Task 3 Flashcards
What is an EEG?
- Electorcenphalography
- records the overall brain activity continuously
- includes endogenous changes in electrical activity as well as changes triggered by events
- signal from each electrode is amplified
- measures the voltage between two electrodes (active + reference)
+ EEG patterns change through processes (but in a predictable manner)
+ normal EEG patterns are well known -> allows detection of abnormal activity
- insights from EEG are limited because recordings represent the brain´s global activity
where are the electrodes placed in EEG?
- electrodes can measure the sum of charges in their surrounding
- only, when they are close to either side of the dipole
- when the electrodes are equally far away from both dipoles, they measure a net neutral
What is special about the EEG electrodes?
- measure the activity of large populations of neurons, when active together
- electrodes meaasure the difference in voltage between the recording electrode and the reference electrode
- voltage can be measured on the outside, because tissue of the brain, skull and scalp passively conduct electrical currents
What is spatial resolution
- where in the brain is the activity happening
- is a clear image possible
- how clear is the structure of the brain depicted
- how detailed can activity be allocated?
What is temporal resolution?
- how long does it take to see the image effect?
- if an image takes a couple of seconds, the temporal resolution is bad
What kind of spatial + temporal resolution does EEG have?
Spatial: quite low: on the order of a few cm3
Temporal: High (ms or better)
What are the neural origins of EEG?
- arises from synchronised synaptic activity in populations of cortical neurons
- extracts ERP
What are postsynaptic potentials?
- PSPs
- origin of ERPS (event-related-potentials), which occur during neurotransmission when the binding of neurotransmitters to receptors change the flow of ions across the cel membrane
What are dipoles?
- A region of positive charge (= source) separated from a region of negative charge (= sink)
- the sum of many individual dipoles (= neurons) in an area is measurable as a single dipole
- the magnitude of the dipole reflects the number of neurons, whose dipoles are summing together
- in order for a measurement ≠ 0
> parallel
> synchronously active (to achieve net charge) - polarity of the signal measure depends on the dipole´s orientation
What are radial dipoles?
- oriented vertical to the scalp surface
- produce deflections in roughly one direction
- I
What are tangential dipoles?
- oriented parallely to the scalp surface
- produce deflections in both directions
- __
what is Volume conduction?
a pool of ions repels nearby ions of the same charge, resulting in a wave of change that travels through the extracellular space
-> how EEG travels within the brain
What is ECOG?
What is the signal to noise ratio?
measure of how much signal the system measures, compared to how much noise is measured
-> higher SNR refelcts a better quality signal that is contaminated by less noise
What are sources of noise?
- external noise: influences the measurement prooportionally to their distance from the electrodess (EEG sytsems may solve the problem of external noise through - passive shielding - active electrodes)
- internal noise: arises within the body and thus cannot be eliminated during data collection (might be minimised by controlling environmental factors and participant behaviour -> filtering and artefact detection)
What are artifacts?
- artifacts influence the outcome -> confounder (= noise)
- artefact detection is applied post processing to filter out internal noise
- examples: eye movements, eye blinks
- solution: instruct participants to maintain fixation and minimise eye blinks (! might distract from task and change brain activity)
What are ERPs?
- ERP graphs show the average of EEG waves time-locked to specific events
- useful for:
> Answering questions about the timing of mental processes
> Determining which cognitive processes are influenced by experimental manipulation
> assessing the anticipatory processes that occur before a stimulus
> covertly monitoring mental activity in the absence of a behavioural response
what are the major ERP components?
- exogenous components: triggered by a stimulus and may be modulated by top-down processes
- endogenous components: neural processes that are entirely task dependent
- motor components: accompany the preparation and execution of a motor response
What are endogenous ERP components?
- the most common endogenous ERP component
- much larger for infrequently occuring stimulus categories than for frequent ones (> in oddball)
- P3b (most common): sensitive to task defined probability -> larger from improbable stimuli only, when the task requires sorting the stimuli in a way, that makes a given stimulus category improbable -> central and parietal brain regions
- P3a (novelty): elicited by an improbable stimulus, even when the task does not require the discrimination of the stimulus -> frontal brain regions)
What are motor ERP components?
- if one creates averaged ERP waveforms, time-locked to a motor response, rather than time-locked to a stimulus, it is possible to see ERP components reflecting the processes that lead up to the response
- readiness potential
What is N170?
- negative wave of visual cortex -> peaks at 170ms post-stimulus onset
- larger when elicited by a facial stimulus
- differentiation is first visible in 150ms post stimulus onset
- brain can differentiate between faces and objects within 150 ms
- abnormal in children with ASD
What is P3/ P300?
- seen when an attended stimulus is presented, expecially if the stimulus is relatively rare
What is a readiness potential?
a large negative voltage, building over the motor cortex prior to a response
What is a lateralised readiness potential?
a portion of the RP is larger over the hemisphere contralateral to the response than over the ipsilateral hemisphere
-> can be used to differentiate between RP and components reflecting stimulus processing (background activity)
what is a capacitor?
- two pools of charges separated by an insulating layer preventing ions from mixing
- if ions would mix: emergence of a neutrally charged pool
- example: electrode gel (removes air)
- the sequence of layers in the brain function like a stack of capacitors = forming a series of conductive volumes, separated by insulating layers
what is the inverse dipole modelling?
- model of a spherical head is created by a computer and a dipole is placed somehwer ein this model
- forward selection is calculated to determine the distribuion of voltages that this dipole would create on the surface
- the predicted pattern is compared to the actual data
- this is repeated until a good match is found
- often more than 1 dipole is used
–> ERPs are the result of processing in multiple brain areas (! inverse problem)
What is the inverse problem?
Given a particular pattern of electricla charge on the surface of the sphere, it is impossible to determine the distribution of charge within the sphere that is caused
what are solutions to the inverse problem?
- modelling techniques are used: physics of the brain need to be simplified
- important assumption: neural generates can be odelled as electrical dipoles
What are exogenous sensory ERP components?
- different ERP components for audiory vs. visual stimuli
- ERO responses following visdual stimuli begin later than those for auditory ones
- Odball paradigm (= most common ERP paradigm)
What is the oddball paradigm?
- part of exogenous sensory ERP components
- standard stimulus (frequently) and oddball stimulus (infrequently) are presented briefly
- participants count/ make a manual response to the oddball stimulus
- the initial response to both stimuli: C1 wave (primary visual cortex) -> influenced by sensor factors but not task
- P1 wave (extrastriate areas of visual cortex) -> influenced by sensory factors, attention, arousal
- N1 wave (different brain areas) including sub-components involved in discriminating the identity of the stimulus influenced by attention
What are the neural origins of ERP?
- originate as postsynaptic potentials (during neurotransmission, when the binding of neurotransmitters to receptors changes the flow of ions across the cell membrane)
- ERPS provide a direct and instantaneous, millisecond resolution measure of neurotransmission-mediated neural activity
- main neurons measured as ERPs are the pyramidal cells, being the main input-output cells of the cortex
What influences the polarity of ERP components?
- orientation of the neurons (concerning the recording electrodes)
- the location of the reference electrode
- the part of the cell in which the neurotransmission is occurring
- whether neurotransmission is excitatory or inhibitory?
