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HD-tDCS of primary and higher-order motor cortex affects action word processing
Poster B32 in Poster Session B and Reception, Thursday, October 6, 6:30 - 8:30 pm EDT, Millennium Hall
Karim Johari1, Nicholas Riccardi2, Svetlana Malyutina3, Mirage Modi4, Rutvik Desai2; 1Louisiana State University, 2University of South Carolina, 3National Research University Higher School of Economics, Moscow, Russia, 4Northwestern University
Introduction: Multiple brain areas represent lexical-semantic knowledge. To what extent action-perception areas (e.g., motor cortex) contribute to processing of action-related language (to throw) remains controversial. Here, we used high-definition transcranial direct current stimulation (HD-tDCS) to examine the role of left-hemisphere hand-motor area (HMA) and anterior inferior parietal lobe (aIPL) in action-related word processing compared to non-action words. Further, we investigated stimulation-related effects at three levels of semantic processing: subliminal, implicit, and explicit. Based on previous studies, we expected that HD-tDCS of HMA and aIPL would facilitate action word processing compared to non-action. Methods: Forty-two volunteers participated in the study (27 females; mean age 21.3 years). Participants were randomly assigned to either the HMA or aIPL experiment. For each location, cathodal and sham stimulations were in two counterbalanced sessions for 20 minutes each. Following stimulation subjects completed two tasks: primed lexical decision (LD; measuring subliminal and implicit processing) and semantic similarity judgment (SSJ; measuring explicit processing) task. LD consisted of verbs, nouns, and pronounceable nonwords. Primes and target words were presented one-at-a-time, and participants indicated via button-press whether the target word was real. The prime was either identical to the target word or was a consonant string, presented for 50 milliseconds to measure subliminal priming. SSJ consisted of verb and noun trials, wherein each trial consisted of three words in a triangular arrangement. Participants indicated via button-press which of the bottom two words was most similar in meaning to the top word. For both tasks, half of the verbs were hand/arm actions (to tie), and half were non-action (to view). For nouns, half were manipulable (the ball), and half were nonmanipulable (the cabin). Conditions were matched on numerous psycholinguistic variables. Stimulation-related effects for response times (RT) and accuracies were calculated by subtracting sham from active stimulation performance. One-tailed t-tests were used to examine action vs. non-action verbs and manipulable vs. nonmanipulable nouns, with stimulation-related accuracy, RT, and priming (identity prime minus consonant prime). HMA Results: In LD, stimulation significantly decreased net accuracy for non-action verbs compared to action verbs (p=0.025) and improved net accuracy of primed action verbs compared to primed non-action verbs (p=0.034). In SSJ, stimulation accelerated RTs for manipulable compared to non-manipulable nouns (p=0.001). No other contrasts were significant. aIPL Results: In LD, stimulation significantly improved accuracy of action compared to non-action conditions for both nouns (p=0.02) and verbs (p=0.021). There was a trend of faster RTs for primed manipulable vs. primed nonmanipulable nouns (p=0.1). No other contrasts were significant. Conclusions: Broadly, HMA and aIPL stimulation facilitated action-related word processing compared to non-action. HMA stimulation facilitated action verbs in sub-explicit contexts, suggesting that HMA represents action verbs in semantically shallow tasks. HMA stimulation facilitated manipulable nouns in an explicit task, suggesting that HMA represents manipulable nouns when demands are high. aIPL stimulation facilitated manipulable nouns and action verbs during an implicit, but not explicit task. This suggests that, while aIPL may help represent action language specifically, aIPL and adjacent areas may also subserve general processes related to semantic control/representation.
Topic Areas: Meaning: Lexical Semantics, Reading