Rethinking motor region role in verb processing: Insights from a neurolinguistic study of noun-verb dissociation in amyotrophic lateral sclerosis

Motor regions atrophy has been traditionally assumed to account for selective deficits of verb (V) vs. noun (N) processing in amyotrophic lateral sclerosis (ALS) patients via action semantic impairment (Embodied Cognition Theory, ECT). Nonetheless, accounts pertaining to both motor region sensitivity to the morpho-phonological structure of Vs and executive functioning (EF) contribution (task difficulty) have not been evenly endorsed. This study aimed at assessing neurocognitive underpinnings of N and V processing in ALS patients. ALS patients were compared to healthy participants (HPs) on tasks evaluating N and V semantic and post-semantic processing. Effects of semantic motor content (actionality) of Ns and Vs and of argument structure complexity of Vs were assessed. EF measures were put into relation to language tasks in patients. Both groups performed worse in V than in N lexical retrieval. Patients had difficulties in retrieving low-actionality Vs, being also sensitive to both transitive Vs and those with 2 and 3 arguments. Patients differed form HPs on both object- and action-semantic measures. EF measures were highly related to patients’ V-retrieval performance. ECT-framed explanations for N > V discrepancies in ALS patients are inconsistent with the present results. Indeed, ALS patients showed semantic deficits that are not limited to the action domain and retrieved more easily high-actionality Vs. N > V discrepancies in ALS patients would thus reflect a magnification of a differential processing demand for Vs and Ns, which is likely intrinsic to the neurocognitive system. Nonetheless, ALS patients being sensitive to argument structure complexity of Vs might imply motor regions involvement in V post-semantic processing, possibly at the lemma level.