Hemispheric Contributions to Figurative Language Comprehension After Unilateral Brain Damage: A Meta-Analysis of Lesion Side and Conventionality Interaction.

TL;DR

Brain damage affects how we understand idioms and metaphors differently depending on which hemisphere is damaged and how conventional the expression is, but there's no simple "left vs. right" story. This meta-analysis of 45 studies finds that both hemispheres contribute to figurative language, with their relative importance shifting based on context.

What the Study Found

For decades, neuroscience has operated on a relatively tidy assumption: the left hemisphere handles literal language and grammar, while the right hemisphere manages the fuzzy pragmatic stuff like metaphors, idioms, and sarcasm. This new meta-analysis of 45 studies involving over 1,600 brain-damaged patients challenges that oversimplification, revealing instead that hemispheric contributions are dynamic and depend heavily on how conventional an expression is.

The researchers synthesized data comparing left-hemisphere-damaged (LBD) patients, right-hemisphere-damaged (RBD) patients, and healthy controls on figurative language comprehension tasks. The headline finding: no significant overall difference emerged between LBD and RBD patients in their ability to understand figurative language. This alone contradicts the traditional right-hemisphere dominance model. However, when the researchers looked at how conventionality (the degree to which an expression follows standard, predictable meanings) moderated these patterns, a more nuanced picture emerged. At high conventionality levels, LBD patients showed greater deficits than RBD patients. At low conventionality levels, the pattern reversed, with RBD patients showing greater deficits. This suggests the left hemisphere becomes more critical as expressions become more established and rule-bound, while the right hemisphere grows increasingly important for novel, unpredictable figurative uses.

The meta-regression analysis identified task type and sample size as significant moderators of hemispheric differences, meaning how researchers measured figurative language comprehension and how many participants they included substantially influenced the results. Aphasia status (whether language impairment accompanied the brain damage) and the number of trials in each task did not significantly moderate the findings, suggesting the core pattern holds across different patient populations and experimental designs. Both LBD and RBD patients were significantly impaired relative to healthy controls across all conventionality levels, indicating that figurative language comprehension genuinely depends on intact neural tissue in both hemispheres, not just one.

The authors argue these findings reshape our understanding of figurative language as a fundamentally bilateral process rather than a right-hemisphere specialty. Highly conventional expressions like "raining cats and dogs" or "the ball is in your court" appear to leverage the left hemisphere's strength in accessing stored, rule-governed meanings. Novel or low-conventionality expressions, by contrast, demand the right hemisphere's capacity to derive new meaning from unusual combinations. This distribution of labor makes computational sense: the left hemisphere can efficiently retrieve well-learned associations, while the right hemisphere flexibly generates new interpretations.

What This Means for You

If you've experienced a stroke or other brain injury affecting language, this research underscores that figurative language deficits are not a monolithic problem tied to one side of the brain. Recovery and compensation strategies may benefit from recognizing that different types of figurative expressions engage different neural systems. Speech-language pathologists designing rehabilitation protocols might consider tailoring tasks based on expression conventionality rather than assuming all metaphors or idioms require identical retraining approaches.

For the broader neuroscience community, the findings argue for moving beyond hemisphere stereotypes in clinical assessment and theory. When evaluating figurative language abilities in brain-damaged populations, clinicians should attend to both the conventionality of test materials and the specific task demands. A patient who struggles with novel metaphors but understands standard idioms presents a different clinical picture than someone showing the reverse pattern, and the hemispheric interpretation of that difference matters for prognosis and intervention.

The methodological moderators (task type and sample size) also serve as a practical reminder that how research is conducted shapes what we conclude about brain function. Large, well-controlled studies using standardized task designs may yield different estimates of hemispheric contributions than smaller studies using varied methods. This doesn't invalidate earlier work, but it highlights the importance of considering research quality and design when applying neuroscience findings to individual cases.

Study Details

Sources

Hemispheric Contributions to Figurative Language Comprehension After Unilateral Brain Damage: A Meta-Analysis of Lesion Side and Conventionality Interaction. Brain and Behavior. PubMed: 42844808