P3a
Encyclopedia
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.
reflects. At the time, several researchers suggested that there needed to be active attention
towards the target stimuli in order to elicit a P300, in part because stimuli that were ignored resulted in a P300
with a smaller amplitude or no P300
at all. On the other hand, some research had shown that subjects exhibit a P300
to unpredictable stimuli in an ongoing repetitive series of stimuli, even when the stimuli were classified as irrelevant and subjects were asked to ignore them while completing another task (i.e. reading a book). It was intriguing that you could elicit a P300
in conditions with active attention and those of non-attention. Upon further investigation it turned out that when comparing the two types of P300 potentials, they differed in latency and scalp topography. This led Squires et al. to suggest that there were two distinct psycho-physiological entities that had been referred to collectively as the P300
More specifically, Squires et al. recorded EEG during an auditory odd-ball paradigm with various conditions. The two types of stimuli were 90 dB
and 70 db tone bursts that occurred 1.1 sec apart. Loud tones occurred with a probability of .9, .5, or .1, while the soft tones occurred with complementary probability. In addition, subjects completed blocks of stimuli under instruction to count the number of loud tones, count the number of soft tones, or ignore the tones and quietly read. Therefore, each set of instructions was performed at each of the probability combinations. Squires et al. found that when subjects were told to ignore the tones, the less frequent or rare tone (probability of .1) elicited a positive-going potential which occurred between 220 and 280 ms. They termed this potential the P3a in order to distinguish it from its relative, the P3b
, which was a positive-going potential that occurred at 310-380 ms when the infrequent tones were attended to. Scalp distribution helped them differentiate the two potentials as well. The newly-coined “P3a” had a peak amplitude occurring at frontal midline sites while the P3b peak amplitude occurred over parietal midline sites.
. With now-extensive research, it is also possible to dissociate these components even when the experimental context is different and/or less well-studied. P3a amplitudes tend to be maximal over frontal/central sites on the scalp, such as FCZ/CZ in the international 10-20 system, which is the standard electrode placement system of many ERP labs around the world. P3b amplitudes are generally greater at sites like PZ. Latency is another distinguishing characteristic. While many things can affect the latency of the P3b, P3a latencies often occur 75-100 ms earlier than P3b peak latencies, and around 250-280 ms.. Finally, the two responses have different functional sensitivities and associated psychological correlates.
is one of the primary paradigms used to elicit a prominent P3a. As the name implies, the paradigm includes three types of stimuli: frequent, attended "standards", less frequent, attended "target" stimuli and a third "deviant" stimulus type. This paradigm is a modification of the oddball task that is used to elicit a P3b. In this task, infrequent-nontarget stimuli are dispersed throughout a sequence of task-relevant target and standard stimuli. When these infrequent, novel stimuli (for example, the sound of dog barks or color forms) are presented in the series of more typical target and standard stimuli (for example, tones or letters of the alphabet), a P3a that is larger over the frontal and central areas of the brain is produced in response to auditory
, visual, and somatosensory stimuli Deviant stimuli from auditory, visual, and somatosensory modalities are all sufficient for eliciting a P3a. For example, Grillon and colleagues used this design when they tested for the effects of rare non-target (deviant) auditory stimuli on subjects' EEG activity. They used 1600 Hz
tones as the standard stimuli, while a 900 Hz tone represented the rare target stimuli. In the “Novel” condition, they added a rare non-target tone at 700 Hz. In their results it was apparent that the P300 they recorded to the rare non-target tones was in fact a P3a. The rare non-target tones resulted in a P300 (P3a) with a shorter latency that was distributed more towards the front of the scalp when compared to the P300 (P3b) elicited by rare target stimuli.
The 3 stimulus oddball paradigm provides a flexible way to examine the P3a across stimulus modality and tasks. Yamaguchi and Knight conducted a study using mechanical tactile stimuli (finger taps) and electric shocks to the wrist within a 3-stimulus oddball paradigm. They were interested in seeing if subjects would elicit a P3a to novel somatosensory stimuli. They devised a design wherein subjects would receive finger taps to hand digits 2-5 and electric shocks to the wrist. Taps on the 2nd finger were considered standards (76% probability) while taps on the 5th finger were targets (12% prob.). Taps occurring on the 3rd and 4th digits were considered “tactile novel” stimuli (6% prob.) and electric shocks to the wrist were shock novels (6% prob.). They found that both types of the novel somatosensory stimuli did in fact produce P3a’s that had a more frontal distribution than responses to target stimuli. Shock novels also resulted in a significantly shorter P3a latency.
of the P3a include habituation
and target discrimination. One major difference between the P3b and the P3a is that only the P3a habituates with repeated presentation. The habituation indicates that some sort of memory
encoding for the event has been created, and for this reason the event no longer generates a response when repeated. Each time a novel event is experienced, it is compared to the previously created neural representation, and, if it is sufficiently deviant, then the process begins again. If this event is not sufficiently deviant (i.e., it is the same) then habituation
occurs. The P3a's rapid amplitude reduction with exposure to repeated trials of novel stimuli supports the idea that the P3a is the electrophysiological representation of the orienting response
(which also habituates in behavior) . For example, Grillon and colleagues used a 3 stimulus odd-ball paradigm wherein they presented subjects with a condition in which the deviant stimuli were constant and a condition in which the deviant stimuli were always novel. Their results showed the largest P3a amplitude in response to deviant stimuli that were novel.
Another factor that affects P3a amplitude is target discrimination. It is interesting that although the P3a is elicited by non-target deviant stimuli, the nature of the target stimuli affect the P3a response. It seems that the amplitude of the P3a may be affected by an individual’s ability to distinguish target stimuli from standard stimuli. When this discrimination is easy, non-target deviant stimuli produce a P300 that is smaller than the target P3b and is largest over parietal sites. However, if target discrimination is difficult, the P3a to non-target stimuli is larger and more frontally-skewed with a shorter latency—in other words, the more "canonical" P3a response
Although the P3a has been dissociated from the P3b, its amplitude and latency may be affected by factors that also modulate the P3b. Some of these factors include stimulus probability, stimulus evaluation difficulty, natural state variables (such as circadian and menstrual cycles), and environmentally induced state variables (such as drugs and exercise). John Polich and Albert Kok have written up an extensive review that covers many of these variables.
Neural sources of the P3a have been hypothesized to arise from frontal lobe
functioning and to involve frontal lobe attention mechanisms. Magnetic resonance imaging
(MRI) studies looking at gray matter volume and P3a amplitude show stronger correlations when non-target, startling stimuli are viewed.. Lesion studies indicate that prefrontal and temporal-parietal cortex contribute to auditory P3a generation. The P3a is suspected to also reflect interactions between the frontal lobe
and the hippocampus
, as patients with focal hippocampal lesions have reduced P3a amplitude from novel distracters.
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.
History
In 1975 Squires and colleagues conducted a study attempting to resolve some of the questions surrounding what neural process the P300P300 (neuroscience)
The P300 wave is an event related potential 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...
reflects. At the time, several researchers suggested that there needed to be active 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....
towards the target stimuli in order to elicit a P300, in part because stimuli that were ignored resulted in a P300
P300 (neuroscience)
The P300 wave is an event related potential 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...
with a smaller amplitude or no P300
P300 (neuroscience)
The P300 wave is an event related potential 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...
at all. On the other hand, some research had shown that subjects exhibit a P300
P300 (neuroscience)
The P300 wave is an event related potential 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...
to unpredictable stimuli in an ongoing repetitive series of stimuli, even when the stimuli were classified as irrelevant and subjects were asked to ignore them while completing another task (i.e. reading a book). It was intriguing that you could elicit a P300
P300 (neuroscience)
The P300 wave is an event related potential 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...
in conditions with active attention and those of non-attention. Upon further investigation it turned out that when comparing the two types of P300 potentials, they differed in latency and scalp topography. This led Squires et al. to suggest that there were two distinct psycho-physiological entities that had been referred to collectively as the P300
P300 (neuroscience)
The P300 wave is an event related potential 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...
More specifically, Squires et al. recorded EEG during an auditory odd-ball paradigm with various conditions. The two types of stimuli were 90 dB
Decibel
The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
and 70 db tone bursts that occurred 1.1 sec apart. Loud tones occurred with a probability of .9, .5, or .1, while the soft tones occurred with complementary probability. In addition, subjects completed blocks of stimuli under instruction to count the number of loud tones, count the number of soft tones, or ignore the tones and quietly read. Therefore, each set of instructions was performed at each of the probability combinations. Squires et al. found that when subjects were told to ignore the tones, the less frequent or rare tone (probability of .1) elicited a positive-going potential which occurred between 220 and 280 ms. They termed this potential the P3a in order to distinguish it from its relative, the 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...
, which was a positive-going potential that occurred at 310-380 ms when the infrequent tones were attended to. Scalp distribution helped them differentiate the two potentials as well. The newly-coined “P3a” had a peak amplitude occurring at frontal midline sites while the P3b peak amplitude occurred over parietal midline sites.
Component characteristics
Consistent with this historical separation of the two components, typically if a stimulus is a rare non-target then the recorded EEG waveform has characteristics associated with the P3a, whereas attended targets elicit a P3bP3b
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...
. With now-extensive research, it is also possible to dissociate these components even when the experimental context is different and/or less well-studied. P3a amplitudes tend to be maximal over frontal/central sites on the scalp, such as FCZ/CZ in the international 10-20 system, which is the standard electrode placement system of many ERP labs around the world. P3b amplitudes are generally greater at sites like PZ. Latency is another distinguishing characteristic. While many things can affect the latency of the P3b, P3a latencies often occur 75-100 ms earlier than P3b peak latencies, and around 250-280 ms.. Finally, the two responses have different functional sensitivities and associated psychological correlates.
Main paradigms
The 3-stimulus oddball paradigmOddball 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...
is one of the primary paradigms used to elicit a prominent P3a. As the name implies, the paradigm includes three types of stimuli: frequent, attended "standards", less frequent, attended "target" stimuli and a third "deviant" stimulus type. This paradigm is a modification of the oddball task that is used to elicit a P3b. In this task, infrequent-nontarget stimuli are dispersed throughout a sequence of task-relevant target and standard stimuli. When these infrequent, novel stimuli (for example, the sound of dog barks or color forms) are presented in the series of more typical target and standard stimuli (for example, tones or letters of the alphabet), a P3a that is larger over the frontal and central areas of the brain is produced in response to auditory
Auditory system
The auditory system is the sensory system for the sense of hearing.- Outer ear :The folds of cartilage surrounding the ear canal are called the pinna...
, visual, and somatosensory stimuli Deviant stimuli from auditory, visual, and somatosensory modalities are all sufficient for eliciting a P3a. For example, Grillon and colleagues used this design when they tested for the effects of rare non-target (deviant) auditory stimuli on subjects' EEG activity. They used 1600 Hz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
tones as the standard stimuli, while a 900 Hz tone represented the rare target stimuli. In the “Novel” condition, they added a rare non-target tone at 700 Hz. In their results it was apparent that the P300 they recorded to the rare non-target tones was in fact a P3a. The rare non-target tones resulted in a P300 (P3a) with a shorter latency that was distributed more towards the front of the scalp when compared to the P300 (P3b) elicited by rare target stimuli.
The 3 stimulus oddball paradigm provides a flexible way to examine the P3a across stimulus modality and tasks. Yamaguchi and Knight conducted a study using mechanical tactile stimuli (finger taps) and electric shocks to the wrist within a 3-stimulus oddball paradigm. They were interested in seeing if subjects would elicit a P3a to novel somatosensory stimuli. They devised a design wherein subjects would receive finger taps to hand digits 2-5 and electric shocks to the wrist. Taps on the 2nd finger were considered standards (76% probability) while taps on the 5th finger were targets (12% prob.). Taps occurring on the 3rd and 4th digits were considered “tactile novel” stimuli (6% prob.) and electric shocks to the wrist were shock novels (6% prob.). They found that both types of the novel somatosensory stimuli did in fact produce P3a’s that had a more frontal distribution than responses to target stimuli. Shock novels also resulted in a significantly shorter P3a latency.
Functional sensitivity
Two important factors for determining the amplitudeAmplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
of the P3a include habituation
Habituation
Habituation can be defined as a process or as a procedure. As a process it is defined as a decrease in an elicited behavior resulting from the repeated presentation of an eliciting stimulus...
and target discrimination. One major difference between the P3b and the P3a is that only the P3a habituates with repeated presentation. The habituation indicates that some sort of memory
Memory
In psychology, memory is an organism's ability to store, retain, and recall information and experiences. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing memory....
encoding for the event has been created, and for this reason the event no longer generates a response when repeated. Each time a novel event is experienced, it is compared to the previously created neural representation, and, if it is sufficiently deviant, then the process begins again. If this event is not sufficiently deviant (i.e., it is the same) then habituation
Habituation
Habituation can be defined as a process or as a procedure. As a process it is defined as a decrease in an elicited behavior resulting from the repeated presentation of an eliciting stimulus...
occurs. The P3a's rapid amplitude reduction with exposure to repeated trials of novel stimuli supports the idea that the P3a is the electrophysiological representation of the orienting response
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...
(which also habituates in behavior) . For example, Grillon and colleagues used a 3 stimulus odd-ball paradigm wherein they presented subjects with a condition in which the deviant stimuli were constant and a condition in which the deviant stimuli were always novel. Their results showed the largest P3a amplitude in response to deviant stimuli that were novel.
Another factor that affects P3a amplitude is target discrimination. It is interesting that although the P3a is elicited by non-target deviant stimuli, the nature of the target stimuli affect the P3a response. It seems that the amplitude of the P3a may be affected by an individual’s ability to distinguish target stimuli from standard stimuli. When this discrimination is easy, non-target deviant stimuli produce a P300 that is smaller than the target P3b and is largest over parietal sites. However, if target discrimination is difficult, the P3a to non-target stimuli is larger and more frontally-skewed with a shorter latency—in other words, the more "canonical" P3a response
Although the P3a has been dissociated from the P3b, its amplitude and latency may be affected by factors that also modulate the P3b. Some of these factors include stimulus probability, stimulus evaluation difficulty, natural state variables (such as circadian and menstrual cycles), and environmentally induced state variables (such as drugs and exercise). John Polich and Albert Kok have written up an extensive review that covers many of these variables.
Theory
The P3a has been linked with novelty or orienting and involuntary shifts to changes in the environment. Some have suggested that the P3a and P3b are variants of the same ERP response that varies in scalp topography as a function of attention and task demands.. In other cases, however, the two can be dissociated: for example, patients with temporal-parietal lesions and an absent visual P3a response have partial preservation of their visual target P3b. These results indicate that at least partially non-overlapping neural circuits may be engaged during P3a and P3b generation.Neural sources of the P3a have been hypothesized to arise from frontal lobe
Frontal lobe
The frontal lobe is an area in the brain of humans and other mammals, located at the front of each cerebral hemisphere and positioned anterior to the parietal lobe and superior and anterior to the temporal lobes...
functioning and to involve frontal lobe attention mechanisms. Magnetic resonance imaging
Magnetic resonance imaging
Magnetic resonance imaging , nuclear magnetic resonance imaging , or magnetic resonance tomography is a medical imaging technique used in radiology to visualize detailed internal structures...
(MRI) studies looking at gray matter volume and P3a amplitude show stronger correlations when non-target, startling stimuli are viewed.. Lesion studies indicate that prefrontal and temporal-parietal cortex contribute to auditory P3a generation. The P3a is suspected to also reflect interactions between the frontal lobe
Frontal lobe
The frontal lobe is an area in the brain of humans and other mammals, located at the front of each cerebral hemisphere and positioned anterior to the parietal lobe and superior and anterior to the temporal lobes...
and the hippocampus
Hippocampus
The hippocampus is a major component of the brains of humans and other vertebrates. It belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation. Humans and other mammals have two hippocampi, one in...
, as patients with focal hippocampal lesions have reduced P3a amplitude from novel distracters.
See also
- 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
- P300 (neuroscience)P300 (neuroscience)The P300 wave is an event related potential 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...
- 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...