Neuroscientists at MIT say they have uncovered why relatively minor details of an episode are sometimes inexplicably linked to long-term memories.     Even irrelevant information that follows the relevant event (rather than precedes it) is more likely to be integrated into long-term memory.

Are they engrams, biophysical or biochemical changes?  Dendritic spines in the cerebral cortex undergo structural remodeling when there are external stimuli, getting larger in response to repeated activity within the brain, and that might be the basis for learning and memory, so figuring out trace memories could help us understand learning.