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EEG/ERP (Electroencephalography) (ERP --> Event related potentials -…

- - - - averaged because:
        --> ERP = tiny signal (2 micro volts while EEG 20micro volts)
        --> eliminates noise (brain activity not related to stimulus + measurement errors)
        --> by averaging all the other activity out :3!!
        --> leads to constant signal
    - - .
  - - - Positive deflection (P)
        --> positive rising wave is deflected at the peak into negative
      - Negative deflection (N)
        --> Negative wave is deflected into the positive :3 !!
    - - eg. N100 = negative deflection 100 ms after stimulus onset
        P250 = positive deflection 250ms after stimulus onset
      - Sometimes they are just labeled according to order of occurrence instead of time
        --> eg. N1 happens first, then P2, then P3
  - - - choose attentition where it will be = endogenous
      - unexected light stimulus = exxogenous
  - - - 10ms (auditory brainstem responses)
        --> roman numbers, eg IV
        
        10 to 60ms midlatency repsonse
        
        50 to 150ms long latency repsonse
        --> p50(P1), n100(N1), p160(P2)
        N170 (Part of N100/N1 component)--> bigger for faces than not faces --> bigger for birds than non birds in bird experts etc..
      - Exogenous cuing
        --> attention task = fixate on cross, arrow will indicate side where stimulus will be
        --> Now instead of arrow, a lightning flash will be displayed randomly in either visual field. --> this grabs their attention toward that location for 50 to 200ms and makes reaction time to stimulus faster , if it appears i that location within that timeframe (<-- because flash random and gives us no cue where stimulus will be it is exogenous, we don't have time to think about :)!)
        ---> if stimulus shown after 50 to 200 ms, the reaction time to stimulus if shown in same area as flash is Inhibited / slower (inhibition of return) #
        --> affects P100/P1 component
    - - P3b
        --> high activity for task relevant stimuli!!
        --> low activity for stimuli that are task irrelevant :3 !!
        --> central parietal distribution
      - P3a
        --> occurs for unexpected stimuli (like in the oddball task)
        ----> e.g. count all the X's (80% X's) But also 20%O's then P3a activity super high for the unexpected O's :3!! (also basically thats the oddball task :3 !!
        --> frontal scalp distribution
      - --> in schizophrenia P300 delayed by 75ms and 50% weaker than healthy controls (slower response selection!!)
      - Endogenous cuing
        --> attention task = fixate on cross, arrow will indicate which side stimulus will be
        ---> RT faster if stimulus will be in indicated spot, cue gave us the idea where it will be so we could consciously ready ourselves to react to it (Conscious processing = endogenous )!! #
        **--> affects P100/P1 and N100/N1 (amplitude bigger if attended location is wehre stimulus will be )
  - - - Sphere model
        if head was a sphere, and all tissue/ bones equally conductive it would be easy to know how the voltage from a signle dipole would distribute at the scalp (thus easier to reverse engineer back to source (dipole))
        ----> BUT' head is not a sphere and tissues and bone structure conduct electricity differently
      - Finite element models
        --> needs MRI picture for voxels and measurments of how big different structures in voxels for accurate resitance rating
        
        Assumptions
        --> resistance / conductivity in each voxel is assumed to be same (ok since they are super small)
        --> resistance/ conductivity between the voxels differs!!
        
        Dividing head in hundret / thousands of voxels
        
        resistance / conductance is calculated for each individual voxel
        --> based on resistance values obtained from dead tissue or animal tissue
        
        Now follows real shape of head and real resistance values
        --> distribution of electric activity of dipole on scalp can be easily calculated :3 !!
      - Boundary model
        --> needs MRI picture for boundaries
        
        Assumptions!!:
        --> most boundaries have different resistance /conductivity (brain, dura, skull, skin)
        --> within the boundary the resistance/conductivity = same
        
        Dividing head based on boundaries
        --> brain, dura, skull, skin
        
        resistance / conductance is calculated for each individual boundary
        --> based on resistance values obtained from dead tissue or animal tissue
    - - Equivalent current dipole technique
        
        Brain Electrical Source Analysis (BESA)
        
        assumptions:
        --> voltage/ current distribution can be modeld by less than 20 dipoles
        --> each dipole = fixed but strength varies over time ?? <-- nope cause orientation = also adjusted
        
        arbitrarily choosing whatever amount of dipoles researcher pleases and putting them in whatever position and location the researcher pleases , strength of dipoles also randomly chosen
        --> also two biggest limitations here lol
        
        calculates how well the model compares to actual measured voltage distribution
        --> adjust orientation and strength slightly to aproximate it more
        --> test again
        --> adjusts again
        --> etc until good fit found (UNIQUENESS PROBLEM!!)
        
        Limitations
        --> Uniqueness problem (infinite possibilities)
        --> arbitrary starting points
        --> noise could make incorrect model correct
        --> no way of knowing if well fit model is actually the accurate one
        #
        
        -------> study where EEG voltage (by 10 dipoles unknown to participants) given to participant researchers, each supposed to find out how many and where localized.
        --------------> semi accurate placement within 1 to 2 cm, but either too many or way too little actual dipoles set 6 to 12 lol)
        --> shows no unique / validifyable way of localizing dipoles!
        
        actual process
        --> assumed that processing begins in sensory areas, 1 or 2 dipols put htere
        --> once model fits there, time window increased and 1 mor edipole added, until it fits
        --> then time window increased again, 1 more dipole added etc
        --> limitation: processing doesnt start with just 1 or 2 dipoles, as much as over 60 are prosbs active jsut for sensory processing alone !!
        other one just using preexistant knowledge to detrmine location and amoutn of dipoles (seeded dipoles)
      - Distributed sources
        
        General approach
        --> brain divided in 100 voxels
        --> each voxel = 3 dipoles (up, forward, lateral)
        --> strenght of the voxels (thus dipoles) adjusted until voltage distribution matches the measured one
        --> limitation:
        
        100*3dipoles= 300 dipoles = 300 electrodes needed
        
        high processing power
        
        dipoles not independent of each other
        
        cortcally constraint
        --> brain surface = divided into tiny triangles with 1 dipole each (perpendicular to cortical surface)
        --> strength of triangles (dipoles) adjusted until voltage distribution matches measured one
        --> limitations:
        --> in sulci, dipoles can become parallel to each other, thus if oriented oposingly could cancel each other out or magnify each other which distorts model
        --> non uniqueness!!
        
        Minimum norm solution
        --> builds on cortical constraint
        --> says model that best conforms to measured one, but with least amount of high peaks = best one (so basically ignores some activity in sulk where the spikes occur tbecause of being parallel to each other thus stronger signal!)
        ---> ACTUALLY always finds a unique solution to inverse problem
        --> limitations:
        ---> biased toward dipoles closest to scalp
        -------> can be corrected with depth weigthed minimum norm # #
        
        low resolution electromagnetic tomography (LORETA)
        --> says that voltage emitted by dipoles decreases gradually as it travels through the brain and towards scalp
        --> thus selects the voltage distribution of high activities that is as smooth as possible.
        ---> limitation:
        --> boundaries between areas blurred, <-- thus only usable for finding center of activation, where ERP originated
- - - - compared to reference electrode
        --> mastoid bone, vertex etc
        --> limitation = if recorded close to that location eg mastoid bone, the deflections will be smaller because the reference electrode will pick up part of the same activity !! #
      - noise reduced by electrode at eye muscle
        --> eye blinks usually interfere with alpha activity and have 7.5Hz
      - there is no perfect reference electrode
        -->at skull always close to other electrodes,
        -->below skull too much interference from muscles (especially heart muscle)
      - alternative methods to reference electrodes
        
        average reference approach
        --> averaging activity by all electrodes and use that to compare activity if each electrode agains <-- average reference approach
        
        hjorth method / source derivation
        --> averaging the 4 nearest electrodes and compare the single electrode to that average, this happens for all electrodes (so their nearest 4 average is what their activity is begin compared against :3) --> hjorth method / source derivation #
    - - 10/20 system
        --> old system 19 electrodes
        --> placed 10 and 20% from 4 trusted points:
        
        nasion
        
        inion
        
        left mastoid + right mastoid (so 2 electrodes)
      - 10/10 system
        --> also includes electrodes betwwen the position of the 10 /20 system
      - 5/5 system
        --> even more electrodes between the 10/10 system
        --> diminished return in accuracy increase after 100 electrodes
      - even distribution of electrodes = very important regardless of system!!
    - - linear (nearest neighbor)
        --> uses a weighted average of the nearest electrode to make up for missing electrode value
      - spline
        --> Uses the average of all electrodes on scalp to make up the missing electrode value #
  - - - Beta waves (13 to 30Hz)
        --> increases with attention
        --> frontal ventral activation
        --> replaces alpha during cognitive activity / focused attention #
      - Gamma waves (36 to 44Hz)
        --> usually 30hz to 80hz, but
        more difficult to measure cause skull mutes the high frequency signal + lots of noise
        --> directly associated with increase in brain activation
        --> object recognition, top-down processing of sensory input, perceptual binding (integrating various aspects of stimulus into one)
      - Theta waves (4 to 8Hz)
        --> seen at beginning of sleep
        
        2 types during wakefulness
        --> presumed gating function for limbic system because of these two lol
        
        Widepsread scalp distribution
        --> drowsiness
        --> less information processing (lol cause drowsieness lool xD sleepy kitty ahw !!)
        
        frontal midline
        --> focused attention
        --> mental processing
        --> effective stimulus processing :) !
      - Alpha waves (8 to 13Hz)
        --> predominant during resting (especially with eyes closed)
        ------> first eye closed, then sudden eye opening = disrupts alpha waves (alpha desynchronization)
        --> greatest above posterior occipital/temporal regions and parietal regions
        --> associated with information processing
        --> 8 to 10Hz = associated with stimulus-unspecific and task-unspecific increases in attenttion
      - Delta (1-4Hz)
        --> sleep + anesthesia
        --> Increase in proximity to lesions
        --> thought (and thought only cause no proof) to be inhibitory
    - - Spectral analyses provides important information about the frequency compositions of EEG oscillations.
        
        --> BUT!!! does not provide temporal information as to when frequencies occur
      - assumptions of stationary signal
        --> thus only 3.5second long snippets , but 60 seconds in total to reduce second to second variability in the signal !!
      - absolute vs relative power
        --> absolute = amount of frequency in whole EEG
        --> relative = amount of activity in frequency divided / by the the total power of the frequency wave
      - Limitation
        --> BUT!!! does not provide temporal information as to when frequencies occur
  - - - tangential
        --> from sulci
        --> parallel/horizontal to the inside of the skull (like lying flat on their side and are paralallel to the inside of the skull like a blanket :3 !!
        --> cant be measured by electrodes directly above (cause positive negative extracellular fluid cancel each other out)
        ---> thus can only be measured by electrodes to the side of it :3 !!
      - radial
        --> from gyri
        --> radiating from center of brain outwards towards the skull
        --> perpendicular to the curved shape of the skull (like standing on it upright!)
    - - --> If excitatory input at soma (or inhibitory input at dendrites), the neuron depolarizes (positive ions form extracellular fluid rush into the cell, leaving this extracellular fluid negatively charged.
        --> at the same time positively charged ions leave the dendrite end, passing on the action potential, charging the extracellular fluid with positive ions
        
        Because in case of radial dipole, the dendrites (including their extracellular fluid closest to scalp, the EEG electrode will pick up this positive charge
        --> happens because same charges (++ or --) repel each other (Volume conductance)
        --> so the positive ions in extracellular fluid at dendrite end push other positive ions towards the dura layer, where they collect
        --> the more collect there the stronger they repel other positive ions on the other side of dura layer, which then collect at inner side of skull, and the more positive ions collect there the more they repel the ones on other side of skull and so forth, until the positive ions reach the electrodes :)
        
        Which then leads to a positive deflection on the EEG
      - --> if inhibitory input at soma (or excitatory input at dendrites), then the concentration of positive charged ions outside the neuron/cell would be higher than the one inside it, hence --> extracellular be positively charged
        --> at the same the dendrite side would be negatively charged, because there would be more positive ions inside the cell of the dendrites than outside, --> extracellullar negative
        
        because of dendrites with negatively charged extracellular fluid closest to scalp the negative charge will be picked up by EEG
        --> cause same charges repel each other (++ or -- ) (Volume conductance)
        --> negative ions repel at dendrite repel other negatively charged ions, pushing them towards dura
        --> the more negatively charged ions collect at dura, the more they repell other negatively charged ions on other side of dura
        etc etc as above :3 !!
        
        which leads to a negative deflection
    - - Dipoles must be parallel and oriented the same way
        --> parallel in the way they are excited
        
        --> if not parallel but random order = no clear dipole possible = no signal
        
        --> if not oriented the same way = positive and negative extracellular fluids mix and cancel each other out = no signal
      - Dipoles must be active at the same time (Synchronous)
        --> otherwise the signal will be too weak for the electrodes to pick it up
        --> mediated by thalamocortical / corticocortical connections
      - Electrodes
        
        Measures every 1 to 2ms / 0.5 to 1khz
        --> and compares it to reference electrode
        
        measurement (positive / negative deflection) represents the amount of ions (positive negative) at the electrode at that time pushed or pulled there by the dipoles
        --> Dipoles exude influence in all direction
        
        How quickly the amount of electrodes switch from positive to negative at the electrode, determines the frequency
        --> usually 1 to 30 Hz (Gamme 30 to 80 hz)
        
        Spatial smeering!
        
        Dipoles exude influence (repell / attract in all direction)
        --> 1 dipole will affect multiple areas of the scalp!
        --> this is called Spatial smeering!
        --> happens for main eeg range 0.1 to 30hz, for higher frequency it doesnt happen
        
        the stronger the dipole the further they can "send" information
        --> possible electrodes could pick up deeper cortical tissue activation (theoretically as dipole output weakens exponentially over distance)
      - Enhancing measurements
        --> by shielding the electrodes / cables from outer influences
        --> by filtering out muscle movements from eye eg. or breathing contraction
        --> both reduce noise :3!!
- - - - just that better signal cause magnetic field not diminished by skull (even though almost same signal source as EEG)
      - can only pick up tangential dipoles ( EEG tangential + radial)
      - Events are timelocked just like ERP's (averaged before = baseline, average after simulus = compared to baseline
      - same signal source = dipoles
    - - need to be chilled to 4 kelvin / -269 degrees to work
        ---> means they have to have certain distance to skull which increases signal to noise ratio
      - most modern scanners 306 SQUIDS
        --> translates to 102 measurement sites
      - 3 tranformer coils
        --> 2 for X-> Y axis
        --> 1 for Z (vertical axis)
    - - Assumptions:
        --> brain = spherical (which it isnt duh!)
        --> all active areas can be represented as a single dipole (also not the case!!)
      - minimum current estimate
        --> makes no assumtions