Closed and Open Electrical Fields
Single pyramidal cell in cortex
For brain electrical activity
to be detectable through skull,
must be strong signal summed
over 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 have
the right properties
Outer Surface of Human Brain
Axial Slice through Human Brain
Front
Gray Matter
Gyri
= Neuron cell bodies
& dendrites
Cortex
= Outer layer of
gray matter
Sulci
White Matter
= Myelin (=fat)covered axons
Back
Pyramidal Cell Orientation in Cortex
Radial Dipole
Tangential
Dipole
Pyramidal Cells mostly
oriented in same direction
relative to cortical surface,
pointing out toward surface
- Electrical activity from gyri
more detectable at scalp
than activity from sulci
Electrical and Magnetic Fields Generated by Dipoles
Electrical Field
Magnetic Field
Alpha Waves
Easy Cap
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Eye Movement Correction
Recording Brainwaves
= Electroencephalography (EEG)
= Timing of critical “events”
- Used in extracting
Event-Related Potentials
(ERPs) from ongoing EEG
Digitize = Take samples of continuous
input at regular intervals
(e.g., every 5 msec = 200 Hz)
Event-Related Activity
(We record continuously
& pull epochs out later)
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
Averaging Event-Related Potentials (ERPs)
Alpha Waves
(8-12 Hz)
“Oddball” Paradigm = 1 stimulus appears
more often (X) than
than another (O)
Typically see large “P3
(P300)” in response to
“oddball” stimulus
P300 can sometimes
be big enough to see
in individual trials
- Not true for N400
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 thalamus
volume-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