Explicit encoding vs. fast mapping of novel spoken words: Electrophysiological and behavioural evidence of diverging mechanisms.

It has been claimed that two major neurocognitive mechanisms - instruction-based explicit encoding (EE) and inference-driven fast mapping (FM) may be involved in rapid acquisition of novel words, but their exact neural underpinnings remain poorly understood. To address this, we trained 36 adult participants with 20 novel spoken words in an audio-visual task, carefully balanced between the EE and FM conditions for physical, psycholinguistic and pragmatic properties as well as the overall task setup. To assess the neural dynamics associated with novel word acquisition, we recorded event-related potentials (ERPs) elicited by these words before and after training, and analysed their relationship with the behavioural learning outcomes, measured in a semantic matching task. Both learning regimes led to successful acquisition, which was somewhat more efficient for EE than FM, as indicated by higher accuracy in the behavioural task. We also found that, whereas words learnt via both EE and FM protocols elicited most pronounced ERP peaks at ∼196 and ∼280 ms, these two phases of activity diverged with respect to the learning type. Multiple linear regression and correlation analyses indicated that the learning-induced amplitude dynamics in the earlier peak was significantly related to behavioural performance for FM-learned items, which may possibly be explained by FM's stronger reliance on early automatic mechanisms of word processing. Performance on EE words was, in turn, significantly linked to the amplitude of the second peak only, potentially due to the involvement of later, top-down controlled processes in this type of word acquisition. Grand-average ERP-based source analysis indicated a left-lateralised activity in the anterior-temporal lobe for FM learning, and a bilateral activation for EE. The results confirm the existence of partially diverging neurocognitive systems for word acquisition and suggest that the configuration of newly established word memory circuits depends on the mode of their acquisition.

Electrophysiological Evidence of Dissociation Between Explicit Encoding and Fast Mapping of Novel Spoken Words.

Existing behavioral, neuropsychological and functional neuroimaging data suggest that at least two major 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 at both behavioral and neurophysiological levels, not least due to confounds related to diverging experimental settings. Furthermore, the neural dynamics underpinning these two putative processes remain 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 auditory and visual features and overall task demands. We tested word acquisition using a range of behavioral measures as well as passive event-related potential (ERP) responses, an established measure of word memory trace activation, and compared brain activity elicited by novel FM and EE words before and after the learning session. Behavioral data obtained in free recall, recognition and semantic word-picture matching tasks indicated successful acquisition of new words after just 10 exposures. Crucially, we found no behavioral evidence of different acquisition outcomes between FM and EE learning. ERP data, which exhibited the main response peaks at ~170, 250, and 520 ms, also 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. Furthermore, already at the earliest peak, we found different topographic distributions for the two learning types, with left-lateralized FM dynamics, suggestive of core language system involvement, and more diffuse activity for EE items, possibly suggesting the role of attention/executive control network. A similar effect also manifested later, at ~520 ms. Our data suggest that while both EE and FM learning can be successful for rapid word acquisition at the behavioral level, the diverging electrophysiological patterns suggest a dissociation between the neural systems underpinning these learning strategies.