The role of the left Inferior Frontal Gyrus in word selection processing in left- and right-sided Parkinson disease patients

AIMS: Patients with Parkinson’s Disease (PD) show a verb production deficit. Different hypotheses have been advanced, morphological difficulties of verbs compared to nouns, as well as degradation of action representation. However, in an experimental condition in which nouns must be selected from a larger set of alternatives, Silveri et al. (2018) found that nouns became more difficult to produce, probably for the dysexecutive disorder related to the fronto-striatal damage typical of PD. Neuroimaging studies suggest that the left Inferior Frontal Gyrus (IFG) is crucial in the word selection processes. The purpose of the present study was to explored if the ability to select words is related to the cortical thickness of the left IFG. MATERIALS: Twelve right-sided PD with nigrostriatal hypofunctionality in the left hemisphere (RPD-LH), 9 left-sided PD with nigrostriatal hypofunctionality in the right hemisphere (LPD-RH) and 19 healthy controls (HC) were asked to produce nouns and verbs in derivation and generation morphology tasks. METHODS: Participants were requested to produce Nouns from Verbs (VN: ‘osservazione’ from ‘osservare’) [‘observation’ from ‘to observe’], a morphology derivation task in which nouns must be selected from a set of alternatives larger, and to generate Verbs from Nouns (NV: ‘fallire’ from ‘fallimento’) [‘to fail’ from ‘failure’], a morphology generation task with only one alternative. Both accuracy and RTs were collected. All subjects received a structural MRI examination. Cortical thickness for IFG subregions and volumetric measurements for subcortical regions (caudate, putamen, pallidus) were computed using FreeSurfer. Non-parametric analyses were carried out on accuracy and RTs, to assess differences among the three groups in the two tasks. Partial correlations were computed between NV and VN tasks and thickness of cortical regions. RESULTS: RPD-LH patients were less accurate than LPD-RH patients in the VN derivation task (accuracy: 66 % vs. 77 %), but no difference emerged in RTs. Both noun and verb production showed significant correlations with the thickness of the left IFG - pars triangularis only in RPD-LH. DISCUSSION: When attentional resources decay, as in PD, word production is penalised in relation to the number of alternatives among which the selection is made. The linguistic nature of the task and attentional components converge in selection processes. Our data do not disentangle whether the association is a result of executive demands, or whether it emerge because the IFG is also part of Broca’s complex. CONCLUSION: Executive resources and language interact in the left IFG in word production. Moss, H. E., Abdallah, S., Fletcher, P., Bright, P., Pilgrim, L., Acres, K., and Tyler, L. K. (2005). Selecting among competing alternatives: selection and retrieval in the left inferior frontal gyrus. Cerebral Cortex. 15:11, 1723-1735. doi: 10.1093/cercor/bhi049 Silveri, M. C., Traficante, D., Monaco, M. R. L., Iori, L., Sarchioni, F., and Burani, C. (2018). Word selection processing in Parkinson’s Disease: When nouns are more difficult than verbs. Cortex. doi: 10.1016/j.cortex.2017.05.023 Siri, S., Tettamanti, M., Cappa, S. F., Rosa, P. D., Saccuman, C., Scifo, P., and Vigliocco, G. (2007). The neural substrate of naming events: effects of processing demands but not of grammatical class. Cerebral Cortex. 18:1, 171-177. doi.org/10.1093/cercor/bhm043