ERP components

First, a participant is shown a stimulus.

We see here that each time the participant is shown that stimulus, the corresponding brain waves look very different each time. This is because the brain is doing many things at one point in time, and the EEG records all of it. So each of these segments represent the combined brain activity of everything it is doing in that moment.

So how do we isolate the brain activity for just the stimulus? For that, we do something called averaging.

Here, we take all the segments and average them out, which removes the other brain activity, leaving behind the consistent brain response that our brain has for that stimulus. ERPs are small relative to the noise level and many trials are usually required to accurately measure a given ERP effect. You must have usually 10 to 500 trials per condition in each subject to achieve sufficient statistical power.

Some examples of some common ERP are the P300 and N170. This is a positive deflection in the EEG signal that occurs about 300ms (approximately) after a stimulus. It's associated with decision making, attention, and memory processing. The P300 is often studied using the "oddball paradigm". This is an experimental design where participants are presented with a sequence of stimuli where one type of stimulus (the "oddball") occurs infrequently among more common stimuli. When participants detect the oddball stimulus, it typically elicits a larger P300 response, reflecting the allocation of attention to the rare event.

The N170 is a negative deflection occurring about 170ms after stimulus onset. It's particularly responsive to face stimuli and is thought to reflect early stages of face processing.

It is important to note, however, that not all mental processes have an ERP signature. This can be because the signal may be too deep in the brain to be picked up by surface level electrodes, or the neural process occurs too fast. Hence, it may be difficult to find any clear contribution to the scalp-recorded voltage.