Lecture 2: Neuronal basis of EEG and ERPs

Question 1

Two hypotheses (by Global Neuronal Workspace Theory (GNWT) and by Integrated Information Theory (IIT)) have been formed around consciousness. Explain these two hypotheses.

Answer 1

• Global Neuronal Workspace Theory: consciousness is only possible if several brain regions synchronize and get activated at once.
• Integrated Information Theory: consciousness is also possible when only a specific brain region gets activated (without for example activating the prefrontal cortex).

Question 2

Why is improvement of prevention, diagnosis and therapies for brain disorders so important?

Answer 2

Because brain disorders are responsible for 35% of healthcare costs, but only 50% of brain disorders are treated.

Question 3

What are advantages of EEG?

Answer 3

• Direct reflection of activity
• High temporal resultion
• Greater specificity
• Non-invasive
• Availability
• Inexpensive

Question 4

Answer the following questions about fMRI, PET, MEG and EEG.

1. What technique has the highest temporal resolution (so the least delay)?
2. What technique has the lowest temporal resolution?
3. What technique has the highest spatial resolution?
4. What technique has the lowest spatial resolution?

Answer 4

1. Highest temporal resolution: EEG, it can measure electrical changes within milliseconds (after this MEG).
2. Lowest temporal resolution: PET
3. Highest spatial resolution: fMRI
4. Lowest spatial resolution: EEG, since it can detect minor changes in specific neurons.

Question 5

Question in the powerpoint which he doesn’t discuss and I can’t find the answer for it: Why do you think the temporal resolution for fMRI has such a broad range (is it actually 0.1 or is it 8 seconds..??)?

Answer 5

fMRI’s temporal resolution is limited by hemodynamic response time; typically the BOLD response has a width of ~3s and a peak occurring ~5–6s after the onset of a brief neural stimulus. This is much slower than the underlying neural processes, and temporal information is thereby heavily blurred.

(Answer from internet)

Question 6

Explain how electroencephalography (EEG) works.

Answer 6

When a neurotransmitters arives in the synaptic cleft, it can activate ion (sodium) channels. When these open, (positive) ions will flow from the outside to the inside of the cell. Since positive ions flow inside the cell, the negative charge outside the cell increases while it decreases inside the cell. Causing formation of an electric dipole neuron. Since our brain mostly contains water, electrodes can be used to measure potential differences. (Once positive ions have entered the neuron/cell, they will be released back into the extracellular space).

Question 7

Depicted in this picture is placement of electrodes in the 10-20 system (“10” and “20” refer to the fact that the actual distances between adjacent electrodes are either 10% or 20% of the total front–back or right–left distance of the skull). What do the letters stand for?

Answer 7

• Fp, pre-frontal
• F, frontal
• T, temporal
• P, parietal
• O, occipital
• C, central
• Cz, electrode placed on the midline sagittal plane of the skull ans is a reference point.

Question 8

Depicted in this picture is placement of electrodes in the 10-20 system (“10” and “20” refer to the fact that the actual distances between adjacent electrodes are either 10% or 20% of the total front–back or right–left distance of the skull). What do the numbers stand for?

Answer 8

Even-numbered electrodes refer to electrode placement on the right side of the head, whereas odd numbers refer to those on the left.

Question 9

Depicted in this picture is placement of electrodes in the 10-20 system (“10” and “20” refer to the fact that the actual distances between adjacent electrodes are either 10% or 20% of the total front–back or right–left distance of the skull). What is the meaning of inion and nasion?

Answer 9

• Inion: crest point of back of the skull (bump)
• Nasion: depressed area between the eyes, just above the bridge of the nose.

Question 10

Why is Cz used as reference?

Answer 10

Cz is placed on top of the skull. Here, the electrode is not influenced by e.g. facial muscles.

Question 11

You can’t measure potentials of a single electrode with EEG. Why is this and what do you measure?

Answer 11

Because an electrical potential, is the difference between one place and another, thus one electrode, being one point in space, can’t have a potential. Therefore you measure the potential difference between two electrodes (which is why you need a reference electrode).

Question 12

Just read (will be discussed seperately after): The following factors influence the EEG signal.

Answer 12

1. Position of reference electrode
2. Electrical contact between scalp and electrode
3. Conductivity of the head
4. Number of active neurons
5. Synchronicity
6. Orientation of the cells
7. Distance from source to electrode
8. Orientation of dipole
9. Site of activation on the neuron
10. Excitation vs. inhibition
11. Artifacts

Question 13

Why is it not enough to just measure the difference in potential between the reference electrode Cz and another electrode (this is called the common reference)?

Answer 13

Because we would then consider the potential difference of the reference electrode Cz as 0. This would mean that how further away you get from Cz, the more positive the potential difference will be. This is of course not the case.

Question 14

What is a solution to the common reference?

Answer 14

The average reference. Here you take the average of the common reference whereafter you substract this average reference potential from the different electrodes.

Question 15

Just look at the picture and see if you understand why taking the average reference based on the Cz reference is the best way to display EEG signals.

Answer 15

Ok

Question 16

How can electrical contact between scalp and electrode influence the EEG signal?

Answer 16

Air and grease (greasy hair) are good insulators for electricity. This is why direct contact (as much as possible) between the electrode and the scalp is needed.

Question 17

How can conductivity of the head influence the EEG signal? And what patients are typically best suitable for EEG?

Answer 17

Water is a fairly good (electrical) conductor whereas bone is not. So a thick skull will attenuate electrical fields more than a thin one. Therefore babies are best suitable for EEG, since their skulls haven’t developed fully yet.

Question 18

How can the number of active neurons and synchronicity influence the EEG signal?

Answer 18

Because of the fast decay of electromagnetic fields with distance from souce to sensor, we need mass-neuronal synchronous activity. Imagine if all the neurons fired independently from each other you would get different EEGs with different potentials (negative and positive). These negative and positive potentials would then balance each other out. Therefore synchronized neuronal activation can lead to an amplified EEG signal (macroscopic dipole).

Question 19

How can the orientation of the cells/neurons influence the EEG signal?

Answer 19

Star-shaped interneurons have a closed field configuration, which does not create an electrid field at long distances. Whereas the parallel alignment of the apical dendrites of pyramidal cells optimally supports spatio-temporal summation of currents.

Question 20

How can the distance from source to electrode influence the EEG signal?

Answer 20

The electric field (E) decays with 1/r^2 (r=distance to the dipole). So superficial (cortical) sources are detected better than deep ones (hippocampus, thalamus etc.)

Question 21

So can the amygdala (prime target for neurofeedback for emotional regulation) be measured through EEG?

Answer 21

No, it requires fMRI because it is so deep.

Question 22

How can the orientation of the dipole influence the EEG signal?

Answer 22

If you take the picture as an example: the arrow represents the direction of the dipole. This means positive ions are flowing in the direction of the arrow, which causes most of the negative potential to be on the opposite side of the direction of the arrow. Therefore the signal is only maximal right above the source if the dipole is radial.

Question 23

What is the oscillatory behaviour of a dipole?

Answer 23

It means that the dipole constantly changes direction for a few miliseconds.

Question 24

So it is known that information presented to the left half of the visual field passes to the right hemisphere, whereas the left hemisphere receives signals from the righ half of the visual field. But what paradox became known?

Answer 24

Paradox in the lateralisation of the visual event-related electrical potential (ERP). When you stimulate the left hemisphere, the EEG signals are still at their strongest at the left hemisphere (while actual processing of stimulus happens in the right hemisphere).

Question 25

What explanation is there for the paradox in the lateralisation of the visual event-related electrical potential (ERP)?

Answer 25

The dipole that gets activated in the primary visual cortex generates different potentials across the brain and electrodes: - The electrode that's directly on top of the dipole hardly picks up on any potential (fluctuations). - While in the direction of the arrow (the current goes towards the left hemisphere), it will create large fluctuations in potentials.

Question 26

How can the site of activation on the neuron influence the EEG signal?

Answer 26

A dipole will get activated when an axon delivers a neurotransmitter to the dipole. Where the neurotransmitter arrives (i.e. where an axon activates the dipole) determines where negative potential will be created. So superficial and deep layer activation may lead to opposite EEG polarity (see picture).

Question 27

How can excitation vs inhibiton influence the EEG signal?

Answer 27

With excitation positive ions (like Na+) will flow from the extracellular space to the intracellular space. This creates negative charge outside the cell. With inhibition negative ions (like CI-) will flow from the extracellular space to the intracellular space, which creates a positive charge extracellularly. Since current moves from higher to lower potentials, with inhibiton the currents will run in opposite direction.

Question 28

Radial and tangential dipoles (sulci vs. gyri) result in different EEG topographies. How?

Answer 28

If the dipole is located in/on a gyrus, the dipole will be radial. This will create the highest positive potential on top of the head. If the dipole is located in/on a sulcus, the dipole will be tangential, which creates the highest positive potential on the top left part of the brain (due to spherical shape of the head).

Question 29

Fill in: Because of radial and tangential dipoles, an EEG above a tangential source will be ... and an EEG above a radial source will be ....

Answer 29

Because of radial and tangential dipoles, an EEG above a tangential source will be 0 and an EEG above a radial source will be maximal.

Question 30

What two things can be concluded based of this picture of EEG analysis?

Answer 30

1. If the signal originates from a gyrus or sulcus 2. If the signal originates from a deeper laying structure (or superficial).

Question 31

How can you determine: 1. If the signal originates from a gyrus or sulcus 2. If the signal originates from a deeper laying structure (or superficial).

Answer 31

1. It will originate from a gyrus (and thus is radial) when positive potential slowly fades to negative potential. It will originate from a sulcus (and thus is tangential) when there is a clear positive and negative side. 2. The signal originates from a deeper layer structure when the gradient of the contours are less closely spaced and originates from a more superficial layer when the gradient of the contours are more closely spaced (think of the distance from source to electrode).

Question 32

What is the inverse and forward problem?

Answer 32

So consider data from a scalp topography with potential differences and a 3D-MRI picture of the brain (so folds and location of white matters is known so you also know the conductivity). - The inverse problem is making use of the scalp topography and the 3D-MRI picture to predict where exactly in the brain the signal originates from. - The forward problem is predicting the electric potential (or scalp topography) if a specific region of the brain gets activated.

Question 33

How can artifacts influence the EEG signal?

Answer 33

Artifacts can present itself as brain activity or muscle activity that originates from blinking your eyes or moving your tongue (for example). They intervene with the brain signals you actually want to research.

Question 34

What is MEG and how does it work?

Answer 34

A neuroimaging technique used to measure brain acitivity. Neurons generate magnetic fields (indication for electrical potentials) which can be measured and located by MEG. Since these magnetic fields are very faint, highly senstive magnetic sensors (which only work when there cooled by liquid helium) are used in a shielded room built with thick layers of metal that block other magnetic sources. MEG can then measure absolute electric potentials.

Question 35

Why is the spatial resolution of MEG better than EEG?

Answer 35

Because magnetic fields are hardly disturbed by different tissues while the conductivity of electricity can be influenced by the different structures in the brain. So you don't need to know much details about the brain (location of fissures, white matter etc.) in order to measure brain activity with MEG.

Question 36

What are two categories of EEG?

Answer 36

Event-related response (ERP) and spontaneuous activity.

Question 37

What is an evoked response (or event-related response (ERP))?

Answer 37

Transient reflection in EEG/MEG signals caused by a stimulus, cognitive task, or motor activity (usually an average of many trials and baseline corrected).

Question 38

Place the words latency, baseline and amplitude in the picture (also explain the meaning of the words).

Answer 38

- Baseline: the signal in a pre-stimulus interval of 50-100 ms is defined as zero. - Latency: how long it takes for the stimulus to cause a maximum amplitude/signal. - Amplitude: how strong the signal gets.

Question 39

What can be concluded based on this picture (results based on determining the speed of processing in the visual system by the ability of recognizing animals)

Answer 39

That up to 150 ms (reaction time) it is not possible to distinguish animals from non-animals, since the graphs for animals and non-animals are the same up to 150 ms. After 150 ms it becomes possible to distinguish animals from non-animals.

Question 40

95% of the brain's activity is spontaneous. If we want to research an event-related response, the 95% spontaneous activity can be seen as "noise". So how can we distinguish the 5% of event-related response?

Answer 40

Since noise is superimposed on deterministic response, averaging trials reduces the noise and keeps the systematic components (so exposing patients to the same stimulus several times and then averaging it out). This means a low signal-to-noise ratio requires more trials.

Question 41

So how can you measure auditory ERPs originated from the brain stem (distant and strictly time-locked response)?

Answer 41

By averaging thousands of responses.

Question 42

When looking at the scalp topography of each deflection in an ERP, you will very often appreciate a surprisingly clear dipolar field, which may be related to specific micro-circuit activations in the cortex. How can this be used in clinical setting?

Answer 42

By evoking a event-related response. For example you know that triggering the medial nerve will evoke an event-related response. By doing this you can (1) see where the motor cortex is and (2) if this dipole pattern deviates due to e.g. a tumor.

Question 43

What does the readiness potential say about free will?

Answer 43

That you don't actually have one. If you look at an EEG of someone who performs something out of free will, you can see there's a increase in readiness potential about 1,5 seconds prior to doing something out of free will. So if the brain shows signs if intention before we experience awareness of our intentions to act, how can we then say to have a free will to choose our actions?

Question 44

What does EEG gives us for information?

Answer 44

Location, timing, nature of information processing and provides information of spatio-temporal dynamics.