Implicit and explicit mechanisms of word learning in a narrative context: an event-related potential study

Abstract

The vast majority of word meanings are learned simply by extracting them from context rather than by rote memorization or explicit instruction. Although this skill is remarkable, little is known about the brain mechanisms involved. In the present study, ERPs were recorded as participants read stories in which pseudowords were presented multiple times, embedded in consistent, meaningful contexts (referred to as meaning condition, M+) or inconsistent, meaningless contexts (M-). Word learning was then assessed implicitly using a lexical decision task and explicitly through recall and recognition tasks. Overall, during story reading, M- words elicited a larger N400 than M+ words, suggesting that participants were better able to semantically integrate M+ words than M- words throughout the story. In addition, M+ words whose meanings were subsequently correctly recognized and recalled elicited a more positive ERP in a later time window compared with M+ words whose meanings were incorrectly remembered, consistent with the idea that the late positive component is an index of encoding processes. In the lexical decision task, no behavioral or electrophysiological evidence for implicit priming was found for M+ words. In contrast, during the explicit recognition task, M+ words showed a robust N400 effect. The N400 effect was dependent upon recognition performance, such that only correctly recognized M+ words elicited an N400. This pattern of results provides evidence that the explicit representations of word meanings can develop rapidly, whereas implicit representations may require more extensive exposure or more time to emerge.

Figure 1

Grand average ERPs at the Cz electrode to real, M+, and M− critical words, divided across the ten presentations in the story task.

Figure 2

Mean amplitude plots of the N400 response across all electrodes included in the main factorial ANOVA to real, M+ and M− critical words in the story task, as a function of presentation. Mean amplitude measurements were computed from 300 to 500 msec post-stimulus. Negative is plotted upward. Error bars indicate standard errors of the mean.

Figure 3

Grand average ERPs to M+ critical words at the Cz electrode across the ten presentations of the story task, divided as a function of subsequent recall.

Figure 4

Grand average ERPs to targets in the lexical decision task, at representative right-hemisphere central and parietal sites. Targets were either nonwords or were real English words preceded by either semantically related or unrelated primes. Targets preceded by real English primes are shown on the left, and targets preceded by M+ primes are shown on the right.

Figure 5

Grand average ERPs to targets, depicted at midline sites, and topographical voltage maps in the recognition task. Targets were real English words preceded by either semantically related or unrelated primes. Targets preceded by real English primes are shown at the left, while targets preceded by M+ primes are shown on the right. The effects depicted in the voltage maps were computed by subtracting the ERP to unrelated targets from the ERP to related targets between 300 to 500 ms post-stimulus. The scale for each of these effects is identical.

Figure 6

Grand average ERPs and topographical voltage maps to targets preceded by M+ word primes in the recognition task, divided as a function of subsequent recognition. Targets preceded by M+ primes that were subsequently recognized (in both related and unrelated trials) are shown on the left. Targets preceded by M+ primes that were not subsequently unrecognized in both trials appear on the right. The effects shown in the voltage maps were computed by subtracting the ERP to unrelated targets from the ERP to related targets between 300 to 500 ms post-stimulus. The scale for each effect is identical.