Lecture 2: Neuronal basis of EEG and ERPs Flashcards
Two hypotheses (by Global Neuronal Workspace Theory (GNWT) and by Integrated Information Theory (IIT)) have been formed around consciousness. Explain these two hypotheses.
- 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).
Why is improvement of prevention, diagnosis and therapies for brain disorders so important?
Because brain disorders are responsible for 35% of healthcare costs, but only 50% of brain disorders are treated.
What are advantages of EEG?
- Direct reflection of activity
- High temporal resultion
- Greater specificity
- Non-invasive
- Availability
- Inexpensive
Answer the following questions about fMRI, PET, MEG and EEG.
- What technique has the highest temporal resolution (so the least delay)?
- What technique has the lowest temporal resolution?
- What technique has the highest spatial resolution?
- What technique has the lowest spatial resolution?
- Highest temporal resolution: EEG, it can measure electrical changes within milliseconds (after this MEG).
- Lowest temporal resolution: PET
- Highest spatial resolution: fMRI
- Lowest spatial resolution: EEG, since it can detect minor changes in specific neurons.
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..??)?
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)
Explain how electroencephalography (EEG) works.
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).
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?
- 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.
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?
Even-numbered electrodes refer to electrode placement on the right side of the head, whereas odd numbers refer to those on the left.
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?
- Inion: crest point of back of the skull (bump)
- Nasion: depressed area between the eyes, just above the bridge of the nose.
Why is Cz used as reference?
Cz is placed on top of the skull. Here, the electrode is not influenced by e.g. facial muscles.
You can’t measure potentials of a single electrode with EEG. Why is this and what do you measure?
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).
Just read (will be discussed seperately after): The following factors influence the EEG signal.
- Position of reference electrode
- Electrical contact between scalp and electrode
- Conductivity of the head
- Number of active neurons
- Synchronicity
- Orientation of the cells
- Distance from source to electrode
- Orientation of dipole
- Site of activation on the neuron
- Excitation vs. inhibition
- Artifacts
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)?
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.
What is a solution to the common reference?
The average reference. Here you take the average of the common reference whereafter you substract this average reference potential from the different electrodes.
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.
Ok
How can electrical contact between scalp and electrode influence the EEG signal?
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.
How can conductivity of the head influence the EEG signal? And what patients are typically best suitable for EEG?
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.