W3 - EEG Flashcards

1
Q

What is EEG in one sentence. What do they pick up?

A
  • Detect neural activity using electrodes on scalp
    • Pick up small fluctuations of electrical signals from activity of (mostly cortical) neurons
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2
Q

Is EEG extra or intra-cranial?

A
  • Extra-cranial/Scalp
    • Non-Invasive
  • Intra-Cranial
    • Measure directly at exposed cortex
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3
Q

Who invented EEG? When and how?

A

Hans Berger.

Detected first EEG with wife’s scalp in 1924.

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4
Q

What is the Alpha Rhythm

A
  • Inconsistent electrical signal varying between 8 - 13 Hz.
  • Resting signal when someone closed their eyes.
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5
Q

What is the pros of EEG

A
  • Cheap
  • Good Temporal Resolution
    • ms accuracy
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6
Q

What is the cons of EEG

A

1.) EEG signal biased to Gyri

  • Sulci harder to detect
    • Masked by gyri signals
  1. ) Meninges, CSF and skull “smear” EEG signal, makes localisation dificult
    * Inverse Problem
  2. ) Poor Spatial Resolution
    * 1-10cm (10-100mm)
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7
Q

What is the inverse problem.

A
  • 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
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8
Q

What is EEG signal measured in relation to

A

In relation to a reference electrode, which is either

  • a neutral point like nose
  • average of all scalp electrodes
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9
Q

How is EEG Recorded: What are the 4 tools

A

(1) Electrode Cap > (2) Amplifier > (4) EEG Recording
(3) Experimental Stimulation > (4) EEG Recording

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10
Q

What are the channels in EEG

A

10 – 32 – 64 – 128 – 256 channels

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11
Q

How are numbers on the scalp displayed in EEG

A

Split cortex odd and even

F = frontal P = parietal C = central O = occipital T = temporal

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12
Q

What is the neurophysiology of the EEG Signal.

What is it NOT

A
  • 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

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13
Q

What can the post-synaptic potential be considered as. Can we record one post-synaptic potental?

A
  • 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
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14
Q

Many neurons spatially align > summed potentials add up and create the signals we can record: What is this called? Where is the origin?

A

Pooled activity

  • From large number of similarly oriented neurons from large cortical pyramid cells
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15
Q

What is the functional unit of EEG? i.e. How many Neurons must be spatially aligned to record?

A

The functional unit is >10,000 simultaneously activated neurons

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16
Q

What determines the sign of the recorded potentials. Can all of them be recorded?

A
  • Orientation of the neurons determines the sign of the recorded potentials.
  • Some orientations lead to signals which cannot be recorded.
17
Q

Typical amplitude of EEG and Steps to make it clear

A
  1. ) 10μV to 100μV (Tiny)
  2. ) Amplified by factor of 1,000 to 100,000x
  3. ) Signal is typically digitalized. Typical sample frequency is 256-1024Hz, but can be >4000Hz
  4. ) Signal is band-pass filtered to remove the low (<0.5-1Hz) and high frequencies (typically >35-70Hz) because they cannot reflect brain activity.
18
Q

What is the most relevant step in EEG signal analysis. What are some examples?

A

Artefacts Removal, removing stuff that are not brain signals

  • Sweating
  • Electrical noise (“notch filter”)
  • Eye movements and blinks
19
Q

How does eye movement affect EEG. How do we prevent it?

A
  • 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
20
Q

Despite EEG signals being very noisy, the dominant frequency in the signal can be determined due to

A

The raw signal shows systematic variations, and more of a specific frequency

(Delta, Theta, Alpha, Beta, Gamma) inconsistent characteristic frequency

21
Q

What is wrong with single EEG-trial studies. What should we do then?

A

Noisy: Too much variance (Fluctuations)

(1) Between sessions from same participants
(2) Between participants
* Averaging over lots of trials will reduce noise.

22
Q

How does an ERP look like? What is P and N

A

Positivity is downwards.

Negativity is upwards.

23
Q

What are ways of reading ERP

A
  • 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

24
Q

Woodman and Luck (1999): What is the signal they used and what did it index.

A
  • N2pc as an index of attention.
  • Attending left = Stronger N2pc right hemsphiere
    • N2pc = 2nd Negative Posterior Contralateral
25
Woodman and Luck (1999): Study Aims, Overview and Hypothesis
_Aims_ Parallel / Serial _Visual Search Task_ Search a coloured square _Hypothesis_ **Serial**: Attention switch (N2pc) from one hemifield to the other, until the target is found . **Parallel:** No N2pc Switch
26
Woodman and Luck (1999): Study Methods. How did they get the participant to attend to one hemifield first?
* Manipulated probability specific colour was target (C75 and C25) to get people to attend to one hemifield * Particiapnts attend to C75 and can monitor attention while particiapants visually scanned
27
Woodman and Luck (1999): Study Results and Conclusion
**Target Absent** a) When C75 and C25 (same field), **no** shift in N2pc b) When C75 and C25 (contralteral), shift in N2pc **Target Present** a) When C75 _target_ and C25 (contralateral), **no** shift inN2pc b) When C75 and C25 _target_ (contralteral), shift in N2pc **Conclusion:** People search in serial *note looking at crossover of N2pc*
28
Gehring et al., 1993: Study Aims
Whether there is mechanism for the detection and compensation for errors.
29
Gehring et al., 1993: What is the signal of interest
_ERN_ * Negative deflection of up to 10μV in amplitude observed at central electrodes ~80-100ms after an erroneous response
30
Gehring et al. (1993): Study Methods. What was manipulated? And what is the hypothesis?
_Method_ Flanker-task (Middle Letter) _3 Conditions_ * Emphaise Accuracy * Emphaise Speed * Control H1.) Incongruent displays should lead to more errors H2.) Error detection should only matter in the accuracy condition
31
Gehring et al. (1993): First Hypothesis supported?
Yes. ERN on incorrect trial in comparison to correct trials
32
Gehring et al., 1993: Second Hypothesis supported? What else were they interested in finding out?
Yes. The ERN was strongest when people emphasised accuracy, and weakest for speed _But is the ERN indicative for compensating for errors?_ - If this were true, one would expect that the ERN should also reflect the attempt to break the error
33
Gehring et al., 1993: Further study of ERN compensation for errors (reflective of breaking the error): Method
Investigated how ERNs of different sizes were related to response parameters (might be involved in error correction) * Divided into quartiles from small to XL
34
Gehring et al., 1993: Further study of ERN compensation results
As ERN increases, * Lowe Response Force * Error correction * Higher Probability of right the next trial * Learning * Slower response on next trial * Post-error slowing * Learning