|
There are about 13 billion neurons in human brain; they are communicating using electrochemical connections.
As human brain, skin, cerebral liquid and skull have reasonably good conductivity it is possible to measure electrical activity of synchronous active groups of neurons by placing electrodes on the skull. We need two electrodes to get electric circuit closed; voltage that we measure is a projection of vector that shows directions of neurons activity to vector that is defined with electrodes. Think a bit about this sentence, it is very important to help you to understood what measuring with scull electrodes actually do - it shows that measured voltage is not just a function of number of active neurons, but also a function of they position in brain rather to electrode position.
To get better information about neurons activity it is possible to use more then two electrodes (up to 256), this is used specially in so called brain mapping, when the result is brain image with areas of activity and inactivity.
Brain mapping
To standardize research and diagnostic results there is a standard for electrode positioning called 10-20 standard.
10-20 electrode system
Measuring of brain electrical activity with scull electrodes is called electroencephalography, measurement report (graph) is called electroencephalograph (EEG), and device that does the measurement is called electroencephalogram.
EEG record
Each stimulus causes activation of certain groups of neurons that are specialized for processing a received stimulus type. Those neurons stimulate other groups of neurons that will take care about stimulus reaction.
Each stimulus or mental activity causes different neurons groups to be activated, but same stimulus or mental activity will produce same neurons reaction (will activate approximate same regions for approximate same amount of time) for all people without any brain disorder.
Based on this fact different regularities in EEG have been discovered during different types of stimulation (acoustic, visual, touch) and during different types of mental activity (meditation, relaxation, sleeping, calculating…).
Brain potential segmentation
Spontaneous brain potentials
Alpha, beta, theta and delta are names for typical shape of so called spontaneous brain potentials because they are not effect of some stimulus from outside, but just from inner examinee state (meditation, relaxation, sleeping, calculating…).
I will describe each type of spontaneous brain potentials.
- Delta is the frequency range up to 3 Hz. It is seen normally in adults in slow wave sleep. It is also seen normally in babies. It may be seen over focal lesions or diffusely in encephalopathies.
- Theta is the frequency range from 4 Hz to 7 Hz. Theta is seen normally in young children. It may be seen in drowsiness or arousal in older children and adults; it can also be seen in meditation. Excess theta for age represents abnormal activity.
- Alpha is the frequency range from 8 Hz to 12 Hz. Hans Berger named the first rhythmic EEG activity he saw, the "alpha wave." This is activity in the 8-12 Hz range seen in the posterior head regions when an adult patient is awake but relaxed. It was noted to attenuate with eye opening or mental exertion. This activity is now referred to as "posterior basic rhythm," the "posterior dominant rhythm" or the "posterior alpha rhythm." The posterior basic rhythm is actually slower than 8 Hz in young children (therefore technically in the theta range). In addition to the posterior basic rhythm, there are two other normal alpha rhythms that are typically discussed: the mu rhythm and a temporal "third rhythm." Alpha can be abnormal; for example, an EEG that has diffuse alpha occurring in coma and is not responsive to external stimuli is referred to as "alpha coma."
- Beta is the frequency range from 12 Hz to about 30 Hz. Low amplitude beta with multiple and varying frequencies is often associated with active, busy or anxious thinking and active concentration. Rhythmic beta with a dominant set of frequencies is associated with various pathologies and drug effects, especially benzodiazepines. Activity over about 25 Hz seen in the scalp EEG is rarely cerebral (i.e., it is most often artifactual).
Brain waves: alpha, beta, gama, delta and theta
Event potentials
Event potentials occur is active environment when the examinee responds to environment stimuli.
Evoked potentials
They are brain reaction on stimulus with amplitude up to few hundred times lower then so called backgrounds EEG (voltage measured because of regular brain activity). There are many techniques for evoked potential estimation from EEG records. The most popular is averaging; The stimulus in each evoked potential test is applied many times (one or two thousand times), and since everything else besides the evoked potential is not related to the signal, it happens at various random times relative to the stimulus, whereas the potential that is evoked by the stimulus always occurs at the same time relative to the stimulus. This allows the computer to pick out and amplify the one consistent peak or series of peaks, which are caused by the applied stimulus. Beside averaging there are other so called single trial estimation techniques like Karhunen-Loeve Transform (KL) also known as Principle component analysis (PCA) (they are same for signal with zero mean, like EEG) that I will talk about in future articles.
Different evoked potentials occurs as a result of different stimulus, visual evoked potentials (VEP) are results of visual stimuli (light flashing, or changing colors), acoustic evoked potentials (AEP) are result of acoustic stimuli, Somatosensible Evoked Potentials (SEP) are results of touch stimuli.
Those evoked potentials are showing just brain knowledge about stimuli existence, but not its reaction on it, so they are called exogenous.
Exogenous stimuli diagnostic value is latency of its typical components (for VEP it’s negative component that occurs 75ms after stimulus – N75, positive component that occurs 100ms after stimulus – P100, negative component that occurs 135ms after stimulus – N135).
Visual evoked potential
As opposed then exogenous, endogenous are evoked potentials that occurs when brain analyze and understand stimulus meaning. Those potentials don’t depend on type of stimulus but on its meaning to examinee. Typical example is P300, positive potential that occurs 300ms after the stimulus. Diagnostic value of those stimuli is latency, same as for the exogenous potentials.
Anticipatory potentials
These potentials occur just before the stimulus, or target event if the examinee knows that the stimulus or the event will happen. When the examinee has to react on stimulus (press some button for example) we talk about preparatory potentials, and when reaction is not needed we talk about expectatory potentials.
|
