Closed and Open Electrical Fields. Single pyramidal cell in cortex For brain electrical activity to...
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Transcript of Closed and Open Electrical Fields. Single pyramidal cell in cortex For brain electrical activity to...
Closed and Open Electrical Fields
Single pyramidal cell in cortex
For brain electrical activityto be detectable through skull, must be strong signal summedover many neurons - All behaving similarly at same time - All oriented in same way - So negative and positive don’t cancel each other out when summed
Pyramidal Cells in the cortex havethe right properties
Outer Surface of Human Brain
Gray Matter = Neuron cell bodies & dendrites
White Matter = Myelin (=fat)- covered axons
Cortex = Outer layer of gray matter
Gyri
Sulci
Axial Slice through Human BrainFront
Back
Pyramidal Cell Orientation in Cortex
Pyramidal Cells mostly oriented in same directionrelative to cortical surface,pointing out toward surface - Electrical activity from gyri more detectable at scalp than activity from sulci
TangentialDipole
Radial Dipole
Electrical and Magnetic Fields Generated by Dipoles
Magnetic Field
Electrical Field
Alpha Waves
Easy Cap
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Eye Movement Correction
Recording Brainwaves= Electroencephalography (EEG)
Digitize = Take samples of continuous input at regular intervals (e.g., every 5 msec = 200 Hz)
= Timing of critical “events” - Used in extracting Event-Related Potentials (ERPs) from ongoing EEG
Event-Related Activity
Recording “epochs”Scale & Polarity(Negative Up!!!)
- Input range = 40-50 microvolts (uV)- Activity of interest often only 1-5 uV, so buried in higher amplitude activity that’s not of interest
(We record continuously & pull epochs out later)
Averaging Event-Related Potentials (ERPs)
“Oddball” Paradigm = 1 stimulus appears more often (X) than than another (O)
Typically see large “P3 (P300)” in response to “oddball” stimulus
P300 can sometimesbe big enough to seein individual trials - Not true for N400
Alpha Waves (8-12 Hz)
Idealized Auditory ERPs
“Inverse Problem” in EEG Source Localization
Electrical fields are “volume-conducted” throughout head Conducted better through some kinds of tissue
E.g., Cerebrospinal fluid vs bone
And summate Both boosting and canceling
So cannot assume activity generated directly under where it’s detected on scalp
Patterns detected at scalp could be produced by many different combos of generators So insufficient by itself to reveal locations of
sources
Electrical potential from a single source in left thalamusvolume-conducted through white matter
Some ERP Peaks Related to Language
N400 = ERP “component” related to meaning - Bigger when word’s meaning doesn’t fit context - Bigger for unfamiliar words - May reflect amount of work required to integrate with context
P600 = ERP “component” related to form - Bigger when word not of expected type for a position in a sentence - May be a type of P300 - Sometimes called Syntactic Positive Shift (SPS)
Can evoke both at once