The P3b is a subcomponent of the P300, an event-related potential (ERP) component that can be observed in human scalp recordings of brain electrical activity. The P3b is a positive-going amplitude from 250 to 500 ms or later depending upon the task and on the individual subject response. Amplitudes are typically highest on the scalp over parietal brain areas.
The P3b has been a prominent tool used to study cognitive processes for several decades. More specifically, this ERP component has played a key role in cognitive psychology research on information processing. 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). The P3b can also be used to measure how demanding a task is on cognitive workload.
History Early observations of the P3b were reported in the mid-1960s. In 1964, researchers Chapman and Bragdon. found that ERP responses to visual stimuli differed depending on whether the stimuli had meaning or not. They showed subjects two kinds of visual stimuli: numbers and flashes of light. Subjects viewed these stimuli one at a time in a sequence. For every two numbers, the subjects were required to make simple decisions, such as telling which of the two numbers was numerically smaller or larger, which came first or second in the sequence, or whether they were equal. When examining evoked potentials to these stimuli (i.e., ERPs), Chapman and Bragdon found that both the numbers and the flashes elicited the expected sensory responses (e.g., visual N1 components), and that the amplitude of these responses varied in an expected fashion with the intensity of the stimuli. They also found that the ERP responses to the numbers, but not to the light flashes, contained a large positivity that peaked around 300 ms after the stimulus appeared. They also noted that the amplitude of this positivity was not affected by the intensity of the stimulus. Chapman and Bragdon speculated that this differential response to the numbers, which came to be known as the P300 response, resulted from the fact that the numbers were meaningful to the participants, based on the task that they were asked to perform.
In 1965, Sutton and colleagues published results from two experiments that further explored this late positivity. They presented subjects with either a cue that indicated whether the following stimulus would be a click or a flash, or a cue which required subjects to guess whether the following stimulus would be a click or a flash. They found that when subjects were required to guess what the following stimulus would be, the amplitude of the "late positive complex" was larger than when they knew what the stimulus would be. In a second experiment, they presented two cue types. For one cue there was a 2 in 3 chance that the following stimulus would be a click and a 1 in 3 chance that the following stimulus would be a flash. The second cue type had probabilities that were the reverse of the first. 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. Another important finding from these studies is that this late positive complex was observed for both the clicks and flashes, indicating that the physical type of the stimulus (auditory or visual) did not matter.
In later studies published in 1967, Sutton and colleagues had subjects guess whether they would hear one click or two clicks. They observed a positivity around 300 ms after the second click occurred or would have occurred in the case of the single click. They also had subjects guess how long the interval between clicks might be, and the late positivity occurred 300 ms after the second click. 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.
Since the initial discovery of this ERP component, research has shown that the P300 is not a unitary phenomenon. Rather, we can distinguish between two subcomponents of the P300: the novelty P3, or P3a, and the classic P3, or P3b. This article focuses on the P3b.
Component characteristics Assuming that a cephalic reference is used (i.e., a reference electrode placed somewhere on the head, such as the tip of the nose or the chin), the P3b is a positive-going ERP whose latency at peak amplitude is usually about 300 ms to simple sensory stimuli. 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. P3b amplitude is generally relatively large (10–20 microvolts), 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 latency of the P3b is usually around 300 ms, though this can vary within a time window of around 250–500 ms (or later) depending on factors such as task conditions and the age of the subjects (see Functional significance: Factors influence latency).
The scalp distribution of P3b is generally larger over parietal areas. 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. Other research, using the International 10-20 System with a left mastoid reference and an oddball task, has shown that with increasing age, the distribution of P3b tends to shift more frontally. Thus, the exact distribution may be dependent upon the task, as well as the gender and age of the subjects.
Main paradigms 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. 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 and Verleger, 1997).
Oddball paradigms Two classic paradigms are the two-stimulus oddball task and the three-stimulus oddball task, the latter of which is used to examine both P3b and P3a. 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. 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. The P3b is typically observed around 300 ms after each presentation of the target (oddball) stimulus.
A three-stimulus oddball task is exactly like the two-stimulus oddball task, except that in addition to the targets and standards, an infrequent, deviant stimulus such as the letter "D" will appear. These are often known as deviant standards, because they are not the target of the task but they differ from the regular standard. The P3b has been shown to respond only to task-relevant stimuli, or the targets that are actively being searched for (in this example, the letter T). Therefore, the deviant standard "D" will not elicit a strong P3b because it is not relevant to the task. However, the deviant standard will still elicit an earlier, positive-going potential that is usually higher over frontal sites known as the P3a. Unlike the P3b, the P3a will habituate with repeated presentations.
Dual-task paradigms Another set of paradigms used to study the P3b are dual task paradigms. There are several variations of the dual task paradigm, and they can be used to study cognitive workload (see Kok, 2001). Workload can be defined as the amount of processing resources required for a particular task. In a dual-task paradigm, participants are given two tasks to perform simultaneously; a primary task and a secondary task. Though the primary task can be of virtually any type, the secondary task should involve some traditional P300 paradigm (e.g., an oddball task). When these tasks are performed concurrently, we would expect to see a reduction in P3b amplitude in response to the secondary task if the primary task requires some stimulus evaluation resources. Furthermore, the extent of this reduction is presumed to reflect the amount of workload associated with the primary task. In fact, there should be a reciprocal relationship between the amplitudes of the P3b response elicited by the primary and secondary tasks, respectively. If the primary task is easier (i.e., requires less stimulus evaluation resources), participants have more resources left over to devote to the secondary task. Conversely, if the primary task is harder (i.e., requires more stimulus evaluation resources), participants have less resources left over to devote to the secondary task.
To give an example, subjects might perform a primary task, such as tracking a visual object on a screen with a joystick, concurrently with a secondary task of mentally counting oddballs in an auditory stream. The difficulty of the primary task is usually manipulated in various ways, and the impacts of these manipulations on the P3b response to the secondary task are examined. For example, in one condition subjects might track the one-dimensional movement of an object (only up and down), and in the more difficult condition they might have to track two dimensional movement (any direction on a computer screen). A motor manipulation like this will typically impact reaction times on the secondary task, but will not impact the P3b response. However, if you increase the demand on working memory or other cognitive resources during the primary task, for example by adding objects to the screen or only having subjects selectively attend to one part of the screen, the amplitude of the P3b in response to oddballs in the secondary task will diminish. The amount that it diminishes can be a measure of how many working memory or stimulus evaluation resources are being used by the primary task.
In another variation of the dual-task paradigm, subjects are presented with a visual stream of items presented one at a time. In this stream there are two targets, each of which requires a separate response. The amount of time or items separating the two targets is varied, and the amplitude of the P3b in response to the second item is examined. Diminished amplitude of the P3b response to the second target would be expected when the first target required more processing or working memory resources.
Functional significance: Factors that influence amplitude
Cognitive variables