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This study explores the structure of the mental lexicon and the processing of Russian verbal morphology by two groups of speakers, adult American learners of Russian and Russian children aged 4-6, and reports the results of two matching experiments conducted at the University of Maryland, USA and St. Petersburg State University, Russia. The theoretical framework for this study comes from research on the structure of the mental lexicon and modularity in morphological processing. So far, there are very few studies investigating the processing of complex verbal morphology, with most of the work done on Icelandic, Norwegian, Italian, and Russian. The current views are shaped predominantly by research on English regular and irregular past-tense inflection, which has been conducted within two competing approaches. This study investigates the processing of verbal morphology in Russian, a language with numerous verb classes differing in size and the number and complexity of conjugation rules. It assumes that instead of a sharp opposition of regular and irregular verb processing, a gradual parameter of regularity may be more appropriate for Russian. Therefore, the issue of symbolic rule application versus associative patterning can take on a new meaning for Russian, possibly, with the distinction between default and non-default processing replacing the regular-irregular distinction.

Functional connectivity between brain areas involved in the processing of complex language forms remains largely unexplored. Contributing to the debate about neural mechanisms underlying regular and irregular inflectional morphology processing in the mental lexicon, we conducted an fMRI experiment in which participants generated forms from different types of Russian verbs and nouns as well as from nonce stimuli. The data were subjected to a whole brain voxel-wise analysis of context dependent changes in functional connectivity [the so-called psychophysiological interaction (PPI) analysis]. Unlike previously reported subtractive results that reveal functional segregation between brain areas, PPI provides complementary information showing how these areas are functionally integrated in a particular task. To date, PPI evidence on inflectional morphology has been scarce and only available for inflectionally impoverished English verbs in a same-different judgment task. Using PPI here in conjunction with a production task in an inflectionally rich language, we found that functional connectivity between the left inferior frontal gyrus (LIFG) and bilateral superior temporal gyri (STG) was significantly greater for regular real verbs than for irregular ones. Furthermore, we observed a significant positive covariance between the number of mistakes in irregular real verb trials and the increase in functional connectivity between the LIFG and the right anterior cingulate cortex in these trails, as compared to regular ones. Our results therefore allow for dissociation between regularity and processing difficulty effects. These results, on the one hand, shed new light on the functional interplay within the LIFG-bilateral STG language-related network and, on the other hand, call for partial reconsideration of some of the previous findings while stressing the role of functional temporo-frontal connectivity in complex morphological processes.

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