Modulo Cona Flashcards
What does the EEG signal reflect?
EEG reflects a summation of post synaptic potentials (PSPs), which are electrical potentials generated in the extracellular fluid of post-synaptic neurons, that have a duration of tens-hundreds ms and are confined to the dendrites and cell bodies of cortical pyramidal cells that are:
• co-activated (synchronous activity)
• co-aligned (similar cortical orientation, parallel to each others )
• aligned perpendicular to the surface of the cortex
Excitation of the postsynaptic neurons creates an extracellular voltage near the neural dendrites that is more negative than elsewhere along the neuron. In this way the neuron become a dipole which came be summed to other dipoles (with some requisites) and give rise to an electrical signal that can be recorded
What’s a dipole?
A region of positive charge separated from a region of negative charge by some distance.
Is it a single neuron’s dipole enough to be measured?
No, a single neuron’s dipole is too small to be measured as far away as the scalp. The sum of many individual dipoles in an area is measurable as a single dipole. Electrodes will measure the sum of both the positive and the negative ends of dipoles in the brain.
Is it always possible to detect neurons’ dipoles?
No, in order to produce a measurable (nonzero) signal, a large number of neurons must be both:
• arranged in a parallel fashion (otherwise they could cancel each other out, or in case of random arrangement, no clear dipole emerges) and perpendicular to the cortex
• synchronously active
Only in this way neurons can create an electrical signals large enough to propagate through the brain, dura, skull, and skin
Which type of connections are thought to drive the synchronous activity that gives rise to EEG?
A combination of thalamocortical and corticocortical
What’s volume conduction?
Volume conduction is the process by which a pool of ions repels nearby ions of the same charge, resulting in a “wave” of charge that travels through the extracellular space
Volume conduction is responsible for the propagation of the EEG signal within the brain.
Once the volume-conducted signal reaches the edge of the volume it is traveling through, volume conduction can no longer occur.
Which are the steps for physiological recording?
• Electrode or transducer placement
• Amplification and filtering
• Analog (polygraph) or digital (computer) recording
What does the electrode measure?
An electrode’s output is a single voltage per measurement typically taken every 1–2 ms.
A measurement of voltage at any electrode on the scalp consists of the sum of influences from many sources of electric fields because the brain is full of dipoles
What’s the impedance?
Impedance is a measure of the opposition that
an electronic component, circuit, or system
offers to the flow of an alternating current at a
particular frequency. It is typically measured in
ohms (Ω)
Which are the characteristics of the electrode?
- Skin surface electrodes
- Good electrical conductors (metal discs)
- Silver-silver chloride (Ag/AgCl) electrodes are
preferable (smaller bias potential and minimal polarization)
How to do the electrode application?
First you have to do skin preparation by cleaning the skin with alcohol or water and rub it a little bit (abrasion) to reduce the impedance to < 5-10 kΩ. Then an electrolyte paste or gel
has to be applied. Since the gel is a conductor, the signal reaches the electrode with less attenuation than if the gel were not present.
Why is Electrode Gel Necessary?
While cerebral spinal fluid and various ion-filled
substances in the brain are very good conductors, they are separated from the electrode by several layers of poor
conductors, including the skull, dead skin cells, hair, and air in the interstices of hair.
The highly conductive electrode gel will saturate the space beneath an electrode, filling in the air pockets between hairs and thus providing a conductive path from scalp to electrode
What’s the International 10/20 System?
A standard system for electrode placement, electrodes are placed at sites 10% and 20% of the total front-back or left-right distance of the skull, based on four anatomical landmarks:
- nasion and inion
- periauricular points
Each electrode’s position is identified with a letter and a number, the letters refer to areas of the brain, while numbers indicate laterality and degree of displacement from the midline (Z).
Which types of montage there are?
Bipolar montage: both electrodes are placed on active sites, used in EKG, EMG. The potential difference between two active sites located in the area of interest is recorded. This configuration is mostly used to record limited changes in potential (e.g. EMG) or in medical tests (e.g. for epilepsy).
Monopolar montage: one electrode is placed on an “inactive” site used as a (common) reference site and one electrode is placed on an active site, used for EEG. The potential difference is recorded between the electrode placed on the active site (of interest) and the electrode placed on the reference site (reference)
Which types of electrodes?
Active electrode that is placed at scalp site where voltage is changing
Reference electrode that should be placed at neutral site.
What are the most neutral sites in our body?
Linked mastoids, earlobes, Cz, Average reference, even if there is no neutral site in our body
How to chosen the reference?
Choice of the reference depends on: the number of electrodes, ERPs
of interests.
Typically:
- 1 electrode on top of the head (Cz)
- Average between electrodes on the two ears
- Average of all connected electrodes (average reference). If many
electrodes
- Average of two mastoid references
- Single mastoid (left or right) reference
-Linked
- Nose reference
New method: REST– reference electrode standardization technique. It’s a
reference that tends to infinite (Yao, 2001)
How does the amplification works?
The amplifier increases the amplitude of the signal to a level (at least 1 V) that can be accepted by the recording system
What’s the amplification factor or gain?
Is the amount by which the very small electrical signals generated by the brain are increased in strength so they can be accurately recorded and analyzed. Is obtained by Vout/Vin
Why is the Amplifier Necessary?
The purposes of an amplifier in an EEG system are to:
1) to maximize the Signal to Noise Ratio of the measured voltage,
2) increase the size of a signal above the size of noise that may be introduced in later elements of the circuit
How does the recording works?
After the analog recording, the signal is converted into a numerical form through
the analog-to-digital (A/D) converter and stored in a computer for further processing
What’s the sampling rate?
The speed at which the converter samples the signal, which should be at least 2-5 times faster than the fastest frequency component in the signal (Nyquist rate)
What is signal?
Signal refers to the portion of measured voltage that reflects the brain
What is noise?
Noise refers to the portion of the voltage that reflects other sources
