Selective Metaphor Impairments After Left, Not Right, Hemisphere Injury

Brain asymmetries in figurative language comprehension

This chapter reviews the literature concerning the neural basis of three types of figurative expressions, namely, idioms, metaphors, and irony. Besides these three forms of language, which are the most investigated, many other types exist, differing in their linguistic structure and, consequently, in the corresponding comprehension processes. After defining the most common figurative expressions and showing how they differ in terms of linguistic properties, the chapter presents early studies that focused on the role of the right hemisphere (RH) in figurative language comprehension in general, without a clear distinction among different forms. Later literature shows how evidence has been accumulated, suggesting that both hemispheres are involved in figurative language processing. Therefore, a sharp distinction between the left hemisphere (LH) and the RH, the first involved in literal language and the second in figurative language, is not tenable. Idioms, metaphors, and irony will be considered separately, demonstrating that different expressions rely on the LH and RH to a different degree.

Relationship Between Cognitive Estimation, Executive Functions, and Theory of Mind in Patients With Prefrontal Cortex Damage

OBJECTIVE

Conflicting evidence has arisen from the few studies that have examined the role of the prefrontal cortex and executive control functions in theory of mind (ToM). Moreover, the involvement of other cognitive domains in the ability to infer mental states is still under debate. This study aims to examine, in addition to the potential contribution of executive functions, the role of cognitive estimation in ToM abilities, given that cognitive estimation processes are strongly associated with some aspects of executive control functions.

METHOD

The cognitive estimation task, along with a set of neuropsychological tasks assessing executive functions, was administered to 30 patients with prefrontal cortex damage and 30 control subjects matched by gender, age, and education level.

RESULTS

Patients with prefrontal cortex damage were impaired in all measures of executive functions, cognitive estimation, and theory of mind compared with control subjects. Regression analysis showed a significant interaction between executive measures and cognitive estimation in predicting ToM performance for patients with prefrontal cortex damage. Additionally, voxel-based lesion analysis identified a partially common bilaterally distributed prefrontal network involved in all three domains, centered within the ventral and dorsomedial areas with extension to the dorsolateral prefrontal cortex.

CONCLUSION

Our findings highlight that, apart from executive functions, cognitive estimation plays a crucial role in the ability to interpret others' cognitive and emotional states in both patients with prefrontal cortex damage and control subjects.

The role of cognitive estimation in understanding the mental states of others

Previous studies have emphasized the critical role of the prefrontal cortex in cognitive estimation and theory of mind, however, none of them has questioned the possible role of cognitive estimation processes in understanding the mental states of others. In this study, we compared 30 patients with focal prefrontal cortex damage and 30 control subjects matched by gender, age, and education level on their performances on a cognitive estimation task and two tasks assessing theory of mind: the "Faux-Pas" task and the Reading the Mind in the Eyes task. The results showed that patients were significantly impaired compared with control subjects on both abilities of cognitive estimation and theory of mind. Moreover, regression analyses showed that performance on theory of mind was predicted by the scores on cognitive estimation. Finally, using voxel-based lesion analysis, we identified a partially common bilaterally distributed prefrontal network involved in both these domains centred within the ventral and dorsomedial areas with extension to the dorsolateral prefrontal cortex.

Exploring Pragmatic Deficits in Relation to Theory of Mind and Executive Functions: Evidence from Individuals with Right Hemisphere Stroke

Research investigating pragmatic deficits in individuals with right hemisphere damage focuses on identifying the potential mechanisms responsible for the nature of these impairments. Nonetheless, the presumed shared cognitive mechanisms that could account for these deficits have not yet been established through data-based evidence from lesion studies. This study aimed to examine the co-occurrence of pragmatic language deficits, Theory of Mind impairments, and executive functions while also exploring their associations with brain lesion sites. Twenty-five patients suffering from unilateral right hemisphere stroke and thirty-seven healthy participants were recruited for this study. The two groups were tested in pragmatics, Theory of Mind, and executive function tasks. Structural imaging data were also obtained for the identification of the lesion sites. The findings of this study suggest a potential convergence among the three aforementioned cognitive mechanisms. Moreover, we postulate a hypothesis for a neural circuitry for communication impairments observed in individuals with right hemisphere damage.

Right hemisphere and metaphor comprehension: A connectionist perspective

Metaphor comprehension is a cognitively complex task, with evidence pointing to the engagement of multiple cerebral areas. In addition, the involvement of the right hemisphere appears to vary with cognitive effort. Therefore, the interconnecting pathways of such distributed cortical centers should be taken into account when studying this topic. Despite this, the potential contribution of white matter fasciculi has received very little attention in the literature to date and is not mentioned in most metaphor comprehension studies. To highlight the probable implications of the right inferior fronto-occipital fasciculus, right superior longitudinal system, and callosal radiations, we bring together findings from different research fields. The aim is to describe important insights enabled by the cross-fertilization of functional neuroimaging, clinical findings, and structural connectivity.

Non-literal language processing is jointly supported by the language and theory of mind networks: Evidence from a novel meta-analytic fMRI approach

Going beyond the literal meaning of language is key to communicative success. However, the mechanisms that support non-literal inferences remain debated. Using a novel meta-analytic approach, we evaluate the contribution of linguistic, social-cognitive, and executive mechanisms to non-literal interpretation. We identified 74 fMRI experiments (n = 1,430 participants) from 2001 to 2021 that contrasted non-literal language comprehension with a literal control condition, spanning ten phenomena (e.g., metaphor, irony, indirect speech). Applying the activation likelihood estimation approach to the 825 activation peaks yielded six left-lateralized clusters. We then evaluated the locations of both the individual-study peaks and the clusters against probabilistic functional atlases (cf. anatomical locations, as is typically done) for three candidate brain networks-the language-selective network (Fedorenko, Behr, & Kanwisher, 2011), which supports language processing, the Theory of Mind (ToM) network (Saxe & Kanwisher, 2003), which supports social inferences, and the domain-general Multiple-Demand (MD) network (Duncan, 2010), which supports executive control. These atlases were created by overlaying individual activation maps of participants who performed robust and extensively validated 'localizer' tasks that selectively target each network in question (n = 806 for language; n = 198 for ToM; n = 691 for MD). We found that both the individual-study peaks and the ALE clusters fell primarily within the language network and the ToM network. These results suggest that non-literal processing is supported by both i) mechanisms that process literal linguistic meaning, and ii) mechanisms that support general social inference. They thus undermine a strong divide between literal and non-literal aspects of language and challenge the claim that non-literal processing requires additional executive resources.

Hemisphere stroke: impact on the semantic lexical aspects of language

OBJECTIVE

To investigate if right hemisphere stroke is associated with decreased lexical-semantic ability.

METHODS

We compared subjects with right hemisphere brain damage after stroke with healthy controls using the validated version of the Montreal Evaluation of Communication to assess lexical-semantic domains of language.

RESULTS

Sixteen patients and thirty-two controls were included. The post-stroke group with right hemispheric brain lesions showed considerable limitations in metaphor interpretation, fluency, and semantic judgment.

CONCLUSION

Individuals with right hemisphere damage after stroke may present with language changes in the lexical-semantic aspects.

Functional anatomy of idiomatic expressions

Idiomatic expressions (IE) are groups of words whose meaning is different from the sum of its components. Neural mechanisms underlying their processing are still debated, especially regarding lateralization, main structures involved, and whether this neural network is independent from the spoken language. To investigate the neural correlates of IE processing in healthy Spanish speakers.Twenty one native speakers of Spanish were asked to select one of 4 possible meanings for IE or literal sentences. fMRI scans were performed in a 3.0T scanner and processed by SPM 12 comparing IE vs. literal sentences. Laterality indices were calculated at the group level. IE activated a bilateral, slightly right-sided network comprising the pars triangularis and areas 9 and 10. In the left hemisphere (LH): the pars orbitalis, superior frontal, angular and fusiform gyrus. In the right hemisphere (RH): anterior insula, middle frontal, and superior temporal gyrus. This network reveals the importance of the RH, besides traditional LH areas, to comprehend IE. This agrees with the semantic coding model: the LH activates narrow semantic fields choosing one single meaning and ignoring others, and the RH detects distant semantic relationships, activating diffuse semantic fields. It is also in line with the configuration hypothesis: both meanings, literal and figurative, are executed simultaneously, until the literal meaning is definitively rejected and the figurative one is accepted. Processing IE requires the activation of fronto-temporal networks in both hemispheres. The results concur with previous studies in other languages, so these networks are independent from the spoken language. Understanding these mechanisms sheds light on IE processing difficulties in different clinical populations and must be considered when planning resective surgery.

Sensitive Measures of Cognition in Mild Cognitive Impairment

BACKGROUND

Sensitive measures of cognition are needed in preclinical and prodromal Alzheimer's disease (AD) to track cognitive change and evaluate potential interventions. Neurofibrillary tangle pathology in AD is first observed in Brodmann Area 35 (BA35), the medial portion of the perirhinal cortex. The importance of the perirhinal cortex for semantic memory may explain early impairments of semantics in preclinical AD. Additionally, our research has tied figurative language impairment to neurodegenerative disease.

OBJECTIVE

We aim to identify tasks that are sensitive to cognitive impairment in individuals with mild cognitive impairment (MCI), and that are sensitive to atrophy in BA35.

METHODS

Individuals with MCI and cognitively normal participants (CN) were tested on productive and receptive experimental measures of semantic memory and experimental tests of figurative language comprehension (including metaphor and verbal analogy). Performance was related to structural imaging and standard neuropsychological assessment.

RESULTS

On the experimental tests of semantics and figurative language, people with MCI performed worse than CN participants. The experimental semantic memory tasks are sensitive and specific; performance on the experimental semantic memory tasks related to medial temporal lobe structural integrity, including BA35, while standard neuropsychological assessments of semantic memory did not, demonstrating the sensitivity of these experimental measures. A visuo-spatial analogy task did not differentiate groups, confirming the specificity of semantic and figurative language tasks.

CONCLUSION

These experimental measures appear sensitive to cognitive change and neurodegeneration early in the AD trajectory and may prove useful in tracking cognitive change in clinical trials aimed at early intervention.

The Neural Basis of Metaphor Comprehension: Evidence from Left Hemisphere Degeneration

Despite the ubiquity of metaphor in cognition and communication, it is absent from standard clinical assessments of language, and the neural systems that support metaphor processing are debated. Previous research shows that patients with focal brain lesions can display selective impairments in processing metaphor, suggesting that figurative language abilities may be disproportionately vulnerable to brain injury. We hypothesized that metaphor processing is especially vulnerable to neurodegenerative disease, and that the left hemisphere is critical for normal metaphor processing. To evaluate these hypotheses, we tested metaphor comprehension in patients with left-hemisphere neurodegeneration, and in demographically matched healthy comparison participants. Stimuli consisted of moderately familiar metaphors and closely matched literal sentences sharing the same source term (e.g., The interview was a painful crawl / The infant's motion was a crawl). Written sentences were presented, followed by four modifier-noun answer choices (one target and three foils). Healthy controls, though reliably better at literal than metaphor trials, comprehended both sentence conditions well. By contrast, participants with left-hemisphere neurodegeneration performed disproportionately poorly on metaphor comprehension. Anatomical analyses show relationships between metaphor accuracy and patient atrophy in the left middle and superior temporal gyri, and the left inferior frontal gyrus, areas that have been implicated in supporting metaphor comprehension in previous imaging research. The behavioral results also suggest deficits of metaphor comprehension may be a sensitive measure of cognitive dysfunction in some forms of neurodegenerative disease.

Impairment in oral medical comprehension in multiple sclerosis

In multiple sclerosis (MS), medical comprehension of website information and informed consent is reported to be impaired. The aim of the present study was to investigate oral medical comprehension of literal, figurative and humorous language in MS through videos of physician-patient exchanges. A group of 35 MS patients was compared to a control group (38 healthy participants). Participants were shown twelve filmed sketches consisting of a patient's question followed by the doctor's response and had to choose the meaning of the physician's response among three possibilities. Group analyses (non-parametric tests, Mann-Whitney) revealed significantly lower scores for MS patients compared to controls for figurative and humorous items. The opposite was observed for literal items. At the individual level, 17% of MS patients were impaired in their comprehension of figurative items and 32% for humorous items. Among them, 20% were impaired in both categories, 20% selectively in figurative comprehension and 60% selectively in humorous comprehension. These preliminary results suggest that a high proportion of MS patients (40%) may be impaired in the non-literal comprehension of simple medical responses. The better performances obtained for literal items suggest a more concrete linguistic decoding in MS. On a theoretical level, the double dissociations shown between figurative and humorous items in patients are not compatible with a serial model of linguistic processing.

Context matters: Novel metaphors in supportive and non-supportive contexts

Creative language is defined as linguistic output that is both novel and appropriate. Metaphors are one such example of creative language in which one concept is used to express another by highlighting relevant semantic features. While novelty is an inherent property of unfamiliar metaphors, appropriateness depends on the context. The current study tests the hypothesis that the context in which metaphors are encountered affects their processing. We examined the neural effects of comprehending metaphors in context by comparing neural activations in response to novel metaphors and literal sentences that were either embedded in a meaningful narrative or in matched jabberwocky contexts. We found that the neural correlates of processing metaphoric sentences and their literal counterparts are indistinguishable when embedded in a narrative: both conditions activate bilateral areas along the anterior temporal poles, middle temporal gyri, superior temporal sulci, and the angular gyri. Metaphors embedded in a narrative as compared to their identical counterparts embedded in jabberwocky show increased responses in sensorimotor areas that correspond to the modality of the literal meaning of the target word, perhaps reflecting deeper semantic processing. Our results confirm that context affects neural mechanisms for understanding creative ideas.

From action to abstraction: The sensorimotor grounding of metaphor in Parkinson's disease

Embodied cognition theories propose that the semantic representations engaged in during language comprehension are partly supported by perceptual and motor systems, via simulation. Activation in modality-specific regions of cortex is associated with the comprehension of literal language that describes the analogous modalities, but studies addressing the grounding of non-literal or figurative language, such as metaphors, have yielded mixed results. Differences in the psycholinguistic characteristics of sentence stimuli across studies have likely contributed to this lack of consensus. Furthermore, previous studies have been largely correlational, whilst patient studies are a critical way of determining if intact sensorimotor function is necessary to understand language drawing on sensorimotor information. We designed a battery of metaphorical and literal sentence stimuli using action and sound words, with an unprecedented level of control over critical psycholinguistic variables, to test hypotheses about the grounding of metaphorical language. In this Registered Report, we assessed the comprehension of these sentences in 41 patients with Parkinson's disease, who were predicted to be disproportionately affected by the action sentences relative to the sound sentences, and compared their performance to that of 39 healthy age-matched controls who were predicted to show no difference in performance due to sensory modality. Using preregistered Bayesian model comparison methods, we found that PD patients' comprehension of literal action sentences was not impaired, while there was some evidence for a slowing of responses to action metaphors. Follow up exploratory analyses suggest that this response time modality effect was driven by one type of metaphor (predicate) and was absent in another (nominal), despite the fact that the action semantics were similar in both syntactic forms. These results suggest that the conditions under which PD patients demonstrate hypothesized embodiment effects are limited. We offer a critical assessment of the PD action language literature and discuss implications for the embodiment debate. In addition, we suggest how future studies could leverage Bayesian statistical methods to provide more convincing evidence for or against embodied cognition effects.