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Modulating Novel Vocabulary Acquisition by Transcranial Direct Current Stimulation
Effects of transcranial direct current stimulation (tDCS) on acquisition of novel vocabulary remain controversial, which could be related to specific neural mechanisms underpinning different language learning strategies. To comprehensively assess tDCS effects on fast mapping (FM) and explicit encoding (EE) of novel words, we modulated tDCS stimulation regime (anodal/cathodal/sham), stimulated hemisphere (left/right), site (Broca/Wernicke/their right-hemispheric homologues), learning type (FM/EE), and active rehearsal (with/without articulation of novel words). Nine groups (n=32 each) of native Russian speakers (age 18–35) received 15 minutes of 1.5mA tDCS before learning eight novel words, presented ten times each in a naturalistic audio-visual word-picture association session. Error rates and reaction times (RTs) were tested immediately after the learning session using recognition and semantic word-picture matching tasks. Learning outcomes were analysed using chi-square (X2) and general linear mixed model (GLMM). Compared with sham (placebo) condition, participants were faster after real tDCS (all p-values<.010), more accurate after cathodal Broca’s area tDCS (X2=5.4; р<.05), cathodal Wernicke’s tDCS (X2=8.3; р<.005), and anodal tDCS of Wernicke’s homologue area (X2=3.8; р<.05). GLMM for RTs did not find significant interaction between learning type and articulation in the placebo group (F(3,389)=2.46, p=.063). However, articulated words were recognised faster than non-articulated ones in EE condition (p<.001). Interaction between articulation, learning type, and stimulation modes was significant for the left (F(15,1628)=5.086; p<.001) and right (F(15,1580)=3.659, p<.001) hemispheres. RTs for articulated words were smaller than for non-articulated ones in EE condition after anodal Wernicke’s area tDCS (p<.01), cathodal Broca’s area tDCS (p<.001) and cathodal tDCS of Broca's right-hemispheric homologue (p<.001). A significant interaction between learning type and articulation was found for error rates for the sham group (F(3,252)=9.537; p<.001), with better performance for articulated than non-articulated words in EE (p<.001), but not in FM condition. Interaction between articulation, learning type and stimulation polarity was significant for both left (F(7,1016)=32.44; p<.001) and right (F(7,1016)=23.74; p<.001) hemisphere. Articulated EE items showed better performance than non-articulated ones in condition after anodal left-hemispheric tDCS (p<.05) as well as anodal (p<.01) and cathodal (p<.001) right-hemispheric stimulation of both zones. Articulated FM words were identified better than non-articulated ones after cathodal tDCS over both zones in both hemispheres (p-values<.001), as opposed to sham. In sum, tDCS of core language areas can enhance both FM and EE types of learning. Moreover, cathodal tDCS over both hemispheres appears to specifically influence FM of novel words.