Memory in early childhood

Possibly the memory It has been the cognitive faculty that has been studied most exhaustively by all the professionals of the neuroscience. In a century that has been characterized by increased life expectancy, a large part of the efforts They have been focused on the study of the decline, normal and pathological, of memory in the elderly population.

However, today I will speak, in broad strokes, of the development of memory in early ages. Being specific, of the development of memory in the fetus (that is, from the 9th week of pregnancy until it is conceived, approximately week 38) and in the neonate.

Memory in childhood

We will probably all agree that babies are super smart and that they already learn in their mother's womb. More than one mom sure could tell us more than one anecdote about it, I'm sure. But does declarative memory really exist? And, if it exists, why do most of us remember nothing of our childhood before the age of three?

In addition, I inform you that if they have any memory from before 2-3 years it is probably a false memory

. This phenomenon is called infantile amnesia. And now we could ask ourselves, if there is infantile amnesia, does it mean that neither the fetus, nor the neonate, nor the child up to 3 years of age have memory? Obviously not. In general, it is assumed that memory occurs in different ways and that each of these presentations involves different brain regions and circuits. Learning involves many memory mechanisms and some of them are not related to the hippocampus (the fundamental structure for the consolidation of new memories).

I'll talk about three fundamental learning mechanisms: the classical conditioning, the operant conditioning and the explicit memory or declarative. I will briefly introduce each of these concepts and show what the main human research on the neurodevelopment of these functions, essential for learning normal of the child.

Classical conditioning

Classical conditioning is a type of associative learning. It was described in the s. XIX by Ivan Pavlov –The widely talked about experiment of the little bell and the salivating dogs. Basically, in classical conditioning a "neutral stimulus" (without any adaptive value for the organism) is associated with an "unconditioned stimulus". That is, a stimulus that innately produces a response (similar to, but not the same, a reflex). Thus, the "neutral stimulus" becomes a "conditioned stimulus" since it will give rise to the same response as the "unconditioned stimulus".

So do babies associate? A small experiment was carried out in which a small breath of air, or "buf", was carried out in the eye (unconditioned stimulus), which entailed a blink response due to air - by way of reflection-. In subsequent tests, the "buf" was performed at the same time as the administration of a specific auditory tone ("neutral stimulus"). After a few trials, the simple production of the tone gave rise to the blink response - it had become a "conditioned stimulus". Therefore, the tone and the "buff" had been associated.

And the fetus, is it capable of associating? Babies have been shown to respond to stimuli that have been presented to them before birth. For this, the heart rate of a melody presented during pregnancy through the mother's abdomen has been measured. Once the baby was born, the cardiac response was compared by presenting new melodies (control melodies) of the previously learned melody. The heart rate was observed to selectively change at the melody presented during pregnancy. Therefore, the fetus is able to associate stimuli.

From a neuroanatomical point of view it is not surprising that babies and fetus generate associations. In these types of associative learning, in which fear or other emotional responses are not involved, one of the main brain structures in charge of it is the cerebellum.

The neurogenesis –The birth of new neurons– of the cerebellar cortex is completed around 18-20 weeks of gestation. In addition, at birth the purkinje cells –Main cells in the cerebellum– show a morphology similar to that of the adult. During the first months after delivery, there are biochemical and neuronal connectivity changes that lead to the cerebellum being fully operational.

Even so, there will be slight variations. In the first months, the most conditioned stimuli are the gustatory and olfactory ones, while in later stages the conditionality to other stimuli increases.. When emotional aspects intervene in classical conditioning, associative learning involves other structures, whose neurodevelopment is more complex, since it is necessary to take into account more factors. Therefore, I will not talk about it today because it would divert the main topic of the text.

Operant conditioning

The operant conditioning or instrumental it is another type of associative learning. The discoverer of it was Edward thorndike, what investigated the memory of rodents using mazes. Basically it is a type of learning that consists in that if the behaviors are followed by pleasant consequences, they will be repeated more, and the unpleasant ones will tend to disappear.

This type of memory is difficult to study in the human fetus, so most current studies have been done in babies under one year of age. An experimental method that has been used is the presentation of a toy to a baby, such as a train that will move if the child pulls a lever. Obviously babies associate pulling the lever with the movement of the train, but in this case we will find significant differences depending on age. In the case of 2-month-old children, if once they have associated the movement of the lever with that of the train, we withdraw the stimulus, then the instrumental learning will last approximately 1-2 days. This basically means that if after about four days we present the stimulus to them, the learning will be forgotten. However, early brain development proceeds at a frantic rate and instead 18-month-olds can sustain instrumental learning for up to 13 weeks later. So we can summarize it by saying that the mnesic gradient of operant conditioning improves with age.

What structures does operant conditioning involve? The main neural substrates are those that form the neostriates –Caudate, Putament and Núcleo Accumbens–. For those who are unaware of this structure, they are basically subcortical gray matter nuclei - that is, below the cortex and above the brainstem. These nuclei regulate the pyramidal motor circuits, responsible for voluntary movement. They also intervene in affective and cognitive functions and there is an important relationship with the limbic system. By the time we are born, the striatum is fully formed and its biochemical pattern matures at 12 months.

Therefore, the possibility that a primitive instrumental conditioning existed in the fetus could be inferred; although the circumstances and context make it difficult to think of effective experimental designs to evaluate this function.

Declarative memory

And now comes the fundamental issue. Do neonates have declarative memory? We should first define the concept of declarative memory and differentiate it from its sister: the implicit memory or procedural.

Declarative memory is toquella that is popularly known as memory, that is, the fixation in our memories of facts and information that are acquired through learning and experience, and to which we consciously access. On the other hand, implicit memory is that which fixes motor patterns and procedures that is revealed by its execution and not so much by its I remember consciously - and if you don't believe me, try to explain all the muscles you use to ride a bike and the specific movements that you perform–.

We will find two fundamental problems in the study of declarative memory in neonates: in First, the baby does not speak and therefore we will not be able to use verbal tests for his evaluation. Secondly, and as a consequence of the previous point, it will be difficult to discriminate the tasks in which the baby makes use of her implicit or explicit memory.

The conclusions on the ontogeny of memory that I will talk about in a few moments, will be from the paradigm of "the preference for novelty." This experimental method is simple and consists of two experimental phases: first, a “familiarization phase” in the one in which the child is shown during a fixed period of time a series of stimuli –generally images of different types– and a second "test phase" in which they are presented with two stimuli: a new one and one that they had previously seen in the test phase. familiarization.

Usually the visual preference for novelty by the baby is observed, by means of different measuring instruments. Therefore, the idea is that if the newborn looks longer at the new stimulus, he means that he recognizes the other. Would, therefore, the recognition of new images be an adequate paradigm for the construct of declarative memory? It has been seen that patients with damage to the medial temporal lobe (LTM) do not show preference for novelty if the period between the familiarization phase and the test is longer than 2 minutes. In primate lesion studies it has also been seen that the LTM and especially the hippocampus are necessary structures for recognition and, therefore, for preference to novelty. Even so, other authors have reported that behavioral measures of novelty preference are more sensitive to hippocampal damage than other recognition tasks. These results would call into question the construct validity of the novelty preference paradigm. However, in general it is considered as a type of pre-explicit memory and a good study paradigm, although not the only one.

Declarative memory characteristics

So that, I will talk about three basic characteristics of declarative memory from this experimental model:

Coding

By coding - not consolidation - we mean the baby's ability to integrate information and fix it. Overall, studies show that 6-month-olds already show a preference for novelty and, therefore, we conclude that they recognize it. Even so, we found significant differences in coding times compared to 12-month-old children, for example, needing these last shorter exposure times in the familiarization phase to encode and fix the stimuli. To be specific, a 6-month-old needs three times as long to show a recognition capacity similar to that of a 12-month-old. However, the differences in relation to age diminish after 12 months of age and there has been seen that children from 1 to 4 years old show equivalent behaviors with similar periods of familiarization. In general, these results suggest that while the beginnings of declarative memory appear in the first year of life, we will find an effect of age in the coding ability that will be especially in the first year of lifetime. These changes can be related to different neurodevelopmental processes that I will talk about later.

Retention

By retention we mean the time or "delay" in which the newborn can maintain information, to later be able to recognize it. Applying it to our paradigm, it would be the time that we allow to pass between the familiarization phase and the test phase. The coding times being equivalent, babies of more months can show higher retention percentages. In an experiment comparing the performance of this function in children aged 6 and 9 months, it was observed that only 9-month-olds could keep the information if a delay was applied between the two phases of the experiment. Instead. The 6-month-old children only showed preference to novelty if the test phase was carried out immediately after the familiarization phase. Broadly speaking, the effects of age on retention have been seen to occur until early childhood.

Recovery or evocation

By evocation we mean the ability to retrieve a memory from long-term memory and make it operational for a purpose. It is the main capacity that we use when we bring our experiences or memories to the present. It is also the most difficult ability to assess in babies due to lack of language. In a study using the paradigm we have discussed, the authors solved the language problem in a rather original way. They made different groups of neonates: 6, 12, 18 and 24 months. In the familiarization phase they were presented with objects on a background with a specific color. When the 4 groups were applied the test phase immediately afterwards, all showed preferences to the similar novelty as long as the background color in the test phase was the same as in the test phase. familiarization. When this was not the case, and a different colored background was applied in the test, only the 18 and 24-month-old babies showed a preference for novelty. This shows that babies' memory is extremely specific. Small changes in the central stimulus or in the context can lead to impaired resilience.

Neurodevelopment of the hippocampus

To understand the neurodevelopment of the hippocampus and relate it to the behavioral events of which we have spoken, we must understand a series of processes in relation to neuronal maturation that are common in all the brain areas.

First of all, we have the bias of thinking that “neurogenesis”, or the birth of new neurons, is all that brain development is summed up to. That is a huge mistake. Maturation also involves "cell migration," by which neurons reach their proper end position. When they have already reached their position, the neurons send their axons to the target regions that they will innervate and, subsequently, these axons will be myelinated. When the cell is already operational, the processes of "dendritic arborization" of the cell body and the axon will begin. In this way, we will obtain a large number of synapses - "Synaptogenesis" - which to a large extent will be eliminated during childhood based on our experiences. In this way, the brain makes sure to leave only those synapses that participate in operational circuits. In more adult stages, "Apoptosis" will also play a very important role, eliminating those neurons that, similar to synapses, do not have a relevant role in neuronal circuits. Therefore, maturing in our brain is not about adding, but rather about subtracting. The brain is a spectacular organ and it always seeks efficiency. Growing up is similar to the task Michelangelo did to sculpt his David from a block of marble. The only difference is that we are sculpted by our experiences, parents, loved ones, etc., to give rise to our phenotype.

With this speech I wanted to say something very simple that now we will quickly understand. If we look at the hippocampal neuroanatomy, we will be surprised to know that most of the structures that are related to it (cortex entorhinal, subiculum, Ammonis horn ...) can already be differentiated at week 10 of gestation, and at week 14-15 they are already differentiated cellularly. Cell migration is also very fast and in the first trimester it already resembles that of an adult. So why, if the hippocampus is already formed and operational three months after the baby is born, do we observe such a difference in our experiments between children of 6 and 12 months, for example? Well, for the same reason that I have already stressed in other posts: the hippocampus is not everything and neither is neurogenesis. The dentate gyrus - a neighboring structure of the hippocampus - requires a much longer development period than the hippocampus and the Authors affirm that its granular cell layers mature at 11 months of birth and would adopt a morphology similar to the adult one year after birth. age. On the other hand, in the hippocampus we find different groups of GABAergic cells - small inhibitory interneurons - which have been shown to play an essential role in the combined processes of memory and attention.

GABAergic cells are the ones that take the longest to mature in our nervous system and it has even been seen that GABA plays opposite roles depending on the age we observe. These cells mature between 2 and 8 years of age. Thus, a large part of the memory gradient that we observe in the capacity of coding, retention and retrieval will be due to the maturation of the connections between the hippocampus and the dentate gyrus and, in addition, to the formation of the circuits inhibitory.

This is not ending here...

As we have seen, declarative memory depends on the medial temporal lobe (LTM) and the maturation of the dentate gyrus explains much of the differences we observe in babies from 1 month to two years. But is that all? There is a question that we have not answered yet. Why does infantile amnesia occur? Or why don't we remember anything before about 3 years old? Once again the question is answered if we leave the hippocampus alone for a little while.

The maturation of the connections between the LTM and the regions of the prefrontal cortex have been related to a large number of memory strategies in the adult child. Declarative memory is in continuous development during childhood and improves thanks to strategies in the capacity of coding, retention and retrieval. Neuroimaging studies have shown that while the ability to recall a story is related to LTM in children aged 7 to 8 years; in children from 10 to 18 years of age it is related to both the LTM and the prefrontal cortex. Therefore, one of the main hypotheses that explain childhood amnesia is the poor functional connections between the prefrontal cortex and the hippocampus and the LTM. Even so there is no definitive conclusion to this question and other molecular hypotheses in this regard are also interesting. But these are points that we will deal with on another occasion.

Conclusions

When we are born, the brain represents 10% of our body weight - when we are adults it is 2% - and it uses up 20% of body oxygen and 25% of glucose - this is more or less the same as an adult. In exchange for this, we are dependent beings who need the care of parents. No baby can survive on its own. We are an easy target in any natural environment. The reason for this “neuro-decompensation” is that the fetus and the baby have a quantity considerable number of learning mechanisms - some of them have not been cited here, such as the ability to of "priming" -. There is something that all grandmothers say and it is true: babies and children are sponges. But they are because our evolution has demanded it. And this not only in humans, but in other mammals.

Therefore, declarative or explicit memory exists in babies, but in an immature way. To mature satisfactorily, it requires the experience and education of the social environment in which we find ourselves involved as gregarious mammals. But why study all this?

In a society that has focused its clinical attention on cancer and Alzheimer's, more rare diseases such as infantile paralysis, autism, various learning disorders, ADHD -which exists gentlemen, if it exists-, epilepsies in children and a long etcetera (I am very sorry if I leave much even more minority without to name); that affect our children. They lead to delays in their school development. They also produce delay and social rejection. And we are not talking about people who have completed their life cycle. We are talking about children whose insertion in society may be at stake.

Understanding normal neurodevelopment is essential to understanding pathological development. And understanding the biological substrate of a pathology is essential to search for pharmacological targets, effective non-pharmacological therapies and to seek early and preventive diagnostic methods. And for this we must not only investigate memory, but all the cognitive faculties that are affected in the aforementioned pathologies: language, normal psychomotor development, attention, executive functions, etc. Understanding this is essential.

Text corrected and edited by Frederic Muniente Peix.

Bibliographic references:

Papers:

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Books:

• Shaffer RS, Kipp K (2007). Developmental Psychology. Childhood and adolescence (7th ed). Mexico: Thomson editores S.A.