Interim summary and discussion

The result of Experiment 1 provides some implications to our research questions.

We found a hint of the frontal negativity effect. However, the effect was much smaller compared to those reported in past research in English. Tanner et al. (2018) pointed out that grand mean ERP waveforms may not always reflect the central tendency of the population, despite the statistically reliable effects. According to their standpoint, it is possible that the grand mean waveforms are subjected to topographic or temporal distortion. Since previous experiments also revealed that the score on verbal fluency is linked with the effect pattern, we thus look into individual data on the basis of the neuropsychological performances. Intriguingly, such typical ambiguity effect was clearly seen on the majority of subjects with high score of reading experience. This result, on the one hand, provides a preliminary support for the view that the interpretation of ambiguous words can be influenced by experience (Rodd et al., 2016, 2013). On the other, it might explain the inference from the grand average data might be illusions of overlapping different ERP effects behind and thus gave rise to the unrecognizable ambiguity effect in grand-average waveforms across all participants.

After reexamining the data on the basis of high score group of neuropsychological tests, we found the prevalent negativity across the scalp when the context selected the subordinate meanings of the homographs. When the context favored the dominate meanings of the homographs, the ambiguity effect was relatively insignificant, which might denote the dominant meaning of the homographs are more likely to be processed as unambiguous words. Consequently, the meaning activation began earlier around 400-500 ms and needed not to sustain for long as compared with the subordinate meaning of homographs which involved in a meaning selection. Perhaps it provides some evidence

for the view that meaning dominance indeed exert influences on lexical ambiguity resolution.

On the other hand, the comprehension task— word recognition task— used between blocks in present experiment might be the possible reason causing such a relatively slight effect. In fact, prior studies applied a semantic relatedness judgement task to facilitate participants to process the meaning of the whole phrases. In comparison, the word recognition task needed not to integrate the syntactic cues and the target word; instead, it could be done by recalling the target words. It seemed to increase participants’ burden on memory load rather to help integrate the visual information. Since the accuracy rate of the task was also significantly lower, word recognition task might not be that ideal as we thought.

To make the above-mentioned potential factors clear, we began to do following follow-up studies like analyzing the individual data by grouping the items based on median split of behavior indexes, modifying the experiment by replacing another online comprehension task. we attempted to look for if these factors really make impacts on the result.

3.6 Follow up analysis: Inter-individual variability analyses

In view of inter-individual differences observed in past research (Lee & Federmeier, 2012), we also set out to explore the individual differences in the overall ambiguity effect across all participants. Figure 3.5 plots the mean amplitude differences during a typical time window for the frontal negativity effect (250-900 ms) post stimuli-onset at a representative channel (FZ) for each participant. This analysis revealed a great amount of individual variation within the interested window, with approximately half of the

participants exhibiting an extended negativity and half a positivity, resulting in the cancellation of a significant overall main effect.

Figure 3.5. Effect sizes per participant for ambiguity manipulation at the representative frontal channel (Fz) within 250-900 ms.

To try to account for the source of individual variations, we analyzed the inter-individual variation depending on several neuropsychological indexes. According to past work, the effect patterns might differ due to participants’ cognitive abilities. Verbal fluency test, for instance, has been widely assessed to measure verbal ability including lexical knowledge and lexical retrieval ability (Cohen et al., 1999; Weckerly et al., 2001;

Federmeier et al., 2010) and executive control ability (Henry & Crawford, 2004;

Fitzpatrick et al., 2013). Some related research has also indicated that better performance on verbal fluency is linked with greater amount of frontal negativity elicited by homographs (Lee & Federmeier, 2011). Motivated by previous findings, we attempted to look at group averages based on a median split of participants’ neuropsychological performance to examine if the unapparent overall ambiguity effect was derived from the individual differences.

Two neuropsychological tests were conducted in this study— reading experience assessed by an author and magazine recognition questionnaire and verbal fluency. As a first step to explore the possible influence of these two types of cognitive abilities, participants were divided into high and low score groups which was created by means of median-split method (see Figure 3.6). The means of low and high score groups in reading experience test were 28% and 51% respectively. For verbal fluency test, the mean of low score group was 105.2 and 140.7 for high. We grouped the ERPs according to the high/low group of two neuropsychological tests and observed relations between the brain responses and the cognitive abilities (see Figure 3.7 & Figure 3.8).

Figure 3.6. High/low score group based on a median split for two neuropsychological tests across twenty participants

Figure 3.7. Grand average waveforms to ambiguous words (red line) and unambiguous words (black line) of the low and high score group for verbal fluency test are plotted at 3 representative midline electrode sites (Fz, Cz, and Pz) to observe the overall effect. There is no statistical significance between ambiguous words and unambiguous words in each group.

Figure 3.8. Grand average waveforms to ambiguous words and unambiguous words of the low and high score group for reading experience test are overlaid at 3 representative midline electrode sites (Fz, Cz, and Pz) to highlight the overall effect. There is a prominent statistical significance (p<.05) in high score group, while there is no difference between ambiguous words and unambiguous words in low score group.

The results showed that higher score groups for both verbal fluency and reading experience showed more frontal negativity effect than did the lower score groups.

However, the between group difference was particularly robust for reading experience.

To inspect if the inter-individual variability did result from the relation with reading experience, we plotted a boxplot to compare the brain response of the high and low score group for reading experience test at a representative channel (Fz) (see Figure 3.9). It showed obviously that most subjects with higher score in reading experience were likely to show a negativity to ambiguous compared to unambiguous words over frontal channels,

Figure 3.9. The boxplot plotted on the basis of the high and low score group for reading experience test at Fz to represent the brain response within two groups.

Motivated by this finding, we continue to investigate whether in the high score group of reading experience, these differences would be larger when the context biases the subordinate meaning of the homographs on the basis of the findings in Lee & Federmeier (2009). Figure 3.10 shows in the high group of reading experience, the waveforms to ambiguous vs. unambiguous words when the context favors dominate and subordinate meaning of the homographs, respectively.

Figure 3.10. In the high group of reading experience, ERPs’ responses at three representative electrodes of ambiguous (red line) vs. unambiguous words (black line) when the context favors dominate meaning (left column) and subordinate meaning (right column) of the homographs. The bottom four isopotential voltage maps show scalp distributions viewed from the top of the head for brain responses in both contexts in two time windows (250-550 ms and 550-900 ms). The statistical significance of the difference between unambiguous and ambiguous words is noted only in subordinate-biasing context.

(p<.05) A clear contrast between groups shows that a notable frontal negative effect is elicited only in subordinate-biasing context.

In accordance with our prediction, in the high score group of reading experience, a robust sustained negativity was elicited when the context favors subordinate meaning of an ambiguous word. The effect was quite widespread, with the effect being only slightly larger in the frontal than in the central and posterior channels. In contrast, there is very little difference between unambiguous words and ambiguous words in the dominate-biasing context, except for the N400 effect.