PBS2 - EEG 1
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What is the most likely source of human EEG activity?
- Electrical activity associated with post-synaptic dendritic currents
- generated in cortical pyramid cells (well aligned and fire together)
- Called the primary current
- Also called equivalent current dipole
- Created by summation of synchronous activity of neurons orientated in the same way.
- Secondary current (or volume conduction) is the electrical current that is propagated through medium in the brain like liquor and tissue.
What is the EEG signal measure in? How much is a typical signal?
- microvolts (μV)
- [requires sensitive electric amplifiers]
Electrode placement on the scalp is standardised. Name one typical standardised method.
The international 10-20 system
What is used to help the conductance during EEG?
- Conductive electrode paste
- Or water solutions
What is ERP and EP?
- ERP - Event-related potential: EEG modulation affected by the stimulus. Recording is time-locked to an event.
- ER - Evoked potential: Similar to ERP I think (ASK)
EEG data recorded to a single stimulus presentation is called...
- single-trial data
- (sometimes called epoch or sweep in literature)
Why is single-trial data not good enough?
- It is fairly noisy due to...
- physiological artifacts (eye blink etc - ASK)
- movement arttifacts
- mental activity unrelated to task
- external noise (eg electric noise from nearby electrical devises)
What is done with EEG in order to compensate for the noise?
- Certain experimental condition is presented many times.
- to extract valid task-relevant signal
- Average the EEG signal in reponse to many presentations of the stimulus
- Assumption is that noise is random, so the averaged ERP will mean the noise will hopefully average to zero.
What is the voltage level before the stimulus is presented called?
- (usually 50 milliseconds before stimulus presentation)
Both __ and __ stimuli elicit a__ of ERP waves(often called ___). The nature of the __ affecting the parameters of the __ helps to understand the __ significance of these waves.
- acoustic and visual
__ can serve as a powerful modulator for various ERP waves. We can thus conclude that __ __ play an important role in generating these waves. The most typical __-related effect is what?
- attention processes
- is that attended stimuli elicit larger ERP amplitudes
For EEG graphs (usually for acoustic and visual I think), the graph is what?
- Negative voltage is up,
- and Positive voltage is down
In case of visual stimulation, stimuli in the __ __ elicit larger ERP amplitudes of the ___ _ and _ waves (sometimes called _) as well as the frontal __ and parietal __ waves (sometimes called _).
- attended hemisphere
- occipital P1 and N1
- N1 and N1
We have to be careful when comparing EEG studies because...
- Labelling of ERP waves can be variable across multiple publications
- as waves differ over different areas of the scalp
What is the phenomenon you have to remember regarding EEG about acoustic stimuli? What is it?
- Mismatch Negativity (MMN)
- reflects the automatic acoustic change detection
- elicited in a passive oddball paradigm - participants are not consciously attending to stimuli
- is the EEG amplitude difference to deviant stimuli (rarely occurring unattended events) relative to standard acoustic stimuli (frequently occurring unattended events)
What is standard and deviant stimuli and use it in an example.
- Standard stimulus - frequently occurring unattended event
- Deviant stimulus - rarely occurring unattended events
- Eg. Participants watching silent video (attending to it) while passively listening to a series of acoustic events. Standards is a syllabus 'ba' (90% of the time) and deviants are stimulus 'ga' (10%).
How will standards and deviants differ in mismatch negativity graphs?
- Standards and deviants elicit ERPs which differ in amplitude
- The amplitude difference can typically be observed between 150-200ms after stimulus presentation.
Why is MMN useful?
- Useful to study acoustic processing without the contamination of attention.
- Automatic, so it can also be used to study acoustic processing in non-verbal participants (eg. infants and coma patients)
What are the two important ERP waves related to higher cognitive functions you should know about?
What is P300 thought to be the reflection of?
- A sum of overlapping waves (why it's sometimes called P300 complex)
- Usually thought to be a reflect...
- 1. stimulus categorization
- 2. working memory updating processes
When does P300 appear?
- to task-relevant and attended stimuli in many different task contexts
- Observed in conscious oddball task
What is the typical latency of P300? What is the latency a reflection of?
- Latency depends on task difficulty (more difficult, the longer the latency)
Why might a wave of latency 300ms and 900ms in two different tasks be both categorized as P300?
- because researchers assume that similar brain functions underlie the generation of the waves in both tasks irrespective of the differences in task difficulty.
- [This illustrates the importance of considering task context when interpreting ERP waves/phenomena].
What are the two frequent potential constituents of the P300 waves?
- [both can be observed in the conscious oddball task situations)
What are the respective topography of the P3a and P3b?
- P3a: a more frontal topography
- P3b: occipito-parietal topography
What do P3a and P3b each reflect?
- P3a: typically appears to unexpected or less typical (but consciously observed stimuli)
- P3b: thought to be the constituent which reflects the categorization and working memory processes related to task-relevant stimuli.
- [Although take these two with a pinch of salt - ASK!]
The distinction of P3a and P3b waves illustrates what about analysis of EEG?
- Illustrates the importance of considering the topography of ERPs when trying to interpret them.
- (They will oftenb fuse together as they have similar amplitude and latency.)
When does N400 appear?
- When participants encounter semantically unexpected stimuli.
- (so when the meaning is weird)
In what kind of a test would N400 be observed in?
- In tests where the participants view sentences word-by-word.
- Often the wave corresponding to the last word is observed.
- When the last word is semantically anomalous with the preceding part of the sentence, then the negative N400 wave appears. (eg. She spread the warm bread with socks.)
What happens when there are anomalous physical stimulus features?
- N400 will not appear (only, it seems, for anomalous semantic stimuli).
- Eg. When the last word is semantically congruent but is written with large caps (SHOES).
- In such case, an enhanced P3b (not P3a? ASK!) rather than N400 appears.
Where, topographically, does the N400 wave maximum amplitude appear?
Over the parieto-occipital scalp.
In many contexts, the __ and __ appear together forming a _____ complex.
- P300/N400 complex (eg. 3+4=.. 9, 7 or 16?)
Why might electric brain potentials related to hand and limb movements be relatively easy to measure?
- Motor cortex is well defined
- Motor cortex is close to the scalp
What is one of the most important motor potentials we should be aware of? What does it mark?
- Lateralized Readiness Potential (LRP)
- It is an event-related potential which marks the lateralized movement preparation in the motor cortex.
- (in other words, preparation for movement on a certain side of the body - limb usually).
Typically, what kind of situation is LRP measured in?
- Where the participant has to respond to stimulation either with a right or left hand response finger.
- Eg. has to press a response button
What is an important property of LRPs?
- LRPs can be detected even before the movement happens
- or if no movement happens at all but the participant is preparing for it.
- Used for observing unconsciously or latently planned movement procedures.
Give an example of a test that works well when trying to observe Lateralized Readiness Potential. What does it show?
- Stroop task
- The LRP can measure a latent movement preparation of a left hand even though she uses the right hand in the end - by inhibiting the automatic response of using the left.
- Shows that the to-be-neglected aspects of the stimuli (the semantic meaning of the word) were not only semantically processed but also resulted in a movement command which did not manifest. Important for some psycho-physiological theories.
What fact is the computation of LRP based on?
The fact that lateralized hand movement result in lateralized ERP phenomena.
What are the electrodes corresponding to the left and right scalp respectively? And what are the computations needed to calculate the LRP?
- C3 - left scalp (right hand movement)
- C4 - right scalp (left hand movement)
- C3minusC4 will give a negative voltage if the right hand moves/planning to move.
- Average of C3-C4 and C4-C3 gives a voltage which is a general index of response hand movement. (This is the LRP).
What is the advantage of LRP?
- The functional interpretation is very clear.
- If the experiment is well-designed to elicit movement related brain activity it is very straightforward to interpret the data.
Contralateral and Ipsilateral.
- Contralateral: opposite side of body
- Ipsilateral: same side of the body
What is an artifact?
- Electrical signals detected along the scalp by an EEG, but that originate from non-cerebral origin.
- Eg. head movement, eye movement
Why do we need to measure the electric activity of eye muscles? What is this called?
- Electro-oculogram (EOG) - to detect Ocular Artifact Propagation
- Very important for detecting eye movement artifacts (helpful when detecting noise when analysing EEG).
What is an assumption about noise?
That it is unbiased and uncorrelated with signal.
What are the different alternatives to EEG?
- Electro Corticography (ECoG) (aka Intracranial EEG)
- LFP spikes
- Single Unite Recordings
ALMOST THERE! Descirbe what the process is of hearing.
- 1. Sound enters ear
- 2. Tiny middle ear bones amplify sound
- 3. Cochlea sorts sounds by frequency
- 4. Nerve passes signal from cochlea to brain stem
- 5. Signal travels through brain getting decoded along the way
- 6. Auditory cortex recognises and processes sounds.
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