Behavioral and neural effects of training and word classin object and action naming in healthy subjects: Evidence from fMRI

Word retrieval deficits are a common problem in patients with stroke-induced brain damage. While the complete recovery of language in chronic aphasia is rare, patients’ naming ability can be significantly improved by speech therapy. A growing number of neuroimaging studies have tried to pinpoint the neural mechanisms associated with successful outcome of naming treatment. Although the picture is complicated by the large variability in lesion and patient characteristics, some advances have been made in this direction. However, one piece of the puzzle is missing. Namely, the brain mechanisms supporting naming practice in the healthy brain have not received enough attention. Yet, understanding these mechanisms is crucial for teasing them apart from functional reorganization resulting from brain damage. In this thesis I present an fMRI study of intensive naming practice in healthy monolingual adults. Subjects were trained on naming objects and actions from pictures for ten consecutive days and were scanned twice, before and after training. Training of noun and verb production was associated with a similar set of BOLD activation changes, encompassing both anterior and posterior regions of the left hemisphere. Interestingly, while the anterior regions (posterior inferior frontal gyrus, anterior insula, anterior cingulate cortex) showed significant practice-related BOLD decreases, activity in the posterior regions (precuneus, posterior cingulate cortex, angular gyrus, posterior middle temporal gyrus) increased following training. We argue that while the activation decreases were likely associated with the facilitation of different aspects of word production, the greater engagement of parietal and temporal cortices after training potentially reflects retrieval of knowledge pertaining to trained items from episodic (and potentially semantic) memory. Additionally, using the data from the pre-training fMRI session, we investigated the putative dissociation between neural representations of nouns and verbs. In line with previous reports, we found that action naming recruited bilateral lateral occipitotemporal and posterior parietal and inferior frontal cortices (predominantly left-lateralized) to a greater degree than object naming. Our second experiment aimed to further scrutinize categorical distinctions in lexical representation by tackling the effects of verb transitivity. Corroborating previous findings, we observed greater recruitment of several regions in a fronto-temporo-parietal network for transitive verbs. However, in alternative to previous reports that accounted for the results in terms of linguistic processes, we interpreted the observed effects in terms of perceptual and conceptual differences between transitive and intransitive verbs. We outline the major questions that should be addressed by future studies. From the methodological perspective, we validated the use of a recently introduced multivariate searchlight pattern classification method for the analysis of training effects in language studies. We found that, compared to the standard GLM method, the searchlight analysis has comparable and, in some cases, greater sensitivity in localizing BOLD signal changes, and thus it represents a promising complementary tool in studies of training.