Semantic elaboration: ERPs reveal rapid transition from novel to known

Abstract

Like language, semantic memory is productive: It extends itself through self-derivation of new information through logical processes such as analogy, deduction, and induction, for example. Though it is clear these productive processes occur, little is known about the time course over which newly self-derived information becomes incorporated into semantic knowledge. In the present research, we used event-related potentials to examine this dynamic process. Subjects were presented with separate but related facts that, when integrated with one another, supported generation of new information (Integration facts). After 2 400-ms presentations, P600 responses to Integration facts differed from responses to Novel facts and did not differ from responses to Well-known facts, suggesting that the newly self-derived information had been incorporated into the knowledge base. The finding of rapid transition from newly self-derived to well known helps explain the richness of semantic memory. By implication, it also may contribute to the absence of episodic information specifying when and where semantic contents were acquired.

Figure 1

During encoding, sentences were presented one at a time on a computer. Duration of presentation was based on the number of characters in the sentence and ranged from 3–7 sec (100 ms per character). After reading each sentence, subjects verbally indicated when in school the fact in the sentence would be learned. Sentences were presented in one of 12 random orders, each used approximately equally often (Panel A). During the ERP test (Experiment 1), new sentences were presented one word at a time. ERPs were time locked to the target word, which always was the last word in the sentence. After each sentence, subjects used a game controller to determine when the fact would be learned. Trials were presented in one of 4 random orders, each used approximately equally often (Panel B). During behavioral testing (Experiment 2), new sentences were presented one word at a time. The final word was omitted and followed by a decision screen with four choices, one of which was correct. Subjects registered their choices with a game-controller. Trials were presented in one of 8 random orders, each used approximately equally often (Panel C).

Figure 2

Representation of the montage of electrodes with the location of the centro-parietal cluster (P3/P4, CP1/CP2, CP5/CP6) outlined.

Figure 3

Scalp ERP of central and posterior sites that demonstrate widespread patterns of P200 and N400 in the first presentation (Panel A) and P600 in the second presentation (Panel B). The centro-parietal cluster (electrodes P3/P4, CP1/CP2, CP5/CP6) was the focus of analysis.

Figure 4

In response to the first 400 ms presentation of the stimuli, analysis of the centro-parietal cluster (P3/P4, CP1/CP2, CP5/CP6) revealed P200 (light shading) and N400 (dark shading), effects. Topographic maps reflect mean amplitudes to Well-known, Integration, and Novel facts at 250 ms (P200) and 400 ms (N400) post stimulus onset (Panel A). In response to the second 400 ms presentation of the stimuli, analysis of the centro-parietal cluster revealed dampened (and nonsignificant) P200 and N400 effects. The P600 (shaded area) differed for Novel relative to Well-known and Integration facts. Topographic maps reflect mean amplitudes at 750 ms post stimulus onset (Panel B). In both panels, ERP waveforms are from electrode site Cz. The raw EEG data were bandpass filtered with half-power cutoffs of 0.1 and 30 Hz; analyses were conducted on these data. For presentation purposes, the data were refiltered with half-power cutoffs of 0.1 and 10 Hz.

Figure 5

In response to the first 400 ms presentation of the stimuli, analysis of the centro-parietal cluster (P3/P4, CP1/CP2, CP5/CP6) revealed P200 (light shading) and N400 (dark shading), effects (Panel A). In response to the second 400 ms presentation of the stimuli, analysis of the centro-parietal cluster revealed dampened P200 and N400 effects. The P600 (shaded area) differed for the Well-known relative to the Novel and Pseudo-Integration fact conditions which did not differ from one another. In both panels, ERP waveforms are from electrode site Cz. The raw EEG data were bandpass filtered with half-power cutoffs of 0.1 and 30 Hz; analyses were conducted on these data. For presentation purposes, the data were refiltered with half-power cutoffs of 0.1 and 10 Hz.

Figure 6

Mean number of correct responses in the Well-known, Integration, and Novel conditions of behavioral testing in Experiment 2 (error bars reflect standard error).