TDCS to premotor cortex changes action verb understanding: Complementary effects of inhibitory and excitatory stimulation

Tom Gijssels,Richard B. Ivry,Daniel Casasanto

doi:10.1038/s41598-018-29600-6

Abstract

Processing the meaning of action language correlates with somatotopic activity in premotor cortex (PMC). A previous neurostimulation study supported a causal contribution of PMC activity to action verb understanding, but the direction of the effect was unexpected: inhibiting PMC made participants respond faster to action verbs. Here we investigated the effects of PMC excitation and inhibition on action verb understanding using tDCS. Right-handed participants received tDCS stimulation with the anodal electrode (presumed to be excitatory) and cathodal electrode (presumed to be inhibitory) placed over left and right PMC, respectively, or with the reverse configuration. After completing the stimulation protocol, participants made lexical decisions on unimanual action verbs (e.g., throw) and abstract verbs (e.g., think). tDCS configuration selectively affected how accurately participants responded to unimanual action verbs. When the anode was positioned over left PMC we observed a relative impairment in performance for right-hand responses (i.e. the hand with which these participants typically perform unimanual actions). By contrast, when the cathode was positioned over left PMC we observed a relative improvement. tDCS configuration did not differentially affect responses to abstract verbs. These complementary effects of excitatory and inhibitory tDCS clarify the functional role of premotor hand areas in understanding action language.

Highlights

Processing the meaning of action language correlates with somatotopic activity in premotor cortex (PMC)
In addition to the predicted 3-way interaction, we found the predicted qualitative pattern of results for the constituent 2-way interactions. This qualitative pattern showed that inhibitory left PMC stimulation tended to cause a relative improvement in performance for right-hand responses, whereas excitatory left PMC stimulation tended to cause a relative impairment; by contrast, transcranial Direct Current Stimulation (tDCS) polarity had no systematic effect on accuracy for abstract verbs
We used two tDCS configurations to alter the state of excitability in hand areas of premotor regions

Summary

Introduction

Processing the meaning of action language correlates with somatotopic activity in premotor cortex (PMC). A previous neurostimulation study supported a causal contribution of PMC activity to action verb understanding, but the direction of the effect was unexpected: inhibiting PMC made participants respond faster to action verbs. TDCS configuration did not differentially affect responses to abstract verbs These complementary effects of excitatory and inhibitory tDCS clarify the functional role of premotor hand areas in understanding action language. FMRI data support this proposal: processing action verbs correlates mainly with activity in motor planning areas (e.g., premotor cortex; PMC), rather than activity in motor execution areas (e.g., primary motor cortex[4,7,8]; but see[3]). CTBS to left vs right PMC did not differentially affect responses to nonmanual action verbs. CTBS to left vs. right PMC did not differentially affect responses to nonmanual action verbs9. fMRI evidence suggests that these effects were likely driven by cTBS to dominant hand areas in left PMC: When right-handers processed unimanual action verbs in the scanner, they showed BOLD modulation in right-hand areas in left PMC, but not in left-hand areas in right PMC10

Methods

Results

Conclusion