A tool to probe domain-general syntax: Simple and complex actions with a tool improve syntactic comprehension in language

• Language and motor processes share a domain-general syntactic function • Motor training can produce a learning transfer to the language domain • Performance in language improved after tool-use training compared to hand training • Complex actions combined with tool-use might enhance this effect • Tool-use dexterity is a good predictor of syntactic performance in language Language is a complex faculty made up of different components, many of which are based on sensorimotor processes. In this study, we focused on syntax, which is known for its role in sentence construction, but also in goal-directed actions like toolmaking or tool-use. It has been shown that syntactic processes in language and tool-use share common neural resources, therefore benefiting one from the other. Here, we investigated behaviorally the relationship between manual action and language comprehension. We varied the complexity of manual action along two independent dimensions: the length of the action sequence and whether participants used their hand or a tool. A cohort of 80 adult participants underwent a learning transfer protocol consisting of a syntactic comprehension test before and after being trained in the motor task. Our results showed that the use of a tool is crucial for allowing a learning transfer to occur from the motor to the language domains: regardless of the complexity of action sequences, participants performed significantly better in the syntactic comprehension task after tool-use training, but not after bare hand training. Our results also suggested that complex actions combined with tool-use might enhance this effect and that inter-individual tool-use dexterity is a good predictor of syntactic performance in language.