ERPs can be reliably measured using electroencephalography (EEG), a procedure that measures electrical activity of the brain through the skull and scalp. As the EEG reflects thousands of simultaneously ongoing brain processes, the brain response to a certain stimulus or event of interest is usually not visible in the EEG. One of the most robust features of the ERP response is a response to unpredictable stimuli. This response-known as the P300 (or simply "P3")-manifests as a positive deflection in voltage approximately 300 milliseconds after the stimulus is presented.
In actual recording situations, it is difficult to see an ERP after the presentation of a single stimulus. Rather the most robust ERPs are seen after many dozens or hundreds of individual presentations are averaged together. This technique cancels out noise in the data allowing only the voltage response to the stimulus to stand out clearly.
While evoked potentials reflect the processing of the physical stimulus, event-related potentials are caused by the "higher" processes, that might involve memory, expectation, attention, or changes in the mental state, among others.
Though some ERP components are referred to with acronyms (e.g., left anterior negativity - LAN), most components are referred to by a preceding letter indicating polarity followed by the typical latency in milliseconds. Thus, the N400 ERP component is described as a negative voltage deflection occurring approximately 400ms after stimulus onset, whereas the P600 component describes a positive voltage deflection 600ms after stimulus onset. The stated latencies for ERP components are often quite variable; for example, the N400 component may exhibit a latency between 300ms - 500ms.
Due to the consistency of the P300 response to novel stimuli, a brain-computer interface can be constructed which relies on it. By arranging many signals in a grid, randomly flashing the rows of the grid as in the previous paradigm, and observing the P300 responses of a subject staring at the grid, the subject may communicate which stimulus he is looking at, and thus slowly "type" words.