Abstract
Previous behavioural and neuroimaging research suggested distinct cortical systems involved in processing abstract and concrete semantics; however, there is a dearth of causal evidence to support this. To address this, we applied anodal, cathodal, or sham (placebo) tDCS over Wernicke’s area before a session of contextual learning of novel concrete and abstract words (n = 10 each), presented five times in short stories. Learning effects were assessed at lexical and semantic levels immediately after the training and, to attest any consolidation effects of overnight sleep, on the next day. We observed successful learning of all items immediately after the session, with decreased performance in Day 2 assessment. Importantly, the results differed between stimulation conditions and tasks. Whereas the accuracy of semantic judgement for abstract words was significantly lower in the sham and anodal groups on Day 2 vs. Day 1, no significant performance drop was observed in the cathodal group. Similarly, the cathodal group showed no significant overnight performance reduction in the free recall task for either of the stimuli, unlike the other two groups. Furthermore, between-group analysis showed an overall better performance of both tDCS groups over the sham group, particularly expressed for abstract semantics and cathodal stimulation. In sum, the results suggest overlapping but diverging brain mechanisms for concrete and abstract semantics and indicate a larger degree of involvement of core language areas in storing abstract knowledge. Furthermore, they demonstrate a possiblity to improve learning outcomes using neuromodulatory techniques.
Highlights
Previous behavioural and neuroimaging research suggested distinct cortical systems involved in processing abstract and concrete semantics; there is a dearth of causal evidence to support this
It was shown that anodal tDCS over dorsolateral prefrontal cortex (DLPFC) or parietal cortex facilitated the retrieval of both semantic types equally[18]
We ran a number of tests (FDR-corrected for multiple comparisons) to check the word acquisition outcomes and found the effects of tDCS in all assessment tasks
Summary
Previous behavioural and neuroimaging research suggested distinct cortical systems involved in processing abstract and concrete semantics; there is a dearth of causal evidence to support this. Neither of these have addressed the core language systems This has recently been done using direct electrical stimulation (DES) during awake surgery, showing that DES applied over the left BA44, which is a part of Broca’s area and inferior frontal gyrus (IFG), decreases the accuracy of semantic judgement task for abstract words, whereas for concrete words similar results were observed when left BA38 in the temporal lobe was s timulated[22]. This is an important finding, one has to be cautious generalising results obtained surgically in neurological patients to healthy population. This is what we aimed at in the current study
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