Much of the following was reviewed in a recent Knowing Neurons blog entry.
Freud was one of the first psychologists to try and explain the phenomenon of "infantile amnesia"; the inability to remember early childhood experiences. As with many of his concepts, he proposed that this was a product of repressing the early psychosexual development in children. More recently, a cellular mechanism by which this occurs has been proposed. Neurogenesis, the production of new neurons, is a process that occurs primarily during development but extends into adulthood in two canonical neurogenic regions; one of which is the dentate gyrus (DG) of the hippocampus. The hippocampus is a structure that is critical for long-term episodic memory storage.
It had been previously suggested that the massive upregulation of neurogenesis in the DG during early childhood development blocks memory formation. When researchers took baby mice and selectively downregulated their DG neurogenesis, their performance on memory tasks improved significantly. Conversely, other studies that upregulated neurogenesis in adult mice after learning a task showed memory defecits. Later studies seemingly contradict this result, as Cre-lox trangenic mice that were induced to increase DG neurogenesis by 3.6 -fold in adult mice showed memory improvement. The key here is the order of events; increasing AN before a task is learned improves memory, whereas after a task was learned interferes with memory. The latter situation is much more akin to what happens in the developing brain which is adding new neurons both before and after and after childhood events occur.
Thus, it seems that during early childhood, the extended period during which human children are unable to form and retain long-term memories is a necessary cost for building a big, complex, and slow-growing brain.
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