N350 Amplitude and Scalp Distribution

Figure 2D shows the scalp-potential distribution of the responses to phonologically legal and illegal stimuli, 350 msec poststimulus over the left and right hemispheres, respectively. Although the scalp distribution of the N350 was apparently more anterior and central than that of the N320, for the purpose of intertask comparisons we have analyzed the same subset of scalp sites as in the phonological/phonetic task. Hence, the mean amplitude of the N350 was calculated between 300 and 400 msec separately for legal and illegal stimuli at TP7, T3, C3, FC5, FC1, F3, and F7 over the left hemisphere and the corresponding sites over the right hemisphere. These data were analyzed using a Stimulus Category × □Site × □Hemisphere within-subject ANOVA. The analysis showed that all three main effects were significant (F(1, 23) = 85.6, p < 0.001, F(6, 138) = 12.8, p < 0.001, GG epsilon = 0.41 and F(1, 23) = 7.1, p < 0.015 for the Stimulus Category, Site, and Hemisphere, respectively). The interaction between Stimulus Type and Site and that between Stimulus Type and Hemisphere and the three-way interaction between Stimulus Type, Site, and Hemisphere were also significant (F(6, 138) = 16.5, p < 0.001, GG epsilon = 0.37, F(1, 23) = 8.5, p < 0.01, and F(6, 138) = 8.5, p < 0.025, GG epsilon = 0.38, respectively). The distribution of the N350 was examined with a Site × □Hemisphere ANOVA. This analysis showed that the N350 was larger (i.e., more negative) at left (□1.58 μV) than at right (□0.91 μV) hemisphere sites (F(91, 23) = 15.6, p < 0.005). Across hemispheres, its amplitude varied significantly (F(6, 138) = 2.9 p < 0.01, GG epsilon = 0.4). Post hoc univariate contrasts revealed that, like the N320, the N350 was largest at T3/4 (□1.75 μV). However, in contrast to the N320, its amplitude was not significantly smaller at FC5/6 (-1.6 μV) than at T3/4, and it was only slightly reduced at F7/8 (-1.34 μV). The difference between the amplitude of the N350 at these three sites was not significant. In contrast, the amplitude of the N350 at the TP5/6 (-1.24 μV) sites, which were immediately posterior to the T3/4, was significantly smaller than at T3/4 (F(1, 23 = 12.9, p□< 0.01). These results confirmed a midtemporal and dorsotemporal scalp distribution of the N350, with ramifications in the anterior temporal lobes. This distribution is different from that of the N320. Yet, given the spatial and temporal proximity of the N320 and the N350, we cannot exclude the possibility that the topography observed in Figure 3D was influenced by an overlap of N320 and N350. As previously, none of the interactions with the participant’s gender were significant.