N450 Amplitude and Scalp Distribution

Figure 2E shows the scalp potential distribution of the responses to pseudowords and words at 450 msec poststimulus onset, at the left and right hemisphere sites.

Words elicited a well-circumscribed bilateral negativity peaking at more anterior sites than that elicited by words in the lexical decision task. Pseudowords display two negative maxima, more evident over the left than over the right hemisphere: one, centered between F7, FC5, and F3 had a topography that contained the areas activated by the N350 but also more anterior regions (Figure 2D and 2E). The second, centered around FC1, corresponds to the N450 shown in Figure 5 and was not observed in the lexical decision task. As in the previous experimental sessions, a negative activity was also observed above the occipital areas.

The scalp potential distribution of the N450 was assessed by a Stimulus Type (words, pseudowords) × Site (TP, T, C, FC5/6, FC1/2, F3/4, and F7/8) × Hemisphere (left, right) ANOVA. This analysis showed that the N450 elicited by pseudowords (-1.0 μV) was significantly more negative than that elicited by words, which, across sites, was positive (0.15 μV) (F(1, 23) = 18.4, p□< 0.001); it was significantly more negative over the left hemisphere (-0.60 μV) than over the right hemisphere (-0.22 μV) (F(1, 23) = 4.9, p□< 0.05) and differed significantly among scalp sites (F(6, 138) = 9.5, p□< 0.001, GG □epsilon = 0.42).

The interaction between Stimulus Type and Site effects was significant (F(6, 138) = 13.2, p□< 0.001, GG □epsilon = 0.33), revealing that the Stimulus Type effect was not significant at the most anterior electrode sites (F7 and F8), whereas it was significant at all other sites, which did not differ among themselves. No other interactions were significant. Post hoc contrasts examining the site effect revealed that, across words and pseudowords, the N450 was significantly larger (more negative) at F7/8 (-1.2 μV) than at all other sites (the difference between F7/8 and the second largest N450 at FC5/6 was significant, F(1, 23) = 8.1, p < 0.01), negative at the anterior supratemporal FC5/6 (□0.68 μV), F3/4 (□0.65 μV) FC1/2 (□0.22 μV) and midtemporal sites T3/4 (□0.36 μV) (which did not differ significantly among themselves), and positive at the centro-lateral C3/4 (0.06 μV) and posterior-temporal sites TP7/8 (0.20 μV). The difference between the N450 elicited at FC5/6 and F3/4 was significant (F(1, 23) = 5.7 p < 0.05). This distribution validates the anterior-temporal and anterior-supratemporal scalp distribution of the N4503.

Notes
3.

The significantly larger N450 at F7 and F8 may, however, reflect the absence of the stimulus type effect at these sites. Nonetheless, for pseudowords as well as for words, the largest N450 was found at F7 (-1.58 and —1.19 μV for pseudowords and words, respectively).