What Do New Cells Contribute to a Brain That Preserves Memories?

When we think about how memories persist, it is tempting to look for something that remains unchanged. Yet preserving past experiences alone is not enough to adapt to new situations. How does the brain reconcile maintaining memories with updating them?

A study published in Nature Medicine on August 21 analyzed postmortem brains from people with depression and suggested that the process of generating new neurons in the hippocampus may have stalled. Using gene expression and other cellular data, the researchers examined differences in the composition of stem-cell-like populations and neuroblasts at a later stage. This was not an experiment that tracked the birth of cells in a living brain, and a causal relationship with depression has not yet been established. Original paper by Peng and colleagues ↗

An earlier mouse study offers a way to interpret this finding. Sahay and colleagues showed that an intervention that increased the survival of newly generated hippocampal neurons improved performance on a task requiring mice to distinguish between two very similar situations. Memory did not improve uniformly across the board. Sahay and colleagues, Nature, 2011 ↗

The distinction worth considering here is whether new cells contribute only to how much can be remembered, or whether they help similar experiences be treated as distinct. Both drawing on past experience and recognizing that the present situation differs from the past are necessary.

In the previous issue, we considered how memories can persist even as neural wiring and activity change. This time, we want to approach the question from the other direction: what is lost when necessary changes become harder to make? The ability to retain old memories might remain intact, while distinguishing new situations or revising earlier judgments becomes more difficult. This is not a conclusion of the current paper, but a hypothesis to test next.

To understand the effects of changing neurogenesis, we need to know which cells, at which stages, enter which circuits and contribute to which behaviors. Understanding those relationships would let us think more specifically about what needs to be restored, rather than simply aiming to “increase brain plasticity.” We will follow this line of research by looking for separate measures of memory stability and the ability to learn anew.