User Tutorial:Introduction to the P300 Response
Since the 1960s, it has been known that presentation of infrequent stimuli evokes a positive deflection in the EEG over parietal cortex about 300 ms after stimulus presentation. This response has been termed the "P300" or "oddball" potential.
A P300 is usually elicited if four conditions are met:
- A random sequence of stimulus events must be presented.
- A classification rule that separates the series of events into two categories must be applied.
- The user's task must require using the rule.
- One category of events must be presented infrequently.
The spatial amplitude distribution of the P300 potential is symmetric around Cz. It is largest at the parietal electrode sites and is attenuated as the recording sites move to central and frontal locations.
Temporally, a typical P300 response has a width of 150-200 ms, and a triangular shape. The peak potential of a P300 is typically 2-5 , which is less than the brain's background activity. Thus, a single P300's signal-to-noise ratio is low, and is typically enhanced by averaging over multiple responses.
NOTE: In the images B and C above, stimulus presentation did not occur at time offset 200ms rather than 0. The additional offset is due to technical reasons, and always corresponds to the duration of a data block in BCI2000.
The P300 potential has been used as the basis for a BCI system in many studies. The classical format developed by Donchin and colleagues presents the user with a matrix of characters:
The rows and columns in this matrix flash successively and randomly at a rapid rate (e.g., eight flashes per second). The user selects a character by focusing attention on it and counting how many times it flashes. The row or column that contains this character evoke a P300 response, whereas all others do not. After averaging several responses, the computer can determine the desired row and column (i.e., the row/column with the highest P300 amplitude), and thus the desired character.
As a next step, we will set up the BCI2000 system for P300 BCI experiments.