Lecture 5 - Electroencephalography (EEG) Flashcards
What does EEG stand for?
Electroencephalography.
Explain what EEG is and how it is used to understand brain activity and function.
EEG involves placing electrodes on the scalp that are able to pick up the electrical signals given off by neural activity of cortical pyramidal cells.
EEG scans can give us information about general areas of neural activation for certain motive or cognitive tasks as well as timing of activation of these areas. It can also give us information about certain events, such as epileptic fits, and is a key diagnostic tool for epilepsy.
What are the pros and cons of EEG?
Hint: list three pros and one con.
The pros of EEG scans are that it is a non-invasive, inexpensive approach to brain imaging. It also has high temporal resolution.
A con of EEG is that it has poor spatial resolution.
In regards to EEG traces/readings what are artefacts, where do they come from, and how are they dealt with?
Artefacts are electrical signals picked up by the electrodes that are not caused by the neural activity of interest. These electrical signal artefacts may come from eye movement, sweat on the skin/skin potential artefacts, and or muscle movement.
These artefacts can be accounted for and dealt with in various ways; by mathematically accounting for them, manually removing them, and or excluding trials that are too “contaminated”
In regards to EEG traces/scans how do we ensure that the electrical activity that we are measuring is from the neural activity of cognitive tasks and not from artefacts?
Artefacts are accounted for manually or mathematically. One way this is done is by manipulating the output of the electrodes. This is done by band-pass and notch filtering the signals and also amplify them so that the signal is large enough so that the results are actually meaningful.
If the electrical activity of a single neuron is too small to be picked up by an EEG electrode what signal is being measured and what is meant by a positive and negative singal?
Hint: pyramid cells and dipoles.
Neurons act as dipoles give the ionic distributions generated within and around them due to action potentials as well as post-synaptic potentials caused by neurotransmitter activity. Cortical pyramid cells tend to clump together in a manner such that their dipoles are spatially aligned. This aligned allows for a combined signal that is big enough for an electrode placed on the skull to pick up.
It is the orientation of these clumps of cells that determines whether the signal read by the electrode is positive or negative. A functional unit of spatially aligned cortical pyramid cells that allows for the generation of signal that can be read by an electrode is >10,000 cells.
How does the structure of the cortex impair the accuracy and spatial resolution of EEG traces/scans?
Because the gyri are closer to the skull the EEG signal is biased toward cells in these structures, whereas the neural activity from cells in the sulci is harder to pick up accurately, as the signal is impacted by the signal from neurons in the gyri, as well as the fact that these cells are further away from the skull.
Furthermore, the meninges, skull, and cerebrospinal fluid “smear” the signal so that the signal obtained in an EEG cannot accurately be attributed to a specific area.
What is an EEG trace called?
A spectrogram.
What information can be obtained from a spectrogram?
The frequency of brain activity, which gives us insight into the brain “state”, e.g alpha, and the timing of brain activity, which gives us insight into the timing of cognitive processes and transition into and out of different states.
EEG caps can have different numbers of….which alters the…..of the EEG trace.
Caps can have different numbers of electrodes. The more electrodes a cap has the higher the resolution/accuracy of the trace.
What are some of the artefacts that can influence an EEG trace and how are they accounted for?
Muscle movement, eye movement, skin potential artefacts all influence the EEG trace. Mathematical algorithms can be used to account for these.
Where does the majority of the EEG signal come from?
Post-synaptic neurons as opposed to action potentials.
What is a rough estimate of the size of a functional unit of cortex measurable by an EEG electrode?
Around 10,000 spatially aligned cortical pyramidal cells with their dendrites aligned with the scalp.
Which areas of the brain is the EEG trace biased to?
EEG electrodes pick up signals that are biased to the signals from the neurons in the gyri bordering the skull. The signals from the sulci are either interfered with by the signals from the gyri, or they are too far away for the electrodes to pick them up.
What does frequency data from an EEG spectrogram tell us about cognitive states?
The frequency information obtained from EEG spectrograms can give an indication of what cognitive state someone is in, e.g alpha waves.
