#REFERENTIAL MONTAGE DEFINITION SERIES#
The speed at which the paper moves on will also affect the appearance of the waveforms.Ī digital EEG system converts the waveform into a series of numerical values. For example a sensitivity of 10 µV/mm means that a signal with an amplitude of 100 µ V will produce a 1 cm vertical deflection. The sensitivity controls the size of the activity displayed. The high and low frequency filter values will set the window within which the EEG activity is recorded. The amplifier output is controlled by high and low frequency filters and sensitivity controls. The pen draws the trace onto paper moving below it. A pen mounted on the galvanometer moves up and down each time the coil moves. The output signal from the amplifier passes through the wire causing the coil to oscillate. A galvanometer is a coil of wire inside a magnetic field. For example the first amplifier may have electrodes FP1 and F3 connected to it and the second amplifier F3 and C3 connected to it.Ĭonventional analogue instruments consist of an amplifier, a galvanometer and a writing device. These sequentially link electrodes together usually in straight lines from the front to the back of the head or transversely across the head. All EEG systems will allow the user to choose which electrodes are to be included in this calculation. The resulting signal is then used as a reference electrode and connected to input 2 of each amplifier and is essentially inactive. Electrodes frequently used as the reference electrode are A1, A2, the ear electrodes, or A1 and A2 linked together.Īctivity from all the electrodes are measured, summed together and averaged before being passed through a high value resistor.
The same reference electrode is used for all channels. Each montage will use one of three standard recording derivations, common reference, average reference or bipolar.Įach amplifier records the difference between a scalp electrode and a reference electrode. The manner in which pairs of electrodes are connected to each amplifier of the EEG machine is called a montage. The resulting signal is amplified and then displayed as a channel of EEG activity. An electrode is connected to each of the inputs.ĭifferential amplifiers measure the voltage difference between the two signals at each of its inputs. Alternative methods consist of a cap in which the electrodes are already imbedded.ĮEG machines use a differential amplifier to produce each channel or trace of activity. The majority are small discs of stainless steel, tin, gold or silver covered with a silver chloride coating. There are a great variety of electrodes that can be used. Odd numbers denaote the left side of the head.Even numbers denote the right side of the head.The letter refers to the area of brain underlying the electrode The electrodes are then placed at points that are 10% and 20% of these distances.Įach electrode site is labelled with a letter and a number. This relies on taking measurements between certain fixed points on the head. These positions are identified by the recordist who measures the head using the International 10/20 System. Small metal discs called electrodes are placed on the scalp in special positions. The brain normally works hard to keep an equal amount of each of these neurotransmitters in the brain.ĮEG activity is quite small, measured in microvolts (µV) with the main frequencies of interest up to approximately 30 Hertz (Hz). There are two types of neurotransmitters, one will help the action potential to move to the next cell, the other will stop it moving to another nerve cell. Special chemicals called neurotransmitters help the signals to move across the gap. These action potentials move from one cell to another across a gap called the synapse. The nerve cells in the brain produce signals that are called action potentials. This activity is influenced by the electrical activity from the brain structures underneath the cortex. The waveforms recorded are thought to reflect the activity of the surface of the brain, the cortex. The first recordings were made by Hans Berger in 1929 although similar studies had been carried out in animals as early as 1870. The electroencephalogram ( EEG) is a recording of the electrical activity of the brain from the scalp.
0 kommentar(er)
