Task 3 EEG Flashcards
What is EEG/What does it measure?
- Electroencephalography
- measures electrical activity of the brain using surface electrodes embedded in elastic cap
- measures electrical activity genereated by synchronized synaptic activity in cortical neurons –> measured on scalp using electrodes
What is the neural source of EEG?
- EEG originates from synchronized synaptic activity in populations of cortical neurons
- must be organised in columns –> pyramidal neurons
- EEG electrodes detect sum of pos. and neg. charges in vicinity –> if electrode is equidistant from source and sink of dipole it will measure net neutral
- EEG is sensitive to both tangential and radial dioles –> limitation is size of electric
What are dipoles?
- excitation of postsynaptic neurons creates extracellular voltage near neural dendrites that is more negative than elsewhere along neuron: region of positive charge separated from negative charge (“dipole”)
- pos. charge: source
- neg. charge: sink
Radial vs. tangential dipoles
- radial dipoles: oriented perpendicular to surface
- tangential dipoles; oriented parallel to scalp surface
Location of electrodes
- 20-256 electrodes placed on scalp
- one reference electrode placed at neutral spot (earlobe, mastoid process, tip of nose) for all measurement electrodes
Volume conduction
- ions repel ions of same charge –> chain reaction results in “wave” of charge traveling through extracellular space –> allows signal to propagate through wire/extracellular space/conductive volumes
- brain is not homogenous:
ions cannot travel through myelin-coated nerve tracts or physical barriers - signal from larger dipole travels further than signal from small dipole
Signal, noise, artefacts
signal: voltage reflecting brain activity
noise: voltage reflecting other sources a) externally or b) internally
artifacts: disruptions that are not constant and are removed from averaged waves
What are event-related potentials?
- electrical potentials generated by brain that are related to specific internal/external events
Categories of ERP components
a) exogenous: triggered by presence of stimulus (top-down processes)
b) endogenous: reflect neural processes that are task-dependent
c) motor components: accompany preparation and execution of given motor responses
Exo: N170
- most common exogenous component
- negative wave over V1
- peaks around 170 ms after stimulus onset
- larger when stimulus is a face compared to object –> can be used to assess ability of preverbal infants to discriminate between faces and objects
Endo: P300
- most common endogenous component
- much larger for infrequently occurring stimulus categories than for frequently occurring –> observed in oddball paradigm
Two subcomponents:
a) P3b: sensitive to task-defined probability: task-irrelevant stimuli generate little P3b amplitude
b) P3a: elicited by highly distinctive improbable stimuli, even when task does not require discrimination of these stimuli
Motor components
- Readiness potential
- accompany motor response
Attention
- voluntary attention: ability to intentionally attend to something –> goal-driven process: goals are used in information processing
- reflexive attention: bottom-up, stimulus-driven process: sensory event captures attention
Orientation of attention:
- overt attention: turning head towards stimulus
- covert attention: attention without turning head
Cocktail Party Effect & Bottleneck
- selective auditory attention allows to participate in conversation in busy environment while ignoring the rest –> information processing bottleneck
Early vs. late selection models
- early selection: stimulus can be selected for further processing or tossed out before perceptual analysis of stimulus is complete
- late selection: all inputs are processed equally and selection follows to determine what will undergo additional processing and get represented in awareness
