EEG Flashcards
Benefits of EEG…
• Excellent time resolution
– Cognitive, perceptual, linguistic, emotional and motor processes are fast and dynamic
> For example, consider theta band (4-8 Hz), a ‘slow’ rhythm but quite ‘fast’ for our conscious experience
– Or consider gamma (30-80 Hz)
- Direct indicator of neuronal activity
- Multidimensional (time, space, frequency, power, phase (temporal), connectivity etc)
- Portability (observing brain in action)
- Relatively inexpensive
+ adanvaced analyis techniques on time series e.g. single trial classifcation methods using fourier transform.
Limitations of EEG…
• It is not well-suited for precise functional localization
• It is not well-suited for measuring deep brain structures (e.g., putamen, thalamus, nucleus accumbens)
Sub-optimal method: where in the brain does process X occur or is information Y stored
• It is also not very well-suited to study very slowly fluctuating process ‘infra-slow’ with uncertain and variable time course (but fMRI is)
from Joy
Interpretation issues:
1) suffers with interpretation of null results – absence of proof is not the proof of absence.
> ERP does not reveal all of EEG information (single trial)
> ERP does not capture non-phase locked responses
2) ERP limited opportunity for linking results to actual neurophysiological dynamics
> ERP less understood than oscillatory (is formed) and synchronous
What type of neuronal activity does EEG capture?
postsynaptic potentials
as opposed to action potential
what does EEG reflect?
EEG reflects the differences of electrical potential over time, created by the current flows originating from neuronal populations
what are chemical synapses?
Chemical synapses cause local changes in postsynaptic membrane potentials, through neurotransmitters. Information transmits with some delay on the order of a millisecond.
what are electrical synapses?
electrical synapses, or gap junctions. Ions flow directly through large channels into adjacent cells, with no time delay.
What is a Post-Synaptic Potential (PSP)?
An electrical potential initiated at a postsynpatic site that can vary in amplitude and spreads passively across the cell membrane, decreasing in strength with time and distance
how is a post-synaptic potential generated?
– When AP reaches presynaptic axon end, a neurotransmitter is released into the synaptic cleft
– The neurotransmitter binds to the receptor of the postsynaptic neuron by opening or closing an ion channels
– This lead to a graded change in membrane potential
what two types of post-synaptic potential are there?
Two types of PSP
– Excitatory PSP (for excitatory synapse) – Inhibitory PSP (for inhibitor synapse)
what is required for a post-synaptic neuron to fire?
A postsynaptic neuron will fire an action potential if a depolarization that exceeds threshold reaches its axon hillock.
Generally the combined effect of many excitatory synapses is required for a post-synaptic neuron to fire.
What are the 2 types of summation ?
spatial and temporal
what is spatial summation?
is the summing of potentials that come from different parts of the cell.
If the overall sum – of EPSPs and IPSPs – can depolarize the cell at the axon hillock, an action potential will occur.
what is temporal summation?
Temporal summation is the summing of potentials that arrive at the axon hillock at different times.
The closer together in time that they arrive, the greater the summation and possibility of an action potential.
what are EEG signals then?
EEG signals are primarily produced by summation of postsynaptic potentials of millions of neurons
summed millions of neurons/ firing in phase
aggregated millions of PSP’s
note: EEG does not measure action potential !!!!
how are the AP alligned?
geormetrically and in phase
EEG is less sensitive to….
It is less sensitive to deep brain structures
– Field strength decreases exponentially with distance
– Neuronal populations in deeper structures are not arranged in a geometrically parallel fashion
EEG cannot mesure …
It cannot measure individual molecular or synaptic events nor it can isolate events that are produced by a specific neurotransmitter or neuromodulator
It is not very suitable to measure to very slow (< 0.1 Hz) or very high (> 100 Hz) fluctuations
what are electrodes made from?
– Ag/AgCl Electrodes (Silver electrodes with a thin coating of silver- chloride
– Tin Electrodes
– Goldcap Electrodes
The conductivity should be good between the electrode and the scalp, how?
gel to reduce the impedance/resistance
– Impedance below 5 Kilo Ohms
– Scalp preparation (removal of dead skin cells)
also:
Active Electrodes
– Integrated pre-amplifier
– Faster preparation time
How many electrodes ?
Traditional 19
Standard 32-64 (sufficient)
High-density 128-256 (or more)
What are the pros to having more elctrodes?
better spatial sampling
better source reconstruction
What are the cons to having more elctrodes?
long prep time
electrolyte bridge
poorer signal quality
what is electrolyte bridge?
when the gel creates short circuit between closely placed electrodes
the signal is amplified, why, and using what?
- The signal is amplified from a few μVolts to a few Volts.
* The amplification is done by Differential Amplifiers
what electrodes are associated with amplification?
Three electrodes: Active Electrode (A) placed at the desired site Reference Electrode (R) placed elsewhere on the scalp Ground Electrode (G) placed elsewhere on the scalp/body
• Elimination of ambient noise • Works best when impedances are same (low) for A and R
• Amplifier gain: 5-10 K
• Optimal gain depends on the
input potential and output range
What are the usual reference sites?
Preferably a ‘neutral’ site (tip of the nose, the earlobes, the mastoids, the chin etc)
• Three practical criteria
– Choose a site that is convenient and comfortable
– Choose a site that does not induce hemispheric bias – Choose a site used by other researchers in your field
• Mostly used ‘neutral’ references: – average of two earlobes
– average of two mastoids
• Other referencing scheme:
– Average of all electrodes
– Current source density maps • Reference free method • Requires high density recording • Less accurate for boundary electrodes • Insensitive to deep sources
–Laplacian
what is aliasing ?
When we are sampling a system (brain) with a sampling freq less than twice the maximum freq of interest.
Because we monitor frame by frame - but what rate?
If it is not as fast as the original, then it is a POOR representation, as we haven’t sampled enough to capture the information in the actual sample.
Nyquist Criterion - sample at least twice as fast as the maximum freq
(Sampling frequency (fs) should satisfy Nyquist Criterion fs > 2 fmax (fmax = max. frequency of interest)
This can be something like x5 the maximum - if we wanna do alpha etc.
why would we need to filter the signal?
to reduce artifacts
what filters are applicable ?
Low pass
High Pass
Band pass
band stop, notch
what is high pass filter?
0.5 Hz (or 0.1 Hz for slow brain responses)
what is low pass filter?
100 Hz
what is a notch filter (band stop)
50 Hz (for removing power line noise; 60 Hz in USA)
We need to xxx analogue to xxxx
convert - digital
what resolution is EEG
16/24 bit Resolution (216 or 16192 different voltage values can be coded by the ADC)
what is aliasing?
When we are sampling a system (brain) with a sampling freq less than twice the maximum freq of interest.
Because we monitor frame by frame - but what rate?
If it is not as fast as the original, then it is a POOR representation, as we haven’t sampled enough to capture the information in the actual sample.
Nyquist Criterion - sample at least twice as fast as the maximum freq
(Sampling frequency (fs) should satisfy Nyquist Criterion fs > 2 fmax (fmax = max. frequency of interest)
This can be something like x5 the maximum - if we wanna do alpha etc.
whay should we avoid aliasing?
to get a faithful representation of our sample
