Closed and Open Electrical Fields. Single pyramidal cell in cortex For brain electrical activity to be detectable through skull, must be strong signal.

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

Leftfronttoback

Rightfronttoback

Leftfronttoback

Rightfronttoback

Midlinefronttoback

Leftfronttoback

Rightfronttoback

Leftfronttoback

Rightfronttoback

Midlinefronttoback

Leftfronttoback

Rightfronttoback

Leftfronttoback

Rightfronttoback

Midlinefronttoback

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