
information before performing the action, at around age 6 or 7 years,
egocentric speech is almost completely internalized as verbal thought
(Joseph, 1993). Myelination of the

Corpus Callosum
The appearance and eventual internalization of egocentric speech occurs in
conjunction with maturational changes in the brain. During the first years
of life, maturation increases the influence of both hemispheres over the
subcortical areas. Little communication occurs, however, between the
hemispheres before age three and communication remains very limited until
age five (Joseph, 1982). This is thought to be due to the immaturity of the
corpus callosum, which connects the two hemispheres and is not fully
myelinated until the end of the first decade. Egocentric speech is
explained by Joseph (1982; 1993) as an intermediary between impulse and
comprehension, that enables the left hemisphere to label, associate, and
interpret information from action initiated by the right hemisphere,
information that it has no direct access to. Egocentric speech is a
function of the left hemisphere's attempt to make sense of behavior
initiated by the limbic system or the right half of the brain, by verbally
labeling it (Maclean, 1990). With maturation of the corpus callosal fibers,
information flows more freely between and within the two hemispheres. The
left hemisphere then uses language to linguistically organize its own
experience as well as the information received directly across the callosum
from the right hemisphere. As the connections between the hemispheres
myelinate, the left hemisphere is increasingly able to gain access to this
information internally rather than through external observation, and the
child begins to create linguistic organization internally as well. The
ability to think thoughts, as well as speaking them, develops (Joseph,
1993). Transmission of information between hemispheres allows the left
hemisphere access to the impulses-to-action originating in the right
hemisphere before the action occurs. Through linguistic labeling,
associating, and organizing, the analytical, sequential and reasoning
"thought" abilities of the left hemisphere can be used to anticipate and
influence limbic and right brain activity rather than simply making sense
of it after its completion. Through this thought process, the child
develops greater understanding and eventually increased control over
behavior. Through thought, the fully mature neocortex linguistically
organizes sensory-limbic right hemisphere initiated behaviors and impulses,
as well as impulses originating in the left hemisphere, so that they may be
carried out motorically in the most efficient manner. It is best to keep in
mind, however, that even fully developed interhemispheric communication is
never complete and that the ancient limbic system can override the
neocortex at times. Even a mature and controlled human being can
occasionally respond to pain with automatic rage (complete with limbic
speech in the form of utterances or curses) as the limbic system
overrides/bypasses higher levels of control.

Because of the previously described inability of the two hemispheres to
fully understand each other, we sometimes respond to limbic or right brain
stimulation in ways that are not accessible to conscious or verbal thought.
This leaves us saying "I don't know what came over me," and searching the
left hemisphere for ways to understand and explain our own behavior to
ourselves (Joseph, 1992; 1993).


As previously discussed, the most basic component of the psychic system,
the hypothalamus, is functioning at birth, and the first breath initiates
the development of the cortex. The newborn is capable of responding to
internal sensory stimulation with emotional responses indicative of the
positive or negative nature of the stimuli, but because of lack of higher
control, is unable to act to change the stimulation except through the
response of the caretaker. As the caretaker responds, and the young
infant's amygdala and hippocampus differentiate and myelinate, associations
between characteristics of events in the external environment and changes
in the internal need state develop. These associations are stored in
memory, and drive the further actions and experiences of the infant. In
this way, infants develop the ability to control their environment through
action, and do so largely in response to the emotional qualities of the
stimulus and the anticipated consequence.

With development of the higher nuclei of the limbic system, further control
and specificity of response to emotional stimuli develops and this
continues with maturation of the cerebral hemispheres and frontal lobes.
The emotional response drives the action which determines the sensory
experience. The characteristics of that sensory experience with associated
visual, auditory, and later verbal, images are stored in the memory of the
developing cortex, in association with the memory of the action. This
information is then used to develop more specific responses that will be
more adaptive in terms of meeting the survival needs of the individual in
the future. Thoughts consist of these stored images, patterns, feelings and
associations that are the organizational strategies of the right cerebral
hemisphere as well as the linguistically organized symbols, labels and
associations of the left. Through thought, the child attempts to understand
or make meaning of ideas, impulses, plans, desires and objects in the
environment so that they can be understood and acted upon in the most
adaptive manner. Once initiated at birth, the cerebral cortex develops
rapidly and the thought processes of the two hemispheres function dually to
interpret, analyze and store information. Maturation of the corpus callosum
which joins the two hemispheres enables the left hemisphere to organize and
understand information from the right directly, and to organize it in
verbal form. This occurs first in the form of spoken language, and as
communication between the hemispheres increases, internally in the form of

So, at the time of my infant's birth, my dog was more advanced in terms of
her ability to make sense of her environment and adapt her behavior to
survive. My daughter's limbic system and neocortex, however, rapidly
"caught up" with those of the shepherd, and the frontal lobes and
differentiated cerebral hemispheres soon surpassed those of the dog. My dog
has a limbic system and neocortex that provide a level of knowing that
enables her to determine that my looking up from my work might mean action
that will lead to pleasure, based on past experience. My return to my work
means only that this likelihood has decreased. For my daughter, the greater
degree of organization and association between events, in terms of meaning
and feeling, results in a more complex response. If I had gone back to my
work without responding to her query about the meaning of my pause, her
associations and schemas would have resulted in a response with a decidedly
emotional component. Previous experiences of my behavior, combined with the
expectations, future goals and anticipation made possible by her frontal
lobes, would have alerted her limbic system to the fact that her needs were
not being met as anticipated and that an adaptive response was required.
This response may have been limbic in nature, but would most likely have
been inhibited and regulated by the higher nuclei such as the amygdala to
follow social rules and maintain the tie with her caretaker (me). Further
modification of the response would have occurred at the neocortical level
as she may have responded verbally in an attempt to organize and understand
the discrepancy between her expectation and my action, to make meaning of
her negative emotion so that she could understand, through thought, that no
threat to her survival or comfort was present.

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