“The Fading Memories of Youth: The Mystery of ‘Infantile Amnesia’ Suggests Memory Works Differently in the Developing Brain”, 2024-03-14 ():
…People generally remember nothing from before age 3, and children’s memory abilities don’t fully mature until about age 7. “It’s a paradox in a sense”, says neuroscientist Flavio Donato of the University of Basel. “In the moment that the brain is learning at a rate it will never show again during the whole lifetime, those memories seem not to stick in the brain.”
…It appears the brain actually can create memories before age 3—although perhaps in a different way from adult memories—and those memories may persist into adulthood. But we can’t consciously access them.
No one is sure why infantile amnesia exists, but studies have shown that many other mammals also experience it, suggesting it’s not linked to language or self-awareness. Instead, this forgetting probably serves some evolutionary purpose, whether that’s helping young brains learn how to attach the proper importance to events or developing a framework for the memory systems they will use throughout life. “We’ve kind of just accepted [infantile amnesia] as a fact of life, as an unavoidable consequence of brain development”, whereas in truth it might be essential, says neuroscientist Tomás Ryan at Trinity College Dublin. Whatever it’s doing, he says, “it’s going to be something that transcends most of the mammalian kingdom.”…But it might not be advantageous for “precocial” species such as guinea pigs and degus, two rodent species that are more behaviorally independent at birth. Indeed, work from Frankland’s lab suggests these animals don’t experience infantile amnesia at all.
…The main goal, Power says, is to figure out exactly when the developing brain switches on the ability to form accessible long-term memories. “It’s really hard to progress to ask other questions if we don’t know exactly when it happens”, she says. Her early data indicate it’s at about 20 months. Children that age who learned to associate a toy with a certain location in each room can remember the information for up to 6 months, whereas younger children only remember it for about 1 month.
…Intriguingly, infantile amnesia seems to affect only certain kinds of memories, particularly the ones known as contextual memories, which involve connecting cues such as the layout of an environment with events that happen there. In humans, the forgotten memories include episodic memories: conscious recollections of where and when a specific event occurred. In contrast, young brains can recall other types of memories just fine, including semantic memories of the meanings of words and motor memories of skills such as how to draw a circle.
…Yet psychologists have found some evidence that early memories may linger, even if we can’t consciously access them. In one set of experiments, researchers taught babies as young as 2 months old that they could make a mobile over their crib move by kicking their feet. The youngest babies could only remember this for a few days. But 3 & 6-month-old babies remembered to kick their feet if researchers showed them a hint, such as the mobile moving on its own, suggesting the memory was still there but less accessible.
In another study, Newcombe found that 3-year-olds who see a set of images of different animals can’t explicitly remember them 3 months later. But when she blurred the images and brought them slowly into focus, children were faster to identify the animal in images they’d seen months earlier. Newcombe says such findings suggest young children can retain specific information at a subconscious, or implicit, level.
Research with young rats and mice suggests they, too, can access suppressed memories with a little help. In a 2016 study, Cristina Alberini, a neuroscientist at New York University, and her colleagues gave juvenile rats a foot shock when they stepped into a dark compartment within a white box. The young animals learned to stay out of the dangerous compartment, but forgot soon after. Once the animals were older, the researchers found they could jog their memory by showing them the white box and shocking them in a different colored box. Then, when the researchers returned the rats to the original white box, the combination of the two cues made the rodents remember to stay out of its dark compartment.
…His team used baby mice genetically engineered to make a light-sensitive protein in the set of neurons in the hippocampus that fired while the animals were learning to associate a box with a foot shock…A month later, when a mouse had forgotten the memory, the researchers flashed a light in the mouse’s brain through an optical fiber. The light-sensitive protein evidently reactivated the engram: The mouse froze, apparently in anticipation of a shock, even if it wasn’t in the box…mice trained as juveniles to find an escape hole in a box, a less fear-laden task, also appeared to form lasting engrams that could be reawakened though optogenetics.
…As children first begin to form accessible long-term memories, they aren’t very good at it, according to research by Riggins. Her team has found that children 4–8 years old struggle to separate similar patterns—believing they’ve seen a photo of a pencil with an eraser when they actually saw one without an eraser, for example—suggesting their memories may run together. Scanning the children’s brains suggested that as they grow older and become better at this task, certain parts of their hippocampi become smaller, which to Riggins suggests greater efficiency.
Newcombe thinks the ability to make fine distinctions among individual episodic memories just isn’t a high priority for the developing brain when it’s trying to learn so much about the world. “It’s more important to know about cats in general than Curtis the local cat next door”, she says.
As researchers continue to puzzle over the purpose of infantile amnesia, they’re also searching for clues about the underlying mechanisms. Ryan and Frankland propose that the rapid birth of new neurons, known as neurogenesis, in infants might be overwriting memories and that infant amnesia disappears once neurogenesis slows. When Frankland’s team used a drug to suppress neurogenesis in the hippocampus in infant mice, the juveniles performed as well as adults on memory tests. Treating adult animals with drugs or stimuli such as exercise wheels that increase the birth of neurons, meanwhile, caused amnesia. If old engrams are simply preempted by new, more important ones without necessarily breaking existing connections, Ryan says, the brain may never truly forget anything.
…In mice, the switch from amnesia to being able to form lasting memories is shockingly sharp—within a 4-day period. Donato’s lab is currently following specific neurons within engrams to see how they change during this transition. By looking at the brain every few hours, he hopes to figure out whether that transition is due to a change in cellular signaling, the formation of neuronal connections, or something else.
Alberini thinks the switch in memory capability is a part of normal brain development that corresponds with the closing of a “critical period”, a window of time during which a developing brain is especially malleable. Her team has found that when juvenile rats begin to make long-term memories, their hippocampi switch to using different molecular and cellular mechanisms. An accumulation of life experience, she believes, matures the hippocampus and drives this switch.
When her team exposed baby rats and mice to different experiences—a box with foot shocks or a memory test involving a toy placed in different locations—they found that each one caused cells in the hippocampus to take on adult-like molecular characteristics. Moreover, the animals were better at performing tasks related to that particular experience—but not unrelated ones—in the future. Alberini says this suggests that each experience, even though it does not leave a lasting accessible memory, stimulates the infant hippocampus to build a scaffold for later memory formation.
Work from other groups suggests disrupting that process can cause lasting harm. Richardson’s team and others have found that separating baby rats from their mothers or exposing them to stress hormones accelerated maturation of the hippocampus and prevented infantile amnesia. That memory boost came with a downside, though—these rats ended up being more anxious for the rest of their lives. “Having a good memory seems like a good thing, but that’s not normal progression”, Richardson says.
…In a recent study, Ryan and Power treated pregnant mice with a chemical that mimics a viral infection. Their male offspring showed autism-like symptoms, such as repetitive behavior, and never experienced infantile amnesia. Tests showed they were better both at recalling episodic memories and at remembering how to navigate mazes than mice whose mothers had not been treated—and the neurons in their hippocampi were more densely connected to one another, as they would be in a mature brain…Stress or infection in early life might activate microglia at the wrong time or in the wrong ways, potentially leaving the mice with an excess of synapses and an anomalously sharp memory.
Ryan says the same thing might happen in people. “It’s possible that there are humans going around who don’t have infantile amnesia”, he says. “It’s going to be very interesting to identify those people and figure out what is going on there.” [cf. hyperthymesia, Solomon Shereshevsky, savant syndrome?]