Answer: The EEG is the tool that is used to record event related potentials.
Neurons temporarily change their electrical properties when they communicate. This change, when added together across many thousand or millions of cells can be detected at the surface of the scalp by a tool called an electroencephalogram, or an EEG. The EEG may look like a series of pads or a cap that is placed on the head. The EEG has conductive leads that can sense the change in electrical activity, and collects that information for analysis by an external computer. EEGs are harmless, are considered noninvasive, and can be worn for several hours at a time.
An event related potential is a change in EEG signal following some type of stimulus (the event). This stimulus may be a sensory stimulus, like showing a person a picture of a face or brushing their arm with a paintbrush. After these sensory stimuli, the neurons of their brain change in activity, which is then detected by the EEG. This change in activity is the event related potential, or ERP for short.
ERPs are also called a time-locked EEG signal, since the change in EEG signal is predictable after the event. Usually, events are measured and detected for up to 1 second (1000 milliseconds) after the event.
More generally, EEG is often used for clinical applications. For example, people may wear an EEG for a night to help the sleep doctors figure out if they have some kind of sleep disorder like sleep apnea or insomnia. EEGs are given to patients when they undergo anesthesia, or to help figure out the nature of their specific type of epilepsy that cause seizures. In these cases, the EEG recordings are done for several minutes or hours at a time.
On the other hand, ERPs are more often used for research rather than for clinical diagnostics. Research teams may be interested to see how strong a specific event related potential is between two populations of people. Two major features of ERPs are often analyzed. Latency, which is the time between the event and the peak event, and the amplitude of the peak.
Broadly speaking, ERPs are subdivided into two categories. Early events are those that reach their peak around 100 milliseconds after the event. These early events are often characterized as sensory events, since they are most prominent after some kind of sensory activity.
Late events are those that peak after 100 milliseconds. These are also called cognitive events, since these events correspond most with complex cognitive functions such as emotional processing.
Event related potentials are often given names referring to the direction of the electrical activity (positive or negative, denoted P or N) and the time after the event (usually in milliseconds, but this may sometimes be shortened). For example, a commonly studied event related potential is called the P300 wave, which is a positive potential event that often peaks around the 250-400 millisecond mark after the event onset. The P300 wave is also called the P3 wave.
Event related potentials are found to be different among people with various disease diagnoses, including alcohol use disorder, schizophrenia, and bipolar disorder. But, it is not clear exactly how these changes in ERPs relate to the disease, or if there is any causative link between the events.