Brain must be forgotten to grow
We called them
fairy stones . In fact, they were just colored gravel stones — those that can be bought to decorate an aquarium — found in the sandbox where I played in the preschool years. But my classmates and I endowed them with magical properties, hunted them like treasures, and carefully sorted into heaps of sapphires, emeralds and rubies. One of my earliest memories is how I sift sand in search of these mysterious gems. At that time I was not more than three years old. My memories associated with kindergarten are also limited to individual episodes: the circle of letters on paper in pink dotted line; watching a film about the inhabitants of the ocean; my teacher cuts a large roll of paper so that we can draw self-portraits with our fingers.
When I try to remember life before the fifth birthday, only those glimpses come up in my memory - these strikes with a match in the dark. However, I am sure that I felt, thought and learned so much. Where are all these years?
Psychologists call this pronounced forgetfulness "
infantile amnesia ." On average, people's memories extend no further than the age of 3.5 years. All the time before that - the dark abyss. “They have been interested in this phenomenon for a long time,” said Patricia Bauer of Emory University, a leading expert on memory development. “It requires our attention because it is a paradox: very young children show evidence of their having memories of the events of their lives, but as adults, we still have relatively few such memories.”
In the past few years, scientists have finally begun to understand what exactly is happening in the brain at a time when we are giving up memories of our earliest years. “We are now adding a biological basis to this story,” said Paul Frankland, a neuroscientist at the Hospital for Sick Children in Toronto. New evidence suggests that as a necessary step in the path to adulthood, the brain must leave most of its childhood behind.
Sigmund Freud gave infantile amnesia its name at the beginning of the 20th century. He claimed that adults forget their early years as a result of the repression of their disturbing memories of sexual arousal. And although several psychologists supported this statement, most usually believed that this phenomenon was explained by the fact that children did not form stable records until the age of 7 years - although this idea had very little evidence. For almost 100 years, psychologists have thought that memories of infancy do not persist, since they were originally unreliable.
The end of the 1980s marked the beginning of the reformation of child psychology. Bauer and other psychologists began to test the memory of children through a sequence of actions — for example, by building a simple toy gong, and hitting it, and then watching if the child could imitate the actions in the correct order after a period of time from several minutes to several months.
Experiment after experiment demonstrated that memory in children under 3 years of age is actually preserved, although with certain limitations. At the age of 6 months, memories remain at least a day. In 9 months - for a month. By two years - for a year. In a landmark 1991 study [Hamond, NR & Fivush, R. Memories of Mickey Mouse. Cognitive Development 6, 433-448 (1991)] scientists found that children of 4.5 years of age are able to recall in detail the trip to Disney World that took place 18 months before. However, at the age of 6, children begin to forget many of these early memories. In the 2005 experiment conducted by Bauer and her colleagues, children of 5.5 years of age recalled more than 80% of the events that occurred to them at the age of 3 years, and children of the age of 7.5 years remembered less than 40% [Van Abbema, DL & Bauer, Recollections of the recent and distant past. Memory 13, 829-845 (2005)].
This work exposed the controversy in the very center of infantile amnesia: babies can create and evoke memories of the first years of their lives, but most of these memories eventually disappear at a rate exceeding the typical speed of forgetting the past in adults.
Perhaps, some researchers have decided, long-term storage of memories requires language or self-awareness, which we lack in childhood. But, although verbal communication and self-awareness undoubtedly increase a person’s memory, their absence cannot fully explain infantile amnesia. After all, certain animals, with a rather complex and large, with respect to the size of their body, have a brain - for example, mice and rats - there is no language, and, probably, our level of self-awareness, and they also lose their memories in childhood. .
Perhaps, the researchers reasoned, this paradox has a more fundamental physical basis common to humans and other mammals with a large brain. The only question is - what?
Between birth and early adolescence, the brain continues to develop part of the fundamental circuits and to engage in the thickening of electrical pathways with adipose tissue so that their conductivity increases. During the growth spurt, the brain induces countless new bridges between neurons. In fact, in the early years we have much more connections between brain cells than in adulthood; most of them are deleted. All this extra brain mass is raw clay, from which our genes and experience mold a brain suitable for a particular environment. Without such a malleable brain, children could never learn so much in such a short time.
As Bower and colleagues discovered, such adaptability comes at a price. In the process of long-term development of the brain outside the womb, a complex and large network of various brain regions responsible for creating and maintaining memory is in the process of creation, Bauer explains, and it doesn’t manage to form memories as well as it will in an adult. As a result, long-term memories created in the first three years of life turn out to be the least stable of all the memories we have made, and they are very easily destroyed with age [Bauer, PJ The Life I Once Remembered
www.zerotothree.org (2009)].
Earlier this year, Frankland and colleagues published a study showing another way for the brain to lose childhood memories: they not only degrade, but also hide [Akers, KG et al. Hippocampal neurogenesis regulates forgetting during adulthood and infancy. Science 344, 598-602 (2014)]. A few years ago, Frankland and his wife, Sheena Jocelyn — also a working neuroscientist at the Hospital for Sick Children — began to notice that the mice they studied coped worse with certain memory tests after living in a cage with a running wheel.
As it was known to the couple, exercises on the treadmill contribute to
neurogenesis - the growth of new neurons - in the hippocampus, a part of the brain in the shape of a seahorse, which is necessary for memory to work. But if neurogenesis in the adult hippocampus probably contributes to learning and memorizing, Carl Deisseroth of Stanford and others have suggested [Deisseroth, K. et al. Adult excitation-neurogenesis coupling: mechanisms and implications. Stanford University] that he can also contribute to the process of forgetting. Just as a certain maximum number of trees can fit in a forest, a certain maximum number of neurons can fit in the hippocampus. New brain cells flood the area where other neurons are located, or even completely replace them, which can break or change the small contours in which individual memories are stored. Therefore, it is possible that the high rate of neurogenesis in childhood is partly responsible for childhood amnesia.
To test this idea, Frankkland and Jocelyn transferred small mice and adult mice from plastic containers the size of a shoebox, which they have known all their life, into large metal cages that they had never seen before. In the new containers, they slightly beat the paws of mice with a shock. Mice quickly learned to bind metal cells with electric shocks, and in fear we were numb every time they returned to this place.
And while the little mice began to forget about this connection the very next day - and to relax after being transferred to a metal cage - adult mice never forgot about the danger. But when adults, after receiving electric shocks, stumbled upon a running wheel, and began to use it, stimulating neurogenesis, they began to repeat the behavior of the mice, forgetting about the danger.
Prozac had a similar effect, stimulating the growth of nerve cells. Conversely, when researchers suppressed mouse puppy neurogenesis with chemical or genetic means, young animals formed more stable memories.
In order to examine in detail how neurogenesis can change memory, Frankland and Jocelyn used the virus to insert a fluorescent protein gene into the DNA of new mouse brain cells. Glowing paint allowed us to consider that the new cells do not replace the old ones; they are connected to existing contours. This suggests that technically, many small neural circuits that store our early memories, neurogenesis does not erase. Instead, they are fundamentally rearranged, which probably explains the strong difficulty in accessing the original memories. “We think the problem is access,” says Frankland, “but also in semantics, too. If memory cannot be accessed, it is essentially erased. ”
This restructuring of the memory circuits means that although some of our childhood memories do disappear, others remain, but they undergo changes and distortions. Studies show that people are able to extract at least some childhood memories by responding to certain requests — pulling, for example, the earliest memories associated with the word “milk” —or representing a house, school, or a particular place associated with a certain age, which allows related to him the memories pop up on their own.
But even if we can unravel several individual memories that have survived restless growth and wilt cycles in a child’s brain, we cannot fully trust them; some of them may be partially or completely invented. In an advanced study, Elizabeth Loftus of the University of California, Irvine showed that our earliest memories are an inseparable blend of real memories, information from other people's stories and subliminal fictional scenes.
In one of the sets of revolutionary experiments conducted in 1995, Loftus and her colleagues offered volunteers short stories about their childhood, provided by their relatives [Loftus, EF & Pickrell, JE. Psychiatric Annals 25, 720-725 (1995)]. But the participants did not know that one of these stories - about how they got lost in the mall in five years - was made up. However, a quarter of volunteers said they remembered that. And even when they were informed that one of the stories they had read was invented, some of them did not understand that the story about the shopping center was invented.
When I was very young, I got lost in Disneyland. This is what I remember: it was December, and I watched a toy train ride around the Christmas village. When I turned around, my parents were gone. The horror of cold molasses dripped down my body. I began to sob and walk in the park in search of parents. A stranger approached me and took me to a giant building on which were television screens that broadcast video from the security cameras of the entire park. Did I see my parents there? I have not seen. We went back to the train, where we found our parents. I ran and rushed into their arms, filled with joy and relief.
Recently, for the first time in a long time, I asked my mother what exactly she remembered about that day at Disneyland. She says that it was spring or summer, and that she and my whole family last saw me near the “Jungle Cruise” boats, and not near the railroad that was at the entrance to the park. As soon as they realized that I was missing, they went to the lost and found office. The park worker really found me and brought me to this center, where I was comforted with the help of ice cream.
It was unpleasant to learn about such a serious contradiction with what I thought was a fairly accurate memory, so I asked my mother to look for some evidence in our family photo albums. But she could only find pictures from an earlier trip. We probably will never get solid evidence of what happened. We will have something more ephemeral: these tiny coals of the past that are in our memory, flickering like the
gold of fools .