July 23, 2019
Written by: Sarah Reitz
Think back to a fun summer weekend you had when you were in elementary school. Maybe you went to the pool with a friend and swam all day, or maybe you went camping with your family and made s’mores by the fire. Now try to remember a summer weekend from when you were 1 or 2 years old. Nearly impossible, right? This inability of adults to remember detailed memories from infancy and early childhood (usually birth to 3 or 4 years old) is called infantile amnesia. Infantile amnesia specifically affects episodic memories, which are memories that involve information about the “who, what, when, and where” of an experience. These memories are sometimes also referred to as declarative memories, since they involve information that must be consciously declared. Experiences in the first years of life are crucial for proper development, so why can’t we remember these events that play such a major role in shaping who we are today?
Scientists have been trying to explain why infantile amnesia occurs since the late 1800s. One early theory suggested that early memories fade quickly because infants and young children haven’t learned to speak yet, and without these language abilities to describe what is happening to or around them, it becomes very difficult to organize and store memories. However, research has shown that infantile amnesia also occurs in animals ranging from mice to monkeys, making a lack of language explanation very unlikely.
Infantile amnesia also cannot be explained by a complete lack of memory formation at early ages. Multiple studies have shown that both human and nonhuman infants learn simple tasks (a form of memory) at a rate similar to adults. While these studies ruled out memory formation as the cause of infantile amnesia, they did reveal a crucial finding that changed how we think about this phenomenon: infants and young children forget much more rapidly than adults.
The difference in memory retention across infancy and into childhood is striking: a 6 month old baby can remember an imitated action for 24 hours, a 9 month old can remember the action for 5 weeks, and a 10 month old can recall the action for up to 3 months1! This rapid forgetting also occurs in many mammals, including mice and rats. While infant rats learned a task just as quickly as adult rats, the infants rapidly forgot the task soon after training, while the adults showed clear memory of the task 42 days later2. Based on these results, scientists now think that it is not an impaired ability to form memories, but instead an increased rate of forgetting that might explain infantile amnesia across multiple species. But what happens in the brains of infants and young children to make them forget so much faster than adults? Is there an actual neurobiological explanation for infantile amnesia?
The hippocampus: our declarative memory hub
To better understand why we forget early memories, we have to understand how memories are formed in the first place. Although many regions of the brain are known to be involved in memory in humans and other mammals, the most important regions for processing declarative memories are the hippocampus and surrounding areas of the medial temporal lobe (Figure 1). The hippocampus and its connected regions encode, consolidate, maintain, and retrieve memories via long-lasting physical changes to neurons, such as an increased ability to transmit signals between other neurons involved in the memory. While the physical trace of some memories eventually shifts to cortical regions, research shows that the ability to recall memories involving autobiographical details may remain dependent on the hippocampus.
Neurogenesis: bad news for early memories?
The brain is constantly developing across childhood, making the task of identifying the underlying cause of infantile amnesia difficult. However, research in the hippocampus revealed that infants have high rates of neurogenesis – or production of new neurons – in this region compared to adults, who produce very few new neurons. Even more, the levels of neurogenesis are inversely related to memory stability across multiple species: ages when memories are rapidly forgotten are the same ages when neurogenesis is highest, and vice versa. When new neurons are introduced to already existing brain circuits, there is a chance that these circuits will be disrupted as the new neurons integrate. Since we know that memories are stored as physical changes in pathways involving hippocampal neurons, perhaps the introduction of new neurons in the maturing hippocampus destabilizes or alters the previously existing memory, preventing it from being remembered.
To test this theory, researchers took two approaches. First, they asked whether reducing neurogenesis in young mice would improve their ability to retain memories. The mice were taught to fear a specific room. Their memory of the room could be tested by measuring the mouse’s fear response, shown by increased freezing in place. Once the mice showed a clear fear memory of the room, researchers gave them a chemotherapy drug to suppress the neuron production that normally occurs in the hippocampus. When these mice were tested 2 weeks later, a time when normal young mice have forgotten the fear memory, they still showed increased freezing behavior, meaning they had retained the original memory3,4!
These results show that halting neurogenesis in young mice decreases infantile amnesia. But to further support the idea that neurogenesis is responsible for this type of forgetting, the researchers also did the reverse experiment: they increased neurogenesis in adult mice and tested whether this impaired their memory retention. Using the same fear task as the young mice, they saw that while normal adult mice showed a clear fear memory 4 weeks later, adults with increased neurogenesis showed fewer signs of fear3,4.
Finally, a different research group also studied infantile amnesia in guinea pigs, an animal that has much lower rates of neurogenesis in the hippocampus compared to mice and rats. Incredibly, unlike rats, infant guinea pigs showed clear memory of trained task up to 75 days after training, remembering just as well as the adults5! While this study is observational, it does support the idea that neurogenesis may be responsible for the high rates of forgetting in early childhood, and that lower levels of neurogenesis allow for more stable memories.
So if we just reduce neurogenesis in infants, they could grow up and remember experiences from their very early lives, right? While this is theoretically possible, we also know that neurogenesis is a crucial part of normal brain development. It is highly likely that preventing neurogenesis, while maybe allowing us to remember early memories, could also make it more difficult to form and retain new memories as we got older. If you ask me, I’ll keep my working adult memory and stick to looking through the boxes and boxes of old photos at my parents’ house to show me what my earliest years were like.
Cover image by 2081671 from Pixabay. https://pixabay.com/photos/child-fun-family-love-play-baby-3046494/
Figure 1 created with BioRender
- Mullally SL & Maguire EA (2014) Learning to remember: the early ontogeny of episodic memory. Dev Cogn Neurosci 9:12-29
- Campbell BA & Campbell EH (1962) Retention and extinction of learned fear in infant and adult rats. J Comp Physiol Psychol55:1–8
- Josselyn SA & Frankland PW (2012) Infantile amnesia: a neurogenic hypothesis. Learn Mem 19(9):423-433. doi: 10.1101/lm.021311.110.
- Akers KG et al. (2014) Hippocampal neurogenesis regulates forgetting during adulthood and infancy. Science 344(6184):598-602. DOI: 10.1126/science.1248903
- Campbell BA, Misanin JR, White BC, Lytle LD(1974)Species differences in ontogeny of memory: Indirect support for neural maturation as a determinant of forgetting. J Comp Physiol Psychol 87: 193–202
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