eli5: Why do most people have no memories from before age 4 or 5 years old?
In: 4721
i took a class on memory about a year ago as an undergrad neurosci major. my professor did research on this specific question. her explanation was that there is a sensitive period during which we develop our ability to make long term memories. before we develop the ability to form longterm memories, we are said to be in a state of infantile amnesia.
a sensitive period is a period of time during which the environment greatly influences our development for certain characteristics such as language or the development of ocular dominance columns in primary visual cortex (this is the classic example in neuroscience, pretty much if you deprive one eye of sensory input during the sensitive period, you develop a lazy eye) in an irreversible way. in rodents (at least rats and mice) the sensitive period for forming longterm memories is from day 17-20 from birth, meaning that rodents before this period can’t remember things a day after learning them. im not sure if we know exactly when this sensitive period is in humans.
my professor’s lab did a series of experiments that showed a few interesting things about this phenomenon
1) learning a task during this sensitive period led to long lasting expression of proteins required for synapse formation and maturation. during the sensitive period, learning causes expression of these proteins that is long lasting and gradually ramps up over hours, as opposed to learning in adult rodents, where the expression of the same proteins peaks roughly 30 minutes after the learning session. the implication here being that much larger scale structural changes are occurring in the young brain during the sensitive period after a learning event than in the adult brain. there are also some differences in which specific proteins being expressed.
2) rodents require two training sessions. one at day 17 during the sensitive period, which develops their ability to form long term memories for a particular type of task, after which a second training session at day 19 will cause them to actually form a longterm memory. if you only provide one training session to a rodent at day 19, they won’t remember the task later on.
3) developing the ability to form long term memories during the sensitive period is task-dependent. training a rodent on one task during the sensitive period develops their ability to form long term memories for similar tasks but not tasks that are very different from the original task.
there’s a number of limitations here, such as there being so many different types of memory and so many different tasks. it’s unclear if this generalizes to all types of memory. the specific types of memory tested here are contextual fear conditioning and novel object location. its also unclear just how integral this sensitive period is for the general ability of rodents to learn later on as much older adults (rats that don’t undergo the specific task training during the sensitive period are presumably able to learn the task as full adults?). and there is also the big caveat that rodents are much different from humans. despite the limitations however, the hippocampal memory system is a super interesting topic and there’s a lot of work being done right now dissecting the specific mechanisms for how it develops.
link for the paper
https://www.nature.com/articles/s41467-020-14461-3#Sec2
Latest Answers