P300
Encyclopedia
The P300 wave is an event related potential (ERP) elicited by infrequent, task-relevant stimuli. It is considered to be an endogenous potential as its occurrence links not to the physical attributes of a stimulus but to a person's reaction to the stimulus. More specifically, the P300 is thought to reflect processes involved in stimulus evaluation or categorization. It is usually elicited using the oddball paradigm
in which low-probability target items are inter-mixed with high-probability non-target (or "standard") items.
When recorded by electroencephalography
(EEG
), it surfaces as a positive deflection in voltage with a latency (delay between stimulus and response) of roughly 300 to 600 ms. The signal is typically measured most strongly by the electrodes covering the parietal lobe
. The presence, magnitude, topography and timing of this signal are often used as metrics of cognitive function in decision making processes. While the neural substrates of this ERP still remain hazy, the reproducibility of this signal makes it a common choice for psychological tests in both the clinic and laboratory.
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. 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.
, and the classic P3, or P3b
. The P3a, or novelty P3, is a component of time-locked (EEG)
signals known as event-related potentials (ERP)
. The P3a is a positive-going scalp-recorded brain potential that has a maximum amplitude over frontal/central electrode sites with a peak latency falling in the range of 250-280 ms. The P3a has been associated with brain
activity related to the engagement of attention
(especially orienting
and involuntary shifts to changes in the environment) and the processing of novelty. The P3b is a positive-going ERP amplitude (usually relative to a reference behind the ear or the average of two such references) peaking at around 300 ms, though the peak will vary in latency from 250-500 ms or later depending upon the task. Amplitudes are typically highest on the scalp over parietal brain areas. The P3b has been a prominent tool used to study cognitive processes, especially 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
.
. In a proposed "guilty knowledge test" a subject is interrogated via the oddball paradigm much as they would be in a typical lie-detector situation. This practice has recently enjoyed increased legal permissibility while conventional polygraph
y has seen its use diminish, in part owing to the unconscious and uncontrollable aspects of the P300. The technique relies on reproducible elicitation of the P300 wave, central to the idea of a Memory and Encoding Related Multifaceted Electroencephalographic Response (MERMER) developed by Dr. Lawrence Farwell
.
Applications in brain-computer interfacing have also been proposed. The P300 has a number of desirable qualities that aid in implementation of such systems. First, the waveform is consistently detectable and is elicited in response to precise stimuli. The P300 waveform can also be evoked in nearly all subjects with little variation in measurement techniques, which may help simplify interface designs and permit greater usability. The speed at which an interface is able to operate depends on how detectable the signal is despite “noise.” One negative characteristic of the P300 is that the amplitude of the waveform requires averaging of multiple recordings to isolate the signal. This and other post-recording processing steps determine the overall speed of an interface. The algorithm proposed by Farwell and Donchin provides an example of a simple BCI that relies on the unconscious decision making processes of the P300 to drive a computer. A 6x6 grid of characters is presented to the subject, and various columns or rows are highlighted. When a column or row contains the character a subject desires to communicate, the P300 response is elicited (since this character is “special” it is the target stimulus described in the typical oddball paradigm). The combination of the row and column which evoked the response locates the desired character. A number of such trials must be averaged to clear noise from the EEG. The speed of the highlighting determines the number of characters processed per minute. Results from studies using this setup show that normal subjects could achieve a 95% success rate at 3.4-4.3 chars/min. It remains to be shown whether such systems provide similar results in patients suffering from locked-in syndrome
, the main target population for such brain driven devices.
Scientific research often relies on measurement of the P300 to examine event related potentials, especially with regard to decision making. Because cognitive impairment is often correlated with modifications in the P300, the waveform can be used as a measure for the efficacy of various treatments on cognitive function. Some have suggested its use as a clinical marker for precisely these reasons. There is a broad range of uses for the P300 in clinical research.
Oddball paradigm
The oddball paradigm is a technique used in evoked potential research in which trains of stimuli that are usually auditory or visual are used to assess the neural reactions to unpredictable but recognizable events...
in which low-probability target items are inter-mixed with high-probability non-target (or "standard") items.
When recorded by electroencephalography
Electroencephalography
Electroencephalography is the recording of electrical activity along the scalp. EEG measures voltage fluctuations resulting from ionic current flows within the neurons of the brain...
(EEG
EEG
EEG commonly refers to electroencephalography, a measurement of the electrical activity of the brain.EEG may also refer to:* Emperor Entertainment Group, a Hong Kong-based entertainment company...
), it surfaces as a positive deflection in voltage with a latency (delay between stimulus and response) of roughly 300 to 600 ms. The signal is typically measured most strongly by the electrodes covering the parietal lobe
Parietal lobe
The parietal lobe is a part of the Brain positioned above the occipital lobe and behind the frontal lobe.The parietal lobe integrates sensory information from different modalities, particularly determining spatial sense and navigation. For example, it comprises somatosensory cortex and the...
. The presence, magnitude, topography and timing of this signal are often used as metrics of cognitive function in decision making processes. While the neural substrates of this ERP still remain hazy, the reproducibility of this signal makes it a common choice for psychological tests in both the clinic and laboratory.
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 N1Visual N1
The Visual N1 is a visual evoked potential, a type of event-related electrical potential , that is produced in the brain and recorded on the scalp. The N1 is so named to reflect the polarity and typical timing of the component. The "N" indicates that the polarity of the component is negative with...
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. 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.
P3a and P3b
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 P3aP3a
The P3a, or novelty P3, is a component of time-locked signals known as event-related potentials . The P3a is a positive-going scalp-recorded brain potential that has a maximum amplitude over frontal/central electrode sites with a peak latency falling in the range of 250-280 ms...
, and the classic P3, or P3b
P3b
The P3b is a subcomponent of the P300, an event-related potential component that can be observed in human scalp recordings of brain electrical activity...
. The P3a, or novelty P3, is a component of time-locked (EEG)
Electroencephalography
Electroencephalography is the recording of electrical activity along the scalp. EEG measures voltage fluctuations resulting from ionic current flows within the neurons of the brain...
signals known as event-related potentials (ERP)
Event-related potential
An event-related potential is any measured brain response that is directly the result of a thought or perception. More formally, it is any stereotyped electrophysiological response to an internal or external stimulus....
. The P3a is a positive-going scalp-recorded brain potential that has a maximum amplitude over frontal/central electrode sites with a peak latency falling in the range of 250-280 ms. The P3a has been associated with brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
activity related to the engagement of attention
Attention
Attention is the cognitive process of paying attention to one aspect of the environment while ignoring others. Attention is one of the most intensely studied topics within psychology and cognitive neuroscience....
(especially orienting
Orienting response
The orienting response, also called orienting reflex, is an organism's immediate response to a change in its environment, when that change is not sudden enough to elicit the startle reflex...
and involuntary shifts to changes in the environment) and the processing of novelty. The P3b is a positive-going ERP amplitude (usually relative to a reference behind the ear or the average of two such references) peaking at around 300 ms, though the peak will vary in latency from 250-500 ms or later depending upon the task. Amplitudes are typically highest on the scalp over parietal brain areas. The P3b has been a prominent tool used to study cognitive processes, especially 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
Cognitive load
The term cognitive load is used in cognitive psychology to illustrate the load related to the executive control of working memory . Theories contend that during complex learning activities the amount of information and interactions that must be processed simultaneously can either under-load, or...
.
Applications
Since the mid 1980s, one of the most discussed uses of ERPs such as the P300 is related to lie detectionLie detection
Lie detection is the practice of attempting to determine whether someone is lying. Activities of the body not easily controlled by the conscious mind are compared under different circumstances. Usually this involves asking the subject control questions where the answers are known to the examiner...
. In a proposed "guilty knowledge test" a subject is interrogated via the oddball paradigm much as they would be in a typical lie-detector situation. This practice has recently enjoyed increased legal permissibility while conventional polygraph
Polygraph
A polygraph measures and records several physiological indices such as blood pressure, pulse, respiration, and skin conductivity while the subject is asked and answers a series of questions...
y has seen its use diminish, in part owing to the unconscious and uncontrollable aspects of the P300. The technique relies on reproducible elicitation of the P300 wave, central to the idea of a Memory and Encoding Related Multifaceted Electroencephalographic Response (MERMER) developed by Dr. Lawrence Farwell
Lawrence Farwell
Dr Lawrence Farwell is the inventor of brain fingerprinting which uses a technology that he developed, MERMER. He was formerly a research associate at Harvard University and is currently the chairman and Chief Scientist of Brain Fingerprinting Laboratories, Inc.TIME Magazine named Dr...
.
Applications in brain-computer interfacing have also been proposed. The P300 has a number of desirable qualities that aid in implementation of such systems. First, the waveform is consistently detectable and is elicited in response to precise stimuli. The P300 waveform can also be evoked in nearly all subjects with little variation in measurement techniques, which may help simplify interface designs and permit greater usability. The speed at which an interface is able to operate depends on how detectable the signal is despite “noise.” One negative characteristic of the P300 is that the amplitude of the waveform requires averaging of multiple recordings to isolate the signal. This and other post-recording processing steps determine the overall speed of an interface. The algorithm proposed by Farwell and Donchin provides an example of a simple BCI that relies on the unconscious decision making processes of the P300 to drive a computer. A 6x6 grid of characters is presented to the subject, and various columns or rows are highlighted. When a column or row contains the character a subject desires to communicate, the P300 response is elicited (since this character is “special” it is the target stimulus described in the typical oddball paradigm). The combination of the row and column which evoked the response locates the desired character. A number of such trials must be averaged to clear noise from the EEG. The speed of the highlighting determines the number of characters processed per minute. Results from studies using this setup show that normal subjects could achieve a 95% success rate at 3.4-4.3 chars/min. It remains to be shown whether such systems provide similar results in patients suffering from locked-in syndrome
Locked-In syndrome
Locked-in syndrome is a condition in which a patient is aware and awake but cannot move or communicate verbally due to complete paralysis of nearly all voluntary muscles in the body except for the eyes. Total locked-in syndrome is a version of locked-in syndrome where the eyes are paralyzed as...
, the main target population for such brain driven devices.
Scientific research often relies on measurement of the P300 to examine event related potentials, especially with regard to decision making. Because cognitive impairment is often correlated with modifications in the P300, the waveform can be used as a measure for the efficacy of various treatments on cognitive function. Some have suggested its use as a clinical marker for precisely these reasons. There is a broad range of uses for the P300 in clinical research.
See also
- Brain fingerprintingBrain fingerprintingP300 - Brain Fingerprinting is a discarded and highly controversial forensic science technique that uses electroencephalography to determine whether specific information is stored in a subject’s brain...
- Somatosensory evoked potentialSomatosensory Evoked PotentialSomatosensory Evoked Potentials are a useful, noninvasive means of assessing somatosensory system functioning. By combining SEP recordings at different levels of the somatosensory pathways, it is possible to assess the transmission of the afferent volley from the periphery up to the cortex...
- C1 and P1C1 & P1 (Neuroscience)The C1 and P1 are two human scalp-recorded event-related brain potential components, collected by means of a technique called electroencephalography . The C1 is named so because it was the first component in a series of components found to respond to visual stimuli when it was first discovered...
- Visual N1Visual N1The Visual N1 is a visual evoked potential, a type of event-related electrical potential , that is produced in the brain and recorded on the scalp. The N1 is so named to reflect the polarity and typical timing of the component. The "N" indicates that the polarity of the component is negative with...
- Mismatch negativityMismatch negativityThe mismatch negativity or mismatch field is a component of the event-related potential to an odd stimulus in a sequence of stimuli. It arises from electrical activity in the brain and is studied within the field of cognitive neuroscience and psychology. It can occur in any sensory system, but...
- N100
- N200N200 (neuroscience)The N200, or N2, is an event-related potential component. An ERP can be monitored using a non-invasive electroencephalography cap that is fitted over the scalp on human subjects...
- N2pcN2pcN2pc refers to an ERP component linked to selective attention. The N2pc appears over visual cortex contralateral to the location in space to which subjects are attending; if subjects pay attention to the left side of the visual field, the N2pc appears in the right hemisphere of the brain, and...
- N170N170The N170 is a component of the event-related potential that reflects the neural processing of faces.When potentials evoked by images of faces are compared to those elicited by other visual stimuli, the former show increased negativity 130-200 ms after stimulus presentation...
- P200P200In neuroscience, the visual P200 or P2 is a waveform component or feature of the event-related potential measured at the human scalp. Like other potential changes measurable from the scalp, this effect is believed to reflect the post-synaptic activity of a specific neural process...
- N400
- P3aP3aThe P3a, or novelty P3, is a component of time-locked signals known as event-related potentials . The P3a is a positive-going scalp-recorded brain potential that has a maximum amplitude over frontal/central electrode sites with a peak latency falling in the range of 250-280 ms...
- P3bP3bThe P3b is a subcomponent of the P300, an event-related potential component that can be observed in human scalp recordings of brain electrical activity...
- Late Positive ComponentLate Positive ComponentThe LPC is a positive-going event-related brain potential component that has been important in studies of explicit recognition memory...
- Difference due to MemoryDifference due to MemoryDifference due to Memory indexes differences in neural activity during the study phase of an experiment for items that subsequently are remembered compared to items that are later forgotten...
- Contingent negative variationContingent negative variationThe contingent negative variation was one of the first event-related potential components to be described. The CNV component was first described by Dr. W. Grey Walter and colleagues in an article published in Nature in 1964...
- Error-related negativityError-related negativityError-related negativity , , is a component of an event-related potential . ERPs are electrical activity in the brain as measured through electroencephalography and time-locked to an external event...
- BereitschaftspotentialBereitschaftspotentialIn neurology, the Bereitschaftspotential or BP , also called the pre-motor potential or readiness potential , is a measure of activity in the motor cortex of the brain leading up to voluntary muscle movement. The BP is a manifestation of cortical contribution to the pre-motor planning of volitional...
- Lateralized readiness potentialLateralized readiness potentialIn neuroscience, the lateralized readiness potential is an event-related brain potential, or increase in electrical activity at the surface of the brain, that is thought to reflect the preparation of motor activity on a certain side of the body; in other words, it is a spike in the electrical...
- Early left anterior negativityEarly left anterior negativityThe early left anterior negativity is an event-related potential in electroencephalography , or component of brain activity that occurs in response to a certain kind of stimulus...
- P600P600The P600 is an event-related potential , or peak in electrical brain activity measured by electroencephalography . It is a language-relevant ERP and is thought to be elicited by hearing or reading grammatical errors and other syntactic anomalies...