Most would agree that growing new neurons (brain cells) would be a good thing. However, a new study has reported that the development of new neurons in the hippocampus, an area of the brain where memories are stored) can erase old memories. The findings help explain why early childhood events cannot be recalled by older individuals. The study results were published in the journal Science on May 8 by researchers at the University of Toronto, Toronto, Canada.
The study authors note that in humans and several other mammals, including mice, new neurons are formed in the hippocampus throughout life. The growth is initially quite rapid; however, it slows with age. They note that previous research has found that stimulating the growth of neurons before learning can improve memory formation in adult mice; however, their new study shows that after information is learned, neuron growth can degrade those memories.
Although this memory loss appears detrimental to one’s cognitive function, the disruptive role of these new neurons has some benefit, because erasing old memories makes room for new ones. They note that memories are based on circuits within the brain; thus, the addition of new neurons disrupts old circuits.
For the study, the investigators tested newborn and adult mice on a conditioning task, which trained the rodents to fear an environment in which they received repeated electric shocks. All the mice rapidly learned the task; however, the infant mice remembered the negative experience for only one day after training. In contrast, the adult mice retained the negative memory for several weeks. The researchers noted that this difference appears to correlate with differences in the growth rate of new neurons. They were able to enhance the persistence of memories in the infant mice by genetically and chemically suppressing growth of new neurons after learning. In addition, in adult mice, four to six weeks of regular exercise, which is an activity that is known to promote neuron proliferation, reduced the persistence of the previously learned fear.
The authors caution that the genetic and chemical manipulations cannot be applied readily to humans; therefore, their findings will be difficult to pursue in people. However, they note that both mice and humans have ‘infantile amnesia’, or pronounced forgetting of early life experiences.