Electroencephalography (EEG) Flashcards
What is an EEG?
An Electroencephalography (EEG) is a method used to detect neural activity. This is done by placing electrodes on the scalp.
How does an EEG work?
Electrodes are placed on the scalp and pick up fluctuations of electrical signals that stem from (mostly cortical) neurons. These signals can be utilised to learn about cognition during particular tasks.
Is an EEG only utilised on the scalp?
Not always. An EEG is utilised at the scalp and is considered non-invasive, however it is possible to record intra-cranial EEG by measuring directly the exposed cortex.
Why would researchers use an EEG?
As a brain imaging technique, an EEG is considered as having an excellent temporal resolution, but a poor spatial one. This allows for increasingly narrowed observations regarding timing between the measurement of neural activity and its actual occurence.
True or False: EEG activity reflects action potentials (AP’s).
False. EEG activity does not reflect AP’s but originates mostly from post-synaptic potentials (voltages that arise when neurotransmitters bind to post-synaptic receptors on the membrane of the post-synaptic cell).
Can single cells be recorded with the EEG?
No. Single cells do not provide a strong enough signal to be recorded outside of the head.
What is required for the successful recording of a signal?
Many neurons must spatially align, in short. When this occurs, their summed potentials accrue and create the signals that can be then recorded.
How many neurons is the function unit?
The function unit is >10,000 simultaneously activated neurons.
Can the orientation of a neuron/s determine the sign of the recorded potentials?
Yes. The orientation of the neurons determines the sign of the recorded potentials. Some orientations lead to signals which cannot be recorded.
Name 4 limitations of the EEG.
- EEG is biased to signals generated in superficial layers of cerebral cortex on the gyri (ridges) directly bordering the skull.
- Signals in the sulci are harder to detect than from gyri, and many additionally be masked by the signals from the gyri
- The meninges, cerebrospinal fluid (CSF) and skull ‘smear’ the EEG signal, making it difficult to localise the source.
- This is known as the inverse problem: Mathematically, if the sources are known, the resulting scalp configuration of signals can be reconstructed, however, the reverse is not true- one given scalp configuration of signals can have multiple dipole solutions.
Discuss the signal of the EEG.
- EEG signals measured from the scalp in relation to a reference electrode
- The reference should be a neutral point, but some people reference to the average of all scalp electrodes
- EEG signals have a typical amplitude of 10uV to 100uV- these are tiny signals!
- Consequently they need to be amplified, typically by a factor of 1000 to 100000
- The signal is then (typically) digitalised. Typical sample frequency is 256-1024Hz, but can be >4000Hz
- Signal is band-pass filtered to remove the low and high frequencies bc they cannot reflect brain activity.
Discuss the processing of data collected by the EEG.
- The EEG signal is very noisy
- When looking at frequency information, for example in sleep research, the raw signal might shows systematic variations and more of a specific frequency
- However, when studying cognitive processes, the raw signal has to be ‘cleaned’ before it can be interpreted
- The most relevant steps are related to finding all the artefacts that are not brain signals (i.e. sweating, electrical noise, eye movements).
List a few artefacts/’observations’ in utilising the EEG.
- Eye blinks and movements have a strong impact on the EEG signal because the eye can be regarded as a dipole itself.
- Signals originating from the eye will contaminate the signal of interest and unfortunately will be much larger.
- These signals can be recorded by placing electrodes next to and under the eye to capture horizontal and vertical eye movements.
- Eye related signals can then be removed by excluding contaminated trials or mathematical algorithms such as ICA.
Define an event-related potential (ERP).
An event-related potential (ERP) is the measured brain response that is the direct result of a specific sensory, cognitive, or motor event.
Provide an overview of ERP’s and their applications.
- We want to know whether we can find brain activity that is reliably related to cognitive processes of interest.
- For this, the single-trial EEG in our studies are probably not good and far too noisy.
- There is a lot of variance between sessions from the same participants, but also between participants themselves.
