Hemispheric involvement for metaphor processing

Hemispheric involvement for metaphorical processing has attracted attention in research the past thirty years. This is because metaphors are fundamental elements of speech and thought. Hence, understanding the underlying neural mechanisms for metaphorical processing is relevant to understanding the mechanisms for linguistic processes, verbal creativity and complex cognition. The current thesis draws upon the weak version of the Right Hemisphere Hypothesis for Metaphor (e.g., Bottini et al., 1994), which suggests the co- ordination and divided labour in the two brain hemispheres for different components of semantic processing. That is, both hemispheres contribute to language processing, but the left hemisphere is particularly important for fine (i.e., links between strong and focused semantic features) semantic processing while the right for the coarse (i.e., large semantic fields and links between distant and unusual semantic features) semantic processing (Jung-Beeman, 2005). This ability of the right hemisphere to bring together coarse semantic associations underlies metaphorical processing. However, we acknowledge that some studies failed to support a particular involvement of the right hemisphere for metaphorical processing (e.g., Rapp, Leube, Erb, Grodd, & Kircher, 2004; Stringaris, Medford, Giampietro, Brammer, & David, 2007), and we believe that this, mainly, highlights task- and methodology-related differences.

Right Hemisphere Hypothesis for Metaphor

The first line of evidence for right-hemispheric involvement during metaphorical processing came from research with brain-damaged patients. In Winner and Gardner (1977), normal controls and patients with either right or left hemisphere lesions performed a pictorial and a verbal metaphorical task. Participants listened to a metaphorical phrase (e.g., “he has a heavy heart”). They had to match the meaning of the sentence to the appropriate picture representing the metaphorical meaning (e.g., a picture of someone crying) vs. a picture representing the literal meaning (e.g., a picture of someone carrying a heart). In addition, they had to verbally explain what the phrase means. The patients with right hemisphere lesions selected the literal version of the pictures, but they were able to accurately explain the phrases. On the other hand, patients with left hemisphere lesions accurately selected the metaphorical version of the pictures, but they sometimes literally explained them. This finding suggests that the left hemisphere is not adequate and responsible for the processing of every linguistic message, and the right hemisphere is involved but not adequate for the processing of metaphorical meaning. In another patient study, Brownell, Simpson, Bihrle, Potter, and Gardner (1990) asked patients with right and left lesions, and controls to perform a similarity judgment between triads of words, which included one target ambiguous word (e.g., “warm”), a word related to the metaphorical meaning of the target (e.g., “affectionate”), and a word related to the prior meaning of the target (e.g., “blanket”). The selection of the two metaphorically related words was considered the correct response. Patients with right hemisphere damage performed worse compared to controls and those with left hemisphere damage, suggesting that the right hemisphere is particularly involved for the evaluation of alternate, metaphorical meaning.

literal and metaphorical sentences in a PET scan study. Findings showed that comprehension of metaphorical and literal meaning is associated with similar activations in the left hemisphere, but metaphorical comprehension is associated with additional activations in right hemisphere regions (the prefrontal cortex, the middle temporal gyrus, the precuneus and the posterior cingulate). Thus, brain hemispheres are bilaterally activated for language processing, and the right hemisphere is particularly involved for metaphorical processing. In addition, Mashal and colleagues provided evidence for the Right Hemisphere Hypothesis for metaphor in several brain-imaging studies. In Mashal, Faust, and Hendler (2005) and Mashal, Faust, Hendler, and Jung-Beeman (2007) participants read Hebrew word pairs representing four semantic relationships: literal (e.g., “broken vase”), conventional metaphorical (e.g., “bright student), novel metaphorical (e.g., “crystal river”), and unrelated (e.g., “boot laundry”). They performed a silent semantic judgment task while in the fMRI scan (i.e., decide if the word pairs are literally related, metaphorically related or unrelated). Findings showed shared activation of a core bilateral network for all conditions, and a special role of the right homologue of Wernicke’s area for the novel metaphorical pairs.

Finally, behavioural studies using the divided visual field technique have provided evidence in favour of the Right Hemisphere Hypothesis. Anaki, Faust, and Kravetz (1998) combined the divided visual field technique with the semantic priming paradigm to investigate hemispheric involvement for metaphorical and literal semantic associations in Hebrew. Participants read a prime (e.g., “stinging”) and performed a lexical decision on a target, which would be either literally (e.g., “mosquito”) or metaphorically (e.g., “insult”) related to the prime. Findings for short stimulus onset asynchronies (200ms) suggested facilitation for metaphorically related targets in both hemispheres while literally related targets were facilitated only in the RVF (= left hemisphere). Findings for long stimulus onset asynchronies (800ms) suggested that metaphorically related targets were facilitated in the

right hemisphere, whereas literally related targets were facilitated in the left hemisphere. In addition, Schmidt, DeBuse, and Seger (2007) used the divided visual field technique at sentence level to investigate hemispheric involvement for metaphorical and literal semantic relationships. Participants read sentences with three different endings (e.g., “the camel is a dessert […]” “animal”-literal or “taxi”-metaphorical or “table”-anomalous). They would read the endings presented either in the right-visual field (left hemisphere) or the left-visual field (right hemisphere). They performed a semantic judgment task (i.e., respond whether the ending fits the sentence or not). Findings showed right hemisphere time advantage for metaphorical relationships and left hemisphere advantage for literal relationships. In a second divided visual field experiment, they varied familiarity of stimuli by creating conditions of literal-familiar, literal-unfamiliar and metaphorical sentences for a plausibility judgment task. Findings showed a right hemisphere time advantage for both literal-unfamiliar and metaphorical sentences, but not for literal-familiar. In a third divided visual field experiment, they varied familiarity of metaphorical stimuli only by creating four conditions of “very high, high, low and very low” metaphor familiarity in a sensibility judgment task. Findings showed right hemisphere time advantage for unfamiliar metaphors and left hemisphere advantage for familiar ones.

Some studies failed to provide support for a particular involvement of the right hemisphere during metaphorical processing. For example, in Rapp et al. (2004) participants read short, simply structured German sentences with either a metaphorical (e.g., “the lovers’ words are harp sounds”) or literal (e.g., “the lovers’ words are lies”) meaning. Participants performed an overt connotation judgment task (i.e., they decided if the sentence had a positive or negative connotation) while in the fMRI scanner. Findings showed that metaphors elicited increased BOLD contrasts in the left rather than the right hemisphere. Similarly, in

performed an additional metaphorical judgment task where they decided if the sentence had a metaphorical or literal meaning. Brain activations showed left lateralisation overall, however there were more right hemisphere regions activated for the metaphorical than the connotation judgment (though not significant). In Stringaris et al. (2007), participants read metaphorical (e.g., “some surgeons are butchers”), literal (e.g., “some surgeons are fathers”) or meaningless sentences (e.g., “some surgeons are shelves”). They performed an overt sensibility judgment task while in an fMRI scanner (i.e., they decided if the sentence made sense or not). Contrasts between the literal and metaphorical conditions failed to provide support for a predominant role of right hemispheric structures (e.g., right inferior frontal gyrus). Finally, Coulson and Van Petten (2007) assessed the lateralisation of metaphorical thinking by recording event-related potentials (ERPs). Participants read sentences that ended literally, with either high or low predictability, and metaphorically. The final part of the sentence was presented in either the left or the right visual hemifield. Findings provided no evidence for differential metaphoricity effects between hemifields, and suggested that the integration of metaphoric meanings required similar involvement of the two hemispheres.

To conclude, the empirical evidence about the hemispheric involvement during metaphorical processing reveals a heterogeneous picture. It seems that the right hemisphere has a particular role for metaphorical processing, however its involvement could vary as a function of other factors rather than metaphoricity per se. For example, familiarity of stimuli (novel vs. conventional metaphors), complexity of stimuli (word pairs vs. sentences), task and instructions (overt vs. covert responses; tasks that rely heavily on semantic processing vs. passive reading tasks), and measurements (behavioural vs. neurophysiological) could determine the degree of right hemispheric involvement (see Schmidt, Kranjec, Cardillo, & Chatterjee, 2010 for a review of studies on the neural basis of metaphor). More importantly, whether the semantic, as opposed to syntactic properties of a metaphorically used term drive

the involvement of the one or the other hemisphere has received little attention. To our knowledge, only one study directly compared the role of syntactic and semantic processing for the hemispheric involvement in metaphorical comprehension. In Cardillo, Watson, Schmidt, Kranjec, and Chatterjee (2012), participants passively read in the scanner metaphors that had the same base term used metaphorically but differed in their syntactic structure. Half of the metaphors were nominal (e.g., “the shop display was was a gentle tug”) and half were predicate (e.g., “the urgent letter tugged at her sleeve”) metaphors and crucially the nominal metaphors had a nominalised verb as a metaphor base term. All sentences were carefully matched for lexical and sentential properties (e.g., frequency, imageability, and figurativeness). Results showed no differences in the neuronal activations (e.g., the inferior frontal gyrus on the left hemisphere and its right hemisphere homolog) for the two types of metaphors that differed syntactically but were equated semantically. This result suggested that semantic rather than syntactic features are crucial to the hemispheric involvement for metaphorical comprehension. However, this study did not test a non-metaphor condition, hence interpretation is tenuous.