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LPC (Late Positive Component) is

The Late Positive Component (LPC) is a positive-going event-related brain potential (ERP) component over parietal scalp around 400 – 500 ms post-stimulus onset, particularly reflecting the neural basis of explicit recognition and recall (Friedman and Johnson, 2000). Based on early studies focusing on the ERP responses of recognition (episodic memory), Finnigan and colleagues (2002) indicated the old/new paradigm have been applied to elicit dissociable early and late ERP components which reflect the modulation of the N400 and a subsequent late positive component. They further proposed an issue that the majority of existing studies have given to this late positive component of the ERP old/new effect incongruent names with P3, P300, P600 and late positive complex. Here, we use the term “LPC” in reference to this subsequent late positive component. History Back to the 90s, the LPC was initially, roughly found across studies examining either repetition or recognition effects. For example, regardless of the study paradigm (repetition or recognition), a few early studies found the ERPs to repeated/recognized items differ from those to newly presented ones in two and sometime three features. Regarding the observation of LPC, these studies found an increase in the amplitude of a late positivity between 500 and 800 ms post-stimulus onset, referred to LPC (Paller, Kutas and McIsaac, 1995; Sanquist et al., 1980) but also known as P300 (Donchin and Fabiani, 1991), late positivity (Donaldson and Rugg, 1999) or “parietal old/new effect” (Rugg, et al., 1996). Further, a study from Friedman (1990) examining the ERP differences between correctly detected “old” and “new” words in recognition memory test could be a decent classic research. During 90s, unlike the majority of other ERP studies of memory in which items are first separately presented for study and are subsequently followed by a test series, Friedman (1990) applied novel methods presenting test items in a continuous recognition paradigm study within the same block of trials. In his study, 10 young adult participants were assessed their ERP data in a continuous recognition memory task for words. Results showed that ERPs to old items were characterized by smaller N300 (now called N400) and larger P300 (now called LPC) amplitudes (from about 250 to 700 ms) than those to new items. This is the first study that clearly addressed the ERP old/new effects. What this paper added is that theses ERP differences could be interpreted as primarily reflecting repetition as opposed to semantic priming effects. Accordingly, N400 and LPC became dissociable. In such recognition memory paradigm, processing new words assesses semantic memory whereas processing old words, having been experienced previously, accesses episodic memory (Friedman and Johnson, 2000). Based on the recognition memory paradigm, following studies found that old words elicits a larger LPC over parietal scalp in the interval between 400 – 800 ms than do new words (for reviews see Johnson, 1995a; Rugg, 1995). Main Paradigm Regardless of recent main paradigm for LPC, ERPs acquired during recognition memory tasks differentiate correctly identified old and new items (see Johnson, 1995a; Rugg, 1995 for review). However, in the original publication in which it was both recognition and repetition that revealed both a LPC and N400 response led to a question that whether the LPC might be the neural reflection of repetition or recognition. This question has been simply answered by a well-designed study (Rugg, Mark, Walla, Schloerscheitd, Birch, & Allan, 1998a). Rugg and colleagues (1998a) conducted a direct comparison of implicit and explicit retrieval ERPs. Specifically, participants performed an old/new recognition judgment on a list of words half new and half repeated during the explicit condition. In the implicit condition participants made living/nonliving judgment on the same material, except now repetition was task-irrelevant. Results revealed that repetition modulated LPC in the explicit task but not implicit task. Thus, it is believed that the ERPs respond neural correlates of explicit and implicit memory differ qualitatively in which the old/new effect could be a better paradigm to elicit LPC responses representing explicit memory processes. In fact, in a quality review study (Friedman and John, 2000), they proposed that the current consensus view is that the greater amount of LPC activity elicited by old words, which has been referred to as the parietal old/new words, has a scalp distribution that is asymmetrical for verbal stimuli, being larger over left parietal electrode sits. In addition, they believed that the differential ERP activity elicited by old and new words in episodic memory paradigm is better labeled the “episodic memory effect”. In sum, it has consistently been found that the response to correctly classified old items is more positive than the response to correctly classified new items between 400-800 ms post stimulus. This is called left parietal old/new ERP effect presumed to index recollective processes. The continuous recognition memory task is typically the main paradigm for this parietal old/new ERP effect. Here we address a typical recognition memory paradigm applied from a study (Rugg, Roberts, Potter, Pickles & Nagy, 1991). This paradigm consisted of the presentation of a list of common English words (200 – 300 words). A small number of these (say 20 words) were filler items, which appeared only once and served to 'pad' the end of the lists, while the remainder were each presented twice. The repeated word was separated from its first occurrence by certain intervening items (probably 5-10 words). In this task, participants have to discriminate between words being shown for the first (new items) and second (old items) time by pressing a button with the right index finger for 'old' responses, and with the left index finger for 'new' responses. Regarding the parameter of this paradigm, the inter-stimulus interval (ISI) varied with reaction time (RT) ranged from 3- 8 seconds, particularly with response latencies of less than 250 ms or greater than 5 seconds scored as errors. In this case, results showed that ERPs to detected 'old' words were reliably more positive-going in the interval 300 to 600 ms post-stimulus than were ERPs to 'new' items with a parietal scalp distribution. Component Characteristics As mentioned above, the episodic memory effect (EM) is better labeled for episodic (recognition) memory paradigms. However, as the advent of ERP mapping techniques, researchers began to dissociate the EM effect into its spatio-temporal subcomponents and determine the cognitive processes associated with each. These subcomponents have been named as “Left Medial prefrontal Subcomponent”, “Parietal Subcomponent” (referred to LPC), and “Right Prefrontal Subcomponent”. As for parietal subcomponent, it temporally overlaps the left prefrontal positivity and has been considered an additional subcomponent of the EM effect with its maximum over left parietal-occipital scalp. In Current Source density (CSD) maps, it appears as a widespread positive current density that begins in the 420 -490 ms epoch, and extends for several hundred milliseconds (Friedman and Johnson, 2000). The most significant characteristics are related to LPC’s amplitude, especially much known about its relation to episodic retrieval. Johnson and colleagues (1998a) has indicated that LPC amplitude increases with study-test repetitions. Further, LPC amplitude is larger in association with those items rated as being consciously remembered (Smith, 1993; Smith and Guster, 1993). Rugg and colleagues (1999) also found that LPC amplitude is larger for correctly recognized items that are subsequently recalled. Most consistently, LPC amplitude had been studied and found that its amplitude is larger for words whose study context is correctly retrieved (Trott et al., 1999; Wilding et al., 1995; Wilding and Rugg, 1996). Thus, these data suggested that the LPC amplitude is closely allied with recollection and reflects successful retrieval. Interestingly, the LPC effect is not seen in patients with MTL damage or bilateral hippocampal damage. This leads researchers to believe that the medial temporal lobe plays an important role in the recollection of memories. In an article by Olichney (2000) they study patients with amnesia. In amnesic subjects, an effect of repetition is sometimes seen, but it is always tremendously smaller than that of a normal patient. Interestingly though, the N400 aspect of the memory component is intact. According to Olichney and his colleagues, this indicates that more conscious processes like recollection (LPC) are effected by amnesia while less conscious ones like the N400 (familiarity) may not be. Functional Sensitivity As mentioned above, LPC mainly reflects recognition memory retrieval. However, the recognition could be subdivided different subcomponent, presumably with different neural involvement. The dual processing theory could illustrate those differences. According to this theory, recognition can be due either to familiarity (derived for example from perceptual fluency) or to recollection (Munte, Urbach, Duzel, & Kutas, 2000). Specifically, Duzel et al. (1997) coupled a Remember and Know (R/K) procedure with a false memory paradigm (Roediger and McDermott, 1985). Duzel and colleagues (1997) found that LPC were distinguishable from those correct rejections of new words in which “remember” but not “know” responses showed LPC response. In other words, N400 represented “know” responses. That is, remembering is more clearly reflected in the LPC; Furthermore, LPC amplitude is also sensitive to levels of processing manipulation, being larger for more deeply encoded items (Paller and Kutas, 1992; Paller et al., 1995; Rugg et al., 1998). However, LPC is more known about its retrieval function rather than encoding processing. LPC has been also widely used in patient studies. As mentioned in “Main Paradigm” section, larger LPC amplitude reflected successful retrieval. Using the same paradigm (EM effect), patient studies have supported these differences of magnitude in LPC amplitude because the LPC could not reflect EM effect with patients who impaired left MTL lesions. More recently, Finnigan, Humphreys, Dennis and Geffen (2002) examining the EM effect provided alternative interpretations of LPC amplitude. Finningan and colleagues (2002) extended tradition old/new effect paradigm by presenting new unstudied words and old words which had been presented at study either once (“weak”) or three times (“strong”). The probability of an “old” response was significantly higher for strong than weak words and significantly higher for weak than new words. Comparisons were made initially between ERPs to new, weak and strong words, and subsequently between ERPs associated with six strength-by-response conditions. Results showed the amplitude of LPC effect was sensitive to decision accuracy (and perhaps confidence). Its amplitude was larger in ERPs evoked by words attracting correct versus incorrect recognition decisions. The LPC effect had a left > right, centro-parietal scalp distribution (in ear-referenced ERPs). Therefore, in addition to the majority of studies in which interpreted LPC from the perspective of dual-process models, Finningan et al. (2002) provided alternative interpretations of LPC in terms of memory strength and decisional factors. In a structured review from Rugg and Curran (2007), they concluded that the functional significance of LPC is obscure. As they mentioned, one early suggestion was that the effect reflects processes that contribute to the representation of recollected information (Wilding and Rugg, 1996).Alternatively, the effect might index attentional orienting to recollected information (Rugg and Henson, 2002; Wagner et al., 2005), rather than processes supporting its representation or maintenance. It has recently been argued (Vilberg et al., 2006) that findings indicating that the effect varies according to the amount of information recollected (Vilberg et al., 2006; Wilding, 2000) are more consistent with the first of these two proposals (Rugg and Curran, 2007). Theory/Sources As ERP studies mentioned, the characteristic scalp distribution of the parietal old/new effect suggests that it might reflect neural activity generated in the lateral parietal cortex (see Rugg and Curran, 2007 for review). Consistent with ERP studies, fMRI findings of recollection-sensitive activity in this region (see Ecker et al., 2007; Wagner et al., 2005 for review) supported this suggestion. Furthermore, findings of direct functional parallels between these fMRI and ERP old/new effects (Yonelinas et al., 2005; Woodruff et al., 2006; Herron et al., 2004; Vilberg et al., 2006; Harron, et al., 2003; Vilberg et al., 2007) give additional credence to the hypothesis that the ERP effect reflects activity in a recollection-sensitive region of the lateral parietal cortex. Researchers are currently trying to use the LPC (in combination with other techniques) to prove or disprove the dual process model of memory (recollection and familiarity as distinct processes). There are many different theories with varying evidence however a few main ones are detailed below. One theory put forth by Friedman and Johnson in their review paper is that the LPC has to do with recollection of an episodic memory. This same theory posits that the amplitude of the LPC is related to the success which the brain has retrieving the details of the “episode.” The more successful the brain is (the more information about the event that is recalled) the larger the LPC in response. The same researchers believe that recollection and familiarity exist on a graded continuum but are controlled by the same areas of the brain. They base this assumption upon a study done by Trott and colleagues (1999), where there was no difference in scalp distribution between the remember vs. know categories. Since the scalp distributions were the same, they assumed that the same brain region was controlling both.