EEG Flashcards
Why is it important to shield n form electrical noise in the environment?
• The signal we get with EEG is already tiny
• Noise from the environment is much bigger and would
significantly distort the signal
• shields n from this noise
How does oscillatory activity outside of n affect the signal picked up by the electrode
- Any oscillatory activity produces a magnetic field
- This magnetic field is picked up by the electrode
- Signal electrode picks up form this > than signal we are interested in
Can you describe the layout of the cage?
Cage – describe the layout?
- two screens one is inside the shield (digitization) and one is outside (computer sending stimuli)
- The outside screen sends stimuli into the inside screen
- Key = both screens are linked, stimulus screen sends pulse to digitization screen saying NOW! And links data to the sitimuli
- There is also the electrodes and amplifier in the cage and perhaps a response button pad for n
Why do we need multiple electrodes?
- EEG picks up on voltage, voltage is the electrical potential of current moving from A to B.
- Can’t be measured from one place
How many electrodes are needed in EEG? What are they
• EEG systems are differential amplifiers
• Need a minimum of 3 electrodes to work
> Active electrode – placed at a region of interest
> Ground electrode – placed somewhere convenient, it is linked to the ground circuitry of the amplifier
> Reference electrode – anywhere on the scalp, not region of interest
I though we just need to measure current moving from A to B? Why are 2 electrodes not enough?
- The ground electrode is linked to the ground circuitry of the amplifier
- Every electrical device has a ground circuitry – very noisy!
- We could look at the voltage between A and G and yes this would tell us something about how the current moves from A to B
- But it would be infiltrated by the noise of the ground circuit – we don’t want that
- Adding a third electrode allows us to subtract the noise
I though we just need to measure current moving from A to B? Why are 2 electrodes not enough?
- The ground electrode is linked to the ground circuitry of the amplifier
- Every electrical device has a ground circuitry – very noisy!
- We could look at the voltage between A and G and yes this would tell us something about how the current moves from A to B
- But it would be infiltrated by the noise of the ground circuit – we don’t want that
- Adding a third electrode allows us to subtract the noise
How does adding a reference electrode subtract the noise?
- Amplifier calculates the difference between: A-G and R-G
- Effectively gives us the difference between A and R, removing the noise from the ground circuitry
Describe a channel
- The three electrodes are combined into a single channel
- The waveforms we see on the screen are the potential from A-G and R-G
- So we look at the electrical flow of current in the brain using three electrodes
Why shouldn’t we have the reference site on the body?
- Must be on the scalp
- If on the body – picks up a signal produced by the heartbeat called EKG
- Dipoles we want to measure are tiny, signal from the heartbeat is massive
- Would massively interfere with the signal
Where should we place the reference electrode?
- Placed at neutral site ideally to allow us to see the flow of current towards the region of interest
- This is because the amplifier looks at the difference between the active and reference site, taking activity from both areas in to consideration
- ERP reflects activity from both sites
- We want a site with the least noise – otherwise will influence the results
what are some common reference sites used?
- tip of the nose (annoying for n)
- chin (annoying)
- mastoids (most common)
- earlobes
things to consider when choosing a site for the reference electrode
- Electrically neural as possible
- Convenient for n
- Not biased towards a particular hemisphere
- Are you comparing your findings with another study? Important to use the same reference site
- Where you place it in relation to the active electrode
When chosing a refernce site why do you need to consider how far it is from the active electrode?
- The location of reference affects the size of the thing we are measuring
- if both nearby, on the same hemisphere then the result (the voltage between both areas) wont be so big
- thus the absolute size of the ERP will differ based on the reference site used
Average mastoid reference – why can’t you just use one of the mastoids?
- Left mastoid or right mastoid
- Reference is biased towards one hemisphere
- Would lead to an imbalance In between the active electrodes in the left and right hemisphere
How do we calculate the average mastoid?
- How is this done – during recording you couple the active electrode and one of the mastoids (e.g., left) and reference this with the other (right) – after recording EEG is pretty cool – allows you to now play around with stuff. Thus after the recording is complete we calculate the average mastoid
- Subtract half the difference between the LM and RM from the active electrodes.
- = average mastoid reference
What does digitization do
- converts continuous signal (waveform deflections) into a series of numbers
- the more numbers the better
- the numerical range depends on the resolution of the digitizer
What is a digitizer government name
analogue-to-digital converter (ADC)
how does the digitiser convert a deflection into a number
• Record different voltage values
• Resolution of 12 bites = records 2 to the power of 12 or 4096 voltage values
• waveform goes up and down on the y axis with say 4096 diff points on the y axis (for 12 bite resolution)
• Amplifier is set to measure the difference between -5 and +5 microvolts – divided into 4096 points.
- 4096 is coded as +5 and 0 is coded as -5. The numbers are coded as somewhere between 0 and 4096.
What is “amplifier blocking”
- Something that prevents us from recording values above +5 (4096) or below -5 (0)
- If a voltage is out of this range it will be represented as either 4096 or 0
- Gain of the amplifier is set so these values it not exceeded – set to a range that makes sense. Otherwise, the values we get would not be meaningful. All it would tell us is if a value is smaller or higher than another
- Most EEG systems have 24 bites of resolution so the issue of amplifier blocking isn’t such a problem today
What does the Y axis of a waveform reflect?
- The size of the potential
* Measured in microvolts
What does the x axis of a waveform reflect?
time
How can we convert to a signal into a waveform
- So each time is associated with a value between 0 and 4096 (depending on the resolution of the amplifier), at another time point there is another value between 0 and 4096
- This is the waveform is constructed
Why do we have so much information with EEG waveform
- Because each time point is associated with a value between 0-4096
- And at each time point this isn’t just the value associated with one but 64 channels
- Very quickly becomes a lot of information
How can we reduce the information load of the continuous EEG signal
- Sampling
- continuous EEG signal is converted into s sequence of discrete-time points
- lower number of time points for each of the 64 channels