Reading negative action verbs: one or two-step processing within the primary motor cortex?

Controversy persists regarding the representation of negated actions, specifically concerning activation and inhibitory mechanisms in the motor system, and whether this occurs in one or two steps. We conducted two experiments probing corticospinal excitability (CSE) and short-interval intracortical inhibition (SICI) in the primary motor cortex at different latencies while reading affirmative and negative action sentences. Twenty-six participants read action and non-action sentences in affirmative or negative forms. Using transcranial magnetic stimulation, we probed CSE in hand muscles at rest and at several latencies after verb presentation. We observed a greater CSE for action sentences compared to non-action sentences, regardless of verb form. In experiment two, nineteen participants read affirmative and negative action sentences. We measured CSE and SICI at short and long latencies after verb presentation. CSE was greater for affirmative and negative action sentences at both latencies compared to rest. SICI did not change at the short latency but increased at longer latencies, regardless of verb form. Our results lend partial support for a two-step model, as negated actions showed the same motor excitability as affirmed actions with no additional inhibition at early latencies. Later neural differences between affirmative and negative actions may occur outside the primary motor cortex. Significant statementIn two TMS experiments, we probed corticospinal excitability and short-interval intracortical inhibition in the primary motor cortex at different latencies while subjects read affirmative and negative action sentences. Consistent with an embodied view of language comprehension, our results demonstrate that reading about actions indeed activates the motor system, and this for both negative and affirmative sentences. Our results lend partial support for a two-step model of negation, as negated actions showed the same increase in motor excitability as affirmed actions, with no additional inhibition at early latencies. This suggests that the motor system contributes to comprehension by simulating the negated or affirmed action. Later neural differences between affirmative and negative actions may occur outside the primary motor cortex.