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Differential Neural Timecourse of Fast Mapping and Explicit Encoding of Novel Words: EEG and Behavioural Data
At least two cognitive strategies are used for new word learning: fast mapping (FM) via context-dependent inference and explicit encoding (EE) via direct instruction. However, these distinctions remain debated and the neural dynamics underpinning these processes are poorly understood. To tackle this, we designed a paradigm presenting 20 new spoken words in association with pictures in either FM or EE settings, closely matched for physical features and overall task demands, and tested FM and EE word acquisition in two studies using behavioural measures and passive
ERP responses, an established measure of word memory-trace activation. Behavioural data obtained in free recall, recognition and semantic word-picture matching tasks indicated successful acquisition of new words after just 10 exposures in both experiments. For ERP analysis, Study 1 compared brain activity elicited in adult learners (N=12) by novel FM and EE words before and after the learning session. The main ERP peaks at ~170, 250 and 520 ms indicated successful learning, with statistically different responses between novel and familiar words present only before but not after the training, suggesting rapid formation of new neural memory circuits matching in activation those for previously known words. Already at the earliest peak we found different topographic distributions for the two learning types, with left-lateralised FM dynamics, suggestive of core language system involvement, and more diffuse activity for EE items, possibly suggesting the role of attention/executive control network. To explore relationship between behavioural and neural indices and further scrutinise brain bases of these phenomena, we ran Study
2 (N=36). ERPs indicated the most prominent peaks for novel words at ~196 and ~280 ms. We explored pre- vs. post-learning difference responses at these peaks using multiple linear regression and found significant associations between the accuracy in the semantic wordpicture matching task and ERP dynamics. Namely, the use of implicit
learning strategy was accompanied by a positive-going ERP amplitude change at the first peak (p=0.015). For EE items, however, linear regression revealed a significant contribution by a negative-going dynamics at the second peak (pb0.043). Source reconstruction (sLORETA) indicated anterior-temporal lobes of both hemispheres as the main generators of these dynamics, with left ATL being predominantly activated for FM, and bilateral activity for EE. In
sum, while both learning strategies can be successful for rapid word acquisition, diverging electrophysiological patterns suggest a spatiotemporal dissociation between neural systems underpinning these learning strategies.
Supported by the grant of the Government of RF №14. W03.31.0010
doi:10.1016/j.ijpsycho.2021.07.276