Lecture 5- EEG Flashcards
Who was EEG invented by?
Hans Berger (1873-1941)
When was the first EEG result published?
1929
What unit of electrical activity is measured?
Microvolts (millionth of a volt)
What are ERPs?
measure of potential fluctuations time locked to an event
What is the spectral content of EEG?
analyses the type of neural oscillations that can be observed in EEG signals in the frequency domain
Advantages of EEG
Cheaper Portable Very high temporal resolution (milliseconds) Tolerant to movement Silent Simple to conduct Can detect covert processing Can elucidate stages of processing rather than just the end result
Disadvantages
Low spatial resolution
Poorly measures neural activity occuring below upper layers of brain
Time intensive
signal:noise is poor
Brain have many folds therefore opposite potentials across a sulcus can cancel out (MEG would still work because magnetic field is orthogonal)
Explain volume conduction ?
Ions of a similar charge repel each other
When many ions are pushed out of many neurons at the same time, they push their neighbours and so on, in a wave
How does volume conduction influence the EEG signal?
When the wave reaches electrode, they push or pull on the electrons inside the metal
The difference in push/pull voltage between an electrode and a reference electrodes is measured by a voltmeter = EEG signal
EEG activity reflects the ____ of activity of many neurons with a similar ____ ____
synchronous
spatial orientation
Why might ions from cells not line up and not create a detectable wave?
Because the cells have not have similar spatial orientation
What cells are thought to produce most EEG signal? Why?
Pyramidal neurons - they are well aligned and fire together.
Why is activity from deep sources difficult to detect?
Because voltage field gradients fall off with the square of the distance
Synchronous activity of a large number of neurones can give rise to macroscopic _____
oscillations
What is impedance measure in? How is it different resistance?
ohms
Has a phase component
How to reduce impedance?
Using conductive gel Wiggling electrodes
What’s the recommended max level of impedance
below 5 kOhms is considered good
What is the most common framework for setting up electrodes?
10-20 system
What is the elctrode in the top-middle of the head?
Cz
the Nasion and Inion are equidistant to which electrode?
Cz
High-pass filter fillters out ___ artifact. For example _____
Slow artifacts e.g. electrogalvanic signals and movement artifacts
Low-pass filter filters out ____ signals
electromyographic (signals from muscle movement)
What is applied to remove artifacts caused by electrical power lines?
Notch filter
EEG display can be set up in different ways to represent difference between voltage at 2 electrodes. These are different forms of _____.
Montage (e.g. sequential, referential, average reference, laplacian)
What kind of analysis can help remove artifacts (e.g. blink detection)?
Independent Componant Analysis (ICA)
How are ERPs analysed?
By averaging over many trials/sites
P100?
basic visual response (occipital)
N100
basic auditory response (frontal)
N170
reflects face stimuli
P300
reflects decision making
Lateralized Readiness Potential (LRP)
indicator of motor planning
What part of the ERP is analysed and quantified?
Peak amplitude
Latency
Area under deflection
(And/Ors)
Multivariate Analysis in EEG adds what component?
Temporal
Fourier analysis is the principle that…
Any waveform can be broken down into or constructed from the sum of sine waves of different frequencies
Fourier analysis can tell you the separate ____ and at what _____ composed your waveform
frequencies
amplitudes
What is the Fourier Spectrum?
Plots amplitude as a function of frequency
What are the frequency bands found in EEG?
Delta <4Hz Theta 4-7Hz Alpha 7-14Hz Beta 15-30Hz Gamma >30Hz
Delta
<4Hz – sleep, some attention tasks
Theta
4-7Hz, similar to alpha
Alpha
7-14Hz - resting, tiredness, attentional lapses
Beta
15-30Hz - motor behaviour
Gamma
> 30Hz - long range neural synchronisation
What is steady state?
Use an oscillating stimulus at a frequency that can then make the brain oscillate at that particular frequency.
Measure the steady-state evoked potential - the amplitude at the stimulus frequency
Head and contour plots?
Plots the EEG signal across the scalp
Typically show the distribution of activity across electrodes at specific point in time
The processes of inferring from the distribution of activity across electrodes where in the brain the signal originated is called…?
Source localization
Special equipment is required to combine EEG with MRI or MEG because….
Too much metal!
In regard to the N and P waves of the EEG, N and P stand for…
Positive and negative
An EEG waveform measures the change over time of the ____ between ____ ____ on the scalp
Voltage between two points on the scalp
A contour plot shows…
the distribution of activity across sensors at a specific point in time
If a large number of neurons are all firing at once, this will produce an ___ ____ and an accompanying ____ ___
Electrical dipole
Magnetic field
Nearby electrodes are often ___ ___ with each other
highly correlated
Opposite potentials across a ___ can cancel each other out
sulcus
In EEG we are measuring difference in electrical charge, this is called ___ or ____ _____
Voltage
Potential difference
In EEG there must always be a ____ electrode
reference
The voltages we measure are usually in the order of ___ of ____
tens of microvolts
A microvolt is a _____ of a volt
Millionth
Electrodes are typically made of?
Silver/silver chloride
What does a multimeter do?
Manually measures impedance
Because EEG signals are so small, we need to ___ them
But what else will be ___?
____ also digitise the EEG signals
Amplify
Noise
Amplifiers
The true signal we are interested in can be revealed by?
averaging over multiple trials and/or multiple subjects
AND/OR
average over several electrodes to get a
global signal for a particular region
Blinks show up on the _____ at the bottom of the plot
electro-oculogram (EOG)
The positive and negative components of a waveform are known as ______
deflections
Using steady-state, we can measure the neural response to some stimulus as a function of its ____
intensity
Clinical applications of EEG
Epilepsy Migraine Movement disorders (e.g. Parkinson’s) Psychiatric disorders Diagnosing brain death, coma etc
How can we relate EEG responses to subjective experience?
What can be used to derive an underlying model that explains data from both measures?
Do psychophysical tasks whilst measuring EEG
Computational modelling
Neurofeedback?
Way of monitoring your own internal mental state
May or may not work
Frequency vs time plots
time = x frequency= y amplitude = colour
