6.4.1 Emission and perception of signals
Organisms ranging from microbes to mammals make use of chemicals to find mates, recruit symbionts, deter enemies, and fend off pathogens. Our own human reliance on visual and acoustic signals notwithstanding, it is by way of molecules that most organisms communicate; the vocabulary of living things is overwhelmingly chemical in nature. Recognition of harmful chemical substances is of immediate importance to primitive organisms and does not require a sophisticated organ. The arthropods in general and the insects in particular elaborated much on this recognition of chemical substances. They rely heavily on the sense of smell for perception purposes and are sensitive to an extraordinary degree to the presence of chemical substances in the environment. For example kerosene, in minute quantities, drives lobsters in murderous frenzies, to the point of killing each other.
18.104.22.168 Pheromones in arthropods
Basic communication is established among these animals by the release of chemicals called pheromones into the external environment. Among lobsters and crabs, the female releases into the water a substance that induces the males to adopt premating behavior. This behavior is adopted regardless of whether the female is present or not. Among spiders, mites and ticks, the sex pheromone may be released in some species by the female, in other species by the male.
In primitive insects such as cockroaches, locusts and grasshoppers (Orthoptera), pheromones are responsible for courtship behavior but also for congregation of the individuals, and there is even a primer pheromone that synchronizes the production of adults in very localized populations. Pheromonal communication is also well developed among the fairly primitive termites (Isoptera). There exists for them a trail pheromone, a sex pheromone and a pheromone that regulates the production of queen and king in colonies.
These pheromones are active in minute quantities and effective at several hundred meters from the site of emission. For example, the sex pheromone released by the female Bombix mori 4 can trigger a search action by the male after only two hundred molecules have been perceived. Only one molecule of the attractant is sufficient to activate the male receptor cells. Yet, the specificity of this sex pheromone is such that only this particular attractant will do. The pheromonal way of communication is by no means an individualization factor to the members of the species since any male endowed with the adequate receptor cells will react positively while the slightest departure from the norm, be it in the attractant or the receptor cells, will result in a failure to act.
These chemical substances are usually very restricted and work for one animal species only and sometimes even for only one colony. Social insect colonies are so tightly integrated that they seem to function as single organisms. The colony is viewed as a super organism that protects itself with colony recognition systems based on odors. Among social insects such as ants, termites, bees and wasps, there exists a colony odor whereby members of a colony recognize each other. Yet, this recognition does not extend to the individual members of the colony. It is the colony as a whole that is recognized. An insect colony of Hymenoptera such as bees, ants or wasps is comparable to a diffuse organism as if the members of the colony were the various cells of a vertebrate body. The whole colony is an animal that forages over a fixed territory with a million tiny mouths. The queen-bee controls her colony with a pheromone released by her mandibular glands. Regarding the workers, which receive about 1/10th of a microgram of it per day, this pheromone evokes at least three different behaviors according to the context of its presentation: either rearing no queen-larvae or else not developing ovaries. The third effect is attraction of drones to a virgin queen.
Note that one single gene may determine a complex social behavior in insects. It is commonly assumed that many genes interacting in mysterious ways influence basic behaviors. Monogyne communities of fire ants permit only a single queen, and those with a resident queen kill off any intruding would-be queen. Polygyne colonies can contain as many as 200 queens and accept new queens from nearby nests. These differences depend on only one gene that single handedly determines whether a colony will have one or many queens, by controlling how ants perceive pheromones that tell them who is a queen and who is not.
The specificity of the “pheromonic language” is not always absolute. For example, the alarm pheromone released by an aphid attacked by a predator will cause nearby aphids to drop from their feeding sites on plants. This pheromone works interspecifically for at least ten different species of insects. Also, certain beetles have mastered the pheromonic language of ants and are able to parasite a whole colony with apparent impunity.
The arthropodal system of communication thus relies heavily on the use of chemical substances: releaser pheromones stimulate rapid changes in behavior, primer pheromones result in delayed behavioral changes and information pheromones indicate an animal’s identity or territory. This system is not sufficiently flexible to permit more than the most rudimentary transfer of information. It does not easily allow any modulation of the behavior or induced state. This rigidity is reinforced by the hormonal system in use by insects. This system also is extremely rigid and based only on a “yes or no” information carried by the hormones in use. Individualization of the members of an insect species by the modulating effect of hormones is rudimentary: within an ant colony, nothing is more similar to a worker than another worker. The eye of the arthropods, finally, allows only the most vague perception of the exact contour and shape of objects. However, it has been claimed that the wildly variable patterns of yellow and black stripes on the bodies of a species of wasps (Polistes fuscatus) are used to visually sort out their place in their pecking order within a nest.
22.214.171.124 Pheromones and olfactory indices in chordates
Pheromones and olfactory indices are operand in the chordate phylum. Protochordates have well developed olfactive lobes. Female sea lampreys must travel long distances up streams to reach the nests that their male counterparts have already built. The male lampreys release a bile acid that can signal their location as well as their reproductive status to females over such large migratory distances. Sharks have been found to possess an almost incredible chemosensory acuity. Their optic lobes are overshadowed by the receptors for taste and smell and the associated brain centers thereof. Catfishes have developed a very sophisticated social behavior based on chemical signals that rivals that of higher vertebrates in complexity. Pheromones allow catfishes to recognize members of their own species and a pheromone commands the communal behavior applied when many catfishes are together. The social status of these fishes is expressed through a chemical substance perceived by the other members of the group. The sex is also signaled by a pheromone, so that even blind fishes will allow only a fish of the other sex in their burrow. The fish of their sex fights intruders and the waiting partner accepts the winner of the contest in the burrow. The system of communication used does not allow the individualization of the various members of the group.
The newt, a common pond dweller salamander (batrachians), exudes odors for use as long distance lures and short-distance seducers. The males largely outnumber the females in breeding ponds. A male close to a female emits an odor in the water, which is repelling for other males.
Pheromones are also recognized among birds and mammals, up to the level of the rhesus monkey. There is no doubt that the human species releases chemicals perceived by other animal species. For example, parrots, dogs and monkeys distinguish human races, men from women and children from adults. Also, the menstrual cycle of nubile women living in close contact (mothers and daughters, nuns in a convent) tends to occur at the same time. A pheromone is probably also the reason why 25% of adult men who live in close contact with their pregnant mate also feel the discomforts (nausea, vertigo) of pregnancy, until their mate delivers. On the other hand, the truffles secrete a steroid that is a pheromone. The testicles and salivary glands of boars synthesize it and this is why sows are used in the Perigord (Central France) to discover the truffles. This steroid is also secreted by human testicles and is excreted at the level of the axillary sweat glands. This steroid makes the men more attentive to the charms of women.
Olfactory indices (not necessarily pheromones) allow a suckling to recognize his mother, an adult to differentiate strangers and a husband to identify his wife. Also, the odor of a 2 to 8 years old child revealed only through his shirt, is practically recognized by his mother and brothers and sisters. We have here an example of an individualizing odor that works especially to strengthen family ties. Up till today, the mode of chemical communication in humans is not known: for a pheromonal mode, there is a need to show that the communication is unconscious and proceeds via a specialized structure, the vomeronasal organ, located in the nose. A vomeronasal organ was discovered in Leipzig by Jacobson, in 1813. He found it in the cat, cow, dog, pig, tiger, camel, deer, etc. and was aware of the importance of his find. He communicated it to Cuvier (see fig.12.43), who totally ignored it. In humans, the vomeronasal organ is very reduced, almost nonexistent. This system of communication is almost totally abandoned by the human species and used only in very rare occasions such as intense fear: not only dogs perceive fear in humans when it is intense enough. Note that the monks of Mount Athos, a peninsula in the Aegean Sea populated solely by Orthodox adult male monks, pretend to recognize women solely by their odor.
These examples demonstrate that pheromones allow an interspecific communication. This is because pheromones are not available in unrestricted different molecular species; there is overlapping from one species to the other.
126.96.36.199 Emission and perception of sounds
Pheromones are not the only means used by insects for communication. Bees communicate through dance. Female vinegar flies recognize sexual partners by their song: their song betrays males of the same morphology but of a different species. The abandonment of chemosensory means of intra- and inter-specific communication for the more refined organs of hearing and vision allows the discernment of idiosyncrasies in the way signals are delivered, that are a magnitude higher than the possibilities offered by pheromones. For example, the recognition of sex based on a pheromone does not induce the sexual partners to adopt individualizing features because the mere possession of the adequate chemical substance is enough to signal one’s sexual affiliation. On the contrary, the specificity of the pheromones is such that the slightest departure from the normal substance is enough to be rejected.
Unlike insects and lower vertebrates, birds and mammals are able to recognize and distinguish each other as individuals on the basis of idiosyncrasies in the way signals are delivered. Communication implies the presence of a sender and of a receiver. A sound acquires meaning only by its structural stability, by the link it establishes between its own emission and a given particular situation (danger, food, female, etc.) and by the perceptual capacities of the receiver.
Bird song. Bird songs constitute signals of communication and serve to maintain a social life. The signals are subdivided into shrieks and song. Shrieks are brief signals emitted all year long by males and females. Each shriek is linked to a particular function such as the appearance of a predator, the feeding of the young, copulation. All bird species emit shrieks. Only the most evolved sing. Songs are emitted usually by the male only, at the moment of reproduction. In some species, males and females sing in duo. Each singing species possesses songs particular to the species, which allows recognition without sight and is useful when morphological differences among species are small.
If songs differ according to species, differences also exist at the individual level. Each bird has its own repertoire, and each region produces its dialect. This intra-specific diversity of song has its limits, needed for congeners to recognize each other. Stereotypes are manifest in all species. Precise rules command the construction of the songs in each species: the genetic program controls the learning of the songs. Oral tradition also restricts the variety of the songs. The offspring sings like the parents. Finally, ecological conditions also restrict the diversity of songs: in the forest, the flats are less muted than the sharps and birds of the same species living in these areas have a more deep-toned song than those of the Savannah, whose pitch is higher. If stereotypy is there to characterize the species, diversity is there also to avoid habit.
The song is essentially designed to facilitate copulations and, once a female has been won, to consolidate the bonds with her mate. Territoriality takes second place. Further, the song allows the recognition of direct neighbors, of members singing the same dialects and of members of the same species. In evolved bird species, song is definitely an individualizing agent of communication.
Primate screams. Another well analyzed vocal means of communication is that of primates. In prosimians, the emission of sounds proceeds via the nasal-vocal tractus. In more evolved hominoids, the production of noise tends to become buccal, ending up in the hominids with the differentiation of the pharynx.
Sound is the privileged means of communication among forested species because individual members are not able to see each other. In this case, the emitted sound, the message, must be as unambiguous as possible because it cannot be exactly specified by a visual confirmation: the scream will be “discrete” in order to be confounded by no other. These fundamental screams amount to a dozen to twenty in the most evolved species. This repertoire is, at least in part, shared by all the forested species: it is a common pool on which all the primates draw. Inter-specific screams are mostly found in cases where the identification of the emitting species is not necessary, such as alarm. When the identity of the group and of the species is at stake, or else reproductive isolation, inter-specific screams are banned, but not in an absolute way: they may be identical for two close species but emitted on a different rhythm, or in association with other differentiating sounds. All in all, one may be certain that a genetic transmission of the physical structure of screams is definitely there, at least in monkeys and apes. As with birds, a certain amount of learning takes place, which individualizes the emitter. The voice of adults is more profound than that of juveniles. The receiver is thereby able to judge the age of the emitter: young monkeys tend to scream “alarm” on any occasion. In some species (e.g. the cercopithecs), only the dominant male has a potent, profound voice. The other males are silent. In general, the vocabulary of the male adults becomes richer but is used less frequently. This phenomenon is not apparent with females.
The structure of a social group influences the use of a repertoire of sounds by different classes of individuals. In cercopithecs, where the social group is rigidly defined as a dominant male plus females and youngsters, the use of different specific vocalizations by different classes of individuals is maximal: in this hierarchized social system, the system of communication is simple. The dominant male uses only a very restricted repertoire from all the screams that are available to the species. He becomes specialized and the females do the same. Among chimps, where the social structure is loose and where hierarchical systems are difficult to pinpoint, all the adult males continue to produce the totality of the repertoire of sounds available to the species. This repertoire has been found to be about the same for all the primate species, indicating that the requirements of survival are about the same for all: access to food, protection against predators, alarm, protection of the social unit, reproduction, maintenance of territorial integrity, maintenance of reproductive isolation, coordination of routine activities, maintenance of social relations among all the members of the unit, raising young.
188.8.131.52 Language as an adjunct to reality
Communication over long distances by the use of sounds and hearing is not restricted to the birds and primates. Elephants communicate by ultrasounds that are heard at several kilometers from their place of emission. The cetaceans exploited to the fullest extent their auditory sense together with its almost obligate corollary for efficiency and meaning, vocal abilities. Whales communicate through sounds over distances of 2000 kilometers and dolphins tell each other how to initiate concerted tasks. For cetaceans and primates, roaming over large and deep oceans or immense steppes, the marking of a territory by olfactory labels (urine, musk and droppings) was not feasible. Like evolved birds, they resorted to sounds. This required the development of vocal capacities necessary to ascertain one’s existence and survival in a group. In the process they abandoned heavy reliance on the olfactory system, whose brain structures could be applied to other functions and explains their magnified intelligence.
The proliferation of sounds was initially only an additional tool evolved to control the normal range of behavior within these mammalian groups, as it is by crowing cocks, cicadas, howler monkeys, etc. The initial role of language was not one of communication but, like the other senses, was an adjunct to construct a reality.
The message given by these means is very rigid. Language, when used in this type of communication, is stereotyped. It is reduced to conventional yells, cries and shootings. If language had been devised for communication from the beginning onwards, one may expect it would have required little learning. It would have been as unambiguous as possible, rigid, resting on only a few small neural systems, as is the case for pheromones. The whole species would have spoken the same language but this language would have been poor and a hindrance for further achievements.
The challenge is to determine what was inherited unchanged by humans since we diverged from a common ancestor some 6 million years ago, what has been slightly modified and what is qualitatively new. Three issues are debated. The first is the apparent discontinuity between the grunts of chimpanzees and the rich expressive power of human language. Given this apparent discontinuity, how did humans cross over from “there” to “here”? A second issue revolves around whether the evolution was gradual or saltational: there could have been no discontinuities during human evolution. The last issue is the evolvement of language by a gradual extension of a preexisting communication system or else an adaptation, at least in part, from other functions, as spatial reasoning, social scheming or even tool-making.
184.108.40.206 Human language
The origin of language has been an evanescent topic in the history of ideas for many centuries. Rousseau, in 1749 and 1755, argued that language flows from emotions. In 1772, Herder suggested that language is an expression of human rationality. Thereafter, so many theories of language and evolution appeared that the 19th century Linguistic Society of Paris banned the inconclusive topic of the origin of language. Darwin created an empirical basis for what had been until his time a purely conceptual debate. He suggested that language emerges from more primitive emotional communication abilities in animals. The 20th century contributed little to the subject, by absence of a clear model that delineates what language is, and absence of fossilized evidence.
In the human species, language is the possession of a code of complex acoustic signals that essentially design elements of the external world. Galileo, Descartes and Von Humboldt have explicitly recognized the potential infiniteness of the system. The core property of discrete infinity is familiar: sentences are built up of discrete units, i.e. words, and there is neither longest sentence nor non-arbitrary upper restriction to sentence length. A phrase can always be elongated by embedding it into another sentence (e.g. Mary said that Johan thought that Joseph supposed that Alexis said that etc…). This capacity of language to generate an infinite range of expressions from a finite set of elements is called recursion5. This faculty to master the syntax of a language is the only uniquely human component of the faculty of language.
The learning of this code takes place during childhood and is placed under the control of the cerebral cortex. This is no longer true for subhuman primates for which the vocal behavior is essentially dependent from subcortical areas. The very weak intervention of the cortex signifies that monkeys are unable to exercise a voluntary control over their vocal expression. The young monkey, like the human infant, emits only sounds subcortically determined and will never be able to imitate and reproduce articulated sounds.
Why do infants, with their immature cognitive system, far surpass adults in acquiring a new language? The behavioral psychologist B.F. Skinner sought the answer in 1957. Skinner held that infants learn a language as a rat learns to press a bar in order to receive a reward. The children learn to talk by external reinforcement and careful parental monitoring. This nativist theory was vigorously opposed by Chomsky, who argued that traditional reinforcement learning had little to do with an infant’s ability to acquire language. He postulated that a “language faculty” included innately specified constraints on the possible forms human language could take. The infants possessed innate constraints for a universal grammar and universal phonetics. In Skinner’s view no innate information was necessary: the child learned by reward. In Chomsky’s view, infants possessed an innate knowledge of language. Its development was a growth of a “language module” and language input triggered a particular pattern from among those innately provided. The notion was that the linguistic experience produced loss of nonnative units; detectors stimulated by ambient language were maintained while those not stimulated by language input atrophied. In this view, the children maintain an initial ability. However, the method of investigation applied showed maintenance or decline in native-language abilities but could not demonstrate growth and could not reveal whether infants listening to native-language contrasts demonstrated a pattern of growth between 6 and 12 months of age.
Experience and culture play a unique role in the acquisition and use of language. Infants begin to acquire knowledge of language before ever uttering a word. Infants are not passive auditory receivers but are active participants, equipped with extremely sharp discriminative capacities and the ability to map properties of incoming signals statistically. Without saying a word, they build a language map from scratch as they explore their nascent world and await brain maturation that will allow them to generate speech and a mother-tongue bias. By simply listening to language, infants acquire sophisticated information about its properties. Firstly, infants show an extraordinary ability to detect regularities in language input: they organize input to recognize similarities and form categories. Secondly, infants exploit statistical properties of the input enabling them to detect and use probabilistic properties of the incoming signal. Thirdly, infant perception is altered by exposure to language in a way that promotes perception.
At birth, infants have been shown to prefer the language spoken by their mothers during pregnancy, as opposed to another language. This skill requires infant learning of the stress and intonation pattern characteristic of the language, information that is reliably transmitted through bone-conduction to the womb. The learning of speech patterns commences in utero: there is an infant’s preference for their mother’s voice over another female’s voice at birth, and their preference for stories read by the mother during the last 10 weeks of pregnancy.
By simply listening to ambient language, infants acquire information about the phonetic units employed by their language and the rules for combining sounds into words. They discover likely word candidates by statistically analyzing the serial and temporal aspects of language input. Infants accomplish this before understanding or producing a single word and before conceiving of the fact that objects and events in the world are named. The learning that ensues in the early period before speech alters infant’s perceptual systems, and this enhances the processing of a specific language. The unconscious speaking style that we use when addressing infants and children, “motherese”, not only appears to be preferred by infants but also is beneficial to learning.
Motherese is a very general strategy used by mothers all over the world that involves the exaggeration of the critical features in speech when addressing children. “Motherese” is characterized by a higher pitch, slower tempo and exaggerated intonation contours. This speech is also modified at the phonetic level in a way that aids infant learning. It suggests a much more important role for infant-directed speech than was hypothesized by either of the historical theorists. The consequences to the child are considerable. Exaggerated speech is a strong predictor of higher speech intelligibility. Clear speech makes words more discriminatory for infants, it highlights the critical acoustic parameters employed in the infant’s native language and provides “prototypical” phonetic units for infants. When given a choice, infants prefer to listen to motherese speech rather than adult-directed speech. Infants allowed choosing turn to voices of mothers talking to infants. It is very sad and deplorable that some parents consciously ban “motherese” talk to their infants, figuring unduly that this “motherese” harms their capacity to learn to communicate correctly whereas it is just the contrary that is true.
In humans as in birds, sensory representations of speech serve as a guide for the motor production of speech. In a 20 weeks old baby, perception and action are deeply interconnected at this early age, and the infant’s babbling is not idle play. Babbling and the auditory stimulation it provides allow infants to map the relationship between speech and motor movements and sound, a requirement for vocal imitation.
By nine months of age, infants exhibit a strong preference for the pattern typical of native language words and detect the patterns related to the orderings of phonemes that are permitted for their language. For example, in English the combinations zw or vl are not permitted whereas in Dutch, they are common (e.g. zwaluw i.e. swallow and Vlaanderen, i.e. Flanders). By nine months of age, but not at six months of age, English infants listen longer to English lists, whereas Dutch infants show a listening preference for Dutch lists.
A much larger number of the brain areas appears to be involved in language-processing than previously thought, areas that go well beyond the classic Broca’s and Wernicke’s areas (see fig. 6.10, 6.11 and 6.13). There is not one unified area for language generation but different cortical systems subserve different aspects of language processing and may be activated in parallel. There is a strong primacy of the left hemisphere over the right for the processing of language stimuli. Deaf persons process signed stimuli in the left hemisphere, although visual stimuli are normally processed via the right hemisphere, indicating that language processing by the left hemisphere is independent of the modality through which it is delivered. This bias toward the left hemisphere is not observed at birth but develops rapidly in infancy. The input that is eventually lateralized to the left hemisphere can be either speech or sign, indicating that it is the communicative nature of the signals rather than the specific modality that accounts for the eventual specialization.
In man, basic communication is still established by other means than articulated language. Fear, happiness, deceit, shame, pride, anger, etc. are readily perceptible through body and face attitudes and these attitudes and facial expressions do not usually lie. The deciphering of the mimics of the face was at one time so important that a whole area of the human cortex is dedicated solely to its comprehension. Now that the use of language made it obsolete, one may expect that in further evolution it will be diverted to other purposes. The legendary distraction of scholars is traced to this lost ability to decipher common mimics, attitudes and faces: the areas of the brain devoted to them have been, in the scholar, applied to other purposes. Another positive result of the abandonment of the olfactory means of communication is that the brain structures thereof may be redirected to other tasks, with a net increase in intelligence as a possible result.
6.4.2 Development of the human visual system
An important heritage of the chordates was the development of a system of perception of the external world based on sight rather than smell. Primitive members of the chordate phylum such as sharks still rely heavily on smell. The only other phylum that developed an organ of sight based on the properties of biconvex transparent lenses was that of the cephalopod mollusks (fig. 6.8).
Figure 6.8. The eyes of the cephalopod mollusks and of the vertebrates exploit the physical properties of biconvex transparent lenses. They are completely different from the eye-type evolved by the arthropods.
The human visual system is characterized by three features. Firstly, it is so exquisitely sensitive that, under optimal conditions, one light quantum can be perceived. Secondly, it recognizes colors. Thirdly, it allows a good perception of shapes. This last ability was perfected by binocular vision, which facilitates the evaluation of distances and perception of tri-dimensional objects.
220.127.116.11 The eye
The light-sensitive cells located in the eye of a vertebrate may be of three types. Rod-shaped cells discern shades and darkness. Cone-shaped cells are able to recognize colors. Animal species are endowed with one or both of these cell types. Bony fishes, birds, frogs, monkeys and man are able to recognize colors while cows, mice and cats, whose eye is devoid of cone-shaped cells, are color blind. The third cell type is able to discern the intensity of light received and commands the adaptation of the organism to the diurnal and seasonal cycles.
The presence of cone-shaped cells in bony fishes and their absence among mammalian species as disparate as cats, cows and mice suggest the initial ability of chordates to distinguish colors. Primitive vertebrates are animals engaged in daylight activities. Rod-shaped cells allow night-vision and are a feature that probably was acquired later in the course of evolution. The extraordinary sensitive rod cell perceives the arrival of a single photon of light. This vision was probably developed by the nocturnal and tree-dwelling archaic mammals, as a response to the heat wave occurring at the end of the Cretaceous period: activity was restricted to the night hours. Some mammal species that evolved later, like cats and cows, lost their cone-shaped cells in the process, and these species were thereafter forever colorblind because lost adaptations do not reappear.
This visual system of the chordates is not perfect; yet, this imperfection is hidden by the fact that we enjoy binocular vision. If the reader closes his left eye and looks at the cross of the figure (fig. 6.9) while holding the image at about 20 cm from the eye, he will only barely be able to see the relatively large circle. This is due to the fact that the place where nerves penetrate the eye is devoid of visual cells. For animal species that do not enjoy binocular vision, the existence of the blind spot may be a serious problem.
Figure 6.9. Imperfection of our visual system. The left eye of the reader should be closed. If he looks with his right eye at the small cross, from a distance of 20 cm, he will not see the large white circle. The eye has a “blind spot”.
The Chordate eye has another imperfection. The retina’s receptor cells are linked to a network of ganglion cells. However, these nerves are not located behind the photoreceptors but in front of them, where they screen out some of the incoming light. By contrast, the eye of the squid carries nerve cells hidden behind the photoreceptors, as it should. The eye of the squid is therefore better engineered than the eye of the humans.
The ability to perceive as little as one quantum of light brings about the problem that perception of all the light received on the retina will jam the system. There exists a screening and sorting out of the visual input. This elimination of information takes place at three different levels of the “eye-brain” system. Information about the outside world does not enter the mind as raw data. Our visual perception is filtered through three stages, in which the data are processed in terms of straight parallel lines (see fig. 6.7).
This deletion process leads us to perceive the world in the form of abstract structures. Yet, this transformation occurs at the preconscious level according to a preexisting program of deletion, epuration and arrangement of the information received, over which we have no control. We are endowed with a preexisting mental structure. Primary visual data become meaningful only if transformation can occur according to a pattern that matches the mental structure. Due to this, our mental equipment is most compatible with a geometry based on straight parallel lines and plane surfaces, which are our primary spatial concept. Sculpture in the round is not easy to master (fig 15.6 versus fig. 15.7) and some civilizations, like Islam 6, show a deep-seated inability to conceive three-dimensional forms.
18.104.22.168 Binocular vision
To appreciate three-dimensional forms, binocular vision is needed. The appreciation of three-dimensional forms is essential for a fruitful arboreal life and also for the making of tools.
Most vertebrate species (rabbits, pigeons, chameleons, horses, etc.) have their eyes placed in such a way that they cover a maximum field. The judging of distances with one eye is possible only through the use of indirect cues but is never very accurate or always possible. This initial need for panoramic vision, so necessary to perceive incoming danger, could be lessened with the acquisition of security, be it by taking to trees, as did some serpents and the primates, or by the adoption of predatory habits, such as those of eagles and felines. These animal orders are endowed with binocular vision. Both eyes look in the same direction and their visual fields overlap to a considerable extent.
Binocular vision makes it possible to use a depth cue more direct and accurate than the depth cue available to one eye only. Depth sensation providing precise localization of objects in visual space is the raison d’être of binocular vision. It is most valuable to tree-dwelling animals, which jump from one branch to the other.
This convergence of the eyes into a frontal position is accompanied by a rearrangement of the optic nerve fibers in the brain; Isaac Newton first proposed in 1704 AD that the optic nerve fibers from roughly corresponding regions of each eye converge on a single site in the brain (see fig. 6.7). The exchange of optic fibers takes place in the optic chiasm: the number of uncrossed fibers in this chiasm depends on the amount of overlap of the two visual fields. The number of uncrossed fibers tends to increase as animals evolve with eyes occupying a more frontal position. In the rabbit, most nerve fibers cross over; in the cat, many fibers no longer cross over. In man, where there is an almost complete overlap, 50% of the fibers remain uncrossed. As a result, the visual cortex lying at the back of the brain receives excitatory impulses from both eyes. The difference between the two views obtained by the two eyes, this horizontal disparity existing between the two retinal images (binocular parallax) allows the visual cortex to make an accurate estimation of the distance of an object.
In mammals, binocular vision was a must for primates. Yet, no mammalian trend towards arboreal life would have been successful if suitable angiospermic trees had not been available. Primitive tree species such as pine or spruce trees, i.e. gymnosperms, were definitely inadequate for the initiation of such a life and more evolved trees than angiosperms, such as the palm trees, appear equally unsatisfactory for the emergence of such a life. The adoption of an arboreal life by primates was thus possible due to a unique concurrence of events. Binocular vision and hands were indispensable to attain the level of the Australopithecine, able to make tools. Additional modifications of the brain were, however, needed to gain an evolutive level at which the integration of language in the other neural means of perception and construction of reality could be realized for the attainment of a plastic and flexible means of communication that would allow reading and writing. For this last achievement to be reached, the asymmetric development of the brain of primates, observed also among birds and rats, was exploited.
6.4.3 Human left cerebral dominance: reading and writing
An asymmetry of the brain is by no means restricted to the human species. Birdsong is neurally controlled in a unilateral way. The mountain gorilla is endowed with a gross asymmetry of the skull. An asymmetry of function in the brain of the gorilla is observed during chest beating. One-sided behavior is common in man, who is left- or right-handed. Only one half of the brain controls learned behavior. The development of speaking and reading abilities is in close relation with the left part of the brain. Destruction of specific areas or accidental damage of the left hemisphere lead to disorders of the spoken language, which are only rarely observed when the corresponding areas on the right side are destroyed. The right hemisphere is adapted for the perception of holistic and synthetic relations. The left hemisphere is specialized for analytic and serial processing of incoming information. This left dominance rests on anatomical differences that are not observable in the intact brain but well within the Sylvian fissure (fig. 6.10).
Figure 6.10. Anatomical differences between the two hemispheres of the human brain are found on the upper surface of the temporal lobe, within the Sylvian fissure (see fig.6.11). When the top portion of the brain is cut away by passing a knife through the fissure, the uncovered planum temporale (Wernicke area) is larger in the left hemisphere than in the right.
At that level, one sees that the left “planum temporale” i.e. an extension of the Wernicke area, is one third larger than the right one in 65% of all human brains examined. The right planum temporale is larger in 11% of the human population examined and both hemispheres are equal for 24% of the population. This asymmetry is observable also in infants. It may be more accentuated in some races than in others. It is generally thought that the left hemisphere alone is important because it is known to be the language-bearing side of the brain. However, rhythm, tone and emotional content of speech are processed in the right hemisphere, which also changed considerably during hominid evolution, beginning in Australopithecines, whose frontal lobe began to expand above the nose. All in all, three dramatic bulges are observed in the right hemisphere and two in the left hemisphere of humans, not observed in chimpanzees.
The relation between anatomy and function is not absolute. In children especially, a destruction of parts of the left hemisphere will lead to a taking-over of some speech functions by the right hemisphere. Also, left-handed people show in general milder disorders than expected when the speech region is damaged, although their left hemisphere is also dominant for speech. Behavioral and physiological tests, as opposed to anatomical measures, indicate that 96% of all people tested have left hemisphere speech dominance while music is analyzed in the auditory region of the right hemisphere.
The comprehension of written language requires the establishment of connections from the visual regions to the speech regions, so as to obtain the conversion of a visual stimulus into the appropriate auditory form. To say the name of a seen object involves six different operations (fig. 6.11).
Figure 6.11. Saying the name of a seen object involves six different operations. It starts with a visual stimulus (1) at the visual cortex (2) and ends up on the face area that commands the movements of the muscles of speech (5 and 6).
Writing is also among the most elaborate acts an organism can perform since it entails the transmission of information through a printing of conventional signs and requires for its performance a good development of the eye, the brain and the hand. Only Homo sapiens can accomplish these operations. It is only very late in historical times that the demand to correctly read and write extended to the near totality of the members of developed societies. A sizable proportion of infants, and also of adults, is unable to learn to read correctly. They are dyslexics and usually also left-handed. These people, who frequently lag behind in their school years, are by no means stupid. An anatomical examination of the brains of left-handed people shows that there is no inversion of the asymmetry of the brain but rather a diminution of the current asymmetry. Left-handed people have more symmetric hemispheres, in that the right planum temporale is usually as large as the left.
One hormone that certainly bears a responsibility in this asymmetry is testosterone. A deficiency of this hormone during their fetal life in boys leads to verbal capacities characteristic of females, while girls who received an excess of male hormones during their fetal life demonstrate a mastery of space generally attributed to the male sex. An excess of testosterone during the second half of fetal life would be responsible for the development of the right planum temporale to a size equal to the left one and lead to a symmetry of the brain that may cause left-handedness. An overdose of the hormone leads to the overgrowth of nervous cells, which provoke dyslexia but favor the development of other, desirable capacities.
The capacity to learn to read was not foreseen by human evolution. A departure from the norm, through an excess of testosterone during fetal life, leads on one side to superior abilities such as athletic capacities, mathematical prowess or other, but on the other side brings unending suffering to the gifted child who has to learn to write and read under the rule of teachers and parents who try to normalize the infant. Einstein is an excellent example of this difficulty for a superior brain to adapt to the norms.
6.4.4 Brain evolution and intellectual capacities
Memory is the capacity to stock in the nervous system informations on the surrounding world, enabling to modify one’s subsequent behavior and adapt. It is an essential mental capacity for our daily conduct. What would our daily behavior be without memory? Six different types of memories are known. The most elementary type of memory is accustoming. Accustom to a noise, to an odor, to wind, to food, to what have you. It is accustoming that allows people to sleep along highways and in the vicinity of airports. Accustom is a simple learning process and is found among unicellular organisms, i.e. in an animal phylum devoid of a nervous system. Accustom must, therefore, rely on another mechanism than a neural one.
A more complex type of memory is alternation. It consists in taking a second choice after one has explored a first avenue. For example, if I am walking in the streets of an unknown town, I may take first left and, on a second occasion, take right. This implies that I remember having already explored the left avenue. This memory also is found among unicellular organisms.
Third in complexity is the Pavlovian conditioning. It consists in associating an initially neutral signal (e.g. the ringing of a bell) to a significant stimulus (the giving of food), to produce a defined response (a dog salivating). The dog will salivate on hearing the bell only after the association between ringing the bell and the offering of a meal has been reinforced by repeat association and the creation of a memory. More complex is the Skinnerian conditioning. It consists in adopting a particular conduct that is followed by a reward or a punishment. These two types of conditioning are not assimilated by simple multicellular species as corals and jellyfishes but are found among worms, mollusks, arthropods and vertebrates.
Fifth in complexity is the learning of detours. It requires from the animal to remember the space in which he moves, enabling it to wander around and find its way back, even if it is not the path it took when it left its lair. This type of memory is reserved to the cephalopods (squids) and the vertebrates.
The ultimate complexity, normally reserved only to vertebrates, consists in the memorization of rules of conduct that simplify the life of the animal living under changing conditions. This superior type of memory is strongly associated with intelligence and will be exposed at some length infra.
The belief that intelligence is restricted to the chordates is erroneous, as is the erroneous belief that intelligence is restricted, within the chordates, to those animals which are endowed with a hypertrophied cerebral cortex, i.e. the mammals and more specifically the primates. Some mollusks species and bird species also possess a considerable level of intelligence. Octopuses are evolved mollusks. They are as able to learn as chordates under the influence of reward and punishment. Like all brains of protostomes, the octopus’ brain surrounds the esophagus and consists of nervous ganglions. In the case of the octopus, the development of the nervous system is designed, as usual, to adapt the animal to its environment: its optic lobes are as big as the totality of the median nervous lobes (see fig.5.5).
Learning, by an animal species, is focused on the tasks the animal is likely to encounter. The animal follows an innate guidance that enables it to recognize when it should learn. It is specialized in what cues it should attend to. Animal species are intelligent in the ways natural selection has favored and they are deficient in the ways not foreseen by their life-style. This specialization enables for example a chickadee to remember the locations of hundreds of hidden seeds whereas we, performing the same task, would remember the location of no more than a dozen.
Laboratory tests designed to measure intelligence are to be extrapolated only with the utmost caution since they do not take into account the whole of the environment of the animal under experiment. Also, differences in intelligence among the various members of a race and of a species must also be taken into account. However, the asking of simple questions such as the relation existing between the brain structure of an animal and its capacity to adjust when put into an unforeseen situation can be validly answered provided no extrapolations are made from such observations to the survival potential of the examined species.
22.214.171.124 probability: adjustment to unforeseen conditions
In tests devised to establish the relationship between brain structures and the intellectual capacity of an animal to adjust to unforeseen conditions, the question is not how fast an animal will learn a task such as to push the adequate button in order to get food. In this case, memory plays a big role and memory depends in part on the global size of the brain. The correct question is how long it will take the animal to reverse the acquired habit and push the other button, which will now grant the expected reward. The animal is supposed to solve a visual problem when the two buttons have the same shape but a different color. Spatial problems are those involving identical buttons located in different places of the cage or tank. The capacity of the animal under experiment to master problems involving probability may be evaluated with such a test. This is done by having the expected reward not systematically granted each time the correct button is pushed. It is possible to program a reward only 7 or 8 times out of ten correct actions.
Bony fishes are unable to cope with these problems. This is true for visual as well as spatial problems. Once a habit has been acquired, fishes show no improvement in the course of time in learning to reverse this habit. It will take them as long to reverse the acquired habit the 150th time the reversal occurs, as it took them the first time. Just like the fishes, the turtle shows no improvement when a habit reversal concerns a visual problem. However, the turtle copes with reversal when spatial problems are presented. The contrary occurs with the octopus, which does not master problems of a spatial nature but copes with the reversal of a visual habit. Finally, birds and mammals (pigeons, rats and monkeys) improve on habit reversal for both types of problems.
The improvement of the ability to adjust is not gradual within the chordate phylum. In fact, new brain structures serve to mediate new modes of intellectual adjustments. The ability to cope with visual and spatial problems, especially when probability is involved, gives a species the possibility to liberate itself from the impervious rules of the instinct.
126.96.36.199 Object recognition
Tests designed to evaluate intelligence analyze the ability of the animal to choose a correct object among a set. This test is extremely difficult to learn. The performance of mammals other than primates is consistently very poor. The better developed the cerebral cortex of the primate, the greater its capacity to answer correctly faster: worst performers are the marmosets, then come the New World Cebidae (the capuchins) followed by the Rhesus monkeys. These need about 50 to 60 trials initially to learn to recognize the correct object. Chimps are best. The pigeon is by no means the most intelligent bird that could be used for such an intelligence test. In these tests, the raven, crow, magpie, mynah bird and parrot surpass the pigeon. Yet, the chicken and the pigeon outdo all the non-primate mammals and the chicken outdoes the marmoset.
Another type of intelligence test consists in the recognition of an odd object in a set, as a circle between two squares or a bowl between two shoes. The cat recognizes a circle among two squares and thereafter a square between two circles. It can however not effectuate the transfer of learning to shoes and a bowl. Monkeys and the canary can.
The ability to count is extremely difficult to master. Some primitive human populations cannot count above three or four. It takes about 21,000 trials to teach a monkey to distinguish between the sound of two tones and that of three tones. But pigeons are able to distinguish between 4 stimuli and 5. Ravens, parakeets and Ara parrots can count up to 20. When visual instead of auditory stimuli are used to monitor performance, rhesus monkeys scored almost as well (76%) as American students (94%) in the addition of two small (less than twenty in total) numbers of peanuts put in two bowls.
188.8.131.52 Behavior tests
Finally, behavior tests can be performed where the animals are presented with two bowls, one of the bowls containing food. The two bowls are then transported to two corners and the animals are scored according to the fact that they go to the food, to the empty bowl, or adopt an aimless behavior. In these tests, rabbits are better than pigeons. Hens are superior to rabbits. Crows, magpies and ravens are superior to the cats and almost as good as dogs.
184.108.40.206 The seat of intelligent behavior in birds
The intelligence of certain birds is thus in various fields consistently equal or superior to that of mammals, including primates. In the management of numbers, certain birds are superior to the most intelligent hominid still existing besides man. The seat of superior intelligent behavior does not have to be the cortex, since this part of the brain of birds is very reduced. Through brain surgery, it was found that the seat of intelligent behavior in birds is the hyperstriatum. This hyperstriatum evolved during differentiation of the birds from the reptiles and does not exist in mammals. The more voluminous this hyperstriatum, the more intelligent the behavior of the bird (fig. 6.12).
Figure 6.12. The evolution of the brain followed separate lines among birds and mammals. Both groups stem from reptilian ancestors, here represented by an alligator’s brain. The striatal tissue (corpus striatum) is more abundant than the cortex in this reptile. In birds, this trend climaxes with a hyperstriatum produced from the floor of the olfactive lobes. In mammals, the pallium, emanating from the roof of the olfactive lobe, developed into the cortex.
220.127.116.11 The seat of intelligent behavior in Man
In humans, different regions of the brain undoubtedly specialize in different activities. Speech and spatial ability emanate from the cerebral cortex, emotions from certain structures in the limbic system. An electric “prick” that stimulates the limbic system at the base of the brain may make the patient feel anxious, furious, cheerful or depressed. The area in charge of the higher functions of thinking, imagination, planning for the future, lies in the associative cortex which includes most of the frontal lobes and all of the cortex not devoted to sensing the outside world or moving the body. People with damaged frontal lobes cannot plan ahead, cannot pay attention nor be vigilant.
18.104.22.168 Human Memory
Neuroscience is concerned with two great themes: the brain’s “hard wiring” and its capacity for plasticity. The “hard-wiring” refers to how connections develop between cells, how cells function and communicate, how an organism’s inborn functions are organized – it’s sleep-wake cycles, hunger and thirst, and the ability to perceive the world. Through evolution, the nervous system has inherited many adaptations too important to be left to the vagaries of individual experience. Motor learning depends on the cerebellum, emotional learning depends on the amygdala, habit learning depends on the basal ganglia. These forms of memory along with simple mechanisms for holding information in mind over short periods are shaped by eons of evolution and provide for myriads of unconscious ways of responding to the world. They are evolutionary ancient systems and are observable in simple invertebrates such as Aplasia and Drosophila. These unconscious forms of memory create the mystery of human experience. For here arise the dispositions, habits, attitudes and preferences that are inaccessible to conscious recollection yet are shaped by past events, influence our behavior and our mental life, and are a fundamental part of who we are.
In contrast, the capacity for plasticity refers to the fact that nervous systems can adapt or change as the result of the experiences that occur during an individual lifetime. Experience can modify the nervous system and, as a result, organisms can learn and remember. The hippocampus and adjacent temporal lobe structures support declarative memory, the memory that is concerned with facts and events.
The precision of neural connections poses deep problems for the plasticity of behavior. How does one reconcile the precision and specificity of the brain’s wiring with the known capacity of humans and animals to acquire new knowledge? And how is knowledge retained as long–term memory? A key insight is that the precise connections between neurons are not fixed but are modifiable by experience. Significant personal experience creates long-term memories, stores them, and allows them to be consciously recalled and somehow integrate them. Emotional states and culture determine the memories selected and the occasions of their recall. The specificity of experiences creates a collection of memory stores and modes of recall that are unique to each individual and that change according to context. Short-term memory results from the strengthening of pre-existing synaptic connections through protein modification, and long-term memory results from the growth of new synaptic connections. A class of signaling molecules common to both regulates the transition from one to the other.
Memory underlies the highest functions of the brain, from adding two numbers to developing a sense of individuality. It lifts animal life out of an eternity of unconnected moments to create a sense of continuity, of connection with the past, which culminates in humans with the sense of history and genealogy. All memories come from the world outside the mind. To keep inconsequential sights, random noises and meaningless experiences from cluttering up the brain, there must exist memory gatekeepers that allow only certain impressions to be permanently stored into the cortex. Short-term memory, that lasts usually less than a minute but may extend to a few hours, is useful for day-to-day efficacious adaptation to the demands of life and is of an electrical nature. The keeping in memory of 7 or 8 digits of a telephone number, the memory of the place where a craftsman deposits his tools, the laying of a table are all stored as electrical impulses that can be erased by an electrical shock. Such a shock treatment wipes out memories of events that occur shortly before the blow but leaves old memories intact, safely stored under the form of permanently changed neurons in the cortex.
The seat of the short –term memory seems to be located in the hippocampus in the middle of the brain, within the limbic system. The hippocampus would also play a sizable role in the appreciation and integration of spatial information, allowing a comparison of new events with old familiar situations.
The important memories worth storing for long terms in the cerebral cortex are processed from there with the help of various neurotransmitters such as acetylcholine, vasopressin and noradrenaline, whose effect is to permanently increase the amount of serotonin (see fig. 6.1) stored at the synaptic ends of the memory “neurons” and also connect the memory neurons among themselves like a network as sensitive as a spider’s web: activating one or two neurons in the network triggers all the others. The human brain is large and the neurons that are stacked in it are of a very small diameter, so that the brain may contain as many as between 10 billion and 100 billion of them, each forming bridges amounting to 1 quadrillion (1024) connections in total.
Throughout the 20th century 7, the integrated brain and mind have been discussed with hardly any acknowledgment that emotion does exist. In post-Cartesian approaches of the mind, emotion was regarded as the very antithesis of the excellent faculty called reason. Scientists, with the notable exceptions of Darwin, James and Freud, spent no time attempting to understand the very opposite of a good thing. Besides emotion, several other shortcomings may be listed in the study of the brain.
Firstly, there was the lack of an evolutionary perspective in the study of brain and mind. Cognitive science proceeded as if Darwin never had existed. Second was the disregard for the notion of homeostatic regulation. The scientific progress made in neurophysiology and neurochemistry was not used to modify the view of how the mind or brain worked. Third was the absence of a notion of organism in the science of brain. The brain remained consistently separated from the body and was never considered part of the mesh of body and brain that defines a complex living organism.
The consequences of leaving out emotion were firstly that it was not possible to understand the relation between an organism and the most complex aspects of an environment. Disorders of emotion can kill, in animals or humans. Emotions serve the purpose of survival in animals. The emotions operate along the dimensions of approach or aversion, of appetition or withdrawal. The emotions protect the organism by allowing it to avoid predators or to scare them away, or by leading the organism to food and sex. As such the emotions operate as a basic mechanism for making decisions without the labors of reason, that is without resorting to deliberated considerations of facts, options, outcomes and rules of logic.
Secondly, emotion plays a role in memory and understanding memory is an important goal of cognitive science because memory is a property of living systems that are determined to survive. In complex organisms, emotion and memory are closely coupled and one cannot fully understand the latter without the former. Thirdly, emotion plays a role in reasoning and decision-making. In human organisms, appropriate learning can couple emotion with facts. The coupling of emotion and fact remains in memory in such a way that, when the facts are considered in deliberate reasoning as when a similar situation is revisited, the paired emotion can be reactivated. The recall allows emotion to exert its modified effect. This higher-order role for emotion is still related to the needs of survival, albeit less visibly. But it should be noted that, beyond survival, the impact of emotion in the process of reason affects the quality of survival and can help guide the creative process that best characterizes the human mind. Making sense of the mechanisms behind the finest human achievements –advanced reasoning, ethics, law, creativity- cannot proceed without understanding emotion.
Mind and body are not split. Human culture is the product of human intelligence but does not escape the strictures of evolution and selection, of the limbic system that drives the rest of the animal kingdom. Cognitive function is connected to an underlying neurological system. Cognition and emotion form a common fabric. Discussions on the neurobiology of emotion resolves around the notion of the limbic system, an odd collection of structures among which are the cingulate cortex, the amygdalea and the hypothalamus. However, it is not true that only the limbic system is relevant for emotions. Nor that emotion and feeling are happening within the limbic system. Other structures are involved in the processing of emotion, such as the prefrontal cortices. Damage of the prefrontal cortex specifically associated with humans may maintain high intelligence but can result in defects in moral and social reasoning, in an inability to feel certain emotions, such as remorse, shame or guilt when confronted with the perversity or social inappropriateness of behavior. Without emotions, we cannot understand how the organism maintains homeostasis in the face of environmental challenges and manages to survive in the complex world of society and culture. Emotions, and the feelings that follow emotions, are an integral part of the value systems necessary for laying down long-term memory and for reasoning and conscious decision-making involving life-directing choices.
The ultimate results of emotion are of two kinds. Firstly, there is behavior. The expression of joy or anger or disgust affects interactions with other living creatures. Secondly, there are cognitive representations of emotional states, feelings, which affect the ongoing thinking of the subject, which alters future thinking, future planning and future behavior.
22.214.171.124 Human maturation
The hippocampus is part of the limbic system. This system is located within the two cerebral hemispheres. It is devoted to the control of emotional states and apparently also to the adaptation of the organism to its environment. The hippocampus, loaded with acetylcholine (see fig. 6.1), is abundantly connected with many other parts of the brain. However, its development is quite slow in some mammalian species. At the time of birth of a rat, the hippocampus is poorly developed. During the next 15 to 20 days -and this is a long time in the life of a rat- its development is associated with striking differences in the animal’s behavior. When the hippocampus is fully developed about 2 months after birth, the animal reaches adulthood. The normal hyperactivity of the young rat, evident when the young animal is put into a new environment, gives place to a calmer behavior. This inhibition is related to the development of the hippocampus. Young infants also show sometimes an uncontrollable hyperactivity that is repressed at school and at home. It subsides normally with age and seems to be due to a delay in the evolvement of the hippocampus. The normal ignorance of a young rat that continuously explores the same alleys and avenues of a dead-end labyrinth, gives way in the two-month adult rat to intelligent behavior that makes it explore successively all unknown avenues. The avoidance of the punishment bound to be received after misbehavior is, just as in children, not really sought by young rats. Again and again these young animals commit the same actions that are bound to result in the same punishment (e.g. an electrical shock upon entering a desirable but forbidden shelter). A two-month old adult rat having been punished thinks twice before attempting the same move that brought about the punishment.
These three conducts (hyperactivity, useless persistence and punishable behavior) are directly correlated with the development of the hippocampus. In precocious animal species (guinea pigs), no changes in behavior are observed from the moment of their birth onwards. In species endowed with a slow maturation such as rats, mice and a fortiori humans, the behavior becomes modulated and restrained according to the maturing of the limbic system.