The brain is probably the most amazing physical structure we know. Nowhere else in the universe do we find anything comparable. People have tried to understand it for thousands of years. The ancient Greeks thought that it acts like a radiator cooling the blood. Medieval philosophers believed that it is the abode of the soul and that it could be invaded by spirits. Today, we think that the brain is responsible for all faculties of mind. The human brain is one of the most intensively researched items in biology, yet there are many questions to which we don’t have answers. For example, we don’t know how consciousness arises from the brain. Nevertheless, significant advances were made in brain research during the past few decades. From classical neuroanatomy we know the different parts and structures of the brain. From neuropsychology we know their psychological and behavioural functions. From neurophysiology and neurochemistry we know the workings of neurons (brain cells) and their connections.

You may find that the appearance of the human brain is quite unimposing. It doesn’t really look like one of the world’s wonders, but rather like something you might find washed up on a beach. The human brain is the size of a large grapefruit and weighs 1 – 1.5 kg. The outer visible layer, the cortex, is part of the cerebrum. It comprises two halves, or hemispheres, of highly wrinkled grey matter. The grey matter consists of the cell bodies of neurons, whereas the subjacent white matter consists of nerve fibres (axons) that constitute long distance connections between neurons. The two hemispheres are separated by a deep grove, the longitudinal cerebral fissure. They are connected at the base by the corpus callosum, a thick layer of nerve fibres. At the outer sides of the hemispheres there is another deep grove, the lateral fissure or lateral sulcus, which divides the frontal and parietal lobes from the temporal lobes. Developmentally, the brain can be divided into three main divisions, the hindbrain (rhombencephalon), midbrain (mesencephalon), and forebrain (prosencephalon).

Divisions of the brain.

The three main parts of the brain can be further divided into substructures, as shown in the illustrations. We will first look at these parts from an evolutionary point of view. The brain stem is the oldest part of the brain. It contains the midbrain and the hindbrain minus the cerebellum. It evolved more than 500 million years ago. Because it resembles the brain of a reptile, it is also called the "reptilian brain". The brainstem controls autonomic functions, such as breathing, heart rate, and digestion. The cerebellum, or "little brain", which is attached to the back of the brainstem, is likewise evolutionary ancient. It contains circuits which are similar in all vertebrates, including fish. Its function is to control and adjust posture and to coordinate muscular movement. The expanded human cerebellum also has a role in some cognitive functions, such as attention.

The limbic system is the group of structures located between the brain stem and the cortex. It evolved between 300 and 200 million years ago and –since it is most highly developed in mammals– it is also called the "mammalian brain". The limbic system is involved in emotion and motivation. For example, the amygdala is involved in aggression and fear, the hypothalamus is involved in sexual arousal, and the nucleus accumbens, the brain’s pleasure centre, is involved in reward, pleasure, and addiction. Furthermore the limbic system controls a host of different functions, including heart rate and blood pressure, hunger, thirst, the sleep and wake cycle, memory formation, and decision making. The two key parts of the limbic system are the hypothalamus and the pituitary gland, the "master gland" of the body. The limbic system interacts with the body through the endocrine system and the autonomic nervous systems. Finally, there is the cerebrum, the largest part of the forebrain, which is evolutionary the most recent and also the largest part of the brain. While the forebrain of a frog is a mere bump, it balloons into the large structure of the cerebrum in higher animals covering the brain stem and the limbic system like the head of a mushroom. The most outstanding feature of the cerebrum is the cortex, which is about two millimetres thick and, like a walnut, possesses an intricately folded surface. This is a special characteristic of "higher" mammals. The many grooves (sulci) and ridges (gyri) create a large surface area of 1,5 square metres allowing for maximum packing of neurons. The cortex is involved in many high-level functions, such as visual and verbal symbol processing, perceptual awareness, communication, language, understanding, and rational thought.

Divisions of the cortex.

The cerebral cortex evolved in three stages and the resulting parts are called archicortex, paleocortex, and neocortex. The most recent one is the neocortex which occupies the topmost layer of the cortex; it is especially developed in humans. Generally, the cerebral cortex acts as a processor of sensory input information, which it receives via the thalamus. The cortex of each hemisphere can be divided into several different areas which are called lobes. At the rear of each hemisphere, the occipital lobe deals primarily with vision, hence, it is also called the visual cortex. It processes visual information transmitted from the eye and analyses it for movement, orientation, and position. A person can become blind if the occipital lobe is damaged, even while the eyes and optic nerves remain intact.

The temporal lobes, located at the outer sides of the hemispheres near the temples, have a number of different functions. A part of it is responsible for hearing. This part is called the auditory cortex. The auditory cortex sits at the lateral fissure and has the size of a large coin. The adjacent areas are involved in high-level auditory processing, such as language perception. Wernicke’s area, which is located at the junction of the temporal and parietal lobe, is mainly responsible for the comprehension of spoken language. Additional temporal lobe functions include behavioural expression, the recognition of faces and scenes, as well as episodic and declarative memory, i.e. the memory and retrieval of events and facts as in textbook learning. Damage to the temporal lobes can cause aphasia, the loss of the ability to form and comprehend language. Damage to the right temporal lobe can result in impaired performance of spatial tasks, for example the ability to draw. If the temporal lobe is electrically stimulated, some persons report being present at two places at the same time. They are conscious of the present moment, as well as of another event stored in memory. For example, they might feel they are at the same time in the kitchen of their home, cooking a meal.

The parietal lobe is a relatively large area located at the back of the hemisphere just above the occipital lobe. Much less is known about this lobe than about the other three lobes. It is involved in touch, pain, and taste sensation, visual and spatial perception, and body orientation. It seems that the parietal lobe is where we put our world together. The parietal lobe integrates visual information and constructs maps and coordinate systems that represent how we see the environment. Another function of the parietal lobes is to combine letters into words, and words into sentences. Damage to the left parietal lobe can lead to Gerstmann’s syndrome which includes the confusion of left and right, impairment of with writing (aphasia) and calculation abilities (acalculia), and difficulty with recognising body parts (agnosia). Damage to the right parietal lobe can result in difficulties with spatial perception, such as unilateral neglect, the limited conscious awareness of information coming from one side of the body, and constructional apraxia, the inability to draw or construct simple configurations.

The frontal lobe, just behind the forehead, is the largest of the four cortical lobes. It controls much of the rest of the brain’s functions. In particular, it is responsible for the higher functions, such as reasoning, planning, organising, problem solving, selective attention, and personality. The frontal lobe is highly connected to the limbic system, which suggests that it is involved in emotions. Moreover, it plays a key role in memory, language processing, speech production, and movement. Cognitive maturity in adulthood is associated with the maturation of cerebral fibres in the frontal lobe. The frontal lobe contains a great number of dopamine-sensitive neurons, which are linked to pleasure, motivation, attention, problem solving and long-term memory. Broca’s area, located at the base of the frontal lobe just above the parietal lobe, is thought to be responsible for the production of speech. Brain damage to this area causes expressive aphasia, the inability to form sentences. If the frontal lobes are damaged, the individual may show symptoms of dementia, such as becoming incapable of planning and executing, incapable of comprehending situations and ideas, unable to focus attention, and being distracted by irrelevant stimuli. Other symptoms include impairment of short-term memory, lack of inhibition, and difficulty in learning new information.

The primary motor cortex is located in the precentral gyrus of the frontal lobe, running from the longitudinal fissure at the top of the brain down to the lateral fissure. It controls movements of specific body parts. Electrical stimulation of certain areas of the motor cortex results in movement of the associated body part. From top to bottom, these are feet, legs, hip, trunk, elbows, hands, and face. The areas are not represented in proportion to the size of these body parts. For instance, the areas for the hand and its individual fingers, as well as the area of the face and its different parts are larger than the areas for other body parts. The primary motor cortex receives feedback from the primary somatosensory cortex to which it is intricately linked. The primary somatosensory cortex, located in the postcentral gyrus behind the primary motor cortex, is the main sensory receptive area for the sense of touch. These two areas wok in conjunction with the secondary motor cortex, located before to the primary motor cortex, which prepares movements and combines series of movements into coordinated sequences. Damage to the primary motor cortex disrupts the ability to move one body part (e.g. one finger) independently of another. It can also reduce the speed and accuracy of movements, but it does not cause paralysis.

Lateralisation and the split brain.

The two hemispheres of the cerebrum look almost identical, but at closer inspection we find significant differences. In 1836, a virtually unknown French country doctor found that all of his brain-damaged patients with speech problems suffered injuries to the left side of the brain. This early finding anticipated modern research of brain lateralisation. Clinical evidence suggests that the two sides of the cerebrum serve different functions. Injuries to the left side usually impairs reading, writing, speaking, calculation, and understanding. Injuries to the right side have less dramatic effects, but tend to affect spatial perception and movement. More extensive research has shown that the left and right hemisphere’s involvement in certain functions is disproportionate.

Left Side Dominance	General Function	Right Side Dominance
Words Letters	Vision	Geometric Patterns Faces Emotional Expression
Language Sounds	Audition	Non-language Sounds Music
	Touch	Tactual Patterns (Braille)
Complex Movement	Movement	Spatial Movement Patterns
Verbal Memory	Memory	Nonverbal Memory
Speech Reading Writing Arithmetic	Language	Emotional Content
	Spatial Ability	Geometry Direction Distance Mental Rotation of Shapes

Yet, it would be wrong to speak of compartmentalisation. The hemispheres of the brain work in tandem as a complex whole. In a famous experiment in the 1950s, the American neuropsychologist Roger Sperry separated the corpus callosum, to treat epileptics. The corpus callosum is a strand of approx. 200 million nerve fibres connecting the left and right hemispheres, which the brain uses to transfer signals between the hemispheres. The patients remained largely normal, but each hemisphere worked independently. Human split brain patients seemed to have two independent brains, each with its own abilities, memories, and emotions. Notably, the left hemisphere of split brain patients was capable of speech, whereas the right hemisphere was not.

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