N320 Amplitude and Scalp Distribution

Figure 2C shows the scalp potential distribution of theresponses to pronounceable and nonpronounceablestimuli at 320 msec poststimulus onset on the left andright hemispheres. A wide positive field on the occipito-central areas characterized the responses to nonpro-nounceable stimuli. The potential distribution topronounceable stimuli displayed two voltage patterns: anegative potential field over the temporal areas and a negative/positive pattern over the occipito-parietal region, slightly larger at the left than at the right hemisphere sites. N320 shows a larger amplitude over the left temporal areas (around T3) than over the right temporal areas (around T4) and larger for pronounceable stimuli than for nonpronounceable ones.

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Figure 3. ERPs to nontarget stimuli (words, pseudowords,and nonwords) at the sites ofinterest in the phonologi-cal/phonetic task. The N320 wave was largest at T3—onthe left temporal hemi-sphere—and was muchsmaller for nonwords than for pseudowords and words.

The statistical analysis of these differences was basedon a three-way ANOVA with Stimulus Category (pro-nounceable, nonpronounceable), Site (TP7/8, T3/4, C3/4,FC1/2, FC5/5, F3/4, F7/8), and Hemisphere (left, right) aswithin-subject factors. The dependent variable was themean amplitude of the N320 between 270 and 370 msec from stimulus onset. This analysis revealed that the N320was larger (i.e., more negative) for pronounceable (-0.18μV) than for nonpronounceable stimuli (which in factelicited a positive waveform in the same latency range,0.99 μV) (F(1, 23) = 15.7, MSE□= 14.66, p□< 0.001) and at the left (0.04 μV) than at the right hemisphere sites(0.77 μV) (F(1, 23) = 19.92, MSE□= 4.53, p□< 0.001). Themain effect of site was also significant (F(6, 138) = 16.92,MSE□= 3.57 p□< 0.001). The interaction between theStimulus Category and the Site effects was significant,suggesting that the difference between the pronounce-able and the nonpronounceable stimuli was larger atsome sites than at others (F(6, 138) = 8.82, MSE= 0.37,p□< 0.001). No other interactions were significant. The scalp distribution of the N320 was analyzed by a one-way ANOVA in which the dependent variable was the amplitude of N320 elicited by pronounceable stimuliaveraged across hemispheres. This analysis showed that the N320 varied significantly with site (F(6, 138) = 13.4,p□< 0.001, GG □epsilon = 0.35). Post hoc univariate con-trasts revealed that the amplitude of the N320 was sig-nificantly larger at T3/4 (-1.1 μV) than at any otherlocation and that it was negative at F7/8, TP, and FC56(-0.68, -0.65, and -0.53 μV, respectively) and positive atthe more central and frontal electrodes, C, FC1/2, and F3/4 (0.59, 0.76, and 0.31 μV, respectively). This distribu-tion statistically validates the lower midtemporal distri-bution of the N320. A post hoc analysis of the interactionbetween the stimulus category and the site showed that the difference between pronounceable and nonpro-nounceable stimuli was largest at F7/8 (1.85 μV).The possible interaction of the hemispheric differ-ences with gender was examined for the N320 as for theN170 potential. This analysis showed that neither the Gender × Hemisphere nor the Gender × Stimulus Type by Hemisphere interactions were significant (for both F(1, 22) < 1).