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Laterality: Asymmetries of Body,
Brain and Cognition
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Semantics is crucial for the
right-hemisphere involvement in
metaphor processing: Evidence
from mouth asymmetry during
speaking
a
a
b
Paraskevi Argyriou , Sarah Byfield & Sot aro Kit a
a
School of Psychology, Universit y of Birmingham,
Birmingham, UK
b
Depart ment of Psychology, Universit y of Warwick,
Covent ry, UK
Published online: 30 Aug 2014.
To cite this article: Paraskevi Argyriou, Sarah Byfield & Sot aro Kit a (2014): Semant ics
is crucial for t he right -hemisphere involvement in met aphor processing: Evidence
from mout h asymmet ry during speaking, Lat eralit y: Asymmet ries of Body, Brain and
Cognit ion, DOI: 10.1080/ 1357650X.2014.951654
To link to this article: ht t p:/ / dx.doi.org/ 10.1080/ 1357650X.2014.951654
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I t is e sse n t ia l t h a t you ch e ck t h e lice n se st a t u s of a n y give n Ope n a n d
Ope n Se le ct a r t icle t o con fir m con dit ion s of a cce ss a n d u se .
Laterality, 2014
http://dx.doi.org/10.1080/1357650X.2014.951654
Semantics is crucial for the right-hemisphere
involvement in metaphor processing: Evidence from
mouth asymmetry during speaking
Paraskevi Argyriou1, Sarah Byfield1, and Sotaro Kita2
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1
2
School of Psychology, University of Birmingham, Birmingham, UK
Department of Psychology, University of Warwick, Coventry, UK
Research on the neural basis of metaphor provides contradicting evidence about the role
of right and left hemispheres. We used the mouth-opening asymmetry technique to
investigate the relative involvement of the two hemispheres whilst right-handed healthy
male participants explained the meaning of English phrases. This technique is based on
the contralateral cortical control of the facial musculature and reflects the relative
hemispheric involvement during different cognitive tasks. In particular, right-handers
show a right-sided mouth asymmetry (right side of the mouth opens wider than the left)
during linguistic tasks, thus reflecting the left-hemisphere specialization for language. In
the current study, we compared the right-sided mouth asymmetry during metaphor
explanation (e.g., explain the meaning of the phrase “to spin a yarn”) and concrete
explanation (e.g., explain the meaning of the phrase “to spin a golf ball”) and during the
production of content and function words. The expected right-sided mouth asymmetry
reduced during metaphorical compared to concrete explanations suggesting the relative
right-hemispheric involvement for metaphor processing. Crucially, this right-sided
mouth asymmetry reduction was particularly pronounced for the production of content
words. Thus, we concluded that semantics is crucial to the right-hemispheric
involvement for metaphorical speech production.
Keywords: Metaphorical speech production; Word-class; Mouth asymmetry; Righthemisphere.
Address correspondence to: Paraskevi Argyriou, School of Psychology, University of Birmingham,
Frankland Building, Edgbaston, Birmingham B15 2TT, UK. E-mail: pxa180@bham.ac.uk
We wish to thank Sarah Aldgate and Heather Golden for developing stimuli and running pilot
studies and Sam Westwood for his help with data coding.
This work was supported by a Ph.D. studentship from the Economic and Social Research Council
awarded to Paraskevi Argyriou for studies at the University of Birmingham [grant reference ES/
J50001X1]. We would like to thank the financial support from the School of Psychology at the
University of Birmingham for the study.
© 2014 The Author(s). Published by Taylor & Francis.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License
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reproduction in any medium, provided the original work is properly cited. The moral rights of the
named author(s) have been asserted.
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2
ARGYRIOU, BYFIELD, KITA
There are many studies investigating what determines the neural recruitment of
metaphorical language processing, and several theoretical accounts have been
proposed about the hemispheric lateralization of metaphor (for a review, see
Schmidt, Kranjec, Cardillo, & Chatterjee, 2010). However, there is contradicting
evidence for the involvement of the right hemisphere in metaphor processing
(e.g., Right Hemisphere Hypothesis, Brownell, Simpson, Bihrle, Potter, & Gardner,
1990; for alternative views see Rapp, Leube, Erb, Grodd, & Kircher, 2007).
Furthermore, most of the studies have been focusing on metaphor processing in
comprehension tasks rather than metaphorical speech production. In this study, we
will use the mouth asymmetry technique during speech production, which reflects
relative hemispheric involvement during verbal tasks (for a review, see Graves &
Landis, 1990). Additionally, we will provide evidence that the right hemisphere is
involved during metaphorical speech production and in particular during production
of content words related to metaphor.
According to the right hemisphere hypothesis for metaphor (Brownell et al.,
1990) the right hemisphere has a privileged role in lexical—semantic processes
related to metaphor comprehension. There are several empirical studies
providing evidence in favour of this hypothesis. However, the overall conclusion
based on the findings remains somewhat vague mainly because the studies used
different populations (i.e., patients vs. healthy participants), tasks (i.e., metaphor
judgement vs. plausibility judgement vs. lexical decision) and stimuli (i.e.,
sentences vs. single words; novel vs. familiar metaphors).
The first evidence for the right-hemisphere involvement for metaphor came
mainly from studies of patients with brain damage. For example, Winner and
Gardner (1977) have shown a deficit in appreciation of metaphorical meanings in
patients with right-hemisphere lesions compared to those with left-hemisphere
lesions in a sentence-picture matching task. However, the pattern was reversed
when patients were asked to verbally explain the meanings of the metaphorical
phrases in the sentences; that is, patients with right-hemisphere lesions offered
appropriate metaphorical explanations of the phrases, while patients with lefthemisphere lesions produced literal verbal explanations. They proposed that both
hemispheres contribute to metaphorical competence, but the right hemisphere is
crucially engaged in the “visualization” of metaphors.
In addition, studies with healthy participants have found stronger righthemispheric engagement whilst processing metaphorical compared to literal
stimuli. For example, Anaki, Faust, and Kravetz (1998) used the divided visual
field technique and the word-priming paradigm and showed that initial activation
for metaphorical meanings involves both right and left hemispheres and
maintenance particularly involves the right hemisphere only. Initial activation
and maintenance of literal meanings involved the left hemisphere only. The
findings, though limited to single words, highlight the importance of time course
of each hemisphere’s involvement in processing semantic link between words.
Moreover, a positron emission tomography neuroimaging study (Bottini et al.,
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METAPHOR PRODUCTION AND RIGHT-HEMISPHERE
3
1994) found right-hemispheric activation during judgement of the plausibility of
metaphorical sentences compared to literal ones. Bottini et al. (1994) also
highlighted the importance of the task’s semantic load for the relative
hemispheric involvement during metaphor processing. For example, a lexical
decision task where subjects had to identify non-words embedded within
metaphorical and literal sentences reveals greater right-hemispheric activation
than a metaphorical sentence comprehension task. Furthermore, some studies
suggest that it is not metaphoricity per se which determines the involvement of
each brain hemisphere. It is rather the degree of saliency. An expression is
considered as salient when its meaning is familiar, conventional, highly frequent
and predictable (Giora, Zaidel, Soroker, Batori, & Kasher, 2000). Jung-Beeman
(2005) suggests there is a core, bilateral, neural network which is involved in the
semantic processing of metaphors. Specifically, the right hemisphere is
predominantly involved for the processing of novel metaphors compared to
conventional ones (Ahrens et al., 2007; Cardillo, Watson, Schmidt, Kranjec, &
Chatterjee, 2012; Faust & Mashal, 2007; Mashal, Faust, & Hendler, 2005;
Schmidt, DeBuse, & Seger, 2007), for the processing of non-salient meanings
compared to salient ones (Giora et al., 2000) and for the processing of distant
semantic relationships compared to closely related word meanings (Mashal,
Faust, Hendler, & Jung-Beeman, 2007).
Some functional magnetic resonance imaging (fMRI) studies failed to fully
support the right hemisphere hypothesis for metaphor. For example, Stringaris,
Medford, Giampietro, Brammer, and David (2007) provided neuroimaging data
while participants judged the plausibility of metaphorical and literal sentences
and failed to show a differential activation of the right-inferior frontal gyrus for
the comparison literal vs. metaphorical. Also, Rapp, Leube, Erb, Grodd, &
Kircher (2004) and Rapp et al. (2007) used metaphorical judgement (“is the
sentence metaphorical or literal”) and connotation judgement (“does the sentence
have positive or negative connotations”) of sentences, and they did not find any
activation in the right-hemispheric structures for the metaphorical sentences.
Benedek et al. (2014) investigated production of metaphor, using a paraphrase
task. Participants were presented with a short sentence (e.g., “the lamp is
glaring”) and asked to provide either a literal (“bright”) or a metaphorical
(“a supernova”) word that replaces the adjective without changing the meaning
very much. The regions more activated for the metaphor condition than for the
literal condition are activated either bilaterally or only in the left hemisphere.
Mixed results regarding the right hemisphere hypothesis for metaphor may
relate to various factors. First, different methodologies reveal different aspects of
metaphor processing. For example, the cognitive activity measured in behavioural
experiments (as in reaction times in Anaki et al., 1998) differs from the neural
correlates of the activity captured in brain-imaging studies (as in BOLD signal in
regions of interest in Rapp et al., 2004; Stringaris et al., 2007). Although
equivalence in findings would clearly support a certain hypothesis about how the
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ARGYRIOU, BYFIELD, KITA
two hemispheres contribute to metaphorical and literal interpretations of linguistic
stimuli, different findings from different methodologies are not necessarily
contradictory. If two cognitive tasks (metaphorical vs. literal processing) result
in different reaction times, this does not necessarily mean that they will be
subserved by different neural pathways. Second, the nature of stimuli differs
greatly across studies. For example, in some studies (i.e., Stringaris et al., 2007),
the degree of saliency or novelty of the linguistic expressions has not been
accounted for, whereas it is controlled in others (i.e., Mashal et al., 2005).
Similarly, some studies focus on metaphorical comprehension for single words
(i.e., Anaki et al., 1998) as opposed to sentences (i.e., Rapp et al., 2004, 2007).
Finally, the involvement of each hemisphere during metaphor processing is task
sensitive. For example, plausibility judgement (i.e., Stringaris et al., 2007)
may involve too many cognitive processes that it has washed out the critical
difference between literal and metaphorical stimuli and thus failed to reveal any
metaphor specific activations. To sum up, any study focusing on the hemispheric
involvement during metaphorical processing and using any type of methodologies
needs to carefully account for the role of semantics so that the involvement of the
right hemisphere is neither masked nor marked due to not metaphor-specific
processing demands or linguistic variables.
As the above literature review reveals, the role of the two hemispheres and
that of semantics in metaphor processing remains controversial. In addition, most
of the studies investigated metaphor comprehension, rather than production (as
far as we know, Benedek et al., 2014 is the only production study). Thus, it still
remains unresolved if the right hemisphere is involved in metaphor processing
during speech production and if semantics is crucial for this particular
involvement.
The contributions of the two hemispheres during cognitive processes (e.g.,
linguistic, visual imagery and emotional tasks) have been investigated using
measurement of mouth asymmetry. The foundational assumption of this
measurement is that each side of the lower facial areal is controlled by the
contralateral cortex (Adams, Victor, & Ropper, 1997; Gardner, 1969). Therefore,
if one hemisphere is particularly involved in a task that requires mouth opening,
there will be greater opening on the contralateral side of the mouth.
Several studies validated asymmetries in mouth openings during speech
production as an indicator of the role of the two hemispheres in various speech
production tasks. For example, Graves and Landis (1985, 1990) indicated that
healthy, right-handed speakers open the right side of their mouth wider than the
left during propositional speech (e.g., spontaneous speech, word list generation
and repetition), thus suggesting the left-hemisphere control over speech
production. This pattern is reversed (left side opens wider than the right) during
automatic speech (e.g., singing, counting and reciting the days of the week),
which is considered to be processed by the right hemisphere (see for a review
Lindell, 2006). In addition, Code, Lincoln, and Dredge (2005) compared the
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METAPHOR PRODUCTION AND RIGHT-HEMISPHERE
5
mouth asymmetry patterns during propositional speech production by righthanded stuttering and non-stuttering speakers. They found a bilateral pattern for
stutterers compared to a clear right-sided mouth asymmetry for the nonstutterers. This finding supports models about a distinct hemispheric control of
speech production in stutters and non-stutters, thus further highlighting the
sensitivity of the mouth asymmetry technique.
The mouth asymmetry as an indicator of hemispheric specialization has also
been validated in studies of emotional expressions (e.g., smile). Graves,
Goodglass, and Landis (1982) showed that healthy, right-handed participants
open the right side of the mouth more widely than the left during propositional
speech linguistic tasks compared to spontaneous smiles. This reflects the lefthemisphere cerebral specialization for language and the right-hemisphere
involvement for emotion processing during smiles. Similarly, Wyler, Graves,
and Landis (1987) showed a clear left-sided mouth asymmetry during smiles,
which is particularly apparent during spontaneous compared to posed smiles
(Wylie & Goodale, 1988). Developmental studies with infants have also
successfully used the mouth asymmetry technique to investigate the lateralization
of emotional expressions. For example, Holowka and Petitto (2002) showed that
infants (5–12 months old) open the right side of their mouth wider than the left
when they are babbling (a precursor to speech) compared to smiling. Interestingly, Schuetze and Reid (2005) showed a right-hemispheric control for negative
emotional expressions (left-sided bias in mouth movements of sadness) which
strengthens with age (from 12- to 24-months old), while this pattern was absent
for the control of positive emotional expressions.
The above studies show that the mouth asymmetry technique is sensitive to
differential hemispheric involvement across tasks. In addition, it is a noninvasive, inexpensive and safe technique inferring relative involvement of the
hemispheres in real time, during actual speech production. However, this
technique has not been used to investigate the hemispheric involvement for
metaphorical speech production, which is still a very much-unresolved question.
In a preliminary study, Argyriou and Kita (2013) tested right-handed speakers
(different participants from the current study) and showed that right-sided mouth
asymmetry reduced when they explained metaphorical phrases compared to
concrete ones (e.g., “to spin a yarn” vs. “to spin a golf ball”). This finding is in
line with the relative right-hemispheric involvement during metaphor compared
to concrete explanations. However, what is not clear from this study is whether
semantic processing during metaphorical speech production particularly involved
the right hemisphere. This is an important limitation as semantics is a crucial
component of metaphor theories (e.g., Giora et al., 2000; Lakoff & Johnson,
1980).
The key aim of the present study is to shed light on lateralization of metaphor
processing during speech production rather than comprehension, using the mouth
asymmetry technique, and to investigate the role of semantics in the involvement
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ARGYRIOU, BYFIELD, KITA
of the right hemisphere. More specifically, we investigated whether metaphor
processing particularly involves the right hemisphere such that it reduces the
right-sided mouth asymmetry during metaphorical compared to concrete speech
production. In addition, we investigated whether semantics is crucial to the righthemispheric involvement for metaphorical speech production such that the
decrease of the right-sided mouth asymmetry during metaphorical compared to
concrete speech production is particularly pronounced for production of content
words, which carry meaning.
In order to test the first research question, we manipulated the content of
speech production. That is, participants explained English phrases with either
metaphorical or concrete meanings (e.g., “to spin a yarn”, “to spin a golf ball”,
respectively). We compared the laterality of maximum mouth openings (mouth
opened wider at the right or left side or equally opened) in right-handed, male
participants during metaphorical and concrete explanations. In line with previous
research (Graves & Landis, 1985; Graves et al., 1982), we expect an overall
right-side bias of maximum mouth openings in the explanation of phrases,
suggesting the role of the language-dominant left hemisphere during speech
production. Crucially, we hypothesized that if metaphor production particularly
involves the right hemisphere, the right-side bias of maximum mouth openings
will be reduced when participants explain metaphorical compared to concrete
phrases.
In addition, we investigated whether the relative right-hemispheric involvement during the metaphorical task is particularly pronounced for the production
of content words (e.g., verbs and nouns) compared to function words (e.g.,
conjunctions and determiners). This is plausible; first, because content words
carry relatively more semantic information, thus presumably the meaning related
to metaphor, while function words are less semantically rich, and subserve
structural functions (Bradley & Garrett, 1983; Hinojosa et al., 2001). In addition,
content words are less lateralized than function words. For example, Mohr,
Pulvermuller, and Zaidel (1994) used the divided visual field technique in a
lexical decision task (content and function words, non-words), and showed that
function words presented in the right visual field were processed faster than
when presented in the left. Thus suggesting that the processing of function words
relies heavily on the left hemisphere. On the contrary, a clear visual field
advantage was not found for the processing of content words. In addition,
Bradley and Garrett (1983) showed that content and function words are identified
equally accurately when presented in the right visual field. However, function
words presented in the left visual field were identified less accurately than
content words presented in the same field. These findings suggest that content
words are bilaterally processed in left and right hemispheres, while function
words seem to be strongly left hemispheric lateralized. The present study tested
whether the relative involvement of the right hemisphere during metaphor
production and, thus, the expected reduction in the right-sided mouth asymmetry
METAPHOR PRODUCTION AND RIGHT-HEMISPHERE
7
during metaphor compared to concrete explanations is driven by semantically
rich content words. Crucially, we hypothesized that if semantics is central for the
right-hemispheric involvement for metaphorical speech production, the reduced
right-sided mouth asymmetry in the metaphorical task compared to the concrete
task will be particularly pronounced for the production of content words.
MATERIALS AND METHODS
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Participants
Twenty-eight subjects (age: M = 19.5 years, SD = 1.9) took part in the
experiment for a course credit or payment of £2. All participants were male,
right-handed, native English speakers, monolinguals before the age of 5 years
and students at the University of Birmingham. We focused on males only
because their bilateral representation of language processing is less frequent
compared to females (McGlone, 1980). Handedness was assessed with a 12items questionnaire based on the Edinburgh Handedness Inventory (Oldfield,
1971). Two bimanual items (from Oldfield’s long list) were added to his
recommended 10-items questionnaire to equate the number of unimanual and
bimanual items. Each “left” answer was scored with 0, each “either” answer with
0.5 and each “right” answer with 1. A total score of 8.5 and above determined
right-handedness (M = 10.98, SD = .97). Text S1 in the Supplementary Material
file available online includes the questionnaire. None of the participants had any
previous serious injury to the face or jaw.
Stimuli
The stimuli were three phrases for the metaphorical and three for the concrete
condition. There was one “backup” phrase for each condition in case participants
could not recognize one of the main stimuli. The metaphorical stimuli were
English idiomatic expressions with metaphorical meanings (e.g., “to pour oil
onto the fire”). The concrete stimuli were matched to the metaphorical ones to
refer to a physical event similar to the literal meaning of the metaphorical phrases
(e.g., “to pour oil into the pan”). See Table 1 for the complete list of stimuli. Ten
participants explained the reserve item for the metaphorical and concrete
conditions.
Procedure
Participants were tested individually. They were seated on a chair, which was
located between two tables of the same height (71 cm tall) and were asked to
keep both hands still on specified marks (white sticky dots) on the tables
throughout the task. Hand prohibition was a necessary experimental control in
8
ARGYRIOU, BYFIELD, KITA
TABLE 1
Complete list of stimuli for the metaphorical and concrete conditions. The first three
items in each column are the main items. The items in parentheses are reserve items
used when the participants did not know the main items
Metaphorical phrases
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To pour oil onto the fire
To set your sights higher
To spin a yarn
(To hit the nail on the head)
Concrete phrases
To pour oil into a pan
To put a shelf higher
To spin a golf ball
(To hit someone on the head)
order to collect a laterality measurement without the influence of gestural hand
movements as gestures are sensitive to the division of labour between the two
hemispheres in speaking tasks (Kita, de Condappa, & Mohr, 2007). The
experimenter was standing and facing the participant, and the video camera
recording participants’ responses (Sanyo HD camera) was placed in front of the
experimenter. Video-recording zoomed-in on the face area. Stimuli were
presented one by one on a white sheet of paper (72 Times New Roman),
which was held by the experimenter until the participant started giving their
response.
Participants were instructed to explain the meaning of the phrases as if they
were explaining it to a non-native English speaker. To encourage metaphorical
thinking in the metaphor condition, participants were instructed to include an
explanation as to how the literal meaning can be mapped on to the metaphorical
meaning of the phrase and to give as much detail as possible (e.g., in the
expression “to spin a yarn”, “yarn” refers to a long, complicated story, and
“spinning” refers to creating this story). For the concrete phrases, participants
were instructed to paraphrase the phrase, using synonyms and give as much
detail as possible (see Table 2 for examples of the explanations participants
produced). The order of the conditions (metaphorical–concrete) was counterbalanced across participants. At the end of the task, participants were debriefed
about the purpose of the study.
Maximum mouth openings coding
The video recordings were analysed using ELAN software (developed by the
Max Planck Institute for Psycholinguists, Nijmegen, the Netherlands). Each
video was analysed on a frame-by-frame basis to identify the maximum mouth
openings in each phrase explanation. One maximum opening was defined as the
widest point the mouth opens since the lips open to the lips resting or the lips
meeting completely. We coded the laterality at each maximum mouth opening.
The options for laterality classification were right-side dominant (the right side of
the mouth opens wider than the left), left-side dominant (the left side of the
METAPHOR PRODUCTION AND RIGHT-HEMISPHERE
9
TABLE 2
Examples of produced explanations for each linguistic task
Concrete explanations
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“To spin a golf ball, the golf ball is a ball you hit
and try and get it in the hole, it is a small ball
normally white and to spin it is to rotate it
round”.
“To pour oil into a pan would mean that you take
a bottle of liquid that originated from a kind of
plant or fuel source and you tip the container
intoa pan which is a cooking utensil”.
Metaphorical explanations
“To spin a yarn, that is to tell a story, the
spinning implies you are making it up as you go
along as if you are spinning cotton and the yarn
is the story that you are making up”.
“To pour oil onto the fire, if you pour oil into the
fire it’s going to make it spark up so if there is a
situation where your anger is firing, to pour oil
into the fire would be to stir things up and make
it even more ferocious”.
mouth opens wider than the right) or sides equally open (see Figure 1 for
examples). Maximum openings for filled pauses (e.g. “eerm”) were coded but
not included in the analysis, neither were the ones whilst participants were
repeating the phrase to be explained in the beginning of each trial. We coded 60
maximum mouth openings (or as many as possible if less than 60 were available
for coding because verbal responses were short) per condition per participants (in
total, we coded 1,549 mouth openings in the concrete task and 1,517 in the
metaphorical task). Text S2 in the Supplementary Material file available online
presents the coding manual.
One individual “blind” coder was trained and coded 26% of the data. Mouth
openings from seven randomly selected participants were coded in terms of right,
left or equal sided mouth asymmetry (in total, 798 maximum openings were
double coded). Coding of mouth laterality matched between the two coders 84%
of the time (Cohen’s κ = .705, p < .001).
Word-class coding
The word produced during each maximum mouth opening was coded as being
“content” or a “function” word. The following grammatical classes were used to
determine a content word: verbs (excluding auxiliary verbs), nouns, adjectives
and adverbs. The following grammatical classes were used to determine a
function word: determiners, conjunctions, auxiliary verbs and pronouns (see
Table 3 for examples). Note that we did not include openings produced with and
prepositions in the analysis because of their dual role as both function and
content words (e.g., “want to achieve”, the preposition “to” does not carry
meaning thus is a function word; “add to a situation”, the preposition “to” is a
content word which carries spatial meaning).
10
ARGYRIOU, BYFIELD, KITA
Figure 1. (From left to right) Examples of equal, right-side dominant and left-side dominant maximum
mouth openings. “Right” and “Left” refer to the speakers’ right and left.
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Measurement and design
A right-sided mouth asymmetry index was computed for each participant in each
linguistic task based on the laterality (right-R, left-L and equal-E) of participants’
first 20 maximum mouth openings per trial: (R – L)/(R + L+ E) (adopted from
Holowka & Petitto, 2002). Mean scores were calculated for each task
(metaphorical vs. concrete) and for each word-class (content vs. function).
Thus, a positive mean score indicated more instances of right-side dominant
mouth openings (left-hemispheric lateralization) and a negative mean score
indicated more instances of left-side dominant mouth openings (right-hemispheric lateralization). We compared the right-sided mouth asymmetry index in
the metaphorical and the concrete task and for the production of different wordclasses (content vs. function) in each task.
RESULTS
We coded 3,066 maximum mouth openings across participants (1,549 in concrete
and 1,517 in metaphorical task). On average, for each participant, we coded
55.32 (SD = 7.66) mouth openings in the concrete task and 54.18 (SD = 10.61)
in the metaphorical task. Though we aimed to code 60 mouth openings per
condition per participant, the means were less than 60 because some participants
gave short explanations and thus we could only obtain less than 60 mouth
opening per condition. Furthermore, some mouth openings were excluded from
the analysis due to the low visual clarity of the recording (i.e., 33 mouth
openings in the concrete task and 49 in the metaphorical task were coded as
“unclear”). Out of the 3,066 mouth openings which were coded, we further
TABLE 3
Examples of words classified as content or function words
Word type
Content words
Function words
Examples
Aim, keep, structure, higher, constantly
Determiners: A(n), another, any, some, the
Conjunctions: And, if, or, so
Auxiliary verbs: Are, be, being, could, do
Prounouns: I, it, that (is), those, yourself
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METAPHOR PRODUCTION AND RIGHT-HEMISPHERE
11
excluded from the analysis 253 openings produced with filler pauses (e.g., “eerm”)
and 240 openings produced with prepositions (e.g., up, to). Out of 1,248 maximum
openings which were included in the analysis from the concrete task, 65% were
produced with content and 35% with function words. Similarly, out of the 1,325
maximum mouth openings in the metaphorical task, 67% were produced with
content and 33% with function words (see Table 4 for means). The proportion of
content words in the concrete task did not differ significantly compared to the
proportion of content words in the metaphorical task, t(27) = –1.425, p = .166.
Therefore, the proportion of each word-class (content vs. function) is comparable
for each linguistic task (concrete vs. metaphorical).
First, we compared the number of mouth openings included in the analyses to
follow. The number of mouth openings is comparable for each linguistic task
(concrete vs. metaphorical), t(27) = –1.662, p = .108. See Table 5 for average
proportion of mouth openings coded (equal, left-dominant and right-dominant)
and included for the calculation of the laterality index for each condition
(concrete and metaphorical) and type of word (content and function).
In addition, we compared the mean length of the explanations in each
condition (see Table 4 for means). Explanations produced in the concrete task
were significantly shorter than metaphorical explanations, t(27) = –2.79, p < .05.
However, there was no significant correlation (p > .05) between the right-sided
mouth asymmetry and the length of explanations in either task (concrete and
metaphorical). Therefore, there is no evidence that any mouth asymmetry
difference between the two tasks could be caused by the length of explanations.
We also compared the mean word length (i.e., the number of letters) in each
word-class (see Table 4 for means). As expected (Gordon & Caramazza, 1982)
function words were significantly shorter than content words, t(27) = –16.054,
p < .001. However, there was no significant correlation (p > .05) between the
right-sided mouth asymmetry and the word length in either task (concrete and
metaphorical). Therefore, there is no evidence that any mouth asymmetry
difference between the two word-classes could be caused by word lengths.
TABLE 4
Mean number of words coded in each linguistic task and word class, the mean word
lengths (i.e., the number of letters) for the coded words and the mean word count per
explanation (i.e., the length of explanation) in each linguistic task. The means are all
across participants. The numbers in brackets represent the standard deviation
Concrete task
Content
words
Number of words coded
Word length
Length of explanation (word count)
Function
words
20.07 (6.90)
15.5 (4.09)
5.19 (.59)
3.18 (.57)
37.31 (9.93)
Metaphorical task
Content
words
Function
words
31.85 (7.77) 15.46 (5.85)
6.14 (.61)
3.59 (.76)
44.04 (12.0)
12
ARGYRIOU, BYFIELD, KITA
TABLE 5
Mean proportion of coded mouth openings (equal, left-dominant and right-dominant)
and included in the analyses for each linguistic task (concrete and metaphorical) and
word type (content and function). The means are all across participants. The numbers in
brackets represent the standard deviation
Concrete task
Content words
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Function words
Equal
Left-dominant
Right-dominant
Equal
Left-dominant
Right-dominant
.13
.06
.47
.07
.02
.25
(.09)
(.07)
(.12)
(.07)
(.03)
(.09)
Metaphorical task
.12
.18
.38
.06
.06
.20
(.09)
(.09)
(.14)
(.04)
(.05)
(.07)
Then, we analysed whether mouth openings were right-side dominant. The
right-sided mouth asymmetry index (as described in section Measurement and
design) was significantly larger than zero in the concrete condition for content (t
(27) = 12.726, p < .001) and function words (t(27) = 11.890, p < .001) and in the
metaphorical condition for content (t(27) = 5.089, p < .001) and function words
(t(27) = 7.081, p < .001; see Figure 2 for the means). Thus, speech production, in
general, relies on left-hemisphere processing.
Next, we analysed whether mouth-opening asymmetry differed between the
two linguistic tasks and during the production of the two different word-classes. A
2 × 2 repeated measures within-subjects analysis of variance (ANOVA) was
performed on the right-sided mouth asymmetry index with linguistic task (concrete
vs. metaphorical) and word-class (content vs. function) as the independent
variables. This yielded a significant main effect of linguistic task (concrete
vs. metaphorical), F(1, 27) = 34.638, p < .001, partial η2 = .562. As predicted,
participants demonstrated a significantly lower right-side bias in mouth openings
during metaphorical explanations compared to the concrete ones (see Figure 2). In
addition, there was a significant main effect of word-class (content vs. function),
F(1, 27) = 4.994, p = .034, partial η2 = .156. In particular, participants
demonstrated a significantly lower right-side bias in mouth openings when they
produced content compared to function words (see Figure 2). Finally, there was
significant interaction between linguistic task and word-class, F(1, 27) = 5.322,
p = .029, partial η2 = .165. This indicates that the linguistic task had different effect
on right-sided mouth asymmetry, depending on what class of word (content vs.
function) people produced. Post-hoc t-tests with Bonferroni-corrected alpha level
(p < .0125) between conditions indicated that right-sided mouth asymmetry was
significantly lower in the metaphorical task than the concrete task for content
words (t(27) = –6.679, p < .001) and for function words (t(27) = –3.306,
p = .003); right-sided mouth asymmetry was marginally lower for content
words than function words during the metaphorical task (t(27) = –2.791,
METAPHOR PRODUCTION AND RIGHT-HEMISPHERE
13
Mean right-sided mouth asymmetry index
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0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
Concrete task
Metaphorical task
Content words
Concrete task
Metaphorical task
Function words
Figure 2. Mean right-sided mouth asymmetry index (R – L)/(R + L + E) per linguistic task and wordclass produced, where R = right-side dominant mouth opening, L = left-side dominant, E = lips equally
opened. The larger value indicates stronger right-side dominance in mouth openings, thus stronger lefthemispheric specialization. Error bars represent the standard errors.
p = .010), but not during the concrete task (t(27) = –.181, p = .857; see
Figure 2). Thus, the interaction is because the task effect (i.e., reduced rightsided mouth asymmetry in the metaphorical task) is larger for content words
than for function words. As evident in Table 5, the right-sided mouth
asymmetry is lower in the metaphorical task because the right-side dominant
openings decrease and the left-side dominant openings increase.
The next analysis aimed to further support that the differences in mouth
asymmetry were resulting from the manipulation of the variable in interest
(metaphor vs. concrete) rather than the words produced. Thus, we focused on
words that appeared in both concrete and metaphorical conditions at least once.
The analysis included 613 content word tokens (49% of all content word tokens
produced) and 777 function word tokens (59% of all function word tokens
produced; see Text S3 in Supplementary Material for a full list of the words and
their token frequencies in each condition). The analysis was limited to 682
maximum mouth openings in the concrete task and 708 in the metaphorical task.
Results remained the same. The 2 × 2 repeated measures within-subjects
ANOVA yielded a significant main effect of linguistic task (concrete vs.
metaphorical), F(1, 27) = 24.175, p < .001, partial η2 = .472. Participants
demonstrated a significantly lower right-side bias in mouth openings during
metaphorical explanations compared to the concrete ones. In addition, there was
a marginally significant main effect of word-class (content vs. function), F(1, 27)
= 4.015, p = .05, partial η2 = .129. Participants demonstrated a significantly
lower right-side bias in mouth openings when they produced content compared
14
ARGYRIOU, BYFIELD, KITA
to function words. Finally, there was a significant interaction between linguistic
task and word-class, F(1, 27) = 5.947, p = .022, partial η2 = .181. In summary,
the pattern of results remained the same as in the previous analysis. Thus there is
no evidence that the effects are driven by the words spoken uniquely in the
metaphorical or concrete condition (Figure 3).
The present study investigated whether metaphor processing particularly
involves the right hemisphere such that it reduces the right-side bias in mouth
openings during metaphorical speech production. First, we compared speakers’
mouth asymmetry during explanation of phrases with metaphorical and concrete
meanings. The mouth opened more widely on the right side during speaking in
both the metaphorical and concrete conditions, suggesting the involvement of the
left hemisphere during speech production. However, the right-sided mouth
asymmetry significantly reduced in the metaphor compared to the concrete task.
In addition, we crucially showed that the reduced right-sided mouth asymmetry
during metaphorical explanation, as compared to concrete explanations, is
particularly pronounced during the production of content words than that of
function words. We propose that semantics is crucial for the right-hemispheric
involvement in metaphorical speech production.
0.80
Mean right-sided mouth asymmetry index
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DISCUSSION
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
Concrete task
Metaphorical task
Content words
Concrete task
Metaphorical task
Function words
Figure 3. This analysis included only the words produced at least once in both concrete and metaphorical
task. Mean right-sided mouth asymmetry index (R – L)/(R + L + E) per linguistic task and word-class
produced, where R = right-side dominant mouth opening, L = left-side dominant, E = lips equally opened.
The larger value indicates stronger right-side dominance in mouth openings, thus stronger left-hemispheric
specialization. Error bars represent the standard errors.
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METAPHOR PRODUCTION AND RIGHT-HEMISPHERE
15
The present findings are in line with the right hemisphere hypothesis for
metaphor (e.g., Brownell et al., 1990) according to which the right hemisphere is
predominantly involved in metaphor processing. Although several studies
manipulated linguistic content (literal vs. non-literal stimuli) to assess the neural
basis for metaphor processing (e.g., Anaki et al., 1998; Brownell et al., 1990),
only one study (Benedek et al., 2014) has explored the involvement of the right
hemisphere during metaphorical speech production. The “open-endedness” and
the description of the metaphorical mapping in the current task was effective as it
revealed the differential hemispheric involvement between metaphor and literal
explanations. Participants in this task were free to choose from a wide range of
possible responses. This “semantic exploration” between possible meanings is
crucial for metaphorical processing, which entails the creation of semantic link
between otherwise distant concepts (Jung-Beeman, 2005). Therefore, this task
was sensitive to capture the crucial element for the right-hemispheric involvement for metaphor processing. Furthermore, the study of metaphor production
during an online task (as opposed to passive tasks of covert reading and
comprehension) offers a new approach to how speakers develop new ideas,
which is important to communication per se and theories about creative cognition
(i.e., Benedek et al., 2014; Dietrich & Kanso, 2010).
Moreover, the present study is in accordance with research on the involvement of each hemisphere for the representation of content and function words
(Mohr et al., 1994). For example, the present study found that for content words,
the right-sided mouth asymmetry was significantly smaller during metaphorical
explanations than during concrete explanations. Therefore, suggesting that the
hemispheric involvement for the production of content words can be determined
by the semantic meaning they carry. When content words are produced to
represent concepts related to metaphorical concepts, as opposed to concrete and
literal concepts, the right hemisphere is particularly involved. First, this finding
validates our initial hypothesis that semantics is crucial for the reduced rightsided mouth asymmetry in metaphorical as compared to literal speech production. In addition, it is compatible with observations that content words are
bilaterally represented, thus do not demonstrate a processing advantage when
presented in either (left or right) visual field (e.g., Bradley & Garrett, 1983; Mohr
et al., 1994).
Not only for content words but also for function words, the right-sided mouth
asymmetry reduced for metaphor explanation. This may be because function
words also carry semantic information related to metaphor, albeit less substantially than content words. For example, pronouns classified as function words
may refer to content words in preceding discourse. If the content words’ meaning
has been processed in the right hemisphere, the right hemisphere may also play
an important role in producing a subsequent coreferential pronoun. In addition, it
is possible that left-hemisphere involvement by content words during metaphor
explanations was carried over to the production of function words as well. For
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16
ARGYRIOU, BYFIELD, KITA
example, when a function word is produced in a sequence of content words
within an utterance, it is possible that the right-hemisphere involvement may be
carried over to the function word. This is possible if it incurs processing cost to
switch on and off the right hemisphere’s involvement in speech production.
Then, even if it is not efficient to process a function word with the righthemisphere’s involvement, it may sometimes be overall more efficient to keep
the right-hemisphere’s involvement (not to switch it on and off too often).
In addition, present results are compatible with previous studies on taskdependent mouth asymmetry. Mouth asymmetry studies have shown that tasks
involving right-hemisphere processes (e.g., emotional tasks and automatic
speech) lead to reduced right-sided asymmetry in mouth opening. For example,
right-sided asymmetry was reduced when spontaneously smiling compared to
generating word lists (Graves et al., 1982), and it was also smaller when singing
and counting (serial speech) than naming pictures and spontaneously speaking
(propositional speech; Graves & Landis, 1985). The present study is the first
study to show the same effect for metaphor. Thus, this study further validated
mouth asymmetry as an indicator of lateralization of processes underlying
various communication behaviours.
But what exactly is happening in the two brain hemispheres during
metaphorical explanation? We may speculate based on our current findings and
also in light of metaphor theories. Metaphor is a way of speaking about one
conceptual domain in terms of another (Lakoff & Johnson, 1980). In particular,
during metaphorical explanation, speakers explain the metaphorical mapping of a
concrete concept (source domain of metaphor) onto a more abstract one (target
domain of metaphor; e.g., when explaining the phrase “to spin a yarn”, the
spinning represents the elaborate creation and narration of a story). This specific
process of mapping during metaphorical processing is essentially the speaker’s
effort to bring closer two semantically distant concepts (i.e., the action of
spinning and the action of narrating). Such semantic processes are an instance of
the processing of coarse semantic links, which is more strongly represented in
the right hemisphere than the left (following the fine–coarse coding theory; JungBeeman, 2005). Crucially, the current study found a significant interaction
between linguistic task and word-class. That is, the right-sided mouth asymmetry
was significantly lower when participants explained metaphorical phrases than
concrete phrases, and this difference was particularly pronounced for production
of content words. Presumably, when speakers produced content words for the
explanation of metaphors, they produced words which carry semantic information related to the metaphorical mapping. For example, in the phrase “to spin a
yarn”, a source domain concept, “objects (yarn)”, maps to a target domain
concept, “story”. Through the metaphorical mapping, some attributes can also be
mapped from the source to the target domain. So, “a complicated (content word)
object like a yarn is used to represent a complicated story”. This mapping is
lexically encoded more often with content words compared to function ones
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METAPHOR PRODUCTION AND RIGHT-HEMISPHERE
17
because function words do not carry enough semantic content to allow for the
representation of abstract concepts in the form of concrete senses (GonzálvezGarcía, Peña-Cervel, & Pérez-Hernández, 2013). Therefore, we propose that
during metaphorical speech production semantics might be what determines the
relative involvement of the right hemisphere.
The current study used so-called “frozen metaphors” in idiomatic phrases,
which in some studies did not involve the right hemisphere as well as novel
metaphors (Cardillo et al., 2012; Mashal et al., 2005). We argue that how much
the right hemisphere is involved in metaphor processing depends not only on the
type of stimulus materials but also on the task. The current study showed that,
with frozen metaphors, if the participants were required to explicitly think about
the metaphorical mapping between source and target domains (e.g., in the phrase
“to spin a yarn”, the yarn represents a long complicated story), the right
hemisphere got involved in the process.
In general, in the discussion of the right hemisphere hypothesis for metaphor
(e.g., Brownell et al., 1990), it may be important to carefully examine the nature
of task used in each study. For example, the fMRI study by Rapp et al. (2007)
failed to show activation of the right hemisphere whilst participants silently read
sentences (literal and non-literal) and performed metaphorical judgments (“is it a
metaphor or not?”) and connotation judgments (“does it have a positive or
negative connotation?”). The task did not require processing of the mapping
between source and target domains. For example, the metaphorical judgement
could have been made based on semantic anomaly in the literal interpretation. In
sentences such as “the director was a bulldozer”, participants could judge if the
sentence is literally plausible or not, without thinking about the metaphorical
mapping. If so, these tasks probably did not strongly activate metaphorical
thinking, thus failed to activate the right hemisphere.
The present study validates the effectiveness of the mouth asymmetry
technique and opens new doors for future research. For example, it would be
interesting to observe the sequence of mouth asymmetries as this might reveal
how the two hemispheres collaboratively produce an utterance. Mouth asymmetry is a suitable technique for such questions, which would be difficult to
answer with functional imaging techniques due to low time-resolution (fMRI) or
articulatory movement artefacts (electroencephalography — EEG). Finally,
calculating the mouth asymmetry index during metaphorical tasks could
supplement future studies with an individual-subjects localization approach and
lead to a clearer picture of the neural basis of metaphorical processing.
CONCLUSIONS
In conclusion, the reduced right-sided mouth asymmetry during metaphorical
compared to concrete explanations is particularly driven by the production of
18
ARGYRIOU, BYFIELD, KITA
content words related to metaphor; thus indicating that semantics is crucial for
the relative involvement of the right hemisphere for metaphorical speech
production. The study also validated the sensitivity of the mouth asymmetry
technique to capture the differential hemispheric involvement for different verbal
tasks, and also for different word-classes.
Supplementary material
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Supplementary contents are available via the ‘Supplementary’ tab on the article’s
online page (http://dx.doi.org/10.1080/1357650X.2014.951654).
Manuscript received 10 March
Revised manuscript received 30 July
Revised manuscript accepted 31 July
First published online 29 August
2014
2014
2014
2014
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