Have you ever wondered why it is so difficult to retain memories from your early childhood? Now, by performing experiments in rodents, scientists provide a plausible answer to this question, showing that baby memories may be erased by the generation of new brain cells.
Memory formation is a key process that allows all organisms, vertebrates and invertebrates, to collect information, act on and combine that information and store it for the future. This invaluable process, is basically a filing system in our brain that contains everything we have learnt and experienced in our life so far. Memory formation is important for many reasons, ranging from our ability to communicate by memorizing language and words, to learning and remembering danger in everyday life so that we are able to survive. In other words, memories allow us to live a normal life, without having to learn words faces and ideas over and over again everytime they cross our path.
But, how are memories formed? The hippocampus, named after its similarity in shape to a seahorse, has been shown to play an important role in the transformation of short-term memory to long-term memory. It is part of the limbic system of the brain, a complex set of structures that is associated with many different functions, including emotions and long-term memory formation. Since both sides of the brain are symmetrical, the hippocampus can be found on both sides of the brain. If one side of the hippocampus is damaged, memory will remain normal, as long as the other side is intact. Damage on both sides of the hippocampus can lead to amnesia. Hippocampus function can also be reduced by age. By the time people reach their 80s, they may have lost as much as 20 percent of the nerve connections in the hippocampus. While not all older adults show this neuron loss, those who do show decreased performance on memory tests.
Throughout their life, human mice and other mammals grow new neurons in the hippocampous, rapidly at first, but more slowly with age. Researchers had previously shown that enhancing neural proliferation boosts memory formation in adult mice. However, no one has ever looked at what happens after memory formation and memory sustenance. Now, Sheena Josselyn together with her husband Paul Frankland, from the Hospital for Sick Children in Toronto, Canada, have taken a close look at this interesting question and their exciting and surprising findings offer a possible explanation as to why people may not be able to recall memories from early childhood.
Is infantile amnesia an effect of the growth of new brain cells in the hippocampus?
Sigmund Freud was the first to offer an explanation for human forgetfulness in their early years, more than 100 years ago. According to his theory, adults wanting to suppress sexual urgings and traumas from that period of time, is the cause of infantile amnesia. Since then, other theories have emerged, including the incomplete development of language in young children, the lack of neurological development of the infant brain preventing the formation of long-term memories and emotional development. But, according to Sheena Josselyn, incomplete language development could not be the reason for infantile amnesia, since this phenomenon is also observed in most animals.
In order to examine if infantile amnesia was somehow tied to brain cell formation, Sheena Josselyn and her team turned to rodents. Initially, the researchers examined whether changes in brain cell formation (neurogenesis) could influence the ability of the hippocampus to form long-term memories. Using a fear-conditioning task, in which they trained animals to fear an environment where they received repeated foot electric shocks, scientists observed that adult animals were able to retain that memory and freeze when placed in the same environment after 28 days, in contrast to newborn mice. Newborn mice, however, were able to retain the memory when tested 1 day after training.
Using chemical (neurogenic drugs like memantine and fluoxetine) and genetic manipulations (e.g. deletion of p53, a protein that suppresses cell proliferation) to suppress neuronal growth after learning, Josselyn and her team were able to boost the long-term memory of mice. On the other hand, promoting neuronal proliferation in adults mice through 4 to 6 weeks of regular exercise – an activity known to boost brain cell formation – reduced the memory retention of adult mice.
To obtain evidence for their surprising findings, scientists also tested the ability of guinea pigs and Chilean rodents called degus, to learn fear. Both of these animals have longer (~ 65 days) gestation periods than mice (~ 21 days), so they are more neurologically mature at birth and have reduced hippocampal neurogenesis after birth. Even though these animals, in contrast to mice, do not have infantile amnesia, researchers were able to mimic this effect through exercise and by administering drugs that boost brain cell growth.
Does the ‘amnesia’ caused by new brain cell formation serve a greater purpose?
So far, the notion about neurogenesis has been that it can help make new memories. However, now looking at the effect of neurogenesis on previously formed memories, seems to show that this process may actually be counterproductive for memory formation. In fact, it seems to eradicate previously formed memories. This amazing finding, observed by Sheena Josselyn and her team of researchers, may serve a greater purpose. In fact, this process, could actually act as a ‘spring cleaning’ system, clearing out all the clutter and allowing new memories to form. And, maybe forgetting is not a bad thing. 'Maybe sometimes it is good to clear out some memories and forget some things that are not so important', says Josselyn.
Akers, K. G. et al. Science 344, 598–602 (2014)
Nature News (08 May 2014) | doi:10.1038/nature.2014.15186