Recording, non-invasive, electrical. measures electrical activity of the brain through electrodes placed on the scalp. EEG signals represent the the change in the potential difference between two electrodes placed on the scalp. - The EEG obtained on several trials can be averaged together time locked to the stimulus to form an Event-Related Potential (ERP). ERPs are voltage fluctuations that are associated in time with particular event or stimulus (visual, auditory, olfactory stimuli). - When populations of neurons are active in synchrony, they produce an electric field that can be detected at the scalp (EEG). When many waves are averaged and linked to the onset of the stimulus, then an ERP is obtained