Brain is the most complex organ of the all creatures. All of it happens in the brain. (buraya Einstein lafını aktar beyni anlayabilincek olsa okadar akıllı olmazdık vs vs). This section tries to give an overview of the structure and the functions of the brain.

2.1 Structure of the Brain

Brain is the main part of the central nervous system, which consist of large brain, brainstem and spinal cord. Brainstem is the part that connects large brain and spinal cord. Anatomically brain can be divide part such as hind brain, mid brain and fore brain. Hind brain consists of myelencephalon which means spinal cord, above the mylencephalon Cerebellum and fourth ventricle locates. The second part, mid brain consists of mesencephalon which consists of tectum and tegmentum and cerebral aqueduct. Last part of the brain is fore brain which consist of two part called diencephalon and telencephalon (Nykopp, June 2001).

After a short brief, functional perspective of these parts is more important for us. It can be divided in three parts in terms of functionality. First part is called large brain, which is also known as forebrain or cerebrum. This part controls higher mental activity such as analytical thinking and language consist of diencephalon and telencephalon. Second part is called brainstem. This part is responsible from visual and auditional functions. The brain stem is located in mesencephalon. Third part is the cerebellum and it handles the motor control and movement of the limbs and body. 

Image 1: Major parts of the braın

2.1.1 The Cortex

It is the dominant part of the cerebrum it consist of 1010 to 1012 neurons arranged on different layers. The cortex is 2-3mm thin but its total area is large comparing to the area of skull. This is because different shapes of the cortex surface such as fissures, which are folding that, divide into two hemispheres and to frontal and temporal lobes. 

The left and right hemispheres of the brain are the major parts of the higher cognitive actions. The left part of the brain is related to language and verbal materials and also positive emotions, whereas the right hemisphere is related to Visio-spatial functions and negative emotions. These two hemispheres are connected and communicates via corpus collosum which is contains millions of nerve fibers run across to different hemispheres.    

Soma motor cortex is the center of the bodily functions located on the brain. The key rule in localization of the centers of the organs on the motor cortex is accuracy on the area is related with the area on the brain such as lips and tongue is cover larger area than legs.

Somatosensory cortex is also laid similar to the somatomotor cortex. Representations of these areas are show as a homunculus below (Image 2).

Image 2: Somatomotor and Somatosensory cortex

2.1.2 The Lobes of the Cortex

Human brain can be categorized for four different lobes. These lobes are different for functionality. These lobes are called frontal, temporal, parietal and occipital.

The frontal lobes control complex cognitive action and language programs and execution of the motor patterns. Damage of this area can leads to Parkinson, Alzheimer or Schizophrenia diseases.

Temporal lobe is used for storage of a visual memory, processing of the language and other audio related functions. Hippocampus which is related to memory is also located in temporal lobe.

Brain handles senses and other outputs in parietal area. It is connected all sensing and processing the data of this. It is ancient area for all mammals.

The occipital lobe is the center of all visual related tasks. This is the place where the visual information from eyes transforms directly. It is also plays role on recognition of the object and processing of them.  

2.1.3 Basal Ganglia

Basal ganglia are the place for control body movements by integrating sensory and motor information from other areas of the brain. Diseases such as Parkinson is also originated form that area.

2.1.4 The Hypothalamus and Thalamus

The hypothalamus is responsible for balancing body in terms of temperature, thirst, hunger etc. by controlling hormones. All this balance of the body is called homeostasis.

Thalamus plays role as an relay station of all sensory data. Thalamus integrates and passes on somatosensory and somatomotor information.

2.2. Signal Types (Brain Rhythms)

EEG signal is a complex type signals which is described in terms of rhythmic and transient. The rhythmic activity is divided into bands by frequency. In healthy adults EEG Signals amplitudes and frequencies may vary from different states such as sleeping and wake. Also the characteristic of waves changes with age.

Image 3: Raw EEG Data (Wikipedia)

There are five major brain waves forms distinguish by their different frequency ranges. These frequency bands from low to high frequencies are called alpha (α), theta (θ), beta (β), delta (δ) and gamma (γ).    

2.2.1 Delta Waves

Delta waves lie within the range of 0.5-4 Hz. This wave type is associated with deep sleep and maybe present in waking state. This signals are very likely confuse with the large muscle artifact signals. Delta rhythm is decreases with age and it is not normal to see in healthy people while they are awake.

Image 4: Delta waves (Wikipedia)

2.2.2 Theta Waves

Theta waves lie within the range of 4-7.5 Hz. Theta waves associated with access to unconscious materials, creative thinking and deep meditation. Origin of this waves name comes from Thalamus. Also there is a link between emotions such as disappointment and frustration.

Image 5: Theta waves (Wikipedia)

2.2.3 Alpha Waves

Alpha waves appear on posterior part of the head and usually found over occipital area of the brain. Alpha waves frequency lies within the range 0f 8-13 Hz. These waves are comparatively higher amplitude to other wave types. It is best seen when eyes closed and mentally relax mode. In some cases this waves interfere with mu-rhythm. This wave type is useful to trace mental effort because of its higher amplitude.

Image 6: Alpha waves (Wikipedia)

2.2.4 Beta Waves

Beta waves consist of large range of frequency spectrum. It is between 13-50 Hz practically. Beta rhythm can be found frontal and central regions of brain. The central beta rhythm is related to Rolandic μ-rhythm and can be blocked by motor activity and operation of planning to move.

Image 7: Beta waves (Wikipedia)

2.2.5 Gama Waves

Gama waves sometimes called fast beta rhythm lies within the range of 30Hz and higher frequencies. These waves are very low amplitude so it is very rare to see them. It is used to detect high cognitive activities and gives some clues about mental diseases.

Image 8: Gama waves (Wikipedia)

2.2.6 Mu Rhythm

Mu (μ) Rhythm which is also called Rolantic μ-rhythm is in frequency and amplitude related to posterior alpha rhythm, but its importance is different from alpha waves. In the name of the wave μ stands for motor and this rhythm is strongly connected with motor activities. This rhythm is very anti-symmetric and easy to detect. Most of the times face muscle activities and eye movements can be seen as an artifact of EEG signal. This rhythm is mostly seen 8-11 Hz frequency and easily detects using C3 and C4 electrodes of standard 10-20 system.

Image 9: EEG Rhythms

Bibliography

Drongelen, W. v. Signal Processing for Neuroscientists.

Electroencephalography. (n.d.). Retrieved May 2010, from http://www.bem.fi/book/13/13.htm

Saeid Sanei, J.A. Chambers. (2007). EEG Signal Processing. Wiley.

Wikipedia. (n.d.). Wikipedia. Retrieved March 2010, from Electroencephalography: http://en.wikipedia.org/wiki/Eeg