Semantic Task

In the semantic task, all the stimulus types elicited distinguishable ERPs. In particular, the semantic task differed from the phonological tasks in that the responses to words and pseudowords were also distinct. However, at most sites, the period during which the ERPs elicited by words seem to be different from those elicited by pseudowords began later and was shorter than the period during which the ERPs elicited by nonwords were distinct from the other two categories. Therefore, for the initial analysis of the differences among stimuli types we divided the entire period during which differences were noticeable (270 to 600 msec) into two epochs. The first was from 270 to 350 msec and the second was from 350 to 600 msec. The differential activity was distributed at the fronto-central and anterior-temporal sites (Figure 2E).

Consequently, the initial ANOVA compared the mean amplitude of the ERPs elicited by each stimulus type, during each epoch, at T3, FC5, FC1, F3, F7, and at the correspondent sites over the right hemisphere. This analysis showed significant main effects of Stimulus Type (F(2, 46) = 16.9, p, GG □epsilon = 0.90), Site (F(4, 92) = 15.0, p□< 0.001, GG epsilon = 0.50), and Hemisphere (F(1, 23) = 29.6, p□< 0.001). There was no significant main effect of the epoch (F(1, 23) < 1.00). The interaction between the Stimulus Type effect and the epoch was significant (F(2, 46) = 15.7, p□< 0.001, GG □epsilon = 0.69). The source of this interaction was revealed by separate analyses for each epoch. These analyses, followed by univariate contrasts, revealed that during the first epoch the ERPs elicited by words and pseudowords were not significantly different (F(1, 23) = 2.35, p□= 0.14), both being more negative than those elicited by nonwords (F(1, 23) = 5.0 p□< 0.05, for words versus nonwords). During the second epoch, however, the three stimulus conditions differed significantly from one another (F(1, 23) = 4.9, p□< 0.05 for pseudowords vs. words and F(1, 23) = 45.6, for words vs. nonwords).

The most conspicuous event that distinguished words from pseudowords during the epoch of interest was a negative potential, that peaked at about 450 msec from stimulus onset. At that time the nonwords elicited a positive potential which resembled the potentials elicited by nonwords in the phonological discrimination tasks. Because no N450 was elicited by nonwords, and assuming that a superficial analysis was sufficient to decide that nonwords were not targets, we analyzed the characteristics and the scalp distribution of the N450, including only the ERPs elicited by pseudowords and words.