Neural correlates of morphological processing: An activation likelihood estimation meta-analysis

Background: Morphemes are the smallest building blocks of language that convey meaning or function. A controversial issue in psycho- and neurolinguistics is whether morphologically complex words consisting of multiple morphemes are processed in a combinatorial manner and, if so, which brain regions underlie this process. Relatively less is known about the neural underpinnings of morphological processing compared to other aspects of grammatical competence such as syntax. Purpose: The present study aimed to shed light on the neural correlates of morphological processing by examining functional convergence for inflectional morphology reported in previous neuroimaging studies. Method: A systematic literature search was performed on PubMed with search terms related to morphological complexity and neuroimaging. 16 studies (279 subjects) comparing regular inflection with stems or irregular inflection met the inclusion and exclusion criteria and were subjected to a series of activation likelihood estimation meta-analyses. Results: Significant functional convergence was found in several mainly left frontal regions for processing inflectional morphology. Specifically, the left inferior frontal gyrus (LIFG) was found to be consistently involved in morphological complexity. Diagnostic analyses revealed that involvement of posterior LIFG was robust against potential publication bias and over-influence of individual studies. Furthermore, LIFG involvement was maintained in meta-analyses of subsets of experiments that matched phonological complexity between conditions, although diagnostic analyses suggested that this conclusion may be premature. Conclusion: The findings provide evidence for combinatorial processing of morphologically complex words and inform psycholinguistic accounts of complex word processing. Furthermore, they highlight the role of LIFG in processing inflectional morphology, in addition to syntactic processing as has been emphasized in previous research. In particular, posterior LIFG seems to underlie grammatical functions encompassing inflectional morphology and syntax