6. The Heritages of the Chordate Phylum

6.5 Human cognitive development

What makes humans special as a species? How do we differ from other species? According to Tomasello 8, primates differ from all other mammals in possessing a greater understanding of relational categories in the social domains. For their part, Hominids differ from non-human primates by their capacity to understand physical causality. In addition, non-human primates have difficulties in understanding the intentions and mental states of congeners. Finally, hominids lack the key mental tool that enables humans to create cultures. Hominids are unable to take someone else’s place, understand what someone else feels, understand what their congeners believe, read their emotions, their intentions and their goals, and use this capacity to imitate. Humans alone can learn by imitation and inform by teaching. Human infants are born with a capacity to imitate. Enriched by culture and by social interactions with parents, teachers and friends, this slavish capacity to imitate allows the accumulation of our ancestors’ adaptations and discoveries and leads to culture. Other animal species also show cultural traditions (e.g. hand clasping, grooming, termite fishing, etc.) but the details of the behavior are seldom passed on. A final difference is language, which provides an exact representational system for large numbers: parrots cannot count above 20.

On the macro level, many of the differences between human and other primate brains have long been obvious (fig. 6.13).

Afbeeldingen14

Figure 6.13. comparison between the brains of a human and a chimpanzee.

But size is not the whole story. A big brain is necessary but not sufficient to explain human cognition because Neanderthalers had brains equal in size or even bigger than ours (see fig. 7.7), yet they did not show any artistic development. Nerve cells in human brains possess microanatomical structures and enhancements in their wiring and connectivity that our cousins , the apes, lack. Brain size is one thing, and brain organization is something else. Some small-scale innovations are shared by humans and great apes, implying that they arose after the apes evolved, 15 million years ago, but before humans came on the scene, about 6 million years ago. These novelties show additional differences between apes and humans and these novelties are seen in parts of the brain implicated in advanced functions such as social cognition and language. The human brain is more than just a greater ape brain. In the visual cortex, located in the occipital lobe in the back of the brain, the nerve cells of the human brain are organized in a complex pattern very different from the simple vertical arrays of cells found in chimps and orangutans. Likewise, deep in the center of the human brain, the anterior cingulate cortex possesses long cells not found in such numbers and in such large size in primates. The cingulated cortex is thought to be involved in social cognition, as trust, empathy and feelings of guilt.

Why have the differences not been noticed earlier? It is all the fault of Darwin, who argued that humans were big-brained apes, and no one dared question this assertion. As a result, researchers have overlooked during decennia important differences between humans and apes, differences that they choose not to see.

6.5.1 Emotional self-control and problem-solving capacities

MacLean, in 1949, developed the concept of vertebrate brain evolution that held that the mammalian brain evolved in a series of concentric shells around an ancient reptilian core. The innermost of these shells was paleomammalian and included the limbic lobe, i.e. a broad band of brain tissue wrapping around the corpus callosum and including parts of the ventral forebrain. Broca believed that the great limbic lobe was primarily involved in olfaction, which led to the term rhinencephalon‘s being attached to this assortment of structures; however, later studies indicated that olfactory functions are restricted to only a small portion of the ventral part of the limbic lobe.

The outer shells were neomammalian and consisted of the neocortex. It is now clear that the mammalian cortex is homologous with the dorsal part of the forebrain in reptiles and amphibians and was a specialization derived from this area in the ancestors of mammals.

6.5.1.1 The cingulate cortex

The dorsal part of the great limbic lobe is the cingulate cortex, so named because it forms a cingulum or collar around the corpus callosum. The cingulate cortex was assumed to be a primitive structure that preceded the neocortex in evolution. The anterior cingulate cortex is indeed distinct from much of the neocortex but is similar to the motor areas of this neocortex. The anterior cingulate cortex is in fact a highly evolved and specialized neocortical structure9 rather than a primitive stage of cortical evolution, undermining the prevailing belief that the term limbic entails primitive functioning. As early as 1937, the cingulate cortex was known as the receptive organ for the experiencing of emotions. It has an important role in emotional self-control as well as focused problem-solving, error recognition and adaptive response to changing conditions. These functions are central to intelligent behavior.

In monkeys, the anterior cingulate cortex monitors performance and reward and readjusts behavior so as to optimize payoff. In humans, the anterior cingulate cortex is active when the subject is performing a task requiring focused concentration. The amplitude of the signal increases with task difficulty. When the subject is anxious, the signal disappears; when the anxiety is relieved, the signal is restored. The cingulate cortex is also activated when the subject is aware of having made an error. The anterior cingulate cortex is active when the subject generates word associations. This activation is linked to focused mental effort. The anterior cingulate cortex is involved in the maturation of self-control as the individual progresses from infancy to childhood to adulthood. In the classic condition of lack of self-control, attention deficit hyperactivity disorder (ADHD), the normal response of the anterior cingulate cortex to tasks involving conflicting or confounding information (this is the Stroop task. It consists for example in reporting the number three written four times) is absent. The activity of the anterior cingulate cortex is greater in subjects who have higher levels of social awareness. The discrimination of affects in faces, which contributes also to social awareness, also selectively activates the anterior cingulate cortex.

Cingulate-lesioned patients show reduced levels of spontaneous behavior. They produce fewer verbal utterances during interviews, produce shorter statements in a written task. Thus, the spontaneity of their behavior is reduced.

6.5.1.2 Neuronal spindle cells

The cingulate cortex of humans and other hominids contains a distinctive class of large spindle-shaped neurons. The enlarged prefrontal cortex of humans invades the ancient, so-called limbic cortex with these specialized spindle cells that emerge after birth. These cells integrate emotional and cognitive brain functions and create the ability to focus attention on a complex task.

No spindle cells are detected in 23 species of primates and 30 non primate species. Spindle cells are a novel specialization of the neural circuitry of the anterior cingulate cortex that originated in the common ancestor of humans and great apes, which would have been a dryopithecine ape living about 15 million years ago. This new circuitry probably augmented emotional self-control and focused problem-solving behavior in an ancestor of the great apes and their descendants.

The spindle cells integrate a variety of reward-related emotional and cognitive brain functions. They are involved in emotional self-control, which is particularly important when reasoning is practiced in social circumstances. Their function appears to be increased in persons with greater social insight and maturity.

The average volume of the cell bodies of the spindle cells varies as a function of relative brain size (encephalization) across humans and great apes. If one sets this encephalization arbitrarily at 1.0 for the chimpanzee, then, the size of the brain of the bonobo is 0.98, of the orangutan is 0.9, and of the gorilla is 0.85. The encephalisation of the humans, relative to body weight, is greatest at 1.4. There is a direct correlation between the size of the spindle cells and the relative volume of the brain: gorilla-orangutan-bonobo-chimpanzee, versus humans with far larger cells than the four hominids. In humans, the volume of the cell body of a spindle cell is four times larger than that of the average neural cells. Because cell body size is probably related to the size of the neural arborization, the arborization of the human spindle cells may be extensive and on a scale with encephalization.

This observation suggests that the spindle cells may have widespread connections with other parts of the brain and may serve to coordinate the activity of these diverse parts to achieve self-control and the capacity to focus on difficult problems. The concentration of the spindle cells is greatest in humans and declines with increasing taxonomic distance from humans. Thus, chimpanzees have more than gorillas, which have more than orangutans.

In humans, the spindle cells cannot be discerned at birth and first appear at four months of age. The emergence of the spindle cells in four-month-old infants coincides with the infant’s capacity to hold its head steady, smile spontaneously, track an object visually, and reach for that object. The spindle cells may participate in the neural circuitry responsible for these functions, which are related to focused attention and emotional expression.

The development of the spindle cells may be particularly influenced by environmental factors. These cells become tuned through experience and seem to thrive or suffer depending on stimulation or stress in early infancy. The survival of postnatally originating neurons is enhanced by environmental enrichment while stress diminishes the production of these neurons. The quality of parental care might also affect the survival and development of the spindle cells during infancy, thus influencing adult competence or dysfunction in emotional self-control and problem-solving capacity.

6.5.2 The mental development process

6.5.2.1 At birth: primitive unity of the senses

A fundamental difference exists between man and the rest of the mammals as far as perception of the world is concerned. In humans, there is a primitive unity of the senses: visual variables specify tactile consequences. This primitive unity is built into the structure of the human nervous system. Newborn infants have not had the time to learn the difference between solidity and immateriality. Yet, they touch and grasp objects without any fear or sign of disturbance. Virtual objects can be created in front of infants with polarizing goggles, screens and projection lamps, and these infants show signs of disturbance when the object they now try to grasp reveals itself to be unreal. Likewise, when an object is hidden behind a screen from an infant as young as 20 days old, he apparently knows that the object is still there and such an infant will show no surprise when the hidden object reappears. He will show surprise only when the object has been hidden too long, because he has forgotten its existence. So, solidity and permanence are built-in properties of the human nervous system.

6.5.2.2 Two months after birth: awareness of discrepancies

The cognitive development of humans is, in some of its aspects, controlled by maturation. Biological factors limit the earliest appearance of certain functions that are expressed only after a growth of the central nervous system. The human infant only becomes consistently reactive to discrepancy between two and three months of age. By that time, its interest will readily be roused when faced with objects slightly different from the objects with which it has gained familiarity. This is particularly true for girls and is accompanied by observable biological changes such as the electrical patterns of encephalograms.

6.5.2.3 Four months after birth: the object concept

A tremendous difference exists between infants younger than 16 to 20 weeks and those that are older. Younger infants, although able to recognize features, do not attach any importance to them as far as recognition of objects is concerned. They respond primarily to place and movement. For example, if a blue automobile starts moving, they will regard it as a different yet identical object to the stationary automobile: for them, two automobiles exist, one stationary in one place and this automobile disappeared and one automobile in movement, which they track. Thus, younger infants cope predominantly with spatial problems, not visual ones: if the moving automobile suddenly changes into a red bear, this red bear will not be perceived as a new object, provided the bear moves at the same speed as the automobile it replaces. For such young infants, the world is overcrowded: it is composed of as many objects as there are places and movements. If a young child is exposed to several images of its mother – and this can be done with mirrors – the child will happily react to all of the images. It has as many mothers as it sees. Infants older than 20 weeks recognize objects by their features: for them, the moving blue automobile is the blue stationary automobile and this blue automobile is not the red bear. Their mother, since they now recognize objects by their features, is only one mother. They get quite upset when they suddenly see three or four mothers simultaneously.

Babies less than 16 weeks old thus live in a world that is grossly overpopulated with objects, since an object becomes another object as soon as it moves. Then, suddenly, when 20 weeks old, they identify objects by their features: different features imply different objects. This attainment is of tremendous significance: it transforms the perceptual world of the infant into something very close to the perceptual world of the adult and simplifies this world more than almost any subsequent intellectual advance.

The object concept is not built into the neural system. It is outside its limits and must be learnt. There is evidence that primates never overcome perceptual errors. The primates, therefore, remain much at the level of 16 weeks old infants.

6.5.2.4 One year after birth: the thinking child

Before the first year of age has passed, the infant is capable of generating hypotheses about its experiences. It is at that time that infants abandoned by parents in the presence of strangers will start crying, because they cannot resolve hypotheses about their immediate future. Anticipation of events (such as toys colliding, balls dropping, etc.) is manifest and denotes a drastic increase in mental activity: towards the end of the first year of life, the child begins to think about unusual events, generates simple hypotheses and solves problems. Biological changes within the brain (spindle cells, see supra) have recently been detected, which correlate with the appearance of the thinking child.

6.5.2.5 Adolescence: synaptic interconnections

In bird species where the female does not sing, it is observed that some parts of the male brain are bigger than the female’s. Not that the number of neurons is greater, but each male neuron makes many more connections towards other cells. If a female is treated by testosterone, she begins to sing and the number of dendritic connections emerging from each neuron augments. In males that do not sing during wintertime, the size of some zones of the brain diminishes because the synaptic connections wither away.

From these observations, one postulates that the number of synaptic interconnections established by a neuron is an index of intelligence, since intelligence is, in a broad way, the capacity to establish multiple adaptations. In chimps, the number of cells present in the brain is superior to that present in the brain of humans. However, the synaptic field of these cells is much reduced compared to that of humans and here lies one essential source of human superiority in intelligence and memory. The memory process takes place by reinforcing a preferential passage for the nervous influx from one neuron to the other through the establishment of more synapses. This anatomical structure of a mnesic trace is possible as long as the cells are able to develop new synapses, that is, as long as they are plastic. The human brain increases 4 to 5 fold in size from birth to adulthood. The increase in size of the brain after birth is accompanied by a considerable reshuffling of the neuronal synapses as well as by a destruction of some synaptic connections. Our memory of early events during childhood is dim because the neurons responsible for these memories have disappeared. This observation ruins the Freudian hypothesis of subconscious memories accumulated during infancy: these memories are not repressed, they are nonexistent. This does not mean, of course, that striking early events all go into oblivion.

The human brain seems to be malleable until age 18-20. At that moment it becomes adult. Thereafter, one must use the existing synapses to establish new contacts. In childhood and adolescence, undernourishment and unadapted nutrition such as, for example, the absence of egg-yolk that provides the choline needed to synthesize acetylcholine, are a hindrance to the development of new synaptic contacts but the greatest influence is shown by appropriate stimulations: verbal plays will develop the language brain areas, equilibrium exercises will develop the equilibrium areas, etc. The more learning is accomplished before the age of 18 years, the richer the register of associate combinations and thus a certain aspect of intelligence. One should thus dissociate children and adolescents from “decision making” because they are, before the age 18-20 years, unable to cope with the challenge. This does not mean, of course, that the children should not be trained early in taking decisions, but that these decisions should not be taken into full account, until the training and time required to reach adulthood is completed. The reverse is also true and one should avoid channeling them too early towards a particular education, since until 18 years of age they are able to acquire unforeseen intellectual capabilities.

From that age onward, we begin to loose a few hundred neurons per day: they die and are not replaced. Yet, only those that are not working disappear. Those under constant activity stay alive and indeed work better with the result that the adult brain becomes more specialized by the loss of potentialities available in younger ages and not exploited and more efficacious by the stabilizing of his established intellectual aptitudes.

6.5.2.6 Critical periods

The notion that there is a critical period for learning in the first three years of life burst into public consciousness in April 1997. At that time, a White House conference concluded that, because a baby’s brain is still developing after birth, there was a need for programs to ensure normal learning experiences for underprivileged children. The conference certainly did not emphasize the importance of the 0 to 3 first years or the existence of critical periods. Critical periods are defined as time windows when the brain is not only receptive to acquiring certain information but needs that information for its normal development. There is overwhelming evidence for critical periods in human development but they are not sharply defined but taper off gradually. Critical periods occur in human visual development. The brain also has sensitive periods for different types of learning. Children form an emotional attachment to their mother during the first year of life. The children who are securely attached form better relationships with others later in life and the bonds of attachment must be in place by age 3 for such benefits to occur: during the course of human evolution, attachment would have been such a key to an infant’s survival that a critical period for its formation may be built into brain development. Another study that suggests the importance of the first three years was that children from underprivileged homes enrolled from birth in educational programs showed greater improvement in IQ than did those enrolled in the same programs in their early school years, i.e. from 3 to 5 years, showing that opportunities missed during the first three years cannot entirely be made up later. There also appears to be a sensitive period for language acquisition. Only foreigners who learn a language before the age of 5 speak as well as native speakers. Using brain imaging of people exposed to a foreign language delayed until 4 years after their birth, it is possible to show differences in the brain organization as a response to grammatical discrepancies. In those who learned the language before the age of 4, the response is entirely on the left side of the brain but later learners show more right-hemisphere activity, indicating that a late-learned language is incorporated differently from one learned early. Nevertheless, learning during adulthood is still possible: the time window tapers off but never completely closes.

6.5.3 Conditions for optimal development

6.5.3.1 Love

The mental development of the human child is heavily dependent on love. Emotional deprivation in childhood can lead the infants to an early grave. These deprived infants defecate more readily, diarrhea sets in and the children fail to gain weight. This biological damage in an infant is accompanied by psychological damage denoted by the fact that the child spits its food out to swallow it up again. The children show anxiety, have insomnia alternated by periods of stupor. They become listless, apathetic and depressed. This same phenomenon of depression and sadness can also be observed among highly evolved birds such as parrots, or mammals such as monkeys and dolphins, when held in captivity and neglected by their owners. They usually die very early. Emotionally deprived infants die readily between the age of 7 months and one year. If the child survives, he remains stunted.

In such cases of dwarfism, the anterior pituitary hormone that regulates the secretion of hydrocortisone (ACTH), which in turn regulates the metabolism of sugar, is remarkably low and this causes permanent damage to personality and intellect. Also abnormally low in such children is the level of growth hormone (somatotropin) that is secreted by the pituitary under the influence of insulin. This growth hormone is secreted most abundantly during the first hours of sleep. If the subject remains awake, the growth hormone is not secreted. One has not yet succeeded in establishing a correlation between the sleeping habits of deprived children and the presence of this growth hormone. These relationships are probably extremely intricate. One thing, however, is clear: the infant-parents relationship is crucially important in the development of the child.

6.5.3.2 Environment

The environment within which a developing brain bathes also has an influence on the ability of the brain to respond to stimuli. Distinct changes in brain anatomy and chemistry appear in rats placed in a livelier environment than their barren laboratory cages. Such an environment is provided by objects disseminated among their food, by longer periods of darkness that favor the activity of these animals, by more animals together in the same place, etc. The occipital part of the cerebral cortex of rats and mice that live in a rich and exciting environment is thicker than that of animals kept in dull places: it has greater weight, the cells and their nucleus are larger. These increases are restricted to the cerebral cortex only and are apparent in fully mature rats as well as in young, although it takes a longer time for mature animals to show the maximum effect. Formal learning, as opposed to enriched life, produces changes that are smaller and have a different pattern.

Information about intelligence is also gained by analyzing the richness of the song repertoire of birds. In singing birds, the song is learnt. What are innate are the cerebral structures capable to organize the song mechanism, but the mechanism itself is only the potential, it is triggered through imitation. An isolated bird, a deaf bird has only a poor repertoire of songs.

Captive-bred trouts have smaller brains compared with their wild kin. Wild trout brains outscore those of hatchery fish on seven of eight anatomical measures. The differences are probably due to environmental factors. Domestic fish are raised in an austere environment whereas their wild cousins must cope with everything from predators to unpredictable edibles.

Controlled studies of the type performed on birds and rats cannot be performed on humans. By inference, one should strive to provide the most enriching environment possible to any child. This is precisely what parents deny their offspring in underprivileged homes. Up to six months of age, the differences among infants in motor activities such as turning around, sitting, crawling, standing, etc. as well as cognitive development, are fairly independent of the child’s social class, ethnic origin and aspects of rearing conditions. By one year of age, differences in rearing conditions and experiences seriously affect cognitive functioning. Extreme deprivation is detrimental but the rat experiments do not address whether additional stimulation beyond that found in a normal environment is necessarily better.

6.5.3.3 Hormonal influences

One should, however, not fall into the error of thinking that attitudes, aptitudes and sentiments of adults are shaped in a decisive way by their experiences in the first two years of life. No one has ever shown that particular experiences in the first two years decisively produce a particular adult outcome. Dr. Mengele was loved by his parents, which did not prevent him from conducting gruesome medical experiments on prisoners in concentration camps during World War II.

Several clues indicate that there are biological aspects to entrenched antisocial behavior. Boys with low levels of heart rate and brainwaves are more likely to commit a felony later in life. This is because people with aggressive antisocial personalities have sluggish nervous systems and need to go to extremes to achieve stimulation. In their case, giving in to destructive impulses does not give place to normal feelings of inhibition and fear. In the case of antisocial behavior early in life, it seems that these children have a flawed biological response to stress: persistently aggressive boys have low levels of the stress hormone cortisol. This hormone normally rises with stress levels. Early emotional trauma may be the cause for a permanent alteration of the hormonal system that generates cortisol.

References

8. M. Tomasello: The cultural origins of human cognition. Harvard University Press, Cambridge, MA, 1999

9. J. Allman et al.: The Anterior Cingulate Cortex pp. 107-118, in Unity of Knowledge Eds. Damasio et al. ANYASc. 935, 2001

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