The Brain

The human brain is an incredibly complex structure which until recently was considered incomprehensible. Most of what we now know about the brain was only discovered in the last 50 years and is in part because of the accelerating pace of research in neurological science and the development of new functional mapping techniques. The advent of brain imaging techniques, including positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) has enabled us to peak inside the brain and glance at its inner workings. Using this technology researchers can examine in a non-invasive

manner human brain functions, by study the temporal and spatial changes in the brain as a function of various stimuli.

The brain is composed of three main parts; the cerebrum, cerebellum, and brainstem, see Figure 2.22.

Figure 2.2: Sagittal view of the brain showing three main parts, cerebellum (Purple), cerebrum (Pink) and brainstem (Blue).

2.3.2.1 Cerebellum

The cerebellum is located posterior to the upper part of the brain stem. The cerebellum is relatively small, only accounting for 10% of the brains overall mass, however it contains approximately half of the brains neurons. Functionally the cerebellum plays an assisted role in motor control, while it does not initiate movement the cerebellum assists in the coordination and planning of voluntary movements such as; posture, balance, and speech. In addition, the cerebellum also plays an important role in motor learning, most notably in learning to adjust to dynamic changes in sensorimotor relationships [162]. The cerebellum also appears to be involved in a variety of linguistic functions [81].

2.3.2.2 Brainstem

The brainstem is located in the posterior of the brain, conjoining the brain with the spinal cord. The brainstem includes the midbrain, pons, and medulla. Though relatively small, the brainstem plays the important role of interfacing the brain with the nerve

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Source: Diagram showing some of the main areas of the brain (cropped, colors modified and re- labelled). licensed under Creative Commons Attribution-Share Alike 4.0 International

connections of the motor and sensory systems which spread out into the rest of the body. The brainstem is also responsible for autonomic, involuntary functions of the body including basic vital life functions such as, blood pressure regulation, breathing and heart rate. The brainstem is also involved with a series of complex reflective motor functions including swallowing, chewing, posture adjustment and locomotion [171].

2.3.2.3 Cerebrum

The cerebrum is the largest part of the human brain and is associated with higher brain functionality, including perception, thought, judgement, imagination and decision making. From an evolutionary perspective, the cerebrum is the newest part of the brain and is highly developed in humans compared to other species of mammals. The surface of the cerebrum is known as the cerebral cortex or simply cortex. The cortex is only a few millimetres in thickness and consists of two cortices, separated along the sagittal plane, known as the left and right hemispheres. Although thin, the cortex contains approximately 10 billion neurons and is home to the most complex functions of the human brain. The cortex has a wrinkled texture made up of ridges called gyri (singular gyrus), and grooves or fissures called Sulci (singular sulcus). Because of this configuration two-thirds of the surface of the cortex is hidden, allowing for a much larger concentration of brain matter inside the skull. Functionally, the cerebral cortex is commonly segmented into four key areas; the frontal lobe, parietal lobe, occipital lobe and temporal lobe, see Figure 2.33.

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Figure 2.3: Cerebral cortex showing four main lobes; temporal lobe (Blue), optical lobe (Green), parietal lobe (Purple) and frontal lobe (Brown), Motor areas; primary motor cortex (Grey), premotor cortex (Red), supplementary motor area (Orange) and

somatosensory cortex (Pink).

The frontal lobe is located in the anterior of the brain and is considered our intelli- gence and emotional center and home to our personality. It controls important cognitive skills such as problem solving, long term memory, language skills, judgement, impulse control, emotional expression and sexual behaviour. At the posterior of the frontal lobe, located in the dorsal precentral gyrus, lies the motor cortex, see Figure 2.3. The motor cortex is of primary interest to this thesis as it the area of the cortex responsible for planning, control, and execution of voluntary movements of the body. The motor cortex can be sub divided into three main areas; the primary motor cortex, which generates the signals that control the execution of movement. The premotor cortex, which is re- sponsible for the preparation of movement, sensory guidance of movement and spatial guidance of movement, and the supplementary motor area, which is responsible for the planning of sequences of movement and the coordination of the two sides of the body.

The parietal lobe is located just behind the frontal lobe, above the occipital lobe in the upper mid section of the brain. The parietal lobe contains the somatosensory cortex which is essential to the processing of the body’s senses, such as touch, pressure and pain. The main functionality of the parietal lobe is the integration of sensory information from various parts of the body. The parietal lobe also plays a part in spatial and visual perception, for example in the differentiation of size, shape and color and in language processing.

The temporal lobe is located at the base section of the brain, it contains the primary auditory cortex responsible for interpreting sound and the hippocampus associ- ated with the formulation and storage of memory. The temporal lobe is responsible for processing sensory information in order to make it understandable and comprehensible, for example in the case of deciphering meaningful speech from sound and meaningful visual information from sight. The temporal lobe is also associated with the retention of language and visual memories, and association of emotion.

The occipital lobe is located at the rear portion of the brain, above the brainstem. The occipital contains the primary visual cortex and is the visual processing center of the brain. The occipital lobe is involved in visual-spatial processing including, the discrimination of movement and recognition of color.

Although the brain accounts for only approximately 2% of the total body weight in humans, it requires a large portion of the body’s blood supply, approximately 15- 20%. Normal healthy function of the brain is highly dependent upon adequate supply of oxygen and nutrients, distributed through a network of blood vessels, see Section 2.3.3 on Cerebral circulation. Without oxygen, brain cells die quickly, in the order of minutes after an oxygen-depriving event. Therefore, any decrease in the flow of blood to the brain can be catastrophic and may result in severe damage to the brain, resulting in impairment in function of the affected area.