N170 Amplitude and Scalp Distribution

Figure 2A shows the scalp potential (SP) and scalp-cur-rent-density (SCD) distributions of the responses to or-thographic and nonorthographic stimuli 170 msecpoststimulus. For both stimulus categories, the N170shows bilateral activation centered between PO3, T5, O1,and OM1 over the left hemisphere and PO4, T6, O2, andOM2 on the right hemisphere (Figure 2B).The mean amplitude of the N170 calculated between140 and 200 msec, on the left and right hemispheres wascompared by a three-way ANOVA with the StimulusCategory (orthographic, nonorthographic), Site (OM, O,PO, T), and Hemisphere (left right) as within-subjectfactors. This analysis showed no significant main effectfor either Stimulus Category or Hemisphere (for both,F(1, 23) < 1.0) but a significant effect of Site (F(3, 69) =15.6, MSE□= 4.2, p□< 0.0001, GG □epsilon = 0.59). How-ever, the interaction between Stimulus Category andHemisphere effect was significant (F(1, 23) = 11.2, MSE= 1.1, p□< 0.005), as was the interaction between Stimu-lus Category and Site (F(3, 69) = 2.9, MSE□= 0.4, p□< 0.5,GG □epsilon = 0.45). Posthoc univariate ANOVAs showedthat the mean amplitude of the N170 was larger at thetemporal sites (-3.22 μV) and at OM (-3.13 μV) than atthe parietooccipital (-1.46 μV) and occipital (-2.25 μV) sites. The Stimulus Category × Hemisphere interactionwas due to the fact that, at all sites, the N170 elicited byorthographic stimuli was larger over the left than overthe right hemisphere sites, whereas the N170 elicited bynonorthographic stimuli was larger over the right thanover the left hemisphere. However, the difference be-tween the N170 elicited by orthographic and nonortho-graphic stimuli was not significant, except at T5 (lefthemisphere), where the N170 elicited by orthographicstimuli (-3.53 μV) was significantly larger than that elic-ited by nonorthographic stimuli (-2.67 μV) (F(1, 23) =6.86 MSE□= 2.55, p□< 0.02).Similar analyses performed on the SCD waveforms ledto similar results: There was no effect of Stimulus Cate-gory or Hemisphere on the mean current amplitude ofthe N170 (averaged across the four sites), but a sig-nificant interaction between the two factors (F(1, 23) =11.96, p□< 0.01). The mean current amplitude of N170 tended to be larger over the left occipito-temporal areasfor orthographic stimuli (-0.73 ma/m3) than for nonor-thographic stimuli (-0.61 ma/m3) (p□< 0.10) and waslarger over the right occipito-temporal areas for nonor-thographic stimuli (-0.78 ma/m3) than for orthographicstimuli (-0.54 ma/m3) (p□< 0.05).These results thus show a double dissociation be-tween the interhemispheric distribution of orthographicand nonorthographic stimuli. Orthographic stimuli elic-ited the largest N170 at posterior left hemisphere sites,whereas the N170 elicited by nonorthographic stimuliwas largest at posterior right hemisphere sites. However,only at the left posterior temporal site (T5) was thedifference between the N170 elicited by orthographicand nonorthographic stimuli significant.Although only 8 out of the 24 subjects were males,given the interhemispheric asymmetrical distributionof the N170 on the one hand, and the recent contro-versy regarding gender differences in the interhemi-spheric asymmetry for language processing (Pugh et al.,1996; Shaywitz et al., 1995; but see Frost et al., 1997),on the other, we compared the pattern of the interhemi-spheric asymmetry of the N170 amplitude betweenthe male and the female participants. This comparisonwas based on a mixed model ANOVA with Gender asthe between-subject factor and Stimulus Category andHemisphere as the within-subject factor. This analysisrevealed that, although the pattern of interhemisphericasymmetry tended to be different for men and women¹, neither the interaction between Gender and Hemi-sphere nor the interaction between all three factorswere statistically significant (F(1, 22) < 1.0 for both interactions).