Consolidation of newly learned words with or without meanings: fMRI study on young adults

Declarative memory is considered to entail episodic memory (memory for episodes that are confined to specific spatial and temporal contexts) and semantic memory (memory for generic knowledge or concepts). Although these two types of memories are not independent and they interact extensively, they seem to involve different brain structures at retrieval, with the hippocampus often regarded to be important for retrieving arbitrary associative information encoded in a specific episodic context, whereas widely distributed neocortical areas, especially higher order associative areas, seem to be important in retrieving semantic or conceptual information. In this word-learning study, we asked if there is more involvement of the episodic memory network when retrieval occurs directly after learning, and if there is a shift towards more involvement of the semantic network as the word becomes more de-contextualized with time. Furthermore, we were interested to see the effect of having extra information at encoding, namely, visual information (a picture depicting the word or a definition describing the word) associated with the phonological form of the novel word. Two groups of participants (picture group n=24; definition group n=24) learned phonological novel word forms with meanings (a picture or a definition) or without corresponding meanings (form-only). Participants’ memory for the words was tested in an fMRI scanner directly after training (recent), and again a week later (remote). To test whether novel words were integrated into their lexicon, pause detection and cuedrecall of meaning association tests were administered behaviourally. Retrieval success was greater for meaningful words than for form-only words on both recent and remote tests, with the difference becoming larger at remote test. There was evidence of lexicalization (as measured with the pause detection task) for the meaningful words. In cued recall, although participants were quicker to choose the associated meanings if they were presented in the trained form (identical picture/ definition), there was less slowing down over time for concept associations (similar picture/definition). Imaging results revealed that hippocampal involvement decreased for form-only words in the picture group, whereas for the meaningful words hippocampal involvement was maintained at remote test. Differences between meaningful and form-only words in the remote session were found in a wide range of neocortical areas for successful recognition of the trained words including the fusiform gyrus, medial prefrontal cortex, precuneus and left angular/supramarginal gyrus. Episodic memory decay over time is unavoidable, but meaningful novel words are better retained. These words also interfered more strongly in judgment of similar sounding existing words, and showed less slowing down for cued recall of meaning associations, both indicating more integration and lexicalization for the meaningful novel words. Better memory for meaningful novel words may be due to the use of both the episodic memory network (hippocampus) and the semantic memory network (left fusiform gyrus, left angular/supramarginal gyrus) at remote test