A new study, published in Science, has suggested that newly-generated neurons in the area of the brain involved in memory formation might actually lead to forgetting of established memories, which could help to explain why we struggle to remember things that happened very early on in our lives.
In the majority of animals, the addition of new neurons to the brain is halted shortly after birth. In humans, mice and a few other species, however, the production of new neurons (neurogenesis) continues throughout life within a particular region of the hippocampus called the dentate gyrus. The hippocampus is critical for memory formation and the dentate gyrus encodes memories about space and events. While neurogenesis in this area is rapid early on in life, it steadily declines in rate with age.
Previous studies have demonstrated that boosting neurogenesis in rodents improves performance on certain learning tasks, but at the same time it seemed to make the mice perform worse on other memory tasks, in particular those that required recollection of past events. There has therefore been some uncertainty over the precise role of neurogenesis in memory.
Computational models have suggested that the addition of new neurons to the hippocampus leads to the degradation of existing memories. “More neurons increase the capacity to learn new memories in the future,” says Sheena Josselyn, one of the lead researchers of the study. “But memory is based on a circuit, so if you add to this circuit, it makes sense that it would disrupt it.”
To test this theory, scientists investigated the performance of both newborn and adult mice in a conditioning task. They first briefly placed the animals in a new box and gave them small electric shocks. To test which animals could remember the experience, they regularly placed the animals in the box but without the shock for several weeks and observed their responses. They found that the adult mice exhibited fear for over a month, whereas the young mice forgot the negative experience within a day.
In order to investigate whether neurogenesis was involved in this apparent forgetfulness of young mice, the researchers boosted neuron proliferation in different groups of adult mice either by exercise or treatment with Prozac, both of which are known to increase neurogenesis. They found that enhancing neurogenesis led to an increase in forgetfulness in the shock test and other memory tasks.
They also used these two methods of enhancing neural proliferation in animals that don’t experience much hippocampal growth after birth; degus and guinea pigs. These animals did not exhibit forgetting as infants, but the scientists were able to increase forgetfulness through exercise or treatment with Prozac.
Furthermore, when the scientists employed a technique that decreased neurogenesis by 50%, the young mice were less forgetful and exhibited a fear response for around a week, as opposed to a day, when returned to the box in which they were previously given shocks.
Taken together, these results support the theory that neurogenesis in the hippocampus can lead to the forgetting of previous memories in mice. At this stage, it is uncertain as to whether this is also the case in humans as it is unwise to extrapolate too much from animal models. However, co-lead researcher of the study Paul Frankland speculates that these findings could explain one mechanism of how antidepressants help depressed people, as it is possible they “promote some sort of clearing or forgetting,” although this is just hand waving at the moment.