Blanks for the memories: Why we can’t remember our first years of life.

Recent findings reveal the causes of infantile amnesia.

Do you remember how you learned to walk or speak? Do you have any memories about experiences before the age of three at all? No? Don’t worry, this phenomenon applies to all of us and is known as infantile amnesia.

Infantile amnesia refers to the inability of adults to recall memories of early childhood. This fact seems paradoxical: Firstly, young children are very well able to remember certain events or places, sometimes in astonishing detail. For example, if children are read the same story several times they often remember its exact wording. Secondly, it is widely accepted that early life events can strongly influence the rest of a person’s lifetime, a fact that is usually discussed in the context of mental disorders. So, on the one hand, children seem to have impressive memory abilities, but on the other, almost no early memories are available in adulthood. How can these facts be reconciled?

The scientific breakthrough

There have been multiple attempts to explain why infantile amnesia occurs, most of them focusing on the concept of an immature brain that is not yet completely competent. This idea sounds logical, because human infants are not born with a fully developed brain and thus might need some time to develop cognitive skills such as memory formation. However, until recently, it was not clear what kind of maturation could make the brain capable of memory processing. Moreover, researchers did not even agree on whether the reason for childhood amnesia lies in forming, storing or retrieving memories. But recently, Travaglia and colleagues managed to unravel the mystery.

From electroshocks in rats to amnesia in humans

It should be noted that infantile amnesia is not a uniquely human phenomenon, it has been observed in various other species, including rats. So in their study, Travaglia and colleagues used two groups of rats at different ages. The experimental group consisted of rats that were 17-days-old, comparable to 3-year-old human children in their infantile amnesia period. The rats in the second group were 24 days old, which is comparable to humans at the age of 7 years, when memories can already be maintained for a long period of time.

The authors exposed these rats to electroshocks in order to find out whether they would remember this negative experience. The initial foot shock was delivered when the rats first entered a new compartment, which normally leads to avoidance of this place. After this first experience, the older rats indeed showed avoidance behavior, even some days later. In contrast, the younger rats avoided the compartment right after this experience, but entered it a day later.

As expected, the younger rats had forgotten their initial experience after a short period of time, which is an indicator for infantile amnesia. But when these rats later in life received a reminder of their first experience, i.e. the presentation of both context and foot shock, they avoided the place where they were first exposed to an electroshock.

What do these results tell us? Apparently, the young rats did not remember the electroshocks, because after one day they did not avoid the associated compartment anymore. After a single reminder shock, however, they seemed to recollect their previous experience. Consequently, when they first experienced a shock, some kind of memory trace must have been formed that could be retrieved only after experiencing a reminder shock.

“Thus, this study shows that the mechanism underlying infantile amnesia is a deficiency in memory retrieval, not in memory forming.”

The brain learns how to remember

The same study also experimented with possible neurobiological mechanisms involved in brain maturation. The most important finding was that the maturation of the hippocampus, a brain area typically associated with memory processing, plays a major role in infantile amnesia. The hippocampus undergoes a critical period sometime during development during which this area acquires its final form and function.

Untitled
The hippocampus, a brain structure whose shape resembles a seahorse, is associated with memory processing in general. Therefore, it is not surprising that it seems to be involved in the phenomenon of infantile amnesia.
Pixabay (CCO)
 The study reveals that the development during this critical period, where the hippocampus is “learning” how to process memories efficiently, depends on an interplay between molecular mechanisms and experience. The authors showed that specific proteins injected into the rats’ hippocampus end the critical period and accelerate the acquisition of functional competence. Importantly, the amount of these proteins is modulated by experience. Taken together, the results of this study suggest that the reason for infantile amnesia lies in the maturation of the hippocampus that still needs to acquire its functional competence.

Lessons from infantile amnesia

The study by Travaglia and colleagues provides relevant insights for clinical treatments. The fact that the hippocampus undergoes a critical period of development highly dependent on learning and experience, highlights the importance of early childhood. If children do not get enough appropriate stimulation from their environment the corresponding brain regions and mechanisms may not develop properly. This can later lead to developmental learning disabilities and psychiatric disorders (see also http://neurosciencenews.com/neurodevelopment-memory-learning-4696/).

Other studies have focused on how of aversive life events in early childhood impact memory development. There is compelling evidence that early exposure to stress alters neurophysiological processes and leads to an accelerated maturation of the hippocampus causing the critical period to be shortened. This allows early experiences to be recalled for a longer period of time explaining how early traumatic experiences can impact our lives as adults despite the phenomenon of infantile amnesia.

Why do we forget?

The cause of forgetting in general is also a highly debated topic (see also).

Recent research indicates that the mechanism for memory loss depends on the type of information. But determining whether the act of forgetting has a deeper meaning is much more difficult. Freud would have argued that early memories are suppressed because of their potential traumatic influence.

No matter what the true reason for memory loss might be, we can use our knowledge from studies about infantile amnesia to induce forgetting in contexts where this is desirable, like in the treatment of certain mental disorders. Some anxiety disorders, for example, cause persistent fear in response to specific stimuli, and mechanisms that cause forgetting in infants could also contribute to fear extinction in adults.

An important mechanism that is thought to play a causal role in ending the critical period is the development of perineuronal nets, specific neuronal structures which are considered stable memory storage sites. Degrading perineuronal nets by implanting inhibitory interneurons in specific brain regions could be a potential method for facilitating fear erasure in anxiety patients.

In conclusion, the research about infantile amnesia shows, that the development of our memory system depends on both molecular mechanisms and experiences. These findings can help us to understand the influence of traumatic childhood experiences and to develop treatments to induce forgetting in patients. While the majority of studies about memory focus exclusively on forming memory, the research reported here shows how much we can learn about memory from forgetting.

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