Neural networks involved in retrieval of newly learned words and effect of overnight consolidation - an fMRI study -

Declarative memory appears to involve two separate systems, with more episodically oriented memories coded in a hippocampal network, and more non-episodic or semantic memories coded in a neocortical network. Previous works (e.g. Dumay & Gaskell, 2007) have shown a role of sleep in the lexicalization of novel words. In line with the two-stage model of memory proposed by McClelland and colleagues (1995), the memory traces for novel words are initially dependent on hippocampal structures. However, a shift towards neocortical representations occurs during the first night’s sleep after training. This shift, or integration of newly learned words into the lexicon (lexicalization) can be observed behaviourally as lexical competition, where novel words slow down recognition of phonologically overlapping known words. To extend understanding of how newly learned words are incorporated into the semantic system, we conducted an fMRI study to elucidate the neural processes underlying sleep-dependent lexicalization, with the additional aim of investigating multimodal information integration in word learning. As a first step towards studying the acquisition of multimodal word meanings, we familiarized subjects with the phonological form of 40 novel words, of which 20 were associated with pictures of novel objects (“picture-associated words”) and 20 were not (“form-only words”). Immediately after training (Day1) and on the following day (Day2), we recorded the BOLD response to auditorily presented “trained novel words”,” untrained novel words” and “existing words”, and administered a lexical competition task to test the effect of novel words on phonologically overlapping existing words. Behavioural data showed enhanced performance in recognition and recall of novel words after sleep, with a greater benefit for picture-associated words. However, lexical competition on Day2 was greater for the form-only words. The fMRI data showed more involvement of the hippocampal network for picture-associated words than for form-only words. In contrast, form-only words activated the semantic memory network already on Day1, whereas this was more apparent on Day2 for picture-associated words. This implies that the consolidation/lexicalization process differs depending on the degree of involvement of the two memory systems, with a greater involvement of the hippocampal system for picture-associated words. Stronger episodic memory traces might slow down the overnight shift of the novel picture-associated words to the lexical network relative to the faster integration into this network of the form-only words