Surprise, or prediction error, occurs whenever a person experiences a stimulus that was not expected. The oddball paradigm is one of the most common tasks for examining the neural correlates of surprise because it allows us to measure the brain’s response to unexpected stimuli using scalp EEG. In the classic oddball paradigm, the participant is presented with a sequence of tones or simple visual patterns presented one at a time with a short interval between each presentation. Most of the stimuli in the sequence are the same, and the participant quickly learns to expect this common stimulus. However, every so often, a different stimulus is presented. This oddball stimulus elicits the surprise response in the brain The nature of the brain’s response to the oddball stimulus depends on the task demands. In an active oddball task—where the participant presses a key or makes some other response when a rare stimulus occurs—the typical neural response is a positive going voltage at central-parietal electrodes approximately 300 ms after the onset of the rare stimulus (Soltani & Knight, 2000). This response is known as the P300 or P3B, and it has long been thought of as the neural correlate of surprise (Linden, 2005). However, the fact that there are distinct subcomponents of the P300 is often underappreciated. This is most easily revealed in the passive oddball task, wherein the participant does not have to make any responses to rare stimuli. In this task, instead of the parietal component, a more frontal component known as the P3A is typically observed (Linden). To examine the different subcomponents of the P300 within subjects (n = 18), we uses a three-stimulus version of the oddball task with both visual and auditory modalities. In this paradigm, there are two rare stimuli in each sequence, one of which is a target that the participant must respond to (as in the active oddball task), while the other can be ignored (as in the passive oddball task). This paradigm allows us to separate the influence of novelty, attention, and memory in the generation of P300 signals. To date, only a handful of studies have used this method to examine the subcomponents of the P300, and no studies have examined them with different modalities of stimuli in the same experiment.
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