Noun-verb dissociation in motor neuron disease: theoretical and clinical entailments.

Targets. Selective deficit of verb (V) vs. noun (N) processing has been consistently reported in the motor neuron disease – frontotemporal dementia (MND-FTD) complex. Prerolandic regions atrophy has been traditionally thought to account for N-V dissociation in the MND-FTD complex via selective action semantics impairment (Embodied Cognition Theory, ECT) [1]. Nonetheless, explanations pertaining to both sensitivity of those regions to morpho-phonological structure of Vs [2] and executive functioning (EF) contribution [3] have not been endorsed. The aims of this study was to both assess neurocognitive mechanisms underlying N-V dissociation in MND patients and validate its cognitive diagnostic role in this population. Materials. Thirty consecutive MND patients and 29 healthy controls were recruited. Patients’ clinical features (disease duration; site of onset; bulbar signs) and both neuropsychological (Edinburgh Cognitive and Behavioral ALS Screen, ECAS) and functional outcomes were considered. Methods. The two groups were compared on tasks evaluating N and V lexical retrieval, object- and action-semantics, and V lexical-morphosyntactic implementation, while controlling for EF measures. Effects of motor feature degree (actionality) and lexical-morphosyntactic complexity of items on lexical retrieval were assessed via logistic linear mixed models, and by between-groups comparisons via overdispersed linear models. Clinical relevance of N-V dissociation was investigated by assessing its association with disease-related variables. Results. Both groups performed worse in V than in N naming. Patients performed worse than controls in V naming and both object- and action-semantic tasks. Both groups were comparable on remaining linguistic measures. Low-actionality and transitive Vs were the most demanding for patients. EF measures did not discriminate patients from controls but were mostly related to V naming in patients. V deficit was related to patients’ ECAS scores but not to other disease-related variables. Discussion. Impaired action semantics cannot account for N-V dissociation in MND patients, which would rather reflect a magnification of a differential EF-related processing demand for Vs vs. Ns intrinsic to the neurocognitive system. Nonetheless, since N-V dissociation in MND patients is augmented by psycholinguistic variables, it might imply prerolandic involvement in V lexical processing. Furthermore, V deficit has been shown to be predictive towards patients’ neuropsychological outcome. Conclusions. ECT-framed explanations for V deficit in the MND patients are not valid. On the contrary, interplays of linguistic and extra-linguistic explanations should be considered. Deficit in V naming can be found in MND patients and should be considered as a sensitive marker of cognitive impairment in those patients. York, C., Olm, C., Boller, A., McCluskey, L., Elman, L., Haley, J., Seltzer, E., Chahine, L., Woo, J., Rascovsky, K., McMillan, C., Grossman, M. (2014). Action verb comprehension in amyotrophic lateral sclerosis and Parkinson’s disease. Journal of Neurology, 261, 1073-1079. De Zubicaray, G., Arciuli, J., McMahon, K. (2013). Putting an “end” to the motor cortex representations of action words. Journal of Cognitive Neuroscience, 25, 1957-1974. Papeo, L., Cecchetto, C., Mazzon, G., Granello, G., Cattaruzza, T., Verriello, L., Eleopra, R., Rumiati, R. I. (2014). The processing of action and action-words in amyotrophic lateral sclerosis patients. Cortex, 64, 136-147.