Word imageability and N400 in an incidental memory paradigm.

High imagery words are memorized better than low imagery words. To examine how these words are processed at encoding, event-related potentials (ERPs) were recorded in an incidental memory paradigm. Frequency-matched high and low imagery words (45 words each) were presented in a random order on a computer screen. Twelve university students were asked to rate the imageability of a word's referent on a five-point scale. High imagery words elicited a larger N400 than low imagery words did. The N400 was more left-lateralized for low imagery words than for high imagery words, suggesting that some neural generators (probably in the right hemisphere) were not involved in the processing of low imagery words. Difference waveforms showed that the N400 was followed by a second negativity (N800), which was also larger for high imagery words and had a scalp distribution similar to that of the N400. Subsequent free recall showed a classical imagery effect that high imagery words were recalled better than low imagery words. These results suggest that the superiority of high imagery words over low imagery words in incidental memory results from more extensive activation of a semantic network distributed across the left and right hemispheres, the latter of which probably deals with imagery-related information that is not activated by low imagery words.