And the mammalian brain became the human brain by adding the massive grey matter (neocortex) which envelopes most of the earlier brain and amounts to about 85 per cent of the human brain mass.

This massive addition consists mostly of two hemispheres which are covered by an outer layer and interconnected by a string of nerve fibres.

The brain is actually divided into its 'hemispheres' by a prominent groove. At the base of this groove lies the thick bundle of nerve fibres which enable these two halves of the brain to communicate with each other.

But the left hemisphere usually controls movement and sensation in the right side of the body, while the right hemisphere similarly controls the left side of the body.

We saw that with the mammalian brain emerged feelings such as attachment, fear and anger and associated behavioural response patterns.

And human emotional responses depend on neuronal pathways which link the right hemisphere to the mammalian brain which in turn is linked to the even older reptilian brain.

Fascinating is the way in which work is divided between the two halves of the brain, their different functions and the way in which they supplement and co-operate with each other.

Most people (about 80 per cent) are right-handed <4> and in the vast majority of right-handed people, the ability to organise speech and the ability to speak are predominantly localised in the left side of the brain. But the right side can understand written and spoken language to some extent at least.

"Appreciating spatial perceptions depends more on the right hemisphere, although there is a left hemisphere contribution. This is especially true when handling objects" and concerning abstract geometric shapes and music.

Roger Sperry, Michael Gazzaniga and their colleagues found that, when presented with a stimulus, both hemispheres were active and could recognise the nature of visual stimuli as well as spoken words.

But while the left hemisphere can express itself by verbally describing a stimulus, the right hemisphere can express itself non-verbally by selecting the matching stimulus.

The left hemisphere deals with word choice, rules of grammar, and the meaning of words. The right hemisphere apparently determines the emotional content of speech.

So a general overview of the functional division of activities between the two hemispheres would be:

Left Hemisphere

Communicates by using words, has highly developed verbal abilities, is logical and systematic, concerned with matters as they are.

Right Hemisphere

Communicates using images (pictures), has highly developed spatial abilities, is intuitive and imaginative, concerned with emotions and feelings.

But the two hemispheres are interconnected and communicate, the human mind brings together these abilities and skills into a comprehensive whole whose operation depends on the way in which its parts contribute and co-operate with each other.

The right hemisphere links to the primitive older part of the brain, and I consider that it communicates using images with its primitive 'unconscious' functions. Thinking in pictures is fast. Think of how long it takes to describe a picture, a scene, in words and compare this with the speed of taking it in by looking at it. But images may be described, or transformed into a narrative, by the left hemisphere.

Language is both spoken as well as written, verbal and visual. And speech and language and associated pictures, images and memories appear to be located all over the brain. Cognition of meaning (knowing and understanding sentences, for example) is high level processing which includes both semantic and visual processing. And behaviour involves the integration of activities in many different parts of the brain.

So now the human brain includes the processing and memorising of images and of their components. And the development of language and corresponding mental processing connected with memory and memorising. As well as the development of a wide range of emotions, of feelings, of care and affection, and the capability for objective and logical thinking and evaluation. And the later development of written languages and artificial images.

SLEEP AND SLEEPING

• BODY-TEMPERATURE AND SLEEP RHYTHMS

Day and night alternate over 24 hours due to the rotation of the planet, and the start and length of daylight varies with the seasons.

So internal biological clocks (controllers) evolved for controlling activities related to the environment such as those of cold-blooded animals which need to maintain their body temperature by warming themselves in the sun. Reptiles are cold-blooded animals warmed by the daylight sun and conserve energy by restricting activities when it is dark. And the biological clock which controls their activity-rest cycle is located within the eye.

But about 180 million years ago, warm-blooded mammals evolved from their cold-blooded reptilian ancestors by developing the ability to maintain a constant body temperature by biological processes. This freed them from depending on daylight and the weather for survival. Deep sleep appeared at the same time.

The earlier mammals were reproducing themselves by hatching their young out of eggs. But about 180 to 130 million years ago, many mammals evolved into giving birth directly from the womb, their young being born alive after having been developed for a considerable period within the womb. Their young have to grow and learn much for a long time before they can survive independently, for many years in the case of human beings. The human brain now has much greater learning capacity.

In mammals, information about light and darkness is transmitted from the eye to a biological clock, now situated in the mammalian brain, which controls the sleep-wakefulness rhythm. Another biological clock controls the body-temperature rhythm, and these biological clocks together control the related body-temperature and sleep-wakefulness rhythms.

While the body's temperature is held at a constant level, it varies by about 0.5 deg C from a low at about 05.00 hours to a high at about 18.00 hours. It appears that we tend to go to sleep after our body temperature has began to fall and tend to wake up after it has started to rise.

"The length of the geophysical day is 24 hours. Our sleep-wakefulness rhythm (circadian rhythm) has a duration which varies from individual to individual (usually between 25 and 28 hours) but is always longer than 24 hours. And our biological rhythms are adjusted accordingly, day by day," by these internal biological clocks, to the external geophysical day, to the environment. People sleep, on average, between 6.5 and 8.5 hours.

The body-temperature clock also controls the appearance of REM sleep.

• SLEEPING

There are key mental states each characterised by its own brain wave pattern. When awake we can be attending or concentrating, or we can be relaxed. When asleep we could be in SHALLOW sleep, DEEP sleep, or REM sleep.

Shallow sleep is often referred to as 'Stage 2' sleep, and Deep sleep as 'Stage 4' sleep.

During REM sleep (Rapid-Eye-Movement sleep), the eyes move rapidly and continuously. At times REM sleep is referred to as dreaming sleep and sometimes called paradoxical sleep, or called paradoxical sleep only when referring to animals.

During Deep sleep the body's muscles are relaxed, heart beat and breathing are slow and regular. In REM sleep the body's muscles are paralysed while heart beat and breathing fluctuate as they would during emotional upsets in waking life.

Brain-wave frequency of the different sleep stages we pass through in the course of a night are outlined in Figure 1 'Sleep Pattern: Day-Night-Day'. From being wide awake before going to sleep, we relax, sleep lightly (shallow) for ten to fifteen minutes before sleeping deeply. Following Deep sleep we REM sleep after which we wake up through relaxing to being fully awake.

Deep sleep is followed by REM sleep. In order to achieve this as far as possible within a night, the brain arranges alternating periods of deep sleep followed by REM sleep.

The illustration shows graphically what happens to the brain's electrical activity as the night progresses, illustrated by the frequency of the brain waves. As we progress from being awake through sleep to being awake again, the frequency drops, reaching its lowest point while in deep sleep and then rises again to the wide-awake level.

Amplitude, that is voltage, changes inversely. It increases when the frequency drops, reaching its highest level during deep sleep, and then decreases again to the wide-awake level.

Considering adults, that is excluding the young and the elderly, on the whole we 'Deep Sleep' during the first half of the night, and 'REM Sleep' during the second. But possibly because we cannot be certain how long we will sleep, whether our sleeping period will be interrupted unexpectedly, Deep sleep and REM sleep are divided into shorter sleep periods which alternate.

That we Deep sleep first and that REM sleep follows Deep sleep is clearly shown in Figure 2. This shows how the two kinds of sleep alternate as Deep sleep ends and REM sleep begins and proceeds.

Deep Sleep and REM Sleep each take up about 20 to 25 percent of the night's sleep. The remainder is largely taken up by transition 'Shallow' sleep' periods which enable brain and body to adjust to the next type of sleep, and by occasional brief periods of intermediate 'Stage 1' and 'Stage 3' sleep.