Readers routinely draw inferences with remarkable efficiency and seemingly little cognitive effort. The present study was designed to explore different types of inferences during the course of reading, and the potential effects of differing levels of working memory capacity on the likelihood that inferences would be made. The electroencephalogram (EEG) was recorded from five scalp sites while participants read 90 paragraphs, composed of 60 experimental paragraphs and 30 filler paragraphs. Each experimental paragraph was four sentences long, and the final sentence stated explicitly the inference that readers did or did not make. There were four types of experimental paragraphs: (1) Bridging inference, (2) Elaborative inference, (3) Word-Based Priming control, and (4) No Inference control. Participants were tested using the Daneman and Carpenter (1980) Reading Span Task and categorized as having low or high working memory capacity. The average peaks of the N400 component of the event-related brain potential (EM) were used as a measure of semantic priming and integration, such that the lower the N400 was in response to the explicitly stated inference concept, the more likely it was that the reader made the inference. Results indicate that readers with high working memory capacity made both bridging (necessary) and elaborative (optional) inferences during reading, whereas readers with low working memory capacity made only bridging inferences during reading. We interpret the findings within the framework of the Capacity Constrained Comprehension model of Just and Carpenter (1992).

The N400 component of the human event-related brain potential appears to be related to violations of semantic expectancy during language comprehension. The present experiment investigated whether the N400 is related specifically to activity in a language system or is an index of a conceptual system that is accessed by both pictures and words. Sentences were visually presented one word at a time with the last word being replaced in one condition by a line drawing representing the same concept (eg, the word “socks” was replaced by a picture of socks). The N400 recorded in the Pictures Condition was found to be identical to the N400 generated by words in terms of amplitude, scalp distribution, and latency. These results suggest that the N400 is an index of activity in a conceptual memory that is accessed by both pictures and words.