Also known as an evoked potential, the event related potential (ERP) is what's often being studied when an EEG is taken. Output from an EEG is a graph of brainwaves on a time scale. While useful for telling us the rough brainwave frequency (alpha, beta, etc.) and amplitude, they aren't terribly useful for much else. Measuring event related potentials involves correlating the EEG brainwave response with a stimulus provided by whoever is doing the experiment, and averaging the result of many trials together to get a clear picture of what electrical activity takes place upon presentation of that stimulus.

Recording the EEG with multiple electrodes allows the experimenter to see not only the brainwave produced, but where and when its electrical activity moves over the surface of the cerebral cortex. These can be used to show how parts of the brain process individual syllables of a spoken word, or words of a sentence, etc. when time correlated and turned into event related potentials. This time relation is used as the naming convention for ERP's -- a positive change that happens 300 ms after the stimulus is called a P300, a negative change after 400 ms is a N400, etc. Confusingly, EEG recordings have electrical activity on the Y axis, but the scale goes from positive on bottom to negative on top. Thus, a negative change looks like an upwards spike on the graph, and vice versa.

Since individual parts of the brain are responsible for certain types of processing in all people -- except for people who've recovered abilities after brain damage, -- the same ERP results from the same stimulus across them. For example, the timing and shape of the certain wave produced only when seeing a flashing red light is going to be the same in an African plains tribesman as in a Japanese CEO. This isn't because they "think alike" in any conventional sense, it's because the brain areas responsible for processing flashing red lights are the same across the entire species. Still, it's an interesting result and could possibly eventually be used to determine not what somebody is thinking, but what part of reality (or internal state) somebody is thinking about.

Commonly there is a differentiation made between the early and late components of an ERP. Waves in the first 200 ms after the stimulus are referred to as exogenous, and are a direct result of the presented stimulus. They can, in fact, be correlated with different physical parts of the stimulus -- syllables in a word, color of the presented light, etc. Waves after the 200 ms mark are known as endogenous and are correlated with conceptual manipulation of the stimulus, or in other words, thought about it.

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