Testing motor grounding and somatotopic effects for literal and figurative action language in motor neuron diseases: a between-group and multiple-case analysis

A sample of patients with amyotrophic lateral sclerosis (ALS) and another with the SPG4 variant of hereditary spastic paraplegia (HSP-SPG4), affecting specifically lower limbs, were tested with two tasks employing upper-limb and lower-limb motion verbs either as literal isolated words or in literal and metaphorical sentences, controlling for cognitive impairment. At the group level, the expected disadvantage for action language emerged only in HSP-SPG4 patients, restricted to literal language, with no limb differences. At the single-case level, literal action-language impairment affected up to 50% of HSP-SPG4 patients, with the expected disadvantage for lower-limb verbs, but was negligible in ALS. Figurative action language was impaired in 20% of ALS and 40% of HSP-SPG4 patients, but somatotopic effects were limited. Findings support the motor simulation account at literal and figurative levels. Yet they also highlight the gradual nature of motor grounding disruption in motor neuron diseases, depending on linguistic contexts and clinical profiles.