How is it that memories change and yet seem fairly stable? In a recent paper, Bridge and Voss, report studies on changes to memory. (see citation). They looked at a particular memory, the location of an object. They changed the background associated with the object. How does the memory change after the change of background? It changes in two different ways depending on the situation.
If the object remained in the same place when the background changed, the new background was added to the memory along with the old background. The memory was expanded. The original memory was probably strengthened and stabilized. On the other hand if the object was in a different place with the new background, then the placement of the object was changed in the original memory to match the placement in front of the new background. So the memory was expanded and perhaps strengthened, but in this case changed rather than stabilized. This sort of mechanism for stability verses change would make the memory more useful (as opposed to more faithful to the original event). How would I deal with a garden, which is different every day in some small way – something grows, something dies – if I had to have a new memory every time there was a change? No. I need only a few memories of the garden occasioned by large radical changes and continuous updating of the current state of the garden in memory. Then I know where the carrots are this year not a couple of years before, and I know how big they are this week not last month. This is a memory useful for day to day living.
No doubt things are not so simple and there are multiple versions, ways to fix on a particular memory and protect it, and many other mechanisms at work that complicate matters. However, here is a clear demonstration of one way that memories change.
Memory stability and change are considered opposite outcomes. We tested the counterintuitive notion that both depend on one process: hippocampal binding of memory features to associatively novel information, or associative novelty binding (ANB). Building on the idea that dominant memory features, or “traces,” are most susceptible to modification, we hypothesized that ANB would selectively involve dominant traces. Therefore, memory stability versus change should depend on whether the currently dominant trace is old versus updated; in either case, novel information will be bound with it, causing either maintenance (when old) or change (when updated). People in our experiment studied objects at locations within scenes (contexts). During reactivation in a new context, subjects moved studied objects to new locations either via active location recall or by passively dragging objects to predetermined locations. After active reactivation, the new object location became dominant in memory, whereas after passive reactivation, the old object location maintained dominance. In both cases, hippocampal ANB bound the currently dominant object-location memory with a context with which it was not paired previously (i.e., associatively novel). Stability occurred in the passive condition when ANB united the dominant original location trace with an associatively novel newer context. Change occurred in the active condition when ANB united the dominant updated object location with an associatively novel and older context. Hippocampal ANB of the currently dominant trace with associatively novel contextual information thus provides a single mechanism to support memory stability and change, with shifts in trace dominance during reactivation dictating the outcome.
D.J. Bridge, & J.L. Voss (2014). Hippocampal Binding of Novel Information with Dominant Memory Traces Can Support Both Memory Stability and Change Journal of Neuroscience, 34 (6) DOI: 10.1523/JNEUROSCI.3819-13.2014