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.
Additional information
http://www.neurolang.org/programs/SNL2014_Program_with_Abstracts.pdf
Publication type
PosterPublication date
2014
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