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.
History Luck and Hillyard (1990) first observed the N2pc while seeking to document electrophysiological correlates of focused attention during visual search using ERPs. Subjects viewed arrays containing 4-12 items, one of which was a target on 50% of trials. Compared to the waveform over cortex ipsilateral to the target, 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.
Component characteristics The component's name, N2pc, abbreviates its characteristics. The component belongs to the family of N2 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 search 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.
Functional sensitivity
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