A facilitating role for the primary motor cortex in action sentence processing

The involvement of the motor system in action language comprehension is a hotly debated topic in cognitive neuroscience and psychology. Recent studies suggest that primary motor cortex (M1) response to action language is context-sensitive rather than automatic and necessary. Specifically, semantic polarity (i.e. affirmative/negative valence) appears to modulate the intensity of this response, which is stronger for affirmative action sentences. The aim of our study was to examine further the context sensitivity of M1 response. More specifically, we aimed to determine whether M1 response follows semantic polarity or the core meaning of the sentence using two-part action sentences containing interacting polarities. Modulations of M1 activity were recorded using surface electromyography of the first dorsal interosseous muscle of the right hand in 22 healthy participants. Our results show an increase in M1 activity during the first part of the sentence, regardless of semantic polarity. This response was then modulated by the polarity of the second part of the sentence, which carried crucial information regarding the action. These observations suggest that M1 differentially responds to different aspects of action sentences, one response being automatic and the other following the core meaning of the sentence. Our results thus contribute to clarifying the nature of the motor response to action language, which is key to develop more comprehensive and plausible neurobiological models of language processing.