What is EEG in one sentence? What do they pick up?
- Detect neural activity using electrodes on scalp
- Pick up small fluctuations of electrical signals from activity of (mostly cortical) neurons
Is EEG extra or intra-cranial?
- Extra-cranial/Scalp
- Non-Invasive
- Intra-Cranial
- Measure directly at exposed cortex
Who invented EEG?
When and how?
- Hans Berger.
- Detected first EEG with wife’s scalp in 1924.
What is the Alpha Rhythm?
- Inconsistent electrical signal varying between 8 - 13 Hz.
- Resting signal when someone closed their eyes.
What are the pros of EEG?
- Cheap
- Good Temporal Resolution
- ms accuracy
What are the cons of EEG?
- EEG signal biased to Gyri
- Sulci harder to detec
- Masked by gyri signals
- Meninges, CSF and skull “smear” EEG signal, makes localisation difficult
- Inverse Problem
- Poor Spatial Resolution
- 1-10cm (10-100mm)
What is the inverse problem?
- If the diple solutions are known, the resulting scalp configuration of signals can be reconstructed
- However, one given scalp configuration of signal = Multiple dipole solutions
What is EEG signal measured in relation to?
In relation to a reference electrode, which is either:
- a neutral point like nose
- average of all scalp electrodes
How is EEG Recorded: What are the 4 tools?
- (1) Electrode Cap > (2) Amplifier > (4) EEG Recording
- (3) Experimental Stimulation > (4) EEG Recording
What are the channels in EEG?
10 – 32 – 64 – 128 – 256 channels
How are numbers on the scalp displayed in EEG?
Split cortex odd and even
F = frontal P = parietal C = central O = occipital T = temporal
What is the neurophysiology of the EEG Signal?
What is it NOT?
- EEG activity orginates from post-synaptic potential
- Voltage when NT binds to post-synaptic membrane’s receptor
- Causes ion channels to open/close, leading to graded changes in potential across membrane
Note: EEG does not record action potential
What can the post-synaptic potential be considered as?
Can we record one post-synaptic potential?
- A small dipole
- Signals from single cells are not strong enough to be recorded outside of the head
- If many neurons spatially align, then their summed potentials add up and create the signals we can record
Many neurons spatially align > summed potentials add up and create the signals we can record: What is this called and Where is the origin?
Pooled activity
- From large number of similarly oriented neurons from large cortical pyramid cells
What is the functional unit of EEG? i.e. How many Neurons must be spatially aligned to record?
The functional unit is >10,000 simultaneously activated neurons
What determines the sign of the recorded potentials?
Can all of them be recorded?
- Orientation of the neurons determines the sign of the recorded potentials.
- Some orientations lead to signals which cannot be recorded.
What is the typical amplitude of EEG and what are the steps to make a clear EEG output?
- 10μV to 100μV (Tiny)
- Amplified by factor of 1,000 to 100,000x
- Signal is typically digitalized. Typical sample frequency is 256-1024Hz, but can be >4000Hz
- Signal is band-pass filtered to remove the low (<0.5-1Hz) and high frequencies (typically >35-70Hz) because they cannot reflect brain activity.
What is the most relevant step in EEG signal analysis? What are some examples?
Artefacts Removal, removing stuff that are not brain signals
- Sweating
- Electrical noise (“notch filter”)
- Eye movements and blinks
How does eye movement affect EEG?
How do we prevent it?
- Eye = Dipole
- Signals from eye contimates EEG signal to large degree
- Record eye signal by placing electrodes next to and under the eye to capture horizontal and vertical eye movements
- Remove by excluding contaminated trials, or mathematical algorithms, such as ICA
Despite EEG signals being very noisy, the dominant frequency in the signal can be determined due to…
The raw signal shows systematic variations, and more of a specific frequency
(Delta, Theta, Alpha, Beta, Gamma) inconsistent characteristic frequency
What was wrong with single EEG-trial studies?
What should we do then?
Noisy: Too much variance (Fluctuations)
- Between sessions from same participants
- Between participants
- Averaging over lots of trials will reduce noise.
How does an ERP look like? What is P and N?
- Up and down fluctuations
- Positivity is downwards.
- Negativity is upwards.
What are ways of reading ERP?
- Peak-amplitude
- 70% of studies
- Area-under-the-curve
- 20% of studies
- Peak-to-peak
- 10% of studies
- Onset of component
- Ambiguous
No clear rule. Results will differ across methods
Woodman and Luck (1999): What is the signal they used and what did it index?
- N2pc as an index of attention.
- Attending left = Stronger N2pc right hemsphiere
- N2pc = 2nd Negative Posterior Contralateral
