NEURONAL DEVELOPMENT, EMOTION AND THOUGHT
What is it in the human brain that sets it apart from the lowerlife forms, making its reactions and capabilities so much more complex?How do these differences develop and what are the relationships betweenneural maturation, emotion and thought? The purpose of this paperis 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 unchangedfor approximately forty thousand years (Gazzaniga, 1988), buthow did it get this way? Although theories of the origin of life itself vary,the evolutionof increasingly complex cellular organisms has progressed sinceshortly 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 withsingle-celled organisms that contained a single strand of DNA within theprotoplasm of their cells (Hyman, 1942). If one subscribes to the theoryof evolution, these prokaryotes were the basis for the eventual development ofmulti-celled organisms including birds, plants, cats, dogs, and even humans.The first single-celled prokaryotes reproduced by dividing and producingidentical copies of themselves and their DNA. Despite their cellularsimplicity and lack of consciousness, these cells were capable of sensingtheir surroundings and attending to those features of the environment that werenecessary for their survival. Even beyond this, these single-celled organismswere capable of communicating and cooperating in some fashion, and formedsingle-celled nations which benefited the individuals and the social group.This primitive ability to communicate and cooperate is thought to be due tothe presence of DNA (Joseph, 1993).
The first multi-celled organisms contained double strands ofDNA, 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-celledcreatures, DNA has served as the intellectual and memory center of allcells. It wasn't until one billion years ago that multicellular creatures engagedin sex,thereby indulging in more complex modes of communication (Joseph, 1993).Multicellular creatures ranging from the simple early organismsdescribed above to humans, exhibit the capacity to combine and interchangeDNA from one cell to another. This makes possible the creation of athird organism which, through the recombined DNA of its parents, carries someof the genetic plans and memories of its predecessors (Watson, 1979).
Even in the case of modern-day humans, all the tissues of ourbodies are derived from a single sexually fertilized ovum cell. Thisprimal cell divides through the process of meiosis, as its daughter cellsdivide after it, to form a complex multicellular human being. Based on the DNAinstructions within each nucleus of each daughter cell, cells are sent tospecific locations within the developing body and once there, form specific connectionswhich enable them to carry out certain functions.
The next important step in evolution occurred around 700 millionyears ago, with the cellular metamorphosis that resulted inthe creation of the neuron (Joseph, 1988). Rather than merely dividing in orderto pass on complex memories and life plans through DNA, neurons are capableof creating memories and plans and communicating this informationto other neurons. Neurons eventually developed dendrites and axons whichare highly specialized for reception versus transmission of electricaland chemical messages, making them capable of communicating more specificand differentiated information.
THE NERVE NET
Over the course of evolution, the number of these secretingand transmitting nerve cells increased in higher life forms. Theresulting organisms could now control, coordinate and direct their behaviorsin a much more sophisticated manner because different areas of thebody could communicate together almost simultaneously through what hasbeen called the "nerve net" (Joseph, 1988). As the interconnectionsof 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 efficienttransmission of information from one neuron to another.
The earliest nervous systems were nerve nets that were quiteindiscriminate in their responses. Activation of a single neuronexcited the entire network and the organism responded as a whole. The primitivenetwork evolved into the primitive nerve cord (such as thatseen in flatworms), and soon after, the specialized structure of "thehead" followed (Glezer, Jacobs, and Morgane, 1988). This differentiation ofone end of the nerve cord continued as enlargement and concentration of internalcommunication and internal exchange mechanisms (Mahoney, 1991).
Cell bodies migrated inward and gathered together over time,eventually forming collections of nuclei and the various lobesof the brain. The first two ancient lobes were the result of ganglia of like-mindedcells forming to serve two vital functions. The olfactory lobe developedto analyze olfactory, pheromonal or chemical information, and the opticlobe developed to analyze visual input. The expansion and axonal-dendriticinterconnections of these first two lobes has, over time, ledto the formation of the modern brain (Joseph, 1993).
From the olfactory system developed the limbic system, whichis concerned with basic survival needs: feeding, fighting, fleeing,and fornicating (Barr, 1979). As a growing number of nerve cellsaccumulated for the purpose of analyzing olfactory information, they beganto form layers which became the first layers of cortex. This olfactory-limbiccortical tissue eventually gave rise to the first motor cortex and to the twocerebral hemispheres that completely encase the remainder of the humanbrain. Much of the human brain has evolved from this ancient olfactory-limbicsystem (Maclean, 1990), a fact with much relevance for understandinghuman behavior.
THE DEVELOPMENT OF THE NEOCORTEX
According to the theory of evolution, about a half billionyears ago, many different types of vertebrates swam the ocean and plantsproliferated wildly over the earth's land surface. This was followed bya large variety of insects, amphibians and eventually the first dinosaurs. Overtime, the brain also evolved in response to the effects of the continuallychanging environment (Joseph, 1993; Mahoney, 1991). Up till the arrivalof the true mammals, around 100 million years ago, the sharks, reptomammals,dinosaurs, and birds possessed only two layered cortical motorcenters and limbic system tissue.
With the true mammals the neocortex developed outward, layerby 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 previouslybeen ruled by less intelligent, although more physically powerfullife forms. Over time neocortex continued to accumulate, and gyri were formedso that expanding cortical connections could fit within the confinesof the bony skull. A wide variety of mammals now exist, with varying amounts ofcerebral cortex adapted to meet specific environmental demands.
The limbic cortex (old cortex) is not terribly dissimilar inshape, location and size among mammals. The neocortical mantle thatcovers it, however, expands progressively as one ascends from primitivemammals to primates and then humans (Joseph, 1990). The limbic systemand "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 frontaland other lobes of the cerebrum, that makes our brains uniquely "human."Even our fellow primates who possess considerable neocortex do not have thebrain area that is considered essential for the production of complex spokenlanguage, the angular gyrus (Joseph, 1993).
So I can communicate with my dog, often unconsciously, becausewe possess much of the same limbic brain tissue. The older communicationsystem 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 theseolder systems also, but we can utilize the additional complex associative,anticipatory, planning and language systems made possible by more extensiveneocortex as well.
It has not always been this way, however. During her infancyI could not communicate with my daughter nearly as well as I couldwith 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 thathave on emotion and thought?
BRAIN DEVELOPMENT - WHAT CHANGES?
Cowan (1979) divided the neuronal development process intosix basic stages. First, the cell is generated; second, it migrates fromits birth citeto its terminal location; third, cells within specific brain regionsaggregate; fourth, axons and dendrites grow and cells differentiate; fifth, synapticconnections 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 overlappingphases. First, the main structure of the brain forms, followedby proliferation of brain cells. Next, cells migrate to their final destinationand simultaneously, synaptic connections increase to further elaboratethe neuronal network. Finally, as in Cowan's description, extensivecell death results in loss of about half of the neurons. This loss isthought to serve a fine-tuning function, and to be associated with increasingspecialization of function in different parts of the brain.
Along with the development and differentiation of this complexneuronal network, the myelinization of axonal fibers, and developmentof neurotransmitters occur at different rates in different portionsof the brain. As various fibers become myelinated, the functions that theysubserve 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 migrationof neurons and formation of billions of synapses can not be controlled genetically.Sensory input is thought to play a "driving" role in organizingneuronal development, and lack of relevant experience can have a lastingeffect on brain development. Because of the role of sensory input innormal brain development, the effects of environment (nurture) versus maturation(nature) are nearly impossible to separate out. The developmentalprocesses 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 thebody, the brain experiences its growth spurt during the prenatal periodand first few years of life. Moore (1982) describes how the fetal brain quadruplesin size by the end of the first trimester, and almost triples again bythe end of the third trimester.
The human nervous system begins as a neural groove which closesinto a neural tube by the forth week of prenatal life (Milner,1967). Milner describes how four flexures develop as the tube lengthens anddemarcate five subsections. Each of these subsections acts as a discrete focusof growth for a major nerve center. The lowest differentiates into the spinalcord, the oldest phylogenetically, and lowest functional level of the mammalianneuraxis. The remaining sections differentiate into the medula oblongata,the pons and cerebellum, the mid-brain, the diencephalic centers, and thetop segment of the tube becomes the cerebral hemispheres, which develop last.The brain attains all of its general structural features by the fourthmonth, and the order of the structure's emergence parallels the order of their phylogeneticappearance (Milner, 1967; Persinger, 1987; Joseph, 1993; Gazzaniga,1988). As we saw in the evolution of the brain, the lowest centers differentiatefirst, and the highest centers last. This is consistent even within thecerebral cortex, as the phylogentically oldest limbic lobe and hippocampus beginto differentiate before the neocortex.
As described by Joseph (1982), prior to the development ofcerebral cortex, primitive neuronal cell bodies (neuroblasts) migrateoutward to form an outer cortical zone called the primordial cortex. At aboutthe third month, this zone receives massive migrations of cells from the innerregions of the brain. The six concentric neocortical layers are not formedsimultaneously, so that their functions develop at differential rates. Thefirst layers to develop are the deepest layers, which consist of cortico-spinal tract(pyramidal) cells and subserve motor function. During this period the motor regionsof the frontal lobes and the deep layers of the temporal lobes (includinglimbic areas) begin to form (Milner, 1967). The second wave of migrationof neuroblasts into the primordial cortex results in formationof layers 2, 3, and 4. These layers receive specific sensory fibers directly fromthe thalamus or association fibers from other cortical regions. As such, theyhave predominantly receptive (sensory) functions. The final migratoryblast forms the most superficial layer. Joseph notes that these last threelayers do not differentiate completely until middle childhood.
As described by Milner, the motor (ventral) roots of the spinalcord begin to show myelin between the fourth and fifth months, andmyelination of the sensory pathways in the cord begins a month later. Myelinationof 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. Itmore than doubles in weight in the first year of life and reaches 90%of its adult sizeby age five years (Moore, 1982). Much of the complex process ofbrain development after birth is not well understood, or more intricatethan this discussion warrants. Therefore this account will present generaltrends in neuronal maturation that are believed to affect the developmentof emotion and thought.
In general, the progression of central nervous system developmentcontinues as seen in the prenatal period. Reflexes and feedbackloops (servomechanisms) become progressively more complex. Inhibitorycenters tend to predominate over excitatory centers, damping and modifyingexcitatory impulses to increase the complexity and specificityof responses. Growth, development and maturation begin in the cord and endin the neocortex. A hierarchy of control develops with higher level(later developing) centers exerting an inhibitory effect on lowerlevel centers and increasing the complexity of function (Moore, 1982; Joseph,1993).
At birth, spinal level structures are primarily myelinatedand brain stem structures responsible for maintaining life functionsin homeostasis are not yet fully functional. During the first month, physiologicalfunctions (breathing, EEG regularity, body temperature maintenance, etc.)stabilize and cortical functioning begins (Milner, 1967). The onset of aerobicrespiration at birth is thought to be the trigger for the spread of electricalconduction from subcortical to cortical cells, and once started growth in theneocortical lobesis rapid.
According to Turner's (1950) study of gross structural characteristicsof brain growth, the most rapid increases in cortical surfacearea in the first two years of life occurs in the parietal and frontal lobes. Theselobes are associated with the sensory, motor, language and speech functions thatundergo rapid behavioral change during early childhood. More moderate growthoccurs in the occipital and temporal lobes, which are associated withvisual perception and experiencing of sound, as well as functions of the limbicsystem. Between the second and sixth year, rapid growth is noted in the temporaland frontal lobes and after age six little or no growth occurs in any ofthe 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 progressesupward and outward. From the central life-sustaining functions ofthe spinal cord and brainstem ("reptilian brain") which are presentat birth, develops increasingly complex memory storage and ability of variousportions of the brain to communicate with each other. The brain develops asa series of four separate brains, each with its own memory, motor and otherfunctions (Mahoney, 1991). Each brain elaborates on the preceding leveland adds increasing degrees of organization and self-preservation capacityto the vegetative functions of the hindbrain, midbrain, and spinalcord.
The first "brain" described by Maclean (1990) isthis "reptilian brain." This part of the brain is responsible for primitive levelsof genetically transmitted knowing that result in repetitive and ritualisticmigratory, territoriality, aggression and courtship behaviors. Macleandescribes an important achievement of the reptilian brain as "homing",or the tendency to return to a recognized frame of reference after reaching outfor 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 andrefines life-relevant
behavior patterns (feeding, aggression, and reproduction) andis 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 involvesits own primitive form of reflective intelligence and self-regulatorycontrol.The third, or "neomammalian" brain, also known asthe "neocortex", accounts for 85% of the entire adult human brain. The frontalarea, which is associated with higher level mental organization, intentionality,and self-awareness, is over six times as large as that of non-humanprimates of similar size (Mahoney, 1991). Mahoney cautions against thinking that,because it develops later, the rational intellectual functions of theneocortex enable it to override or control the passions of the limbic brain. Althoughunder inhibitory control of the neocortex, parts of the limbic systemwith their primitive survival functions, can override neocortical controlas will be discussed later (Joseph, 1992; Joseph, 1993; Mahoney, 1991).
The fourth human brain is seen in differentiation of the neocortexinto two separate and independently functioning "higher brains"or cerebral hemispheres. In his original description of "the triunebrain," MacLean denied the need to describe this fourth level of independentbrain functioning, however the majority of modern neuroscientistshave disagreed (Mahoney, 1991). Differentiation of these four brain systemsand concomitant changes in emotion and thought occur primarily during earlychildhood, 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, "tomove." Emotionality is basically protective in function, and is closely related tomovement and action. It either promotes survival of the individual throughfight or flight responses or survival of the species through reproductive orsocial 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 majorsites for elicitation of emotional arousal (Joseph, 1993). The limbicsystem is described as the background of emotional tone (Moore, 1982), and is involvedin: 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 limbicsystem (Joseph, 1992).
The limbic structures receive projections from all sensoryreceptors which enables the individual to judge the appropriate responseto sensory input. In sufficient intensity any sensation (pressure, heat,sound, smell, movement, touch, etc.) will result in emotional characteristicsleading to approach or avoidance.
The limbic structures of primary importance for considerationof the development of emotion are the hypothalamus, amygdala, hippocampus,and the septal nuclei. These limbic nuclei functionally matureat different rates. Corresponding behaviors and capacities appear, overlaypreviously developed capacities, become differentiated, and become suppressedor eliminated as further neuronal development and myelinationoccur (Joseph, 1992).
The hypothalamus emerges and differentiates before all otherlimbic nuclei, and according to Joseph (1992), constitutes the mostprimitive and purely biological aspect of the psyche. It reacts in an on/offmanner to maintain pleasurable or avoid noxious conditions. The hypothalamusis largely concerned with monitoring the internal environmentand maintaining homeostasis in body tissues.
Emotions elicited by the hypothalamus are largely undirected,unconnected with events in the external environment, and consistof feelings such as aversion, rage, hunger, thirst, pleasure and unpleasure.The hypothalamus is functional at birth, however because of itslack of connections with higher order nuclei, has no way to mobilizethe infant for effective action. Newborns first experience or express themost powerful emotionality in response to bodily needs, tactile sensationsand loss of body support (Joseph, 1992). The earliest emotional responses consistof 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 thoughtthat newborns exhibited only undifferentiated emotions. Specificemotions such as fear, anger, and happiness were thought to emerge graduallyas a result of learning and maturation. More recent research reportedly demonstratesthat a range of different discrete emotions are present in earlyinfancy, although they undergo further differentiation with maturity and experience(Mahoney, 1991). With maturation of higher order limbic nuclei, the infantbecomes more aware of external reality, begins to differentiate andassociate externally occurring events, and forms memories. This results in the differentiationof more complex emotional responses such as surprise, fear, oranxiety. The context in which various emotions are elicited also changesover the course of development (Rutter and Rutter, 1993).
One of the nuclei most involved in more differentiated controlof emotion is the amygdala. During the course of evolution, thehypothalamus initially controlled and expressed raw and reflexive emotionalityin response to monitoring of internal homeostasis and basic needs. Thedevelopment of the amygdala enabled the organism to monitor and test the externalemotional features of the environment and to act on them (Joseph,1992; Joseph, 1993).
In the infant, as in phylogenesis, when the amygdala becomesfunctional it hierarchically takes over control of emotionfrom the hypothalamus. At birth, the hypothalamus signals pleasure ordispleasure in response to the infant's internal needs, but because of itsfunctional isolation, has no way to get these needs met. Over the course of the firstfew months of life, the amygdala and then the hippocampus develop. Thesetwo limbic nuclei enable the infant to monitor the external world, whileregistering and remembering events and objects (including people) associatedwith pleasure, or tension reduction.
The amygdala is interconnected with various neocortical andsubcortical regions, so it is capable of monitoring and abstractinginformation from the environment that is of motivational significance tothe infant (Joseph, 1992). The amygdala assigns emotional or motivationalmeaning to that which the infant experiences. The ability to distinguishand express subtle socio-emotional nuances including friendliness, fear,distrust, anger develops during the first several months with maturation ofthe amygdala. Because of the polymodal nature of amygdaloid neurons, thisstructure is involved in attention, learning, and memory as well as emotionaland motivational functioning.
The hippocampus is also associated with learning and memory,and complements and interacts with the amygdala in regard to attentionand the generation of emotional imagery. The left amygdala/hippocampusis thought to be involved in attending to and processing verbalinformation (Joseph, 1992). The right is involved in learning and memoryof motivational, tactile, olfactory, facial, nonverbal, visual-spatial,environmental and emotional information.
So, with the maturation of the amygdala and hippocampus overthe first few months of life, the infant is able to orient andselectively attend to the external environment based on hypothalamically monitoredneeds. He or she is increasingly able to differentiate what occurs externally,to determine what is satisfying, and to remember this information. Oncethese capacities are developed, further associations, memories, differentiationsand more specific and complex emotional responses develop as the infantinteracts with the environment. These emotional responses also determine thebehavior of the infant, and play a key role in the way the infant organizeshis or her experiences (Mahoney, 1991). Izard (1978, p.391) describesemotions as "the principle organizing factors in consciousness."
The septal nuclei, or septum, is interconnected with all regionsof the hippocampus, as well as projecting heavily throughout the hypothalamusand connecting with the amygdala (Joseph, 1992). It appearsto function in an inhibitory manner, dampening and quieting arousal and limbicsystem functioning. As such, it reduces extremes of emotionality andmaintains the individual in a state of quiet readiness to respond. In contrastto the amygdala which promotes social behavior, the septum counterssocializing tendencies (Joseph, 1992). With maturation of the septum, theinfant develops an increasing capacity for controlling emotional responsesbased on information from past or anticipated future experiences. Morespecific emotional reactions are seen in response to various individualsin the infant's environment, with pleasure or comfort associated withfamiliar caretakers and fear or anxiety seen in response to strangers.
Although the limbic system is considered the seat of emotionand emotional control, neocortical areas are also important inthe development of emotional response and regulation. As described previously,the frontal lobes are the last part of the brain to finish developing, and continueto change until adulthood. The frontal lobes allow the predominance oftwo important behaviors that are relevant to the development of emotionalcontrol: the ability to inhibit and the ability to anticipate (Persinger,1987). The ability to inhibit enables us to control the impulses that arise fromthe 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 withina society.
Along with the septal nuclei of the limbic system and the amygdala,the frontal lobes contribute to regulation and modificationof emotional response. As previously noted, the most rapid growth in thefrontal lobe is seen during the first six years of life, when social interactionsresult in understanding of social rules and consequences that are crucialfor developing optimal control over one's impulses. This frontallobe development may also be a contributor to the increased emotionalcontrol seen in the so-called "latency aged" child, and tothe gradual increase in control that develops into early adulthood.
The differential rates of development of the cerebral hemispheresis another factor to be considered in the development of emotion,although it will be covered in more detail in conjunction with the developmentof thought. As the functions of the hemispheres become differentiated,right hemispheric activity is associated with greater emotionalitythan the left (Mahoney, 1991). This is thought to be due to the greater abundanceof reciprocal interconnections between the right hemisphere andthe limbic system (Joseph, 1982). Joseph argues that the left cortex developsbefore the right, although the right may actually start earlier, but developmore slowly and over a more extended period of time. Regardless, the relevantimplication 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 capacityfor motor and verbal functions associated with the left hemisphere.
Even the frontal lobes appears to be split in terms of emotionalrepresentation. There is some evidence that positive emotionsare more commonly associated with activity in the left, and negativeemotions more often related to activity in the right frontal regions (Buck,1986).Emotion and thought are virtually inseparable, and developin an interdependent manner (Mahoney, 1991). With the developmentof cognitive ability, increased memory, and growing associations,children develop more complex emotional responses. For example, olderchildren and adolescents experience emotions such as guilt, envy, andembarrassment, that are not within an infant's emotional repertoire (Rutterand Rutter, 1993). The more complex emotional response known as "guilt"does not develop until shortly after the child's second birthday, when a childis capable of appreciating standards and the expectations of others thatthese be met. It is likely that the capacity for development of this kind of complexemotional response is made possible through maturation of the limbicsystem as previously described. The complexity and specificity of theresponse, however, is dependent upon development of the frontal lobes,right hemisphere, and upon cognitive developmental processes whichcontinue into late childhood, adolescence, and possibly even adulthood.
DEVELOPMENT OF THOUGHT
Thinking is described by Joseph (1982, p. 4) as "a meansof organizing, interpreting, and explaining impulses that arise in the non-linguisticportions of the nervous system so that the language-dependent regionsmay achieve understanding." He also considers thought to be a formof language which exists as an organized hierarchy of symbols, labels and associationsthrough which ideas, impulses, plans, objects in the environment, anddesires can be understood and possibly acted upon or prevented.
Linguistic thinking is therefore a process by which one accessesand organizes information that is possessed within the brain soas to explain it to oneself in language form. Thinking also occurs in feelings,images, musical ideas and mixtures of associations which may be visual, verbalor both. These associations may be coupled, through connections to the limbicsystem, with an emotional tone that directs the entire process (Joseph,1993).
Because of their role in the communication between parts ofthe brain, the following neural changes are relevant to a discussion ofthe development of thought. The development of the "forth brain",as previously discussed, involves the differentiation of two independently functioningcerebral hemispheres, each with its own specialized functions. The increasingmaturation of intra-cortical and subcortical structures andpathways corresponds with the development and internalization of language,and the myelination of the corpus callosum results in increasing informationtransfer between the two hemispheres.
Asymmetry of Cerebral Function
As pointed out by Mahoney (1991), it is now widely acceptedthat one of the cerebral hemispheres (usually the left) specializes inhigher order symbolic processes such as language, mathematics and analyticlogic. The other hemisphere (most often the right) is adapted for dealingwith space-time relationships such as rhythm, form and synthetic operations.Joseph (1982, p. 5) calls this lateralization the "hallmark ofthe human brain."
Although there is considerable overlap of functional representationand expression, these two independent mental systems coexistside by side, each capable of acting on information independently and withoutinterference from the other. They use different strategiesfor analyzing and expressing information, and can transfer information acrossthe corpus callosum for further analysis. The left hemisphere uses predominantlyverbal-analytic strategies and the right uses primarily visual-spatialand sensory-affective associational strategies. Although this specializationresults in increased range and speed of information analysis, Joseph(1982) points out the potential for miscommunication and distortion that existsbecause of the different modes of coding, processing, and storing information.Transfer of information, even in adulthood when the corpus callosum isfully mature, is sometimes inefficient or incomplete.
The development of two kinds of processing abilities in thetwo hemispheres results in different modes of thought and differentlanguage or expressive systems, linked by the slowly myelinating corpuscallosum.
Development and Internalization of Language
Stern (1985) discusses the ability of six to seven month oldinfants to recall memories for affective as well as motor experiences.He proposes that infants can recall affective experiences before the developmentof linguistic encoding vehicles, through other vehicles. This is consistentwith the neurological development of the limbic system, particularlythe maturation of the amygdala and hippocampus in the first few months ofpostnatal life. As previously discussed, these nuclei enable the infant tomonitor the external, as well as internal environment, to form associationsbetween need states and events, objects, or people that bring pleasure ordispleasure, and to remember these experiences. These early memories are thoughtto be stored in the form of images, feelings, and associations that arenot tied to language, or higher level thought, through limbic structures and eventuallythrough interconnections with the right hemisphere (Joseph, 1982).As previously discussed, these emotional or affective memories may drivethe 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 purposefuland intellectual activities develop" (Joseph, 1982, p. 18). Limbic speechis basically concerned with expression of moods, impulses, feelings, desires, etc.and may be expressed in the form of crying, babbling, or later, callingout "mama." This form of communication is primarily emotional, automatic, andyet is symbolic since it serves as a command and/or accompanimentto action. Initially these limbically induced motoric responses do notsignify the specific desire, state, etc. An infant's early cry indicates discomfortand the caretaker figures out the specifics. This limbic speech is social, however,and provides the context for vocalization-experience pairings and the constructionof schemas.
Maturation of the left hemisphere and its sequential, analytical,and motor functions, together with external stimulating activitiesthat enable the infant to interact and develop associations, result in thedevelopment of denotative speech. Denotative speech is concerned with namingand 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 occuruntil much later.
Egocentric speech develops at approximately three years ofage, and consists of the child's self-directed verbal explanation ofhis/her own actions to him/herself (Piaget & Inhelder, 1963). Initially, thiscommentary occurs after the action is performed, and with progressing age, thechild explains the action during its performance and finally, before it occurs.Shortly after the child develops the ability to access this information beforeperforming the action, at around age 6 or 7 years, egocentric speech is almostcompletely internalized as verbal thought (Joseph, 1993).
Myelination of the Corpus Callosum
The appearance and eventual internalization of egocentric speechoccurs in conjunction with maturational changes in the brain.During the first years of life, maturation increases the influence ofboth hemispheres over the subcortical areas. Little communication occurs, however,between the hemispheres before age three and communication remainsvery limited until age five (Joseph, 1982). This is thought to be due tothe immaturity of the corpus callosum, which connects the two hemispheres andis not fully myelinated until the end of the first decade. Egocentric speechis explained by Joseph (1982; 1993) as an intermediary between impulse andcomprehension, that enables the left hemisphere to label, associate, and interpretinformation from action initiated by the right hemisphere, informationthat it has no direct access to. Egocentric speech is a function of the lefthemisphere's attempt to make sense of behavior initiated by the limbic systemor the right half of the brain, by verbally labeling it (Maclean, 1990).
With maturation of the corpus callosal fibers, informationflows more freely between and within the two hemispheres. The left hemispherethen uses language to linguistically organize its own experienceas well as the information received directly across the callosum from theright hemisphere. As the connections between the hemispheres myelinate, the lefthemisphere is increasingly able to gain access to this information internallyrather than through external observation, and the child begins to createlinguistic organization internally as well. The ability to think thoughts,as well as speaking them, develops (Joseph, 1993).
Transmission of information between hemispheres allows theleft hemisphere access to the impulses-to-action originating inthe 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 influencelimbic and right brain activity rather than simply making sense of it afterits completion. Through this thought process, the child develops greater understandingand eventually increased control over behavior.
Through thought, the fully mature neocortex linguisticallyorganizes sensory-limbic right hemisphere initiated behaviors and impulses,as well as impulses originating in the left hemisphere, so that they maybe carried out motorically in the most efficient manner. It is best to keepin mind, however, that even fully developed interhemispheric communication isnever complete and that the ancient limbic system can override theneocortex at times. Even a mature and controlled human being can occasionallyrespond to pain with automatic rage (complete with limbic speech inthe form of utterances or curses) as the limbic system overrides/bypasseshigher levels of control. Because of the previously described inability of thetwo hemispheres to fully understand each other, we sometimes respond to limbicor right brain stimulation in ways that are not accessible to conscious orverbal 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 ownbehavior to ourselves (Joseph, 1992; 1993).
As previously discussed, the most basic component of the psychicsystem, the hypothalamus, is functioning at birth, and thefirst breath initiates the development of the cortex. The newborn is capable of respondingto internal sensory stimulation with emotional responses indicativeof the positive or negative nature of the stimuli, but because oflack of higher control, is unable to act to change the stimulation exceptthrough the response of the caretaker. As the caretaker responds, and theyoung infant's amygdala and hippocampus differentiate and myelinate, associationsbetween characteristics of events in the external environment and changesin the internal need state develop. These associations are storedin memory, and drive the further actions and experiences of the infant. Inthis way, infants develop the ability to control their environment through action,and do so largely in response to the emotional qualities of the stimulusand the anticipated consequence. With development of the higher nucleiof the limbic system, further control and specificity of responseto emotional stimuli develops and this continues with maturation of the cerebralhemispheres and frontal lobes. The emotional response drives the action whichdetermines the sensory experience. The characteristics of that sensoryexperience with associated visual, auditory, and later verbal, images are storedin the memory of the developing cortex, in association with the memory ofthe action. This information is then used to develop more specific responsesthat will be more adaptive in terms of meeting the survival needs of the individualin the future. Thoughts consist of these stored images, patterns,feelings and associations that are the organizational strategies of theright cerebral hemisphere as well as the linguistically organized symbols,labels and associations of the left. Through thought, the child attemptsto understand or make meaning of ideas, impulses, plans, desires and objectsin the environment so that they can be understood and acted upon inthe most adaptive manner.
Once initiated at birth, the cerebral cortex develops rapidlyand the thought processes of the two hemispheres function dually tointerpret, analyze and store information. Maturation of the corpus callosumwhich joins the two hemispheres enables the left hemisphere to organizeand understand information from the right directly, and to organizeit in verbal form. This occurs first in the form of spoken language, andas communication between the hemispheres increases, internallyin the form of thought.
So, at the time of my infant's birth, my dog was more advancedin terms of her ability to make sense of her environment and adapt herbehavior to survive. My daughter's limbic system and neocortex, however,rapidly "caught up" with those of the shepherd, and the frontallobes and differentiated cerebral hemispheres soon surpassed those ofthe dog. My dog has a limbic system and neocortex that provide a level of knowingthat enables her to determine that my looking up from my work mightmean 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 associationbetween events, in terms of meaning and feeling, results in a morecomplex response. If I had gone back to my work without responding to her queryabout the meaning of my pause, her associations and schemas would haveresulted in a response with a decidedly emotional component. Previous experiencesof my behavior, combined with the expectations, future goals andanticipation made possible by her frontal lobes, would have alerted her limbicsystem to the fact that her needs were not being met as anticipated and that anadaptive response was required. This response may have been limbic innature, but would most likely have been inhibited and regulated by thehigher nuclei such as the amygdala to follow social rules and maintain thetie with her caretaker (me). Further modification of the response wouldhave occurred at the neocortical level as she may have responded verbally inan attempt to organize and understand the discrepancy between her expectationand my action, to make meaning of her negative emotion so that shecould understand, through thought, that no threat to her survivalor comfort was present.
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