Electroencephalography lab

Cognitive electrophysiology lab

Electroencephalography (EEG) and event-related potentials (ERP)

What is EEG?

EEG (Electroencephalography) is the measurement on the scalp of electricity originating from the brain. Because this electricity fluctuates over time, the EEG recording is informally referred to as ‘brain waves’. Embedded in these waves are electrical potentials which brain cells generate when they receive and transmit information in large assemblies (networks). It is therefore unsurprising that one can extract from the EEG signals that can give us clues about the current state of the brain (e.g. vigilance, sleep, arousal) and even about the way mental processes are ‘wired’ in the brain.

For several decades now, researchers have employed two complementary approaches to EEG. One is to separate oscillations (waves) of different frequencies. These frequencies, which are commonly labelled using Greek letters (alpha, beta, gamma, delta, etc.) have been associated with different states of the brain or ‘modes’ of information processing.

The second approach is to extract from the EEG stretches time-locked to specific stimuli (or responses) and look for characteristic activity (whatever its frequency) associated with types of stimuli and/or experimental tasks in whose context these stimuli are presented. The latter approach is referred to as Event-Related Potentials (ERPs). Whichever approach one adopts, EEG has very high temporal precision (temporal resolution), because it measures brain cell activity (rather than a metabolic consequence of this activity). The drawback of EEG is that, because the brain conducts electricity in all directions and the measurement is from the scalp (rather than from inside the head), it is usually very difficult to determine where in the brain the signal originates.

Is EEG safe?

Completely. In fact, it is the safest of all brain measurement techniques, because it does not subject the person to radiation, or any kind of stimulation- it simply measures the body’s (brain’s) own electricity. The EEG application has its inconveniences- for instance, one needs to insert conductive gel underneath each EEG sensor to ensure good contact between the sensor and the surface of the head. This gel is completely harmless and easily washed- but it does mean that one needs to wash his/her hair after an EEG session. This is about as ‘invasive’ as EEG gets!

What do we do with EEG?

Our primary tool is the ERP method (see 'What is EEG' box). We try to use the exquisite temporal resolution of EEG to determine the precise sequence of cognitive processes. The temporal precision of the EEG also allows one to look for characteristic time-courses of brain activity (referred to as ‘components’), which are associated with specific experimental manipulations. We have been keen to combine EEG with other techniques. To improve the quality of its spatial inference, we have combined it with fMRI. To relate ERP components to the dynamics of visuo-spatial attention, we have combined EEG with eye-tracking. Finally, we are also combining EEG with TMS to identify the electrophysiological correlates of some of the effects of TMS on behaviour we have been documenting.