2.2.2.1 Features of the N2pc Component
The N2pc component has been extensively studied in literature related to theories of selective attention. It is a negative component (hence the “N” prefix) that generally appears within 170–300 ms of stimulus onset, in the time range of the N2 component. The “pc” suffix denotes its location: posterior contralateral electrode sites (e.g., P7/8 and PO7/8 electrode sites from the 10-20 international system), meaning that it is elicited in electrode sites on the opposite side to the visual field where the target is found. The maximum amplitudes of the N2pc oscillate around 2–3 µV .
The N2pc is an asymmetric component which is best observed by computing the difference waves between the two brain hemispheres, so it is typically described in terms of the difference between the contralateral and the ipsilateral waves. For this, trial averages are performed separately for trials on which the target is ipsilateral or contralateral to a given electrode site [Hopfinger et al., 2004]:
• The ipsilateral waveform includes the average of the left visual field (LVF) targets for the left hemisphere electrodes and the right visual field (RVF) targets for the right hemisphere electrodes.
• The contralateral waveform is calculated as the average of the RVF tar- gets for the left hemisphere electrodes and the LVF targets for the right hemisphere electrodes.
2.2.2.2 Neurophysiology of the N2pc Component
Visual information about the environment enters the brain through the eyes and, more specifically, the fovea, which is the central area of the retina and provides about 2◦of visual angle [Luck,2012]. Before “foveating” to an object (i.e., moving
the gaze so that the object falls in the middle of the fovea; overt attention), it is often necessary to perform a selective processing of the relevant features. This is usually done by means of covert attention (i.e., attending to a non-foveated object) [Luck, 2012].
The most extended theory of selective attention, called the feature-integration theory, was first proposed by Treisman[1969], who hypothesises the existence of specialized modules that automatically detect and code different sensory features (e.g., colours or shapes) in different feature maps. This is one of the first steps in covert attention. During visual search, the sensitivity will be increased for factors specified in the search template and the objects that comply with the pattern will have priority for further processing, producing a shift of covert spatial attention towards their location. Then, if the object still seems to match the search pattern, it may result in the item being “foveated” [Luck, 2012], which would be the first of several steps in overt attention.
The stages described above have been associated to different ERP components of visual perception. In particular, the N2pc component is related to the shift of covert spatial attention and reflects the focusing of attention on a potential target in visual search arrays, before foveating to it (for a review, see [Luck,2012]).
The N2pc is often followed by the so-called Sustained Posterior Contralateral Negativity (SPCN), from which it is usually separated by a positive-going de-
flection (the origin and cause for which is not known). This component starts around 300–400 ms after stimulus onset and is found in visual search experi- ments in which the participants are asked to remember a given aspect of the target [Dell’Acqua et al.,2006; Jolicœur et al.,2006a, 2008; Thiery et al., 2016], but also in tasks that are not defined as memory tasks, but which are supposed to engage visual short term memory as a processing buffer (e.g., if participants are asked to make a choice afterwards) [Jolicœur et al.,2008].
In contrast to the N2pc, the amplitude of the SPCN is sensitive to the memory load, with bigger amplitudes associated with higher loads. Moreover, the SPCN persists for the duration of the retention interval [Jolicœur et al., 2008]. Hence, whereas the N2pc seems to indicate the orienting of attention towards a given hemifield (left or right), the SPCN is believed to indicate storage of the target in the visual short term memory [Dell’Acqua et al., 2006; Jolicœur et al., 2006a,
2008].
Both the N2pc and the SPCN have been reported to be affected (by means of a decrease in amplitude and an increased latency) by the attentional blink [Dell’Acqua et al.,2006;Jolicœur et al.,2006b;Zhang et al.,2009] (see Section2.4.3.1). How- ever, even when the final responses given by participants are wrong, due to this effect, the N2pc has been observed in the EEG epochs [Dell’Acqua et al., 2006;
Woodman & Luck, 2003].
The SPCN is sometimes referred to as a Contralateral Delay Activity (CDA) [Mc- Collough et al., 2007; T¨ollner et al., 2013] or Contralateral Negative Slow Wave (CNSW) [Klaver et al., 1999]. However, it is still unclear whether these three components are actually the same or not [Jolicœur et al., 2008].
2.2.2.3 Variations of the N2pc
The list below summarises the main findings related to the N2pc and the factors that affect its amplitude:
• It is found when the search array contains at least one distracting item (any non-target stimulus) apart from the target. The closer the distractor, the bigger the N2pc amplitude will be [Luck & Hillyard, 1994b].
• It does not appear if the items surrounding the target provide information about it (e.g., when the experimental design requires two or more items to be compared) [Luck & Hillyard, 1994b].
• It is not elicited if there is only one item (regardless of whether it is a target or a distractor) in the search array [Luck & Hillyard,1994b].
• It is elicited both in the presence of “pop out” targets and when dis- crimination is based on several conjuctive features, requiring sequential search [Eimer, 1996].
• It can be elicited not only by physical properties of stimuli, but also by semantic properties [Eimer, 1996].
• In upper vs lower visual field discrimination, the N2pc component is larger in the latter [Luck et al., 1997].
• Distractors that closely resemble the target have been shown to elicit a smaller N2pc (which is not followed by a P300).
• The amplitude of the N2pc is relatively unaffected by target probabil- ity [Luck & Hillyard, 1994a].
• The amplitude of the N2pc is larger and it has a shorter latency when participants are given a target template rather than a target category [Nako et al., 2014, 2015].
• N2pc amplitude is smaller and has a longer latency when multiple targets are found in the search array [Nako et al., 2014].
• Searching repeatedly for a target in different search arrays enhances N2pc amplitude in the later trials [Nako et al., 2015].
Even though the N2pc was first thought of as just a reflection of the attentional selection of target stimuli, the first three bullet points in the list above reflect its filtering function: when several items near the target compete for attention, a bigger effort to suppress the information from the distractors is required, so the N2pc has a larger amplitude (first bullet point). Moreover, this filtering is not necessary when the surrounding items give information that helps in the discrimination of the target (second bullet point) or when there are no competing items (third bullet point). Hence, the amplitude of the N2pc component would reflect the level of suppression of task-irrelevant or conflicting information.
If the feature that distinguishes the target from the distractors on the search array makes it “pop out” (e.g., if the distinctive feature is colour), it will be found faster (as evidenced by reaction times) than when finding the target requires sequential search. However, no statistical differences have been reported in the latency of the N2pc components elicited in these cases [Eimer & Kiss,2007,2008;