Electroencephalography (EEG) measures the electrical activity (field potentials) in the cortex through electrodes attached to the scalp. In the Humanities Lab we use either electrode caps or sensor nets that are placed on top the surface of the skull. Conductive gel or electrolytic water is then applied to the 32-128 electrodes. The electrodes record electrical activity as it happens in the brain, millisecond by millisecond, very much like a stethoscope is used for listening to heartbeats.
In studies performed in the Humanities Lab we are interested in the brain’s processing of certain types of stimuli, such as a word that is semantically congruent or incongruent in a sentence. For these types of studies the EEG is time-locked to the presentation of the specific word. The EEG portion that is analyzed is called the event-related potential (ERP). To measure the small ERPs accurately a large number of trials are needed in order to average out the noise from other EEG activity. When analyzing the ERP we look at polarity, topography, latency, and amplitude. Polarity (plus or minus) and topography (the distribution of the ERP waveforms over the skull) are differences that indicate different neural generators of the electrical activity measured for the particular event, or stimuli. Another ERP analysis parameter is latency (the timecourse of features and the amplitude of the ERPs are quantitative differences that are thought to indicate differences in speed of processing and effort.
The benefits of ERPs compared to behavioural measures are that they have excellent temporal resolution (in milliseconds) and that they don’t require a behavioural response, and can therefore be used for participant groups where a behavioural response is difficult to record e.g., in young children. In addition, ERPs can be measured to very specific stimuli e.g., specific words in sentences instead of at the end of the sentence as in the case of behavioural reaction time studies. In other words, ERPs are a powerful research tool as they measure the brain’s processing of the stimuli as it occurs on a millisecond by millisecond basis.