Chapter 4 Results
4.2 ERP data
4.2.1 N400
Visual inspection of the waveforms indicated that the scalp distribution of the N400 effect not only existed in the central-parietal region, but also existed in the frontal region and had an even distribution across the two hemispheres, corroborating the conclusions in previous literature (Kutas & Federmeier, 2011). Therefore, an ROI consisting of 15 electrodes in the frontal-central-parietal regions was analyzed for the N400 component. The time window between 300 to 600-ms was used to analyze the N400 component.
A three-way repeated-measures ANOVA on the factors of group (Students, Graduates, and Professionals), language (non-switched and switched), and congruity (congruent and incongruent) revealed a main effect of language, F (1, 27) = 7.07, p
< .05, a main effect of congruity, F (1, 27) = 112.83, p < .001, and a marginal main effect of group, F (2, 27) = 3.31, p = .052. For the language main effect, mean amplitudes of switched conditions (M = -0.37μV, SD = 0.38) were more negative than those of non-switched conditions (M = 0.37μV, SD = 0.38). For the congruity main effect, mean amplitudes of incongruent conditions (M = -1.33μV, SD = 0.34) were more negative than those of congruent conditions (M = 1.33μV, SD = 0.4). No interactions were observed.
Although the group main effect was marginal and there were no interactions involving group, in the interest of the present study, separate one-way ANOVAs on the dependent variables of mean amplitudes of the four collapsed conditions
(congruent, incongruent, non-switched, switched) were conducted. Results showed that the groups only differed in the congruent condition, F (2, 27) = 3.75, p < .05.
Post-hoc Tukey’s HSD tests showed that the mean amplitudes of Graduates (M = 0.44μV, SD = 0.7) were significantly more negative than those of Professionals (M = 2.88μV, SD = 0.7), p < .05, while neither the difference between Students and
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Professionals nor the difference between Students and Graduates reached significance.
To test if the congruity effects and switch effects revealed by the three-way ANOVA actually existed within each group, separate pairwise comparisons within each group were followed up. Mean amplitudes of the collapsed congruent and incongruent conditions were compared, and those of collapsed non-switched and switched conditions were also compared. The results (see Table 13) showed that the congruity effect was robust and substantial, while the switch effect was insignificant.
Figures 7, 8, and 9 show the collapsed brainwaves of congruent versus incongruent conditions of Students, Graduates, and Professionals, respectively. Waveforms of the three groups of subjects from electrodes (FZ, CZ, and PZ) that are illustrative of the congruity paradigm are shown in Figure 10. Figure 11, 12 and 13 show the collapsed brainwaves of non-switched versus switched conditions of Students, Graduates, and Professionals, respectively.
Table 13
Pairwise comparisons showing the congruity effect and switch effect in the N400 time window in each group. (Mean amplitudes in μV. Standard deviation in
parentheses)
Experiment conditions
Students Graduates Professionals
Mean
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Figure 7. Collapsed brainwaves of congruent vs incongruent conditions in Students.
Figure 8. Collapsed brainwaves of congruent vs incongruent conditions in
Graduates.Congruent Incongruent
Congruent Incongruent
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Figure 9. Collapsed brainwaves of congruent vs incongruent conditions in
Professionals.Students Graduates Professionals
Figure 10. Representative EEG waveforms from three electrodes delineating the
collapsed congruent vs collapsed incongruent conditions. Time epoch is -200-ms to 1000-ms. The minimum and maximum amplitude on diagram is -4μV to 8μV.FZ
CZ
PZ
Congruent Incongruent
Incongruent Congruent
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Figure 11. Collapsed brainwaves of non-switched vs switched conditions in Students.
Figure 12. Collapsed brainwaves of non-switched vs switched conditions in Graduates.
Non-switched Switched
Non-switched Switched
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Figure 13. Collapsed brainwaves of non-switched vs switched conditions in
Professionals.To further explore the laterality of the congruity effect across the three groups of subjects, one-way ANOVAs on were conducted on two ROIs: the right hemisphere consisting of 5 electrodes (F4, FC4, C4, CP4, and P4), and the left hemisphere consisting of 5 electrodes (F3, FC3, C3, CP3, and P3). No difference was revealed in the right hemisphere, F (2, 27) = 1.28, p = .293, but there was a marginal difference in the left hemisphere, F (2, 27) = 3.12, p = .06. Independent t-tests between the groups were performed, and results revealed that the left hemisphere congruity effect (as indexed by the amplitude difference) of Professionals (M = -3.02μV, SD = 1.59) was significantly larger than that of Students (M = -1.6μV, SD = 0.76), t (18) = 2.56, p
< .05. The difference waveforms in the Figure 14 illustrate these results, as the difference seemed more pronounced in the left hemisphere. The waveforms for three representative electrodes from the left hemisphere are shown in Figure 15.
Non-switched Switched
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Figure 14. Comparing the waveforms of the congruity effect (incongruent minus
congruent) across the three groups of subjects.Students Graduates Professionals
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F3
C3
P3
Figure 15. Representative EEG waveforms from three electrodes illustrating the
congruity effect. Time epoch is -200-ms to 1000-ms. Minimum and maximum amplitude on diagram is -6μV to 4μV.Previous literature suggested that during language comprehension, the left hemisphere could be responsible for a more predictive strategy, while the right
Students Graduates Professionals
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hemisphere a more integrative strategy (Federmeier & Kutas, 1999b). Therefore, a two-way ANOVA on the difference amplitude with the factors of hemisphere and group was conducted. Results revealed a main effect of hemisphere, F (1, 27) = 5.45,
p < .05, with the difference amplitude larger in the right hemisphere (M = -2.72μV, SD = 0.28) than the left hemisphere (M = -2.25μV, SD = 0.24). However, there was
no main effect of group, F (2, 27) = 2.27, p = .122. Laterality analysis within each group was conducted by comparing the congruity effect between both hemispheres, in accordance with the method used by Federmeier and Kutas (1999b). The results of the t-tests are summarized in Table 14. The congruity effect in all three groups did not show significant hemispheric differences. However, the discrepancy between the amplitude difference in the left and right hemispheres of the Professionals seemed to be smaller, suggesting that they were less lateralized than the other two groups.Table 14
Laterality analysis of congruity effect (amplitude in μV).
Congruity effect Amplitude difference (SD)
t (9) p value
Left hemisphere Right hemisphereStudents -1.6 (0.76) -2.11 (1.27) 1.85 .097
Graduates -2.13(1.37) -2.88 (1.54) 2.17 .058
Professionals -3.02 (1.59) -3.18 (1.78) 0.38 .714
The Professionals’ congruity effect seemed to be larger and more sustained (see Figure 15). However, visual inspection of the waveforms comparing the three groups of subjects in congruent (see Figure 16) and incongruent (see Figure 17) conditions suggested that the congruity effect was larger not because of larger negativity in the incongruent condition, but because of a large positivity in the congruent condition that began around 300-ms post stimulus and continued well into the 600 to 900-ms time window. This positive component will be analyzed below.
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FZ
CZ
PZ
Figure 16. Group comparison of the collapsed congruent condition waveforms.
FP 1 FP 2
EEG file: S_Congruent.avg Recorded : 10:21:50 01-Feb-2013 Rate - 1000 Hz, HPF - 0 Hz, LPF - 100 Hz, Notch - off
Neuroscan SCAN 4.5
Printed : 16:49:23 31-May-2013
ms
-200.0 50.0 300.0 550.0 800.0
uV
EEG file: S_Congruent.avg Recorded : 10:21:50 01-Feb-2013 Rate - 1000 Hz, HPF - 0 Hz, LPF - 100 Hz, Notch - off
Neuroscan SCAN 4.5 Printed : 16:49:46 31-May-2013
ms
-200.0 50.0 300.0 550.0 800.0
uV
EEG file: S_Congruent.avg Recorded : 10:21:50 01-Feb-2013 Rate - 1000 Hz, HPF - 0 Hz, LPF - 100 Hz, Notch - off
Neuroscan SCAN 4.5 Printed : 16:49:58 31-May-2013
ms
-200.0 50.0 300.0 550.0 800.0
uV
EEG file: S_Congruent.avg Recorded : 10:21:50 01-Feb-2013 Rate - 1000 Hz, HPF - 0 Hz, LPF - 100 Hz, Notch - off
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FZ
CZ
PZ
Figure 17. Group comparison of the collapsed incongruent condition waveforms.
FP 1 FP 2
EEG file: S_Incongruent.avg Recorded : 10:21:50 01-Feb-2013 Rate - 1000 Hz, HPF - 0 Hz, LPF - 100 Hz, Notch - off
Neuroscan SCAN 4.5
Printed : 16:51:48 31-May-2013
ms
-200.0 50.0 300.0 550.0 800.0
uV
EEG file: S_Incongruent.avg Recorded : 10:21:50 01-Feb-2013 Rate - 1000 Hz, HPF - 0 Hz, LPF - 100 Hz, Notch - off
Neuroscan SCAN 4.5 Printed : 16:52:11 31-May-2013
ms
-200.0 50.0 300.0 550.0 800.0
uV
EEG file: S_Incongruent.avg Recorded : 10:21:50 01-Feb-2013 Rate - 1000 Hz, HPF - 0 Hz, LPF - 100 Hz, Notch - off
Neuroscan SCAN 4.5 Printed : 16:52:22 31-May-2013
ms
-200.0 50.0 300.0 550.0 800.0
uV
EEG file: S_Incongruent.avg Recorded : 10:21:50 01-Feb-2013 Rate - 1000 Hz, HPF - 0 Hz, LPF - 100 Hz, Notch - off
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