In the purely resting state, brain waves often are randomly active. However, when a person perceives or responds to a sensory or cognitive stimulus (e.g., a blue triangle), groups of neurons fire together, and the EEG is no longer random. The activity that is related to processing of the stimulus always occurs at the same time after the stimulus (i.e., is time–locked). This time–locked (i.e., event–related or evoked) activity is embedded in background random EEG that is not related to the stimulus processing. In order to see the tiny event–related activity, the EEG is averaged across multiple identical occurrences or trials (e.g., whenever the blue triangle occurs in a series of red squares); activity that is random with respect to the stimulus cancels out with each presentation of the stimulus, whereas the time–locked activity that occurs at the same time on every trial increases in the average. The waveform produced after averaging across identical trials is called an event–related potential (ERP). If one were to make a movie of the brain activity involved in the mental processing of a stimulus as it happens in real time, one would first see early fast activity that is related to sensory reception (e.g., seeing a visual stimulus) occurring in the visual cortex (occipital lobe [see figure 1]); this would then be followed by slower activity related to higher cognitive function (e.g., identification and attention to a blue triangle in a series of squares), which involves activity in the parietal and frontal lobes (figure 1). The fast and slow neural oscillations that underlie the ERP, called event–related oscillations (EROs), represent sensory and cognitive functions.
http://pubs.niaaa.nih.gov/publications/arh27-2/153-160.htm
