Electrophysiological neuroimaging Flashcards
What was Berger’s EEG recording system?
String galvanometer
Current flows through conducter, creates mag field perpendicular to current flow, if its inside a mag field,
creates an oscillation
The source of current in EEG isnt PA bc they’re too small
What does EEG measure?
Not action potentials
Excitatory post synaptic potential in apical dendrites
Bc the cells are ligned up, we get dipoles that sum together, if neurons werent ligned up, we would get potentials that cancel each other out
What is an electromagnetic field?
Current creates mag field, can also be used as a measurement of mag field (right hand rule), mag field perpendicular to the flow of current
* Electromagnetic field is a physical field produced by electrically charged objects (e.g., a piece of brain tissue).
* It has properties of both electricity and magnetism.
Theres and elect field that runs inside
the neuron and also outside bc brain
fluid is a conductor
Mag field isnt blocked by bio tissues (doesnt get diverted)
Mag field represents the same info as electric field, its just perpendicular
How do we go from neurons to electromagnetic field?
- Electrophysiological activity of neurons generates an electromagnetic field. The field activity due to apical dendritic activity of cortical pyramidal neurons is measurable non-invasively.
- The measurable signals are restricted by cytoarchitecture, gross anatomy of the cortex, and the sensor locations (on/nearby the scalp).
We can measure the signal if enough neurons are active, if neurons are properly aligned and if we put the electrodes in the right spot (as close as possible to the source
What is EEG?
We’re recording a voltage, the current is the driving force and the flow depends on the resistance
We dont normally record EEG relative to ground but relative to a reference, we do this bc there are lots of signals picked up by the electrodes that we’Re not interested in (ex <3 creates very
strong electric signal, electromag activity in a room), reference electrodes pick up all the noise but hopefully not the brain (put it behing ears or on nose) but has to be close enough to
recording electrodes and are affected just as much by noise as recording electrodes are
So when we subtract, what we’re left with is brain activity
What are EEG electrodes?
- Made with conductive materials, e.g., tin and silver
- Made in various shapes
- Used with or without gel (dry electrode) Electrodes just have to be good conductors (as low resistance as possible) If inject metal mesh in the skin, risk of infection but can be in place for long period of time without moving (good for tracking epilepsy)
Other electrodes rely on adhesion with skin and gel for low impedance then impedence and resistance can change
What is contact impedance?
- The contact impedance is electric resistance between an electrode and the scalp. * High impedance means low signal-to-noise ratio, i.e., bad EEG signal. * Electrode gel/paste is applied to decrease the impedance. Can break the epidermis layer to dim impedance
Where do we put electrodes?
Imp to have standardised method so research can be reproductible
Systems based on anatomical landmarks, line from nose to back of head, from tragus to tragus and the divide it up by the 10-20 syst or 10-10 syst (percentages of the distances between
points) then create arcs , odd numbers on left, even # on right, f = frontal, t = temporal, c= central, p = parietal, o = occipital
Where do we put ground and reference electrodes?
Ground on forehead
References on Tip of nose, Nasal cavity, mastoid or earlobe
Nasal cavity = low resistance pathway
Can record EEG from nasal cavity and mastoid but signal is smaller and more contaminated
Why do we use EOG electrodes?
Lots of artifacts come from eye mvmts (causes flux fields and electrical activity in muscles)
Can identify these pts later on and exclude
them or use data analysis to project them out
of the system
Why do we do 3D digitization of electrode locations?
The electrode positions arent the same on everyone, they’re roughly the same but everyone’s skull shape is different so we can make 3D digitizations, collect all the positions of the
electrodes (by infrared, electromag or ultrasound) so it maes source reconstruction better, need knowledge of geometry, need model of the persons head geometry (which electrode
would the signal go most depending on where the signal source was)
Digitization is very imp if we want to do source localisation (inverse prob)
What are 4 types of electrodes?
- Scalp electrodes pick up electric signal from the brain. record signals of interest
- A ground electrode protects participants from current leakage.
- A reference electrode provides a biological baseline. Records electrical activity that isnt coming from brain
- EOG, ECG, and EMG electrodes are attached to monitor the artifacts.
What are 2 types of EEG amplifiers?
- Alternating current (AC) amplifier
- Direct current (DC) amplifier
Measured signal is in microvolts but recorder needs it to be in volts to capt it so have to amplify it
What is analogue to digital conversion?
- Analogue EEG signal is digitized to have time series of voltage values
- Sampling frequency: the rate of digitization
Take continuous recorded signal (analogue) and convert it into numbers (digital)
What is the AD level?
*Amount of information in each sample 24
How to represent the signal in the amplitude domain
Whether or not theres a signal in a certain band (bits), see whether
or not the signal is above a certain threshold
The more bits we can dedicate to a signal, the better that signal will
be represented/recreated (more faithful recreation)
Having more bits requires more power which can be dangerous for the
EEG subject
