W3 - EEG

Method of detecting neural activity via. electrodes on the scalp. Electrodes pick up small fluctuations of electrical signals from activity of (mostly cortical) neurons

• Very noisy and might not look like much, but they are systematically related to cognitive processes > Use these signals to learn something about cognition when people perform tasks

Scalp: Non-Invasive Intra-Cranial: Measure directly at exposed cortex

Hans Berger. Detected first EEG with wife’s scalp in 1924.

Alpha rhythm – When people closed their eyes, - Inconsistent electrical signal varying between 8 - 13 Hz - Used two electrodes (silver wires and later foil), one attached to the front of the head and one to the rear, and recorded the potential/voltage difference.

Cheap Temporal Resolution Poor Spatial Resolution

Electrode Cap > Amplifier > EEG Recording Experimental Stimulation > EEG Recording

10 - 32 - 64 - 128 - 256

Odd and Even. Each corresponds. F = frontal P = parietal C = central O = occipital T = temporal

Post-synaptic potentials (Voltage when NT binds to post-synaptic membrane) > Causes ion channels to open/close, leading to graded changes in potential across membrane Not Action Potential

• Understood as a small “dipole” (magnet) - Signals from single cells are not strong enough - Many neurons spatially align > summed potentials add up and create the signals we can record

Pooled activity from groups of similarly oriented neurons mostly comes from large cortical pyramid cells

The functional unit is >10,000 simultaneously activated neurons

Orientation of the neurons determines the sign of the recorded potentials Some orientations lead to signals which cannot be recorded

1.) Biased to Gyri (Sulci harder to detect / masked by gri signals) 2.) Meninges, CSF and skull “smear” the EEG signal > Localisation difficult - INVERSE PROBLEM: one given scalp configuration of signals can have multiple dipole solutions!

In relation to a reference electrode - Reference either a neutral point or average of all scalp electrodes

1.) 10μV to 100μV (Tiny) 2.) Amplified by 1,000/100,000x 3.) Signal is digitalized. Typical sample frequency is 256-1024Hz, but can be >4000Hz 4.) Signal is band-pass filtered to remove the low (<0.5-1Hz) and high frequencies (typically >35-70Hz) because they cannot reflect brain activity.

Finding all the artefacts that are not brain signals! - sweating - electrical noise (“notch filter”) - eye movements and blinks

• Eye can be regarded as a dipole - Signals originating from the eye will contaminate the signal of interest (And will be much larger)

• Record eye signal by placing electrodes next to and under the eye to capture horizontal and vertical eye movements - Remove by excluding contaminated trials, or mathematical algorithms, such as ICA

Far too noisy. Too many variance between sessions from same participants and between participants

Deflection in amplitude. P = Signal < 0 N = Signal > 0

1.) Peak-amplitude (used in 70%)) 2.) Area-under-the-curve (used in 20%) 3.) peak-to-peak (used in 10%) 4.) on-set of component (ambiguous) No clear rule, and results might differ between measures

N2pc (second negativity, posterior contralateral) > index attention Strongest over the posterior cortex contralateral to where the observer is attending