Gestures, verbs and the Motor Cortex: A TMS study
Creators: name: Mertens, B
Publisher: name: University Of Tasmania
Year: 2023

description: Willems and Hagoort (2007) outline a number of links between language and the motor cortex. Previous researchers report the motor cortex is engaged when processing specific linguistic information such as action words, and even manual communication (e.g., gesture). Using behavioural measures, transcranial magnetic stimulation (TMS), and electroencephalographic (EEG) data, the three empirical studies reported examine the role of the motor cortex in processing both action words and gestural information. Study One was inspired by the work of Pulverm‚àö¬¿ller, Hauk, Nikulin and Ilmoniemi (2005), who reported facilitative effects of single-pulse TMS on reaction time (RT) to action words. Pulverm‚àö¬¿ller et al. reported that RT to hand-action words decreased when TMS was applied over the hand-area of the left motor cortex at 90% of threshold and at 150 ms after the presentation of the word. The current study was designed to investigate the limits of the reported facilitative effect. Twenty-four participants (TMS: n = 16, control: n = 8) performed a word recognition task while single-pulse TMS or sham-TMS was applied at varying latencies (120 ms, 150 ms, 180 ms, 230 ms, and 290 ms) and intensities (90%, 100%, 110%, and 120%) over the hand area of the left motor cortex. RT was compared between hand-action and non-action words. Contrary to the hypothesis, no latency/intensity combinations elicited faster RT to action words and surprisingly, RT was faster to the non-action words compared to action words when TMS was applied at 150 ms/90%. This indicates that these specific TMS parameters do not necessarily specifically speed processing for action words compared to non-action words, and it was concluded that these findings deviate from previous reports due to methodological differences. Study Two examined the role of the motor cortex in gestural processing using a particular repetitive TMS (rTMS) protocol known as continuous theta-burst stimulation (cTBS) which has been shown to temporarily reduce motor cortex excitability (inhibition). It was anticipated that if the motor cortex is involved in gestural processing, inhibiting motor cortex function would impair processing and increase RT to gestural stimuli. cTBS was administered over the hand-area of the left motor cortex in ten participants. Participants then viewed five test stimuli words that were presented multiple times in different modalities: text, speech, gesture, and speech and gesture combined. Contrary to the hypothesis, no difference in RT between cTBS and control was found for any modality. These findings are interpreted in light of recent reports that motor cortex inhibition may not lead to behavioural change, despite being detectable in physiological excitability measures. Study Three further investigated the role of the motor cortex in gestural processing. In addition to utilising cTBS, this study also examined EEG data, specifically event related potentials (ERP) to assist with overcoming the limitations of behavioural data. The N400 is an ERP component is widely reported to be modulated by the integration of semantically conflicting linguistic information. It was anticipated that if the motor cortex were involved in gestural processing, a cTBS-induced inhibition of the left motor cortex may cause perturbations in the N400 component. EEG data were collected from 10 participants who were presented with congruency-matching and mismatching sentence and speech-gesture stimuli. RT and N400 amplitude and latency (at central and parietal sites) were compared for these stimuli between cTBS and control. Contrary to expectations, cTBS led to greater negativity in N400 amplitude to both stimuli types. No specific impacts on N400 amplitude to speech-gesture stimuli were found, however cTBS did have some effect on N400 latency. These results are interpreted in light of the understanding of the motor cortex as part of a network of brain areas that may be involved in cognitive processing, and the non-focal impacts of cTBS. The findings from this series of studies are discussed with reference to the literature on the role of the motor cortex in language processing, the broader cortical impacts of TMS, and the future of research designed to examine cortical involvement in cognitive processes.