Event-related potentials, semantic processes, and expectancy factors in word recognition.

Electrophysiological activity was recorded at 16 scalp locations during a word recognition task in order to investigate the effect of expectancy factors on ERPs. In each of 160 trials two stimuli (S1 and S2) were presented with a stimulus onset asynchrony (SOA) of 1500 msec. There were four experimental conditions. In the word-antonym (W-A) and the word-nonantonym (W-NA) conditions, both S1 and S2 were words. The subjects' task was to think of the antonym to S1 and respond as fast as possible after the presentation of S2 by pressing a "YES" button if S2 was an antonym to S1 (in the W-A trials), or a "NO" button if S2 was not an antonym to S1 (in the W-NA trials). In the nonword-word (NW-W) and nonword-nonword (NW-NW) conditions S1 was a nonword, while S2 was either a word (in NW-W trials) or a nonword (in NW-NW trials). If S1 was not a word, the subjects were instructed to wait for S2, and respond as fast as possible by pressing the "YES" button if it was a word an the "NO" button if it was not a word. EEG was sampled during a time epoch that started 100 msec before the onset of S1 and continued for another 2560 msec. The ERPs were analyzed separately for each experimental condition and for time epochs related to S1, to S2, and to the SOA. Expected antonyms were recognized significantly faster than any other words or nonwords. The RTs to words in the W-NA and NW-W condition, and to nonwords in the NW-NW condition did not differ significantly from each other. The ERP difference between the four conditions following S2 was interpreted in terms of a negative-going potential which appeared prior to the P300, during a time period which started 200 msec and ended 550 msec from stimulus onset. The negativity related to nonwords was significantly larger than the negativity related to words. The negativity related to the expected antonym was almost nonexistent. It is speculated that this negativity has the same origin as N400, and that it might be related to the process of lexical access.