A baby laughs at an elephant at the zoo. A toddler runs across a beach. Small children make memories all the time, but how many will they recall as the years pass? Maybe none at all. The phenomenon is called “infantile amnesia,” and scientists may have pinpointed a reason for why it occurs – neurogenesis.
Researchers at the Hospital for Sick Children in Toronto say that when new brain cells integrate into existing circuitry, they remodel the structure of networks already in place, wiping out the information previously stored there. This process is prevalent in infancy and early childhood because this is the time when new brain cells are being generated faster and more frequently than at any other time in a human being’s life. Humans and other mammals spawn new neurons throughout their lifespans, although the rate of neurogenesis decreases significantly with age.
In their paper, published in Science, the researchers explain how recent studies have focused on how new brain cells can lead to new memories, but the Toronto team speculated that neurogenesis could also wipe away memories. To test their hypothesis, they conducted a series of studies on populations of newborn and adult mice. Neuron development in mice occurs in much the same way as in humans, with rapid cell genesis in infancy that tapers off with age.
One experiment tested whether increased neurogenesis in adult mice would result in forgetting something that was recently learned. The researchers trained mice in a “fear conditioning” scenario, exposing them to an environment where they received foot shocks. Some of the adult mice were then given exercise wheels – since running is known to increase neurogenesis in adulthood. Adult mice that ran for four to six weeks after fear conditioning, did not freeze as readily as other mice, indicating that they had forgotten the previously learned fear.
To examine levels of neurogenesis in the mouse brains, the researchers used the optical fractionator probe in Stereo Investigator to count cells that had been labeled by a green florescent protein, indicating that they were newborn cells.
Then, they used Neurolucida to create 3D reconstructions of the brain circuitry where new neurons had become integrated into existing networks.
“Through a series of studies, we showed that high levels of neurogenesis disrupt established hippocampus-dependent memories. As such, our findings reveal a novel role for neurogenesis in forgetting or memory clearance,” the authors say in their paper.
Akers, K. G., Martinez-Canabal, A., Restivo, L., Yiu, A. P., De Cristofaro, A., Hsiang, H.-L. L., Wheeler, A.L., Guskjolen, A., Niibori, T., Shoji, Y., Hirotaka, S., Ohira, K., Richards, B.A., Miyakawa, H. Josselyn, S.A., Frankland, P.W. (2014). Hippocampal Neurogenesis Regulates Forgetting During Adulthood and Infancy. Science, 344(6184), 598-602.