History/Basis of EEG

Question 1

What did Galvani discover?

Answer 1

That muscles move with the help of electricity. Stimulating the nerves of a dead frog with electricity results in muscle movement. He called this “animal electricity”.

Question 2

What did Berger identify?

Answer 2

That you could measure electric activity from the brain.

Question 3

How do you measure the electrical activity of biological cells and tissue?

Answer 3

Using: 1) intracellular recordings (sharp electrodes), 2) extracellular recordings (single and multi-unit recordings), 3) EEG

Question 4

What did Fritsch and Hitzig (1870) show?

Answer 4

They showed that electrically stimulating the sensory-motor cortex of a dog produced movement. It’s the 1st account that there’s a connection between the two.

Question 5

What did Caton (1875) show?

Answer 5

The existence of electrical activity in exposed rabbit brain. Not one can you stimulate it, but there’s also electrical activity.

Question 6

What are the clinical uses of EEG?

Answer 6

1) Differentiating between different types of epileptic seizures; 2) Differentiating between migraine variants, 3) Identifying movement disorders (psychiatric vs psychological), 4) Test for brain death, 5) Measure depth of anaesthesia, 6) Monitor seizures.

Question 7

How about the cognitive neuroscience applications?

Answer 7

Measure cognitive effects such as consciousness, attention, perception, memory, etc.

Question 8

What does the EEG activity reflect?

Answer 8

The synchronous activity of many many many neurons with high temporal precision . To measure this activity you need a massive collective synchronised activity in order to measure EEG.

Question 9

What are the 4 rhythms and in which conditions do you find them?

Answer 9

1) Beta 14-30Hz (awake with mental activity; 2) alpha 8-13Hz (awake and resting-state); 3) theta 4-7Hz (sleeping); 4) delta <3.5Hz (deep sleep).

Question 10

What is voltage?

Answer 10

The potential of current to flow from one point to another. It’s a relative measure, you measure it in relation to something

Question 11

What is current?

Answer 11

The number of charged particles (electrons, ions) that flow in a give time, it’s the actual flow.

Question 12

What is resistance?

Answer 12

The resistance of movement of charges.

Question 13

Which one do we measure with EEG: voltage, current or resistance?

Answer 13

We measure the voltage.

Question 14

What are the two things that are being generated?

Answer 14

LFP (local field potential- potential differences between all the dendrites and soma) and spiking activity.

Question 15

What does EEG measure?

Answer 15

The source of electrical signal is the cell polarization of pyramidal cells, so pyramidal cells are the thing that generate the EEG.

Question 16

What is spiking activity and local field potential?

Answer 16

LFP is the result of synchronised input activity of many dendrites into neurons. The action potential (spiking) is the output activity of a neuron.

Question 17

What are single and multi-unit recordings?

Answer 17

Directly inserted electrodes into the brain. Usually from only one or two regions (limited extend). Usually measure spiking activity (=action potentials).

Question 18

When measuring EEG, do you look at LFPs or Spiking potentials?

Answer 18

You are measuring the local field potential (LFP), not looking at the spiking potentials. You measure the volt difference between electrode and reference on the scalp. The LFPs generate dipoles and that’s what you measure.

Question 19

What happens when you have a fold?

Answer 19

To measure a dipole you need to have a really huge number of neurons that are positioned in the same position (must have unidirectional voltage fields). Otherwise, you have local cancellation, and you can’t measure anything. This can happen when folding.

Question 20

What’s the source of EEG?

Answer 20

LFPs and not spiking activity.

Question 21

What are open fields? Also, when can you measure EEG, in open fields or closed fields?

Answer 21

All neurons being located in the same direction is called open field. Closed fields don’t measure anything, you need open fields to measure it.

Question 22

When are scalp-recorded potentials possible?

Answer 22

The scalp-recorded potentials are only possible for layered structures with consistent orientations, which are mostly cortical (not subcortical). What’s subcortical is too far hidden.

Question 23

How do voltages spread through the head?

Answer 23

They spread through volume conduction. There’s voltage everywhere except at negative-positive transitions. The skull causes lateral spread (blurring).

Question 24

Is it true that you can tell whether a positive or negative EEG deflection is caused by excitation or inhibition?

Answer 24

No. The orientation of neurons with respect to the electrode is in practice unknown. Whether neural connections are inhibitory or excitatory is not something you can measure with EEG. You don’t know the direction of the neurons under the skull, so you don’t know if positive or negative EEG deflections are caused by inhibition or excitation

Question 25

What are the 3 requirements for EEG?

Answer 25

1. Many LFPs need to occur at the same time to create a sufficiently strong dipole (synchronous activity of many neurons).
2. Dipoles (and thus neurons) need to have the same orientation.
3. Can only measure radial dipoles (perpendicular to the skull). Neurons should not be oriented in parallel to the cortical surface. EEG is particular sensitive to dipoles that are in the radial direction and not tangential.

Question 26

A lot of event-related neural activity does not meet the requirements for EEG. What does that mean for the interpretation of EEG?

Answer 26

It means that we lose a lot of information. It’s an incomplete signal, we interpret only part of it.

Question 27

What are samples?

Answer 27

MEG/EEG is primarily temporal, acquired at discrete moments in time, called samples.

Question 28

What is the sampling rate?

Answer 28

The temporal resolution at which these samples are acquired is called the sampling rate.

Question 29

How do you acquire EEG/MEG?

Answer 29

MEG/EEG is also spatial, acquired across a varying number of electrodes (in MEG and analysis software these are often called channels).

Question 30

How do plot the signal over time?

Answer 30

The signal in each of these channels can be plotted over time, as can be seen in an event related potential (ERP=

Question 31

What is the name of the map in which the signal can be plotted in a particular time point?

Answer 31

The signal across channel for a particular time point can be plotted in a topographical map: the topomap.

Question 32

What are examples of potential analytical approaches of the EEG analysis?

Answer 32

1. Event related potentials (ERPs)
2. Time-frequency representations (TFRs).
   3) Multivariate approaches (e.g. decoding, or forward modelling).

Question 33

What is the inverse problem and what are some solutions? And forward problem?

Answer 33

The inverse problem consists on using a topomap to determine where the brain activity came from, so it is the question of where do the voltage differences come from. This problem is undetermined because more than one pattern of activity can cause the same result (different types of brain activity can give rise to the same pattern of activity/topomap, you can’t solve this problem perfectly with EEG).

Solutions:

1. Limit the amount of sources to only a couple of dipoles- Assume that only one or two sources can give rise to this activity.
2. Limit to sources located on the cortex
3. Limit to sources perpendicular to cortex

Question 34

What does the ECoG consist on?

Answer 34

Electrodes are placed directly on the surface of the brain. Solves some of the inverse problem because it doesn’t have so much of the blurring. But doesn’t solve it completely because electrodes are sitting on top of the brain and not in the brain itself, and also the ECoG is limited in specific regions and it’s invasive so use is limited.

Question 35

What does the MEG consist on?

Answer 35

Measures large-scale magnetic activity using magnetometer coils placed on the head. It’s much more popular as you can measure magnetic signals. It measures small perturbation in magnetic field strength, which are caused by the dipoles.

Right hand grip rule. Only measures magnetic fields that leave the skull, so cannot detect dipoles oriented perpendicular to the scalp (=cannot measure radial dipoles).

Question 36

What is the right hand grip rule (MEG)?

Answer 36

It indicates the relationship between a dipole and a magnetic field. You can measure the opposite thing with MEG from the EEG (here you want it to be parallel to the scalp - tangential).

Question 37

What does fMRI consist on?

Answer 37

As increases in neural activity occur, blood is transported to these areas to provide oxygen to the needy cells. Oxygenated blood is diamagnetic (non-magnetic) and thus generates less signal loss. This signal is called the BOLD response. fMRI is slow but relatively precise spatial location. Has a relatively high spatial resolution, unlike EEG. But it’s much slower, we are looking at the order of seconds to measure BOLD, lower temporal resolution compared to EEG.

Question 38

Temporal vs spatial resolution of EEG compared to fMRI

Answer 38

EEG has a high temporal resolution and a low spatial resolution compared to fMRI.

Question 39

What is the problem with all these techniques?

Answer 39

None allow us to establish a causal relationship.