N2pc
Encyclopedia
N2pc 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 vice-versa. This characteristic makes it a useful tool for directly measuring the general direction of a person's attention (either left or right) with fine-grained temporal resolution
.
correlates of focused attention using ERPs. Subjects viewed pairs of letters on either side of a computer screen. They were instructed to pay attention to either the left or right side of the screen and report (using a button-press) whether or not the two letters on that side were the same. Compared to the waveform over cortex ipsilateral to the direction of subjects' attention, experimenters observed a consistently greater negative deflection of the ERP waveform at approximately 200 ms after the stimulus at posterior sites (i.e., over visual cortex) contralateral to the side of the screen subjects attended.
The N2pc first received its name from Luck and Hillyard (1994), who named the component after its characteristic features. The “N” denotes a negative polarity; “2” describes its latency in the waveform (i.e., the second negative deflection, typically around 200 ms); and “pc” stands for “posterior-contralateral,” as the component appears over posterior electrode sites contralateral to the direction of attention. The experimenters explored what factors would modulate the N2pc using a visual search
paradigm in which subjects had to report the presence of a target object in a display (e.g., a green box or a horizontal bar). They confirmed that the N2pc appeared contralateral to attended stimuli, and furthermore found that it did not appear when subjects saw only one object at a time or had to spread their attention over all the items in the display. These data led the experimenters to believe the N2pc corresponds to a filtering process that occurs whenever people focus attention on one object while ignoring others.
ERP components, a negative deflection in the ERP waveform at a latency of approximately 200-300 ms following a stimulus. The “pc” stands for “posterior-contralateral”, describing the topographic distribution of the component. It appears as a greater negativity at posterior electrode sites contralateral to the attended side of the visual field relative to ipsilateral electrode sites. For example, when a person pays attention to something in the left side of the visual field, an N2pc appears as a greater negativity over the right posterior areas of the brain than the left. MEG has been used to localize the N2pc primarily to lateral extrastriate cortex and inferotemporal visual areas, such as V4.
paradigms to study the deployment of attention over time and test hypotheses of parallel and serial models of visual search.
The first experiments to investigate specifically the N2pc used a visual search paradigm in which subjects reported the presence or absence of a pre-defined target (e.g., a green rectangle) in a display containing one “oddball” stimulus that differed on a single feature from a uniform background of items (e.g., a green square among blue squares). The oddball stimuli would “pop out” and attract attention, but were not necessarily targets. As a result, experimenters knew where subjects directed attention, but could simultaneously manipulate factors orthogonal to the location of attention, such as low-level features or probability of the target appearing. The pop-out oddball would generate an N2pc, as it received focused attention, while stimulus characteristics modulated the amplitude and latency of the component.
Subsequent investigation into the N2pc manipulated the number of items in the array and found that a display with as few as two objects elicits the component. Because an object cannot “pop out” and attract attention in a two-item display, experimenters concluded that the N2pc must reflect top-down, controlled processes of directing attention. The same study also demonstrated that the N2pc does not only occur when attending to visual features, but semantic features as well. In one experiment, subjects had to respond to the words “left” and “right” while ignoring the color words “white” and “brown.” Even in this case of semantically-defined targets, subjects demonstrated an N2pc contralateral to the target word. Together, these results have provided strong evidence that the N2pc reflects the location of covert, consciously directed attention.
The prototypical visual search paradigm for eliciting an N2pc component has subjects attend and respond to a target stimulus to the left or right of fixation. Unlike regular visual search experiments, however, two major criteria most hold when attempting to measure N2pc response. First, the stimuli should be identical in all conditions, and the experimenter should only manipulate instructions for directing attention across conditions; this precludes the possibility that stimulus features drive ERP effects rather than focused attention. Second, the target should be easy to find, usually via “pop-out.” The goal is to minimize the variability in search times and N2pc latency, resulting in a much clearer signal when the waveforms are averaged together over multiple trials.
An example experiment for eliciting the N2pc that follows the critical principles above might proceed as follows: Subjects see an array of upright and inverted T's. One T is red, and one T is green, but the rest are black (thus fulfilling the first criterion of easy-to-find targets). Subjects are told to attend to either the red T's or the green T's at the beginning of the experiment and report whether that letter is upright or inverted (thus fulfilling the second criterion that attention should not be confounded with stimulus characteristics). We should expect to see an N2pc contralateral to the side of the screen the attended letter appeared.
Early investigations of the N2pc critically found that the component was sensitive to the presence of distractors, appearing only when distractors accompanied a target stimulus. Furthermore, N2pc amplitude increases with the number of distractors in the display.
The N2pc also disappears when targets in the visual search task are defined as “any oddball object” rather than by one or more specific features. Luck and Hillyard (1994) have argued that in this case, determining whether a given object is a target requires distributing attention over multiple objects in the array (and determining the common features) rather than filtering them. Consequently, the spatial filtering process is discouraged, and the N2pc therefore does not appear.
Other work has explored further cognitive processes that could be linked to the N2pc. For instance, the classic visual search paradigm that elicits the N2pc could be broken down further into processes of shifting attention, and spatially-based processing of non-target locations. When combining the visual search task with visual cues that drew attention to spatial locations in the display, experimenters found that while the N2pc may not reflect shifts of attention, it may still reflect processing of a location in space that may or may not contain a target.
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....
. The N2pc appears over visual cortex
Visual cortex
The visual cortex of the brain is the part of the cerebral cortex responsible for processing visual information. It is located in the occipital lobe, in the back of the brain....
contralateral to the location in space to which subjects are attending; if subjects pay attention to the left side of the visual field
Visual field
The term visual field is sometimes used as a synonym to field of view, though they do not designate the same thing. The visual field is the "spatial array of visual sensations available to observation in introspectionist psychological experiments", while 'field of view' "refers to the physical...
, the N2pc appears in the right hemisphere of the 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,...
, and vice-versa. This characteristic makes it a useful tool for directly measuring the general direction of a person's attention (either left or right) with fine-grained temporal resolution
Temporal resolution
Temporal resolution refers to the precision of a measurement with respect to time. Often there is a tradeoff between temporal resolution of a measurement and its spatial resolution. This trade-off can be attributed to the finite speed of light and the fact that it takes a certain period of time...
.
History
Researchers first observed the N2pc while seeking to document electrophysiologicalElectrophysiology
Electrophysiology is the study of the electrical properties of biological cells and tissues. It involves measurements of voltage change or electric current on a wide variety of scales from single ion channel proteins to whole organs like the heart...
correlates of focused attention using ERPs. Subjects viewed pairs of letters on either side of a computer screen. They were instructed to pay attention to either the left or right side of the screen and report (using a button-press) whether or not the two letters on that side were the same. Compared to the waveform over cortex ipsilateral to the direction of subjects' attention, experimenters observed a consistently greater negative deflection of the ERP waveform at approximately 200 ms after the stimulus at posterior sites (i.e., over visual cortex) contralateral to the side of the screen subjects attended.
The N2pc first received its name from Luck and Hillyard (1994), who named the component after its characteristic features. The “N” denotes a negative polarity; “2” describes its latency in the waveform (i.e., the second negative deflection, typically around 200 ms); and “pc” stands for “posterior-contralateral,” as the component appears over posterior electrode sites contralateral to the direction of attention. The experimenters explored what factors would modulate the N2pc using a visual search
Visual search
Visual search is a type of perceptual task requiring attention that typically involves an active scan of the visual environment for a particular object or feature among other objects or features . Visual search can take place either with or without eye movements...
paradigm in which subjects had to report the presence of a target object in a display (e.g., a green box or a horizontal bar). They confirmed that the N2pc appeared contralateral to attended stimuli, and furthermore found that it did not appear when subjects saw only one object at a time or had to spread their attention over all the items in the display. These data led the experimenters to believe the N2pc corresponds to a filtering process that occurs whenever people focus attention on one object while ignoring others.
Component Characteristics
The component's name, N2pc, abbreviates its characteristics. The component belongs to the family of N2N200 (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...
ERP components, a negative deflection in the ERP waveform at a latency of approximately 200-300 ms following a stimulus. The “pc” stands for “posterior-contralateral”, describing the topographic distribution of the component. It appears as a greater negativity at posterior electrode sites contralateral to the attended side of the visual field relative to ipsilateral electrode sites. For example, when a person pays attention to something in the left side of the visual field, an N2pc appears as a greater negativity over the right posterior areas of the brain than the left. MEG has been used to localize the N2pc primarily to lateral extrastriate cortex and inferotemporal visual areas, such as V4.
Classic Paradigms and Findings
The N2pc can be used flexibly in nearly any task in which one would like to study the direction and time course of selective attention. However, researchers have primarily used the N2pc in visual searchVisual search
Visual search is a type of perceptual task requiring attention that typically involves an active scan of the visual environment for a particular object or feature among other objects or features . Visual search can take place either with or without eye movements...
paradigms to study the deployment of attention over time and test hypotheses of parallel and serial models of visual search.
The first experiments to investigate specifically the N2pc used a visual search paradigm in which subjects reported the presence or absence of a pre-defined target (e.g., a green rectangle) in a display containing one “oddball” stimulus that differed on a single feature from a uniform background of items (e.g., a green square among blue squares). The oddball stimuli would “pop out” and attract attention, but were not necessarily targets. As a result, experimenters knew where subjects directed attention, but could simultaneously manipulate factors orthogonal to the location of attention, such as low-level features or probability of the target appearing. The pop-out oddball would generate an N2pc, as it received focused attention, while stimulus characteristics modulated the amplitude and latency of the component.
Subsequent investigation into the N2pc manipulated the number of items in the array and found that a display with as few as two objects elicits the component. Because an object cannot “pop out” and attract attention in a two-item display, experimenters concluded that the N2pc must reflect top-down, controlled processes of directing attention. The same study also demonstrated that the N2pc does not only occur when attending to visual features, but semantic features as well. In one experiment, subjects had to respond to the words “left” and “right” while ignoring the color words “white” and “brown.” Even in this case of semantically-defined targets, subjects demonstrated an N2pc contralateral to the target word. Together, these results have provided strong evidence that the N2pc reflects the location of covert, consciously directed attention.
The prototypical visual search paradigm for eliciting an N2pc component has subjects attend and respond to a target stimulus to the left or right of fixation. Unlike regular visual search experiments, however, two major criteria most hold when attempting to measure N2pc response. First, the stimuli should be identical in all conditions, and the experimenter should only manipulate instructions for directing attention across conditions; this precludes the possibility that stimulus features drive ERP effects rather than focused attention. Second, the target should be easy to find, usually via “pop-out.” The goal is to minimize the variability in search times and N2pc latency, resulting in a much clearer signal when the waveforms are averaged together over multiple trials.
An example experiment for eliciting the N2pc that follows the critical principles above might proceed as follows: Subjects see an array of upright and inverted T's. One T is red, and one T is green, but the rest are black (thus fulfilling the first criterion of easy-to-find targets). Subjects are told to attend to either the red T's or the green T's at the beginning of the experiment and report whether that letter is upright or inverted (thus fulfilling the second criterion that attention should not be confounded with stimulus characteristics). We should expect to see an N2pc contralateral to the side of the screen the attended letter appeared.
Amplitude
The N2pc is primarily sensitive to the directional focus of attention over time. However, research has found a variety of factors that modulate N2pc response. N2pc amplitude is sensitive to factors related to increasing demands on focused attention.- When non-target stimuli closely resemble the target (e.g., when targets are defined by size, but the size difference between targets and distractors is very small), they elicit an N2pc of lower amplitude than a target stimulus
- When targets are defined by a conjunction of features (e.g., blue, horizontal bar) rather than a single feature (e.g., blue bar), they elicit a larger N2pc, which may reflect a greater demand on attention to identify the target.
- In a dual-task situation where subjects focus on a demanding primary task while performing target detection as a secondary task, the N2pc only appears in response to detecting targets defined by conjunctions of features. Again, the greater attentional demands of conjunction-based targets relative to feature-based targets may be responsible.
- N2pc amplitude increases as distractors appear closer to the target, which increases the need to focus on the target while filtering the distractors (but also see Mazza et al., 2009, who found conflicting results).
- When subjects have to indicate where a target is located, they exhibit a larger N2pc than when they simply have to report whether or not a target is present
Eliminating the N2pc
Certain experimental conditions can eliminate the N2pc entirely. These results have been used to argue for a spatial filtering hypothesis, which proposes that the N2pc reflects the process of ignoring task-irrelevant (i.e., non-target) stimuli.Early investigations of the N2pc critically found that the component was sensitive to the presence of distractors, appearing only when distractors accompanied a target stimulus. Furthermore, N2pc amplitude increases with the number of distractors in the display.
The N2pc also disappears when targets in the visual search task are defined as “any oddball object” rather than by one or more specific features. Luck and Hillyard (1994) have argued that in this case, determining whether a given object is a target requires distributing attention over multiple objects in the array (and determining the common features) rather than filtering them. Consequently, the spatial filtering process is discouraged, and the N2pc therefore does not appear.
Functional Significance
The N2pc literature agrees on a few functional characteristics of the N2pc. First, the N2pc appears whenever a person focuses attention on an object. Second, it serves as a direct measure for the direction of focused attention, either to the left or right. Finally, the N2pc is generally believed to be tied to a spatial filtering hypothesis (see above: “Eliminating the N2pc”). The last point regarding the functional significance of the N2pc, however, has been challenged. Some have contested the spatial filtering hypothesis, arguing that the N2pc reflects an enhancement of task-relevant stimulus processing rather than a suppression of irrelevant stimuli.Other work has explored further cognitive processes that could be linked to the N2pc. For instance, the classic visual search paradigm that elicits the N2pc could be broken down further into processes of shifting attention, and spatially-based processing of non-target locations. When combining the visual search task with visual cues that drew attention to spatial locations in the display, experimenters found that while the N2pc may not reflect shifts of attention, it may still reflect processing of a location in space that may or may not contain a target.
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
- N2pc
- 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...
- 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...