The 2009 IITA International Conference on Communication Systems, Networks and Applications (ICCSNA 2009)

978-1-4244-4246-1/09/$25.00 ©2009 IEEE (CCCM 2009)

Significant Differences between metaphoric idioms and literal expressions in Chinese comprehension: an ERP proof supporting left-hemisphere activation

gradients and spatial involvement

Hongjun Chen a, b, Yiyuan Tangb, Huili Wanga, Wenyu Liua

a. School of Foreign Languages, Dalian University of Technology, DUT

Dalian, People’s Republic of China chenhj@dlut.edu.cn

Yan Wangb, Si Hab, Li Zhoub, Danni Suib b. Institute of Neuroinformatics,

Dalian University of Technology, DUT Dalian, People’s Republic of China

Yy2100@126.com

Abstract—Event related potentials (ERPs) were used to examine the processing of literal idioms and metaphoric idioms. We compared the patterns of brain electrical activity elicited by processing four-character expressions denoting literal, conventional metaphoric meaning. Participants performed a yes-no judgment task in which they decided whether each idiom conveyed a metaphoric meaning. N400 amplitude after the onset of each idiom were recorded and analyzed. The results show that metaphoric idioms do not necessarily elicit right-hemispheric activation which has been controversial in recent years. However, Conventional metaphor and literal idioms have different brain activity which is consistent with Giora’s ‘Graded Salience Hypothesis’. Cz electrode’s more negative response with metaphor seems to suggest that Lakoff’s metaphorical mapping theory has neural correlates. These findings are consistent with recent brain imaging studies and complement them by adding the spatial analysis capacity involvement.

Keywords-metaphor; ERPs; N400; ;spatial capacity

I. INTRODUCTION Metaphor is pervasive and systematic in our everyday

language [1], in which the intended meaning is not in consistent with the literal meaning. The assumption that metaphor involves reference to one target domain, while vocabulary used to commonly refer to another source domain which is mapped onto the target domain [2,3]. To make it more clear, an exemplary mapping of the metaphor “love is a journey” is provided, where the lovers represent passengers, their love is the means of transport, the welfare of the relationship equals the destination of the journey and possible feuds between partners are like obstacles on the road.

It has often, but not always, been observed that right hemisphere is better at metaphor comprehension [4-8]. Many researchers consider metaphor comprehension is somewhat different from other linguistic aspects of meaning, which the left hemisphere specializes, and it is interpreted more by the right hemisphere as figurative language [3]. The “right hemisphere theory” of metaphor processing is

mainly based on a number of patient studies, in which some right hemisphere-damaged patients have difficulties in understanding non-literal languages [9], and research with patients who suffer from left hemisphere-damaged have shown preserved metaphor comprehension skills, but not for the right hemisphere lesions patients[9,10]. An evidence from the positron emission tomography (PET) also suggest that right hemisphere contributes to metaphor processing, in which the blood flow in prefrontal cortex, the middle temporal gyrus, the precuneus, and the posterior cingulated of right hemisphere increase when comprehending the metaphoric sentences as compare to literal sentences [11]. However, a growing body of studies fails to support the unique role of right hemisphere in metaphor processing [12,13]. In a resent functional magnetic resonance imaging study, results also stand against the theory of right hemisphere advantage during the processing of metaphor and literal control sentences[14]. When subjects reading literal and metaphoric sentences and making their judgments, greater activation in a left lateralized brain regions is revealed by metaphoric sentences than literal sentences, and no differences in right hemisphere are significant. It is also suggested by Lee and Dapretto [15] that left hemisphere involve in the processing of similarly salient metaphoric and literal meaning. As Giora suggest, when understanding metaphoric idioms requires at least two stages relative to the literal idioms, we first refer to their primarily literal interpretation and then replace it with a contextually nonliteral interpretation [4].

The findings of lateralization effects in the processing of metaphors are somewhat conflicting. Some studies explained the differences in the results by methodological factors, stimulus selection and novelty or semantic distance [14,16-18]. Mashal and his associates [19] give some interpretation and they suggest that right hemisphere specialization is involved in novel and unfamiliar metaphor processing regardless of metaphoric or literal, which is supported by the studies of Schmidt et al [20].

Event related brain potentials (ERPs) were frequently used to examine the time course of metaphoric expressions

with its high temporal resolution [21-25]. Among these studies, the N400, a particular ERP component is specially stressed on. N400 is negative deflection that peaks approximately 400 ms after the presentation of a meaningful stimulus, which has been studied extensively on language processing and has been shown to vary systematically with the processing of semantic information [25,26]. Several studies show that N400 is elicited by relevance of the context, regardless of familiarity of metaphoric or literal sentences[21]. Empirical studies conducted by Kazmerski show that metaphors evoke greater N400s than that of literal conditions [23].

In the present study, we use an ERP study to examine the processing of literal idioms and metaphoric idioms. The main objective of this study was to explore (i) if left hemisphere is advantageous for the Chinese four-word expressions denoting conventional metaphoric meaning and, (ii) if larger amplitude N400s for metaphoric than literal words are found since metaphor comprehension is more demanding, (iii) if metaphor comprehension causes the activity in Parietal lobe which supports Lakoff’s mapping theory.

II. METHOD A. Participants

Twenty-six healthy and right handed adults (12 male, 8 female; age 18-23 years, mean=20.3, standard deviation=1.08) joined in this study. All were university students and had normal or corrected-to-normal visual acuity. Subjects had no history of neurological or psychiatric disorder. B. Stimuli and Procedure

The words stimuli are idioms comprised of four characters, and classified into two types: one only has literal meaning without deep meanings; the other has both literal and metaphoric meanings. These idioms are selected by a questionnaire which mainly requires subjects’ familiarities to the idioms. The familiarities to the idioms are ranked 5 levels from “don’t know” to “very familiar”, and the answers are scored from 1 to 5 point. 93 subjects who didn’t participate in the experiment finished this questionnaire, and 160 idioms (80 literal, 80 metaphoric) with the high familiarity were selected to the experiment.

Then the idioms were made into pictures (2cm*8 cm), with white letterform and black background. The screen was put 40-60cm away from the subjects. There will be a white cross showing on the screen, and its duration is randomly

selected from 500-1500ms (to avoid subjects’ expectancy effect). Then the idiom will be present for 4000ms until the subjects respond. The subjects were asked to judge whether the idiom they read on the screen had metaphor meaning. C. ERP data recording

EEG was recorded from 61 Ag/AgCl scalp electrodes recording system (Brain Products GmbH, Munich, Germany). Standard electrode sites followed the International 10-20 System nomenclature. A reference electrode was placed at the center between Cz and Fz. The vertical electrooculogram (VEOG) electrodes were placed above and below the left eye. All inter electrode impedance was maintained < 10k Signals were amplified with 0.1-50 Hz band pass filter and digitized at 500Hz. The EEG data were epoched into periods of 900ms (100 ms prestimulus baseline is included). D. Results Behavioral results

The average participants responded correctly 83.15% (S.D. =8.21%) of the literal idioms. And the metaphoric idioms are 77.65% (S.D. =12.12%) responded. With Paired sample T-test analysis, there is no significant difference between the accurate rates of these two kinds of idioms t (19) =1.40629, P>0.05. And the respond times of literal and metaphoric idioms are 1477.71ms (S.D. =259.98ms); 1544.30ms (S.D. =274.85). Paired sample T-test shows no significant difference t (19) = -1.563, P>0.05.

Event-related potentials

ERPs were gotten from correctly respond trails of each condition and each subject. The waveforms in the present study are characterized by early component P100, and N200. Grand average waveforms are shown in Fig.1, which contains most electrodes waveforms in these two conditions. Analysis contains mean amplitude measurements of time windows 400-600ms, captured the N400 component, especially some electrodes which are focused on in the former studies.

From the Fig.1, we can see that the electrodes in the left anterior and central areas performed the difference of waveforms between literal and metaphoric idioms. To further assess the differences, we group the electrodes into 4 zones and their average voltages of 400-600ms are involved in the repeated-measures ANOVA with three factors: condition (literal/metaphor), hemisphere (right/left), and caudality (anterior/posterior). There is a significant effect

Fig.1-Grand average ERPs for the Literal Idioms (solid line), and Metaphor Idioms (dash line) conditions.

of condition, F(1,19)=8.817, P<0.05. There is also significant interaction of condition by hemisphere, F(1, 19)=5.168, P<0.05. The three-way interaction of condition is by hemisphere and by caudality, F(1, 19)=14.298,

P<0.05. The left anterior zone is more negative, and further analysis shows that the metaphor condition’s left anterior electrodes are more negative.

The analysis on the central electrodes is focused on Cz and Pz. Their waveforms are shown in Fig.2, and Fig.3. The Cz electrode with waveforms from 400-600ms yielded a significant effect of different idioms after the paired t test, t

(19) = 2.104, P<0.05. While, there is no significant difference in the Pz electrode, t (19) =0.603, P>0.05.

Fig.2. Grand average ERPs of Cz

Fig3. Grand average ERPs of Pz

The scalp distributions of the N400 component in the

two conditions are shown in Fig.4. As it is shown in the map, the metaphor condition’s left anterior electrodes are more negative, compared with the literal condition.

III. DISCUSSIONS The results of the present study revealed that people

performed differently when reading four-word expressions denoting literal, conventional metaphoric meaning. Results show the N400 amplitude gradient, significant smaller N400 for literal relations and increasing for metaphors. Furthermore, there is a significant interaction of condition by hemisphere, further analysis show that the left anterior zone is more negative for metaphoric condition as compared to the literal condition.

Our findings show that metaphoric idioms do not necessarily elicit right-hemispheric activation which has been prevailing in recent years. However, some studies suggest that both cerebral hemispheres can benefit from supportive sentence context, but may suggest an important

Fig.4. Scalp distribution over the 400-600ms time

window for the literal (right two) and metaphoric (right two)

role for anterior regions of the left hemisphere in the selection of semantic information in the face of competing alternatives[3]. In addition, left-hemisphere activation gradients and spatial involvement for conventional metaphoric meaning is consistent with the argument of Giora’s ‘graded salience hypothesis’ [4].

This study is conducted on the basis of experiment paradigm and data processing methodology published by Arzouan et al[25]. Consistent with the prior studies[21,25,27], we found that metaphors elicited more negative N400s relative to literal expressions. As Arzouan et al suggested, conventional metaphors were judged as fast as literal relations, yet elicited bigger amplitude on the N400 [25], and the greater N400 amplitudes might reflect the relatively more taxing effort. Consistent with this study, Coulson et al also found larger amplitude N400s for metaphoric than literal words with carefully controlling for lexical factors, supporting the regular comprehension mechanism that metaphor is more difficult[27].

An interesting finding was waveforms during 400-600ms at electrode Cz yielded a significant effect of different idioms that greater N400 amplitudes were found for metaphoric condition relative to literal condition which seems to suggest that Lakoff’s metaphorical mapping theory has neural correlates. This finding also consists with Michelli et al’s suggestion that superior parietal areas are responsible for spatial analysis[28].

This study is significant in that it uses Chinese as

experiment stimuli which hasn’t been seen in recent publications. Besides, our findings help to advance theories of language comprehension, especially metaphor comprehension. However, our research is still fundamental compared with extensive metaphor studies.

ACKNOWLEDGMENT This research was supported by Liaoning Provincial

Humanities Research Fund, NSFC 30670699 and NCET-06-0277. I would like to express my gratitude to all who have ever helped me during the experiment.

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