2 Electroencephalography (EEG) Basic principles & Applications Flashcards
- Why Electroencephalography (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)
2.
List 4 positive aspects of EEG.
- Direct indicator of neuronal activity
- Multidimensional (time, space, frequency, power, phase, connectivity etc)
- Portability (observing brain in action)
- Relatively inexpensive (20k - vs 2million fMRI)
3.
Name some negative qualaties 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 with uncertain and variable time course (fMRI is better)
4.
EEG reflects the differences of ………………………… over
time, created by the ……….. flows originating from
……………………….
EEG reflects the differences of electrical potential over
time, created by the current flows originating from
neuronal populations.
5.
Name the two types of neuronal electrical activity.
(i) Action potential (AP) (an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory)
(ii) Postsynaptic potentials (PSPs) are changes in the membrane potential of the postsynaptic terminal of a chemical synapse. PSPs are graded potentials, and should not be confused with action potentials although their function is to initiate or inhibit action potentials
EEG reads signals from PSPs in groups of millions of neurons.
6.
What are sodium channels (Na+)?
Sodium channels are integral membrane proteins that form ion channels, conducting sodium ions (Na+) through a cell’s plasma membrane
7.
Mediation of the Action Potential by Voltage-Gated Sodium Channels:
List the 6 stages
- Resting State – Both sodium and potassium channels are closed and membrane is in the resting state.
- Threshold - A stimulus opens some sodium Na+ channels. If the influx of sodium Na+ achieves threshold, then more channels are opened, triggering an action potential.
- Depolarization Phase- Activation gates of sodium channels are open, but the potassium K+ channels remain closed. Sodium ions rush into the cell, the interior of the cell becomes more positive
- Repolarization phase- Inactivation gates close channels, and potassium channels open. Potassium ions leave the cell, and the loss of positive charge causes the inside of the cell to become more negative.
- Undershoot - Both gates of the sodium channels are closed, but potassium channels remain open because their relatively slow gates have not had time to respond to repolarization of the membrane. Within another millisecond or two, the resting state is restored and the system is ready to respond again.
- Back to Resting State ready to fire again
8.
Propogation of action potential - how are electrodes placed?
Electrodes may be placed along an axon and will collect data at different times depending on the direction of the action potential.
9.
…………………. cause local changes in postsynaptic membrane potentials, through ……………………….. Information transmits with some delay on the order of a millisecond.
Besides ……………….. there are ……………………. or gap junctions. Ions flow directly through large channels into adjacent cells, with no time delay.
Chemical synapses cause local changes in postsynaptic membrane potentials, through neurotransmitters. Information transmits with some delay on the order of a millisecond.
Besides chemical synapses there are electrical synapses, or gap junctions. Ions flow directly through large channels into adjacent cells, with no time delay.
10.
Generation of Post-Synaptic Potential (PSP)
Generation of PSP:
- When the AP reaches the 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 ion channels
- This leads to a graded change in membrane potential
11.
What are the two types of post-synaptic potential (PSP)?
- Excitatory PSP (for excitatory synapse - positive potential)
- Inhibitory PSP (for inhibitor synapse - negative potential)
A PSP is 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
12.
Recording Postsynaptic Potentials:
Draw an image of both excitatory and inhibitory neurons being read by EEG electrodes.
Red is excitatory (+)
Blue is inhibitory (-)
Depending on the neurotransmitter that is released the post synaptic potential can be either negative or positive.
- Electrical Signals are the Vocabulary of the Nervous System
…………. perform information processing to integrate ………………………..
A …………………. will fire an ……………………….. if a …………………. that exceeds threshold reaches its …………………….
Generally the combined effect of many …………………………….. is required for a …………………… neuron to fire.
Neurons perform information processing to integrate synaptic inputs.
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.
14.
Spatial Summation in a Postsynaptic Cell (Part 1)
…………………………… cause a cell to fire.
Excitatory inputs cause a cell to fire.
15.
Spatial Summation in a Postsynaptic Cell (Part 2)
……………….. also plays a role.
Inhibition also plays a role.
Inhibition counteracts excitation, no AP
16.
There are two types of summations:
What is spatial summation?
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.
17.
There are two types of summations:
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.
18.
…………….. are primarily produced by summation of ……………………… of millions of …………….
EEG signals are primarily produced by summation of postsynaptic potentials of millions of neurons
19.
List 5 things EEG is not:
Include statements on:
meaurement, sensitivity, brain regions and speed.
- It cannot measure all neural events
- It cannot measure individual molecular or synaptic events nor it can isolate events that are produced by a specific neurotransmitter or neuromodulator
- 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
- It is not very suitable to measure to very slow (< 0.1 Hz) or very high (> 100 Hz) fluctuations
20.
Sample MC Question EEG
Signals represent summation of:
A.action potentials
B.post-synaptic potentials
C.a mixture of A and B
D.neither A nor B
B. post-synaptic potentials
21.
Sample Brief Question
(a) Describe briefly how post-synaptic potentials are generated.
(b) Also describe briefly the procedure by which the post-synaptic potentials are integrated
a) When the AP reaches the 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 ion channels
This leads to a graded change in membrane potential
b) Neurons perform information processing to integrate synaptic inputs. Generally the combined effect of many excitatory synapses is required for a post-synaptic neuron to fire. IPSPs and EPSPs are summed together.
22.
Who undertook the first EEG recording in Humans?
Hans Berger (1873-1941)
23.
Draw a diagram of the EEG setup:
24.
Detail the electrondes used in EEG:
• Metal (conductive) –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 –Electrode gel to reduce the impedance/resistance
- –Impedance below 5 Kilo Ohms
- –Scalp preparation (removal of dead skin cells with alchohol cleaner wipes)
•Active Electrodes (relatively new last decade or so)
- –Integrated pre-amplifier
- –Faster preparation time
25.
What locations are used for EEG electrode placement?
- International 10-20 Electrode Placement System Jasper (1958) EEG Clin Neurophysiol
- All electrondes are placed at certain locations related to specific distances - F for frontal - P parietel - T for Temporal
- Odd numbers are for left hemishphere
- Even numbers are for right hemisphere
26.
Electrode placement
Fp = ?
C = ?
O = ?
T = ?
P = ?
Fp = Frontal pole
C = Central
O = Occipital
T = Temporal
P = Parietal
27.
How many electrodes are used in:
a traditional
b Standard
c High density
a) Traditional 19
b) Standard 32-64
c) High density 128-256 (or more)
Pros: Better spatial sampling, Source reconstructions
Cons: Long preparation time, Electrolyte bridge, Poorer signal quality
Rule of Thumb: Unless you expect precisely localized brain activity, 64 electrodes will be sufficient
28.
What is the Three Steps Procedure with Active Electrodes?
- Select cap to suit the size of participant
- Apply gel to each electrode location in cap
- Connect electrodes
No hair is hard - the skin is exposed and the skin gets thickened and not very conductive.
If using passive electrodes, prepare the skin at first and then follow these 3 steps.
29.
The amplification is done by ……………………… The signal is amplified from a few ……… to a few ………..
List the 3 different types of electrodes.
The amplification is done by Differential Amplifiers. The signal is amplified from a few μVolts to a few Volts.
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
–Amplifies AG – RG (whereas AG = A – G; RG = R – G)
30.
Ambient noise reduction in EEG?
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
31.
What is the reference site and what are three paractical criteria to use?
- 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
32.
Most used ‘neutral’ references:
- average of two earlobes
- average of two mastoids
33.
Other referencing scheme:
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
34
Why are analog filters required?
- Avoiding ‘aliasing’ (sampling frequency is less than 2 x maximum frequency)
- Reducing artefacts
35.
What are the 4 types of analog filters?
- Low pass
- High pass
- Band pass
- Band stop, Notch
Typically:
- High pass: 0.5 Hz (or 0.1 Hz for slow brain responses)
- Low pass 100 Hz
- Notch 50 Hz (for removing power line noise; 60 Hz in USA)
36.
Digitization (Analogue to Digital Conversion) is required for EEG what is the resolution and sampling frequency?
16/24 bit Resolution (216 or 16192 different voltage values can be coded by the ADC)
Sampling frequency (fs) should satisfy Nyquist Criterion
- fs > 2 fmax (fmax = max. frequency of interest) In practice, fs > 5fmax
Need to avoid aliasing (slow sample)
Niquist therorem - sampling rate must be at least twice the Hz of sampling frequency.
39.
What are probelms in EEG signals called?
Artefacts
42.
What is Independant Componant Analysis (ICA)
ICA is a quite powerful technique and is able (in principle) to separate independent sources linearly mixed in several sensors. For instance, when recording electroencephalograms (EEG) on the scalp, ICA can separate out artifacts embeded in the data (since they are usually independent of each other).
44.
What are the standard frequency bands?
- Delta: 0 – 4 Hz
- Theta: 4 – 7 Hz (Stage1- 3-7cps)
- Alpha: 8 - 14 Hz (Drowsy - 8 - 12 cps stage 2 12- 14 cps +K complexes)
- Beta: 15 - 30 Hz (Awake low voltage - random fast)
- Gamma > 30 Hz
Note: This division is arbitrary, mostly by convention
45.
Fourier Analysis
Transformation of the EEG into sine functions of various frequencies
50.
ERP Model: Signal Averaging
What is the ERP hypothesis?
ERP Hypothesis: ERP is a signal (s) that appears superimposed and without interaction on the background or ongoing EEG which is considered random noise (n).
Below: (where i being the trial index x-th point of time N is the total trial number)
55.
What is the “baseline” period used for in ERP analysis?
“Baseline” period: In averaging, all trials are set (arithmetically) to have the same zero voltage at stimulus onset, so that only deviations from the baseline voltage are seen in the ERP, after stimulus presentation.
Baseline subtraction (mean of baseline period is subtracted)
56.
Averaging to form a …………….
Averaging to form a VEP (visually evoked potential).
58.
…………………. increases as a function of the square root of the ……………………….
SNR (signal-to-noise-ratio) increases as a function of the square root of the number of trials.
Amount of noise left after averaging = (1/sqrtN)R
R = amount of noise on a single trial
N = total number of trial
62.
List the 4 ERP parameters.
- Latency (ms) (when is the peak reached?)
- Amplitude (μV) (what is the peak value?)
- Polarity (-/+) (is it positive or negative?)
- Distribution over the scalp (how are the electrodes distributed?
63.
Describe the use of Visual Evoked Potentials (VEP) in EEG research.
Presenting stimuli at intervals longer than a sec elicits a transient VEP
Presenting stimuli at faster rate elicit Steady State VEP (SSVEP)
- Provides fine grained information of early visual processing
- Routinely used to assess the integrity of the visual system from retina, via optic chiasm, to primary visual cortex
- Used in several clinical cases:
- Multiple sclerosis
- Visuospatial neglect patients
- Localization of optic lesions
64.
Describe the use of Auditory Evoked Potentials (AEP) in EEG research.
List:
- Early AEP,
- Middle latency Potentials
- Late Latency Potentials
1) Early AEP/ Brainstem AEP
- Testing the integrity of primary auditory pathways
- Localizations of deficits/damages
- Detection of early hearing loss in infants
2) Middle Latency Potentials
- 10 ms
- Possibly thalamic and very early cortical responses
- Earliest attention related effect (P1)
- Monitoring purpose in anesthetized conditions
3) Late Latency Potentials
- Cortical origin
- Reflect both the sensory processing and ongoing cognitive processing (i.e. attention)
- N1-P2 complex
- Developmental language disorders, dyslexia
65.
Describe the use of Somatosensory Evoked Potentials (SEP) in EEG research.
Various Sub-components
- Early N10 reflect action potentials from the peripheral nerves
- Subcortical (thalamic) components
- Cortical components for later latencies
Applications
- Tracking the somatosensory pathways from periphery to cortex
- Estimation of peripheral conduction velocities
- Patients with spinal chord injury, traumatic lesions, multiple sclerosis
- Classification of patients (i.e. apallic syndrome from locked-in states)
- Neuromonitoring purpose (spinal chord) during surgery
66.
Describe the use of Chemosensory Evoked Potentials in EEG research.
- Issue of timing
- No clear presence of early ERP components
- Amplitude is modulated by the concentration of odorant
- Useful for ageing study (olfactory discrimination degrades with normal ageing)
- Early detection of Alzheimers (olfactory impairment precedes cognitive impairment in AD)
67.
Describe the use of Contingency Negative Variation (CNV) in EEG research.
- Indicator of learning paired stimuli (Get Set – Go)
- Reflection of attention, concentration & readiness to S2
- Index of neuronal excitability
Interesting fact that if asked to move a finger the desision is activated subconsiously before we move - big implications on free will.
68.
N400 in Semantic Violations Kutas and Hilyard (1980) Science
A classic study where participants are exposed to semantically meaningless sentances.
When semantically wrong the N400 has a peak.
70.
Music & Language: Semantical Priming Koelsch et al. (2004) Nat Neuro
Similar semantic priming effect in language and music
Medial temporal gyrus (neighbour to superior temporal sulcus STS)
- Center for semantic integration
N400 again implicated.
71.
Music: Rule Violation Koelsch et al (2000)
J Cogn Neurosci
- Music, like language, has “rules” – called syntax
- A listener expects specific musical events according to a preceding musical context
- Ex: Chord functions with final chord as regular (harmonically expected) or irregular (unexpected) Remember: The last chord itself is a valid musical entity, only the prior context makes the difference!
73.
Dynamic Brain Oscillations
……………………….. have strict phase relationship to the stimulus
……………………. do not have strict phase relationship, Thus, ERP represents only ……………. not ………………..
Evoked oscillations have strict phase relationship w.r.to the stimulus.
Induced oscillations do not have strict phase relationship Thus, ERP represents only evoked activity not induced activity.
75.
Visual Feature Binding in Adults
Tallon-Baudry et al (1996) J Neurosci
Three stages: Illusionary triangle - real triangle and No-triangle.
76.
Visual Feature Binding in Infants
Csibra et al (2000) Science
77.
Oscillations in Complex Cognition
Sandkuhler, Bhattacharya (2008) PLoS ONE
78.
Neuronal precurser to Aha!
Posterior gamma for sudden solutions
81.
Dense Brain Network in Musicians during Attentive Listening to Music
Higher Gamma Band Synchrony in Musicians
