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

Abstract

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.

Electrophysiology of Word Learning

“…Such an amplitude increase was not found either for the familiar words or for acoustically similar untrained control stimuli. Similar findings were obtained for implicit learning of novel words that appeared in ten different questions asked about graphical images featuring novel objects [68,69]; remarkably, in these studies the effects were found not only in the N400 time range, but also much earlier, at 170-200 ms in passive auditory ERP recordings, indicating enhanced lexical (or lexicosemantic) access of novel words after a short exposure in meaningful audio-visual contexts. In line with this, very early (from 150 ms) modulations of ERP responses to new words learned implicitly from short visually presented stories (in which only five instances of a novel word were incorporated) were also shown [70] to reflect the semantics of these novel words: concrete vs. abstract.…”

“…Some studies have used audio-visual associative learning designs to directly compare explicit and implicit learning. Shtyrov and colleagues [68,69] reported that ERP patterns elicited in a passive listening paradigm by novel words immediately after the training session differed as early as at ~170 ms, and indicated a more diffuse activity for explicitly learnt words (suggestive of the executive system's involvement), as opposed to the core language areas (particularly in the temporal lobe) contributing to the implicit word acquisition from context. Moreover, behaviorally measured performance on novel words in a semantic task showed a correlation with the amplitude of an earlier ERP peak around 190 ms (putatively linked to automatic processing stages) for implicit learning, and only with a later peak (~280 ms) for the explicit acquisition, the latter being in line with the time interval of attention-related P3/P300 response [69].…”

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“…Other studies showed no significant differences between the efficiency of two learning strategies (Warren et al, 2016;Himmer et al, 2017). Our own previous results (Shtyrov et al, 2021;Perikova et al, 2022) demonstrated very similar levels of performance for both strategies in a free recall task implemented immediately after a word learning session.…”

“…Indeed, differences between the two strategies could be found at the neurophysiological level. For instance, a recent electroencephalography study found the acquisition of novel word forms to be equally successful in both conditions, whilst the patterns of ERPs elicited by items learned in EE and FM conditions differed in their scalp topography and temporal dynamics (Shtyrov et al, 2021).…”

Passive Voice Comprehension during Thematic-Role Assignment in Russian-Speaking Children Aged 4–6 Is Reflected in the Sensitivity of ERP to Noun Inflections

“…The aim of our project is determined by four points: (i) children tend to rely on semantics, not grammar, during sentence comprehension until they are nine years old [ 16 ]; (ii) at the age of three to six years, there is intensive maturation of the neural networks providing syntactic processes [ 40 ]; (iii) skill improvement leads to an intensive restructuring of the neural network and affects the ERP [ 61 ]; and (iv) passive voice comprehension in Russian children is the most intensive at the age of four to six [ 27 ]. The questions are: is it possible to find specific markers of the passive voice processing?…”