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 Back Gray Matter = Neuron cell bodies & dendrites Gyri Cortex = Outer layer of gray matter Sulci White Matter = Myelin (=fat)- covered axons

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

Left front to back Right Midline

Left front to back Right Midline

Left front to back Right Midline

Eye Movement Correction

= Electroencephalography (EEG) 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