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.