Learning with the wave of the hand: Kinematic and TMS evidence of primary motor cortex role in category-specific encoding of word meaning

Language processing recruits a core fronto-temporal cortical network, which is complemented by a distributed network of modality-specific areas (such as the motor cortex) to encode referential aspects of meaning. Since most studies typically focus on already fully-formed adult vocabulary, it remains unclear how and when exactly modality-specific areas become involved in language processing. Here, we addressed this question using a 3D virtual environment game to teach adult participants new action verbs and object nouns. To test the role of primary motor cortex (M1) in selectively encoding aspects of action verb meaning early on in the process of word learning, we delivered theta-burst stimulation to three groups of participants prior to learning: M1 TMS, active control TMS, and sham TMS. Our results show that TMS of M1 (but not active or sham controls) interferes with the learning process, as indexed by measures of movement kinematics and a higher number of errors during training. Thus, TMS disruption of M1 degrades learning outcomes when motor information is an integral part of lexico-semantic encoding. This effect was corroborated in a subsequent lexical decision task, which showed significant group- and word-category RT differences, suggesting category-specific effects of TMS on word learning. Overall, our study demonstrates the M1's causal involvement in the earliest phases of word learning and rapid encoding of semantic motor information.