#SfN13 Adult neurogenesis and the fluidity of memory flow


This is part of a series of blog posts for the 2013 Society for Neuroscience Annual Conference. 

Poster III32. Adult neurogenesis protects against proactive interference.

JR Epp, R Silva Mera, LCP Botly, AC Gianlorenco, S Kohler, SA Josselyn, PW Frankland. Hospital For Sick Children, Toronto, ON, Canada; Federal Univ. of Sao Carlos, Sao Carlos; Univ. of Western Ontario, London, ON, Canada

Think about the last time you started up iTunes in search of a song. Every flick of your finger brought a new, dazzling piece of cover art into view. With one goal in mind, you fixed your gaze steadily on the centre of the screen, barely noticing as previous covers gradually drifted from your sight and disappeared from your mind.

This is how I picture memory. As we go through our daily lives, new memories silently replace older ones that are similar in context and scheme. The neurobiology behind this “refreshing” of memory is mind boggling in complexity and not well understood. Now, researchers from the Hospital for Sick Children in Toronto, Canada have uncovered a potential, if somewhat surprising, mechanism – adult neurogenesis.

Throughout our adult lives, the olfactory bulb and the dentate gyrus constantly produce new neurons; rapidly in infancy then gradually slowing as we age. Increasing adult neurogenesis in the dentate gyrus, either through drugs or exercise, helps the brain encode and differentiate between two or more similar memories. As such, high rates of neurogenesis have always been considered a “good” thing: more computational power, better memory.

Yet as new neurons reach out and connect into an existing neural network, it inevitably disrupts old information stored within. Following this line of thought, could adult neurogenesis paradoxically deteriorate existing memories?

Researchers first trained adult mice on a spatial recognition task. Picture a large pool filled with murky water with a platform hidden just beneath the surface. Mice are efficient swimmers, but they prefer to relax on the platform (who doesn’t?) if given the chance. A few training sessions later, all the mice managed to find and remember the location of their resting spot. Researchers then separated them into two groups: the “couch potato” group was housed in a standard cage; the “runner” group was given a running wheel. Mice are like your average pet gerbils – give them a wheel and they’ll happily go at it for hours.

4 weeks later, compared to the couch potatoes, the runners had significantly more new neurons in their dentate gyrus. When challenged with the same water maze, a clear difference in performance emerged. Compared to sedentary controls, the runners had a hard time homing in on the right spot. They spent much less of their time circling waters close to the platform, suggesting that their memory of the location had deteriorated by a greater extent. On the contrary, when researchers used a chemical-genetic method to eliminate new neurons in another cohort of water maze-trained mice, they remembered the platform location even better than sedentary controls.

Before you go and throw your running shoes out the window, pause and consider this: far from disadvantaged, the runners' memories were more "flexible". When researchers covertly moved the platform to a new location, runners learned much faster than their sedentary peers. Conversely, mice with disrupted neurogenesis stubbornly clung onto their old outdated memory, taking longer than controls to find the new platform location in every single training session. These changes in memory flexibility weren’t a generic effect of running but a specific result of neurogenesis: when runners had their numerous new neurons ablated, they behaved just like sedentary controls – better retention, slower relearning.

Incredibly, neurogenesis only seems to help with learning new information that is highly similar to that previously remembered. Researchers trained mice to discriminate between two boxes: one was shaded on one side and smelled of coffee; the other was striped with a hint of cinnamon. A month later, researchers swapped the odours between the two boxes. Once again, mice that ran during the interim learned the reversed odour-box pairings faster than their sitting peers. However, when researchers trained these mice in a similar task with two completely new odour (ginger, thyme)-box pairings, both groups learned at the same rate.

Taken together, it seems that adult neurogenesis after learning weakens old memories, which in turn facilitates learning of new but similar information. This is not to say that adult neurogenesis induces a state of tabula rasa; the data clearly shows that existing memories are weakened, not completely wiped clean. In a sense, adult neurogenesis tips the static-fluid memory scale just enough so that new information about the environment can be incorporated, either through altering the original memory trace or the formation of a new one. Hence, we live and thrive in the present.

There are cases where we resist: when learning a new language, you often automatically refer to a more familiar language for guidance - a headache dubbed "proactive interference" by psychologists. This study suggests that maybe you should close your books and go for a run. Come back later and who knows? You just might learn that second language faster.

My previous posts on adult neurogenesis can be found here.