Perturbation of old knowledge precedes integration of new knowledge.

The importance of memory consolidation in integrating new knowledge has received much recent attention in the field of word learning. Less examined is the change in existing word knowledge as a result of learning, which we hypothesize to occur prior to the opportunity for consolidation. To test this, we had participants learn new meanings for known words and novel words. Then they performed a one-back task on a list of words that included the trained words followed by words that probed either their new or original meanings while EEGs were recorded. A probe word related to the new meaning of the preceding trained word did not show an N400 reduction compared to an unrelated word, suggesting that the new meaning had not been fully integrated, consistent with one account of complementary learning systems. However, when the probe word was related to the original meaning of the preceding trained word a perturbation effect was observed, indicated by a larger negativity at the central midline cluster (Cz) within 500-700 ms when the trained word had a new meaning than when presented as an exposure control. The perturbation effect suggests that even before a new meaning has become integrated with a word form, the attempt to learn a new meaning temporarily makes the original meaning of a word less accessible.