Of course, any experimental paradigm in which participants are instructed to attend to and evaluate stimuli should elicit a P3b component, including selective attention tasks, explicit memory tasks, and visual search tasks (for thorough reviews of experimental paradigms which have been used to elicit this component, see Kok, 2001 In a classic two-stimulus oddball task, a sequence of visual stimuli is presented.
For example, subjects might see a string of letters presented one at a time.
However, using a 15-electrode setup with a linked-earlobe reference and an oddball task (described below), researchers have also found that the positivity increased moving from frontal to parietal sites, and that females have a greater increase than males.
A less frequent "target" or "oddball" stimulus such as the letter T is presented along with more frequent "standard" stimuli, such as the letter S.
The subject is typically instructed to respond in some way (such as with a button press) only to the targets, and to ignore the standards.
Generally speaking, improbable events will elicit a P3b, and the less probable the event, the larger the P3b.
However, in order to elicit a P3b, the improbable event must be related to the task at hand in some way (for example, the improbable event could be an infrequent target letter in a stream of letters, to which a subject might respond with a button press).
This shows two important findings: first that this late positivity occurred when the uncertainty about the type of click was resolved, and second that even an absence of a stimulus, when it was relevant to the task, would elicit the late positive complex.
These early studies encouraged the use of ERP methods to study cognition and provided a foundation for the extensive work on the P3b in the decades that followed.
Amplitude has been defined as the difference between the mean pre-stimulus baseline voltage and the voltage of the largest (in this case, positive-going) peak of the ERP waveform in a specific time window.), but varies systematically as a function of a number of important factors (see Functional significance: Factors that influence amplitude).
Latency has been defined as the time from the onset of the stimulus (or whatever the desired point of measurement might be) to the point of maximum amplitude.
The P3b can be observed in a variety of experimental contexts.
The most common paradigms will either present infrequent, task-relevant stimuli as a way to elicit a P3b, or they will employ two tasks at the same time to use P3b as a measure of cognitive workload.
They found that the amplitude of the positive complex was larger in response to the less probable stimuli, or the one that only had a 1 in 3 chance of appearing.