Electroencephalogram (EEG) measures the electrical activity (field potentials) in the cortex by attaching electrodes to the scalp. In the Humanities Lab we use either electrode caps or sensor nets that are placed on top of the scull. Conductive gel or electrolytic water is then applied to the 32-128 electrodes. The electrodes record brain activity as it happens in the brain, millisecond by millisecond, very much like a stethoscope is used to 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 need to be included to average out the noise from the larger EEG.
When analyzing the ERP we look at polarity, typography, latency, and amplitude. Polarity (plus or minus) and typography (the distribution of the ERP waveforms over the scull) are qualitative differences that indicate different neural generators of the electrical activity measured for the particular event, or stimuli. On the other hand, latency (how early the effect onsets or peaks) and the amplitude of the ERPs are quantitative differences that are thought to indicate differences in speed of processing and effort.
The benefits to the continuous ERPs compared to behavioural measures include the excellent temporal resolution (in milliseconds) and that it doesn’t require a behavioural response, such that it can easily be used for participant groups where a behavioural response is difficult to record e.g., in young children. In addition, ERPs are measured to e.g., specific words in sentences instead of at the end of the sentence as in the case of reaction time studies. That is, ERPs measure the brain’s processing of the stimuli as it occurs in the brain millisecond by millisecond.
Print Nov. 23, 2017 of http://www.humlab.lu.se/en/facilities/eeg/