Semantic elaboration: ERPs reveal rapid transition from novel to known.

Semantic memory is an open system to which new information is added regularly. Some new entries come about through direct observation or through explicit learning or instruction. Others are self-derived, through logical processes such as analogy (e.g., Gentner, 1983), deduction (e.g., Johnson-Laird & Byrne, 1991), and induction (e.g., Holland, Holyoak, Nisbett, & Thagard, 1989). Semantic memory also can be elaborated by self-derivation of new knowledge through integration of information acquired in separate yet related learning episodes. For example, children presented with separate passages of text in which they learn that “dolphins live in groups called pods,” and “dolphins talk by clicking and squeaking” subsequently generate the novel understanding that “pods talk by clicking and squeaking” (Bauer, King, Larkina, Varga, & White, 2012; Bauer & San Souci, 2010; Bauer, Varga, King, Nolen, & White, in press; Varga & Bauer, 2013). As a result of the productive nature of semantic memory, the boundaries of the conceptual knowledge network extend far beyond what has been directly learned. Indeed, this feature of semantic memory may help explain the absence of episodic information specifying when and where its contents were acquired—some entries in the semantic knowledge network were never directly experienced and thus there is no event to locate in time and place. Though we know these elaborative processes occur, we know little about the time course over which newly self-derived information becomes incorporated into the semantic knowledge base. In the present research, we used event-related potentials (ERPs) to examine the dynamic process of transition of information from newly self-derived to well-known. Productive self-derivation of new knowledge is pervasive in adults (e.g., Johnson-Laird, 2006) and children (e.g., Moshman, 2004) alike. In some cases, the newly-derived information may be fleeting, such as when separate premises are combined in the service of comprehending a text (e.g., Yule, 2010). For example, the premises “Mary entered the room” and “She turned,” can be integrated to derive the information that it was Mary who turned. This type of self-derivation is fast-acting—happening on the order of milliseconds—and the resulting inference likely survives in working memory only long enough to ensure adequate comprehension (McKoon & Ratcliff, 1992). In other cases, the products of self-derivation may become integrated into the knowledge base and endure over time. In scientific reasoning, for example, separate empirically derived observations may be combined to reach logical conclusions. The observations that “the movement of tectonic plates produces jagged mountains,” and “the Appalachians are jagged mountains,” may lead to the logical conclusion that “the Appalachians were produced by the movement of tectonic plates.” In such cases, information that was neither directly observed nor explicitly learned may become part of the more-or-less permanent store of semantic or conceptual knowledge. To date, the transition of information from newly self-derived to known has been assumed but not observed. Because they are measures of information processing in real time, ERPs are ideally suited to examination of this transition. ERPs are scalp-recorded electrical activity that reflect the summation of the postsynaptic potentials of large populations of neurons activated in response to specific stimuli. Several ERP components are sensitive to the status of semantic knowledge and thus may illuminate the process of transition from newly self-derived to known. One especially robust effect is the N400 (Kutas & Hillyard, 1980): a pronounced change in neural activity 300–500 ms after stimulus onset, that is maximal at 400 ms (for reviews, see for e.g., Federmeier & Laszlo, 2009; Kutas & Federmeier, 2011). When the stimulus is novel, the response is a marked negative deflection, relative to the baseline (i.e., pre-stimulus) level of neural activity. When the stimulus is well-known, the neural response is a substantially smaller change. Across stimulus classes, when stimuli are repeated, the amplitude of the N400 decreases (e.g., Besson & Kutas, 1993), presumably reflecting facilitated semantic processing resulting from the previous presentation of the item (e.g., Olichney, Van Petten, Paller, Salmon, Iragui, & Kutas, 2000). Neural activity both before and after the N400 also can be revealing of the knowledge status of stimuli. The P200—a positive deflection 150–300 ms after stimulus onset—is sensitive to comparisons between sensory inputs and stored memory representations and generally is larger when they match (e.g., Evans & Federmeier, 2007; Federmeier, Mai, & Kutas, 2005; Wlotko & Federmeier, 2007). The P200 thus can be expected to differentiate novel from known information. A third component, termed the P600 (e.g., Rugg, Roberts, Potter, Pickles, & Nagy, 1991) or Late Positive Component (e.g., Olichney, Morris, Ochoa, Salmon, Thal, Kutas, et al., 2002), reflects ongoing processing of a stimulus. It is marked by positive activity beginning around 500 ms and extending for several hundred ms, peaking around 600 ms (e.g., Coulson, King, & Kutas, 1998; Hagoort, Brown, & Osterhout, 1999). The P600 is associated with a variety of processes, including language comprehension and production (Hagoort et al., 1999; Osterhout & Holcomb, 1993), reading music (e.g., Patel, Gibson, Ratner, Besson, & Holcomb, 1998), and abstract pattern completion (e.g., Lelekov-Boissard & Dominey, 2002). In these contexts, the P600 is thought to reflect the effort associated with extracting meaning from stimuli, especially stimuli that are unexpected or improbable (Hagoort, 2003; Osterhout & Holcomb, 1993). Positive activity that peaks around 600 ms also is reflective of the status of contents in long-term memory (e.g., Besson, Kutas, & Van Petten, 1992; Olichney et al., 2000, 2002). Upon initial presentation of a stimulus, a larger P600 (or LPC) is associated with better subsequent memory (e.g., Rugg, Mark, Walla, Schloerscheidt, Birch, & Allan, 1998). Like the N400, the P600 changes with stimulus repetition. Unlike the N400, the effect is different for different stimulus classes. When the repeated stimulus is semantically predictable from prior context (i.e., such as a sentence or category label), a reduction in the size of the P600 is observed. When the repeated stimulus is not semantically predictable, a smaller or no change in the P600 is apparent (e.g., Besson et al., 1992; Olichney et al., 2000, 2002; Van Petten, Kutas, Kluender, Mitchiner, & McIsaac, 1991). Van Petten and colleagues (1991) have interpreted this patterns to indicate that the first presentation initiates an attempt to retrieve relevant contents from long-term memory, resulting in a high amplitude response. The second presentation is associated with a decrease in the amplitude of the P600 to the extent that the initial retrieval attempt was successful, rendering the relevant material more readily accessible upon the second presentation, thus requiring less effortful processing of the stimulus the second time. Due to the apparent sensitivity of the P600 to the existing contents of memory, it may be especially informative of a change in knowledge state from novel to known. In the present research, we capitalized on the sensitivity of ERPs to the status of semantic knowledge to observe the transition from novel to known of newly self-derived information. In two experiments, we arranged self-derivation of information by providing subjects with separate pieces of information that could be integrated with one another to yield new knowledge (Bauer et al., 2012, in press; Bauer & San Souci, 2010; Varga & Bauer, 2013). For example, when presented with the information that “apple seeds are called pips,” and “pips contain the compound cyanide,” subjects could be expected to generate the new information that “apple seeds contain the compound cyanide.” In Experiment 1a, we recorded ERPs as subjects processed facts derived through integration. We predicted that as the new information was processed, it would transition from being treated as novel (small P200, large N400, large P600) to known (reduced P600). As summarized above, each of these ERP components has been found to be sensitive to the status of the contents of memory. In the present research, we adopted the novel approach of using them as temporal markers of change in the knowledge status of information. Experiment 1b was an ERP control experiment to eliminate priming as an explanation for the observed pattern. In Experiment 2, we used a behavioral paradigm to verify that the integration facts were newly self-derived, based on integration of separate sources from a prior learning episode.