Distinctive features of human brain function

Human brains are bigger relative to body size, and much more complicated in terms of numbers of synaptic connections, than the brains of other mammals. However, being bigger and more multiply connected does not mean that they are better at everything than the brains of other species. For example, dogs process olfactory information far more efficiently and elaborately than humans. Salient examples of human capabilities are hand control, facial recognition and language; we discussed hand control and its evolutionary significance earlier.

An obvious prerequisite for facial recognition is vision. A good deal of the cerebral cortex, particularly the occipital lobe and part of the parietal lobe, is devoted to vision. Brains have no control areas or synchronisers; they rely on dialogue between circuits among which the work-load is democratically divided. Information from the retina is sent to different areas in the primary visual cortex so that form, motion and colour are recognised separately. Each primary visual area sends signals to other parts of the brain. The secondary visual cortex (in the parietal lobe) integrates them into meaningful messages. Other regions of the cortex correlate them with simultaneous inputs from other senses; and in the limbic area and other regions below the upper cortex, emotional responses are organised. Subjectively, the experience of seeing something or someone and responding emotionally is unitary. In fact, diverse brain areas are involved, each performing a distinct task.

The human brain is apparently hard-wired to attend and respond to the sight of human faces. As a result, it is able to learn very early in life to respond to particular faces, especially those of adult carers. The memories of particular faces seem to be stored towards the front of the brain in the temporal lobes. However, actual recognition of a face within the field of vision involves a region called the fusiform gyrus, where the occipital and temporal lobes meet. Therefore, matching current visual information to memory requires communication between quite separate brain areas.

From an evolutionary point of view, our skill in recognising and distinguishing faces probably served two main purposes. First, it facilitated social bonding, particularly between child and parent - a prerequisite for the prolonged learning period that became more and more necessary for survival as hominid evolution progressed. Second, it afforded a channel for communicating emotions. Communication of emotions by facial expression (which rapidly incites particular forms of behaviour and can be of great survival value) is not confined to humans. Its prevalence in mammals was the subject of Darwin's final book and has since been a major topic in ethology. However, it is particularly refined and developed in humans.

Like visual perception itself, our emotional responses to facial expression involve several different brain areas. An expression of disgust activates a region of the midbrain called the inula, which lies near the taste centres. Stimulation of the inula depresses appetite for food. The advantage is obvious: if you eat something nasty then your face expresses disgust, and no one who sees your expression feels hungry any more. Expressions of fear and anger, on the other hand, activate the amygdala, an almond-sized area that lies under the temporal lobes. Anger and fear in a tone of voice also stimulate the amygdala, which then sends signals to a region near the midbrain aqueduct called the preaqueductal grey. This initiates defensive body postures and movements, increases the pain threshold and instigates an adrenaline surge - all appropriate responses to a physical threat. However, the amygdala is not involved in recognising other emotions, or in integrating other visual and auditory signals.

Just as human brains are hard-wired for facial recognition, so they are hard-wired for acquiring language. Near the front of one temporal lobe -usually the left - is a region (Broca's area) that is necessary for speech production. Further back in the temporal lobe is a region (Wernicke's area) necessary for interpreting spoken language. Near Wernicke's area are regions involved in the processing and retrieval of verbal memories. Damage to these areas impairs recollection of what has been heard or read, but has no effect on language-using skills per se (or on "intelligence").

Language sounds and the meanings of words are stored - separately from one another - in parts of the left temporal cortex near the secondary auditory area. Some brain injuries damage the phonological stores without harming the lexical ones, and vice-versa. Different categories of words (parts of speech, types of noun and adverb, etc.) are processed separately in the lexical stores. Signals from these enter brain areas around the Sylvian fissure (between the temporal and frontal lobes) that are active in language production and particularly in the recall of nouns. The fine division of labour between these different parts of the brain reflects different facets of language (sounds, words etc.) and the distinct processes of recognition, interpretation, recall and production.

As with other characteristically human attainments, most of our language skills are learned rather than innate. The brain of the newborn infant is constructed to be good at language acquisition, but of course it has not yet acquired language. Its ability to do so seems to depend on recognition of repeated sound patterns. When strings of nonsense syllables are read monotonously to young infants (so that changes of intonation have no effect), the infants very quickly pay attention to two- and three-syllable strings that recur. The infant brain seems to be able to compute the probabilities of sound sequences and respond accordingly, treating repeated patterns as significant.

Language involves many other skills than sound recognition. It is unlikely that all these skills evolved simultaneously. Martin Nowak and his colleagues used evolutionary game theory to construct a three-step model for language evolution. First, the vocalisations of the common ancestor of humans and chimpanzees developed into a more elaborate repertoire of sounds, each sound associated with a specific object. According to Nowak's model, sound-object associations were likely to arise in a highly social species with an elaborate life-style, because the resulting ability to communicate information benefited both "speaker" and listener. Presumably the vocal apparatus became more elaborate at this time. Perhaps it happened during the proposed semi-aquatic phase of human evolution.

However, the increasing complexity of hominid life outpaced the increase of vocalisation repertoire, which has an upper limit; in all human languages, the total number of distinct phonemes is quite small. A second stage of language evolution ensued; brains that could combine sounds into words became advantageous. A repertoire of words rather than single sounds allowed a virtually unlimited number of objects to be represented distinctly. The third step, syntax, enabled individuals, actions and relationships to be specified uniquely. This became essential when each individual in the community had to meet a complicated range of expectations. Any reasonably advanced social learning must have required syntax. It is generally supposed that language complete with syntax did not exist before H sapiens, but the evolutionary conditions for syntax according to the Nowak model might have obtained earlier. The skull contours of erectus and even habilis suggest advanced temporal lobe development and therefore, conceivably, language development.

The most distinctive feature of sapiens, perhaps related to the increased size of the frontal lobe, seems to be the capacity for abstract thought and abstract associations. A fully developed capacity for language might have been a prerequisite for abstraction. Abstraction was in turn a prerequisite for symbolic representation, which is why the 50,000 year old Australian rock art alluded to earlier was surely the work of H sapiens.

Africa today is home to a large number of language families. In other parts of the world, individual language families such as Semitic and Indo-European tend to cover bigger land areas. Does this pattern reflect the migrations of H sapiens, originating in Africa, or the migrations of H erectus; or neither? The finding that human genetic variance within Africa exceeds that in the rest of the world indicates the African origin of our species. In the same way, the finding that Africa is home to the widest variety of language families might indicate that human language capacity, including syntax, was complete before our ancestors migrated out of the continent.

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