Action verbs and the primary motor cortex: A comparative TMS study of silent reading, frequency judgments, and motor imagery

Single pulse transcranial magnetic stimulation (TMS) was applied to the hand area of the left primary motor cortex or, as a control, to the vertex (STIMULATION: TMSM1 vs. TMSvertex) while right-handed volunteers silently read verbs related to hand actions. We examined three different tasks and time points for stimulation within the same experiment: subjects indicated with their left foot when they (i) had finished reading, (ii) had judged whether the corresponding movement involved a hand rotation after simulating the hand movement, and (iii) had judged whether they would frequently encounter the action verb in a newspaper (TASK: silent reading, motor imagery, and frequency judgment). Response times were compared between TMSM1 and TMSvertex both applied at different time points after stimulus onset (DELAY: 150, 300, 450, 600, and 750 ms). TMSM1 differentially modulated task performance: there was a significant facilitatory effect of TMSM1 for the imagery task only (about 88 ms), with subjects responding about 10% faster (compared to TMSvertex). In contrast, response times for silent reading and frequency judgments were unaffected by TMSM1. No differential effect of the time point of TMSM1 was observed. The differential effect of TMSM1 when subjects performed a motor imagery task (relative to performing silent reading or frequency judgments with the same set of verbs) suggests that the primary motor cortex is critically involved in processing action verbs only when subjects are simulating the corresponding movement. This task-dependent effect of hand motor cortex TMS on the processing of hand-related action verbs is discussed with respect to the notion of embodied cognition and the associationist theory. (c) 2008 Elsevier Ltd. All rights reserved.