![]() To test this theory, researchers took two approaches. 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. 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. 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. 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. ![]() The brain is constantly developing across childhood, making the task of identifying the underlying cause of infantile amnesia difficult. Neurogenesis: bad news for early memories? ![]() 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. ![]() 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. 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). To better understand why we forget early memories, we have to understand how memories are formed in the first place. Together with other areas of the medial temporal lobe, the hippocampus works to encode, consolidate, maintain, and retrieve memories. The hippocampus: our declarative memory hub Figure 1: The hippocampus (shown in teal) is the main memory hub in the brain. 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? 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. 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 later 2. 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 months 1! This rapid forgetting also occurs in many mammals, including mice and rats. 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. Multiple studies have shown that both human and nonhuman infants learn simple tasks (a form of memory) at a rate similar to adults. Infantile amnesia also cannot be explained by a complete lack of memory formation at early ages. However, research has shown that infantile amnesia also occurs in animals ranging from mice to monkeys, making a lack of language explanation very unlikely. 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. Scientists have been trying to explain why infantile amnesia occurs since the late 1800s. 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? These memories are sometimes also referred to as declarative memories, since they involve information that must be consciously declared. Infantile amnesia specifically affects episodic memories, which are memories that involve information about the “who, what, when, and where” of an experience. 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. Now try to remember a summer weekend from when you were 1 or 2 years old. 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. Think back to a fun summer weekend you had when you were in elementary school.
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