When electrodes are placed on the scalp, it is possible to record the rhythms of electrical activity in the underlying cerebral cortex - this is known as the ElectroEncephaloGram (EEG). There is international agreement on the placement of electrodes, and the potential difference between different electrodes is recorded as a series of waves ('brain waves') that relate to the electrical activity of the region of underlying cerebral cortex.
Electrodes over the occipital region record activity from the underlying visual cortex are particularly affected by changes in patterns of light falling on the retina.
The EEG is a sequence of brain waves that occur at different frequencies: a number of cycles per second
The normal rhythm in an awake individual is the alpha rhythm, and consists of large amplitude waves when the eyes are closed.
When the subject thinks or opens his eyes, the amplitude gets smaller and the waves of electrical activity become more frequent: this is the beta rhythm.
Beta waves are observed in all age groups, and are small in amplitude. Certain drugs, such as benzodiazepines, increase the frequency of beta waves. Theta and Delta rhythms are present during sleep, and the deeper the sleep the slower and larger are the waves.
The waveforms of the EEG is due to rhythmical activity in cortical neurones mediated by neural circuits (reverberating circuits) linking the cortex and the thalamus in both directions (i.e. thalamic neurones that excite cortical ones, that then re-excite thalamic neurones). The frequency of this synchromised activity during sleep is determined by the reticular nucleus of the thalamus (RNT) - see below.
|The Reticular Nucleus of the Thalamus (RNT) and Rhythms in the EEG|
The Reticular Nucleus of the Thalamus (RNT) is a nonspecific nucleus shaped like a skin that covers the outside of the whole thalamus and has connections with all areas of the cortex.
The RNT acts as a switch that determines the frequency of cortical activity during sleep. Slow waves in the EEG are synchronised with the intrinsic rhythm of RNT neurones, which also reduces the power of sensory inputs to influence cortical activity by inhibiting the specific thalamic sensory (relay) nuclei.
RNT neurones have some special characteristics:
Cortico-Thalamic circuits, 'reverberating circuits', are formed by the axons of pyramidal cells that thalamic nuclei. Activity in cortical pyramidal cells increases the excitability of relay nuclei using this pathway - but can be moderated by the tonic inhibitory activity of the reticular nucleus.
Consequently there is a switch in the thalamus: cortical activity keeps the pathway from the relay nuclei active, but the NRT can inhibit activity in these reverberating circuits - which it does particularly during sleep, when cortical neurones all adopt the rhythm of the NRT, characterisic of the EEG during sleeping.
When the ascending reticular activating system (ARAS) - the arousal system of the sleep-waking cycle - becomes active, it excites the thalamic nuclei and the cortex. This input disrupts the existing rhythm generated by the NRT, and leads to desynchronisation of the EEG.
The reticular nucleus is responsible for synchronising many cortical columns in the absence of external stimuli, such as during sleep; but excitatory inputs from sensory systems or the Ascending Reticular Activating System (ARAS) inhibit the NRT and result in Arousal and desynchronisation of the EEG.
Epilepsy is a common condition in which a variety of causes may give rise to seizures (uncontrolled movements, fits, absences) that originate in the brain. The most severe seizures consist of generalised convulsions that affect many muscle groups in the body and are accompanied by unconsciousness (a grand mal attack). Other patients experience a transient trance-like state (petit mal).
Epilepsy often begins in childhood, and is normally diagnosed after a person has their first seizure.
There are two main causes of epilepsy: brain injuries (including birth traumas, infections of the brain or meninges, and congenital abnormalities) and metabolic imbalances in the brain caused by drugs or alcohol, tumours, ischaemia or changes in blood chemistry (e.g. ionised calcium, glucose).
Low blood glucose, poor oxygen supply and disturbances of blood electrolytes such as sodium or calcium ions also can precipitate seizures in patients who are susceptible.
During the EEG test, patients may be asked to overbreath because the alkalosis associated with overbreathing causes blood levels of ionised calcium to fall transiently, and early asymptomatic signs of epileptic activity may be observed in the electrical recordings.
During hyperventilation, a patient had a brief absence during which high voltage 3 Hz generalized spike wave discharges were recorded during 6 s on EEG. Time base: 30 mm/s, sensitivity (of original recording): 300 µV/cm, high cut: 35.0 Hz, low cut: 0.5 Hz.
Hyperventialtion can be used to induce changes in serum ionised calcium, which can induce seizures in susceptible individuals.
Some patients with epilepsy are susceptible to flashing lights which may precipitate a seizure; EEG changes induced by flashing lights may indicate neuronal hyperexcitability.
|EEG and Sleep|
Awake relaxed people have alpha waves in most areas of cortex; mental arithmetic, thinking and attention cause an increase in frequency (beta waves).
Sleep is associated with a slowing of the rhythm (delta rhythm: less than 4Hz) and in deep sleep the frequency drops frther. The amplitude of the voltage changes become larger due to synchronous activity in cortical neurones. These changes are associated with the synchronous activity in reverberating circuits between the cortex and the nucleus reticularis of the thalamus.
Arousal causes desynchronisation of the EEG: the frequency of the brain waves becomes faster and the amplitude of the waves is decreased. This is due to the activity of the arousal system, including the cholinergic neurones of the basal forebrain. These changes are discussed in the section on the Ascending Reticular Activating System
Sleep architecture is a measure of the amounts of time spent in the different phases of sleep, and certain sleep disorders can be diagnosed by observing changes in these patterns.
|EEG and the Brain-Computer Interface|
The frequencies of EEG waves recorded simultaneously from many sites on the scalp can be analysed using computers with the objective of controlling robots, such as robotic arms.
Following some brain disorders, patients can be trained to produce specific patterns of EEG activity may be recognised by a computer and used to give the patient a measure of control over machines.
The topic of brain controlled robotics is in its infancy, but has had some initial successes in allowing some people to have a measure of control over robots.