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

introductionPrevious studies revealed that individuals with brain damage exhibit distinctive patterns in figurative language comprehension. However, a systematic investigation of the mechanisms between figurative language deficits and moderating factors such as lesion side remains lacking. Moreover, while the right hemisphere has traditionally been emphasized in pragmatic processing, accumulating evidence now suggests that the contributions of the left and right hemispheres may depend on linguistic factors such as conventionality. A recent systematic review on idiom comprehension in aphasia has highlighted the critical role of stimulus characteristics, yet no quantitative synthesis has examined how conventionality modulates hemispheric contributions to figurative language processing after unilateral brain damage. This meta-analysis aims to: (1) quantify the overall difference between left- and right-hemisphere damaged (LBD and RBD) patients in figurative language comprehension accuracy; (2) examine whether conventionality moderates hemispheric differences; (3) explore potential moderators-including aphasia status, task type, and sample size-that may influence the LBD-RBD difference; and (4) compare impairment patterns of LBD and RBD patients relative to healthy controls across different conventionality levels.

methodsFollowing PRISMA guidelines, a systematic literature search was conducted across Web of Science, PubMed, and ProQuest. Studies comparing LBD and/or RBD patients to healthy controls on accuracy in figurative language comprehension tasks were included. Multi-level meta-analyses were performed to estimate overall effect sizes, examine moderating effects of conventionality, and compare impairment patterns. Meta-regressions explored potential moderators including aphasia status, sample size, task types, and number of trials.

resultsForty-five articles met inclusion criteria, with 13 articles providing direct LBD-RBD comparisons (516 LBD; 476 RBD patients). Overall, no significant main effect of hemisphere emerged. Although the interaction between conventionality and lesion side did not reach statistical significance, the pattern of effect sizes across conventionality levels revealed meaningful trends. At high conventionality, LBD patients showed greater deficits than RBD patients, while at low conventionality, RBD patients showed greater deficits. Moderator analyses further revealed that task type and sample size significantly modulated the overall LBD-RBD difference, whereas aphasia status and number of trials did not. Additional comparisons including 70 LBD versus HC studies (1120 LBD patients, 1342 HC) and 68 RBD versus HC studies (1135 RBD patients, 1291 HC) confirmed that both patient groups are significantly impaired relative to healthy controls across all conventionality levels.

conclusionHemispheric contributions to figurative language comprehension are dynamic and conventionality-dependent. The left hemisphere is particularly crucial for highly conventionalized expressions, while both hemispheres contribute to novel expressions. These findings are further qualified by methodological moderators: task type and sample size significantly modulated the observed hemispheric differences, whereas aphasia status and number of trials did not. The results have implications for neurocognitive models of figurative language and for the design of clinical assessments, highlighting the need to consider both conventionality and task demands when evaluating figurative language abilities in brain-damaged populations.