NEURONAL DEVELOPMENT, EMOTION AND THOUGHT

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 responses.

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).

SINGLE-CELLED TO MULTI-CELLED ORGANISMS

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.

THE NERVE NET

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.

ANCIENT LOBES

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).

CORTICAL DEVELOPMENT

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.

THE DEVELOPMENT OF THE NEOCORTEX

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?

BRAIN DEVELOPMENT - WHAT CHANGES?

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.

DEVELOPMENT OF EMOTION

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).

Hypothalamus.

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).

Amygdala.

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 functioning.

Hippocampus.

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.

DEVELOPMENT OF THOUGHT

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).

SUMMARY

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 thought.

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.

References

Barr, M. (1979). The Human nervous system. Baltimore: Harper & Row. Brown, J. W. (1990). Preliminaries for a theory of mind. In E. Goldberg (Ed.), Contemporary neuropsychology and the legacy of Luria. Hillsdale: Lawrence Erlbaum Associates, Publishers. Buck, R. (1986). The psychology of emotion. In J.E. LeBoux and W. Hirst, (Eds.), Mind and brain: dialogues in cognitive neuroscience, (pp.275- 300). Cambridge: Cambridge University Press. Cowan, W. M. (1979). The development of the brain. Scientific American, 241, 112-33. Gazzaniga, M. S. (1988). Mind matters: how mind and brain interact to create our conscious lives. Boston: Houghton Mifflin Co. Glezer, I.I., Jacobs, M., & Morgane, P.J. (1988). Implications of the initial brain concept for brain evolution in Cetacea. Behavioral and Brain Sciences, 11, 75-116. Hyman, L.H. (1942). The transition from unicellular to the multicellular individual. Biological Symposiums, 8, 27-42. Izard, C. E. (1978). On the ontogenesis of emotions and emotion- cognition relationships in infancy. In M. Lewis and L.A. Rosenblum, (Eds.), The Development of affect. (pp. 389-413). N.Y.: Plenum. Joseph, R. (1993). The Naked neuron.

N.Y.: Plenum Press. Joseph, R. (1992). The limbic system: emotion, laterality, and unconscious mind. Psychoanalytic Review, 3, 406-456. Joseph, R. (1990). Neuropsychology, neuropsychiatry, and behavioral neurology. N.Y.: Plenum Press. Joseph, R. (1988). The right cerebral hemisphere. Language, music, emotion, visual-spatial skills, body image, dreams, and awareness. Journal of Clinical Psychology, 44, 630-673. Joseph, R. (1982). The neuropsychology of development: hemispheric laterality, limbic language, and the origin of thought. Journal of Clinical Psychology, 38, 4-28. Maclean, P. (1973). The Evolution of the triune Brain. N.Y.: Plenum Press. Mahoney, M.J. (1991). Human change processes - the scientific foundations of Psychotherapy. N.Y.: Basic Books. Milner, E. (1967). Human neural and behavioral development. Springfield: Charles Thomas. Moore, J. (1982). Neurobehavioral sciences and their relationship to rehabilitation.

Rockville: The American Occupational Therapy Association. Piaget, J. & Inhelder, B. (1963). The Psychology of the child. N.Y.: Basic Books. Persinger, M. (1987). Neuropsychological bases of God beliefs. N.Y.: Praeger. Rutter, M., & Rutter, M. (1993). Developing minds- challenge and continuity across the life span. N.Y.: Basic Books. Stern, D. N. (1985). The interpersonal world of the infant. N.Y.: Basic Books. Turner, O.A. (1950). Post-natal growth changes in the cortical surface area. Archives of Neurological Psychiatry, 64, 378-84. Watson, J.D. (1979). The Double helix.

N.Y.: Penguin Press.