Memory has perhaps been the most studied cognitive faculty by all neuroscientists. In a century characterized by increasing life expectancy, much of the effort has focused on studying the decline, normal and pathological, of memory in the elderly.
However, today I will talk roughly about the development of memory in the early ages. Be specific, of the development of memory in the fetus (that is, from the 9th week of pregnancy until conception, week 38 approximately) and in the newborn.
Memory in childhood
We will probably all agree that babies are super smart and are already learning in their mother’s womb. More than one mother could certainly tell us more than one anecdote on this, I’m sure. But does declarative memory really exist? And, if there is, why do most of us not remember anything from our childhood until the age of three?
Also, I inform you that if they have memories from before 2-3 years, it is probably a false memory. This phenomenon is called infantile amnesia. And now we might ask ourselves: Does infantile amnesia mean that neither the fetus, nor the newborn, nor the child up to the age of 3 have a memory? Obviously not. It is generally assumed that memory is given in different ways and that each of these presentations involves different regions and circuits of the brain. Learning involves many memory mechanisms and some of them are unrelated to the hippocampus (the fundamental structure for consolidating new memories).
I’m going to speak about three fundamental learning mechanisms: Classical conditioning, operant conditioning and explicit or declarative memory. I will briefly present each of these concepts and show what postulates the main research in humans on the neurodevelopment of these functions, essential for normal learning in children.
Classical conditioning is a type of associative learning. It was described in para. XIX by Ivan Pavlov -the widely spoken experience of the bell and salivating dogs-. Basically, in classical conditioning, a “neutral stimulus” (without any fitness value for the organism) is associated with an “unconditioned stimulus”. That is, a stimulus that naturally produces a response (similarly, but not equally, to a reflex). Thus, the “neutral stimulus” becomes a “conditioned stimulus” because it will result in the same response as the “unconditioned stimulus”.
So, do babies pair up? A small experiment was performed in which a small puff of air, or “puff”, was performed on the eye (unconditioned stimulus), which involved a blinking response due to the air – in the form of a reflex -. In subsequent trials, the “buf” was performed concurrently with the administration of a specific auditory tone (“neutral stimulus”). After a few repetitions, the mere production of the tone gave rise to the blinking response – it had become a “conditioned stimulus” -. Therefore, the tone and “buf” had been associated.
And the fetus, is it able to associate? Babies have been shown to respond to stimuli presented to them before birth. For this, the heart rate of a melody presented during pregnancy through the mother’s abdomen was measured. After the baby was born, the heart response was compared by presenting new melodies (control melodies) from the previously learned melody. It has been observed that the heart rate selectively changes depending on 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 the 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 is the cerebellum.
Neurogenesis – the birth of new neurons – of the cerebellar cortex ends around 18-20 weeks of gestation. In addition, at birth, Purkinje cells – the main cells of the cerebellum – exhibit a morphology similar to that of adults. During the first months after childbirth, changes in biochemistry and neural connectivity lead to the cerebellum being fully operational.
Still, there will be small variations. In the first few months, the most conditionable stimuli are taste and olfactory stimuli, while in the later stages, the conditionability to other stimuli is increased.. When emotional aspects are involved in classical conditioning, associative learning involves other structures, neurodevelopment is more complex, as more factors need to be taken into account. Therefore, I will not speak about it today because it would deviate from the main theme of the text.
Another type of associative learning is operant or instrumental conditioning. Its discoverer was Edward Thorndike, who investigated the memory of rodents through labyrinths. It is basically a type of learning which, if the behaviors are followed by pleasant consequences, will repeat themselves more, and the unpleasant ones will tend to disappear.
This type of memory is difficult to study in the human fetus, which is why most of the current studies have been done on infants under one year of age. One experimental method that has been used is presenting a toy to a baby, such as a train that will move if the child pulls a lever. Obviously, babies associate the pull of the lever with the movement of the train, but in this case we will find significant differences according to age. In the case of 2 month olds, if once they match the movement of the lever with that of the train, we remove the stimulus, then instrumental learning will take about 1 to 2 days. Essentially, this means that if we present the stimulus to them after about four days, the learning will have been forgotten. However, brain development at an early age progresses at a breakneck pace, while subjects as young as 18 months can sustain instrumental learning for up to 13 weeks later. So we can sum it up by saying that the memory gradient of operative conditioning improves with age.
What structures does operative conditioning involve? The main neuronal substrates are those which form the neostriatum -Caudado, Putament and Nucli Accumbens-. For those unfamiliar with this structure, they are essentially 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 are also involved in affective and cognitive functions and there is an important relationship with the limbic system. By the time we are born, the striatum is already fully formed and its biochemical pattern matures at 12 months.
So, one could infer the possibility that there was a primitive instrumental conditioning in the fetus; although the circumstances and context make it difficult to think of effective experimental models to assess this function.
And now comes the basic question. Do newborns have declarative memory? We must first define the concept of declarative memory and differentiate it from its sister: implicit or procedural memory.
Declarative memory is awhat is popularly known as memory, i.e. the fixation on our memories of facts and information acquired through learning and experience, And which we consciously access. Instead, implicit memory is the one that defines models and procedures motor skills that are evident by its execution and not so much by its conscious memory – and if you don’t believe me, try to explain all the muscles you use to ride a bike and the specific movements you perform.
We will find two fundamental problems in the study of declarative memory in newborns: first, the baby does not speak and therefore we will not be able to use verbal evidence for its evaluation. Second, and due to the previous point, it will be difficult to discriminate between tasks in which the baby uses his implicit or explicit memory.
The conclusions on the ontogeny of memory which I will speak about briefly, they will be of the paradigm of “preference for novelty”. This experimental method is simple and consists of two experimental phases: first, a “familiarization phase” in which the child is shown for a fixed period of time a series of stimuli – usually images of different types – and a second ” test phase “in which two stimuli are presented to him: a new one and one that they had seen before in the familiarization phase.
in general visual preference for novelty on the part of the baby is observed, by means of different measuring instruments. Therefore, the idea is that if the newborn looks at the new stimulus for a longer time, it means that he recognizes the other. Would the recognition of new files therefore be an appropriate paradigm for the construction of declarative memory? It has been shown that patients with medial temporal lobe (MTL) lesions show no preference for novelty if the period between the familiarization phase and the test is longer than 2 minutes. In primate injury studies it has also been seen that LTM and in particular the hippocampus are necessary structures for recognition and therefore novelty preference. Thus, other authors have reported that novelty-preference behavioral measures are more sensitive to hippocampal damage than other recognition tasks. These results would question the conceptual validity of the novelty preference paradigm. However, it is generally considered a type of pre-explicit memory and a good paradigm for study, but not the only one.
Characteristics of declarative memory
Therefore, I will talk about three basic characteristics of declarative memory from this experimental model:
By coding – and not by consolidation – we mean the baby’s ability to integrate and correct information. Overall, studies show that 6 month olds already show a preference for novelty and so we concluded that they recognize. However, we found significant differences in coding times compared to 12 month olds, for example, needing the latter lower exposure times in the familiarization phase to code and fix stimuli. To be precise, a 6 month old needs three times as long to show similar recognition ability as a 12 month old. However, the age-related differences diminish from 12 months and children aged 1 to 4 years showed equivalent behaviors with similar familiarization periods. In general, these results suggest that if the beginnings of declarative memory appear in the first year of life, we will find an effect of age on coding ability that will occur mostly in the first year of life. We can relate these changes to different neurodevelopmental processes that I will talk about later.
By retention we mean when or “how long” the newborn can keep the information, To be able to recognize it afterwards. Applying to our paradigm would be the time that we will let pass between the familiarization phase and the test phase. Because coding times are equivalent, babies over a few months old may have higher retention rates. In an experiment comparing the performance of this function in 6 and 9 month old children, it was observed that only 9 month old children could retain information if a “delay” was applied between the two phases of the experiment. . However. Children of 6 months showed a preference for novelty only if the testing phase was carried out immediately after the familiarization phase. In general, the effects of age on retention are evident from early childhood.
Recovery or evocation
By evocation we mean the ability to save a memory from long-term memory and make it operational for a specific purpose. It is the main ability that we use when we bring our experiences or memories to it. It is also the most difficult ability to assess in infants due to lack of language. In a study using the paradigm we talked about, the authors solved the language problem in a rather original way. They made different groups of newborns: 6, 12, 18 and 24 months. In the familiarization phase, they were presented with objects in a background with a specific color. When all 4 groups were applied to the testing phase immediately after, all showed similar novelty preferences as long as the background color in the testing phases was the same as in the familiarization phase. When this was not the case and a background of another color was applied in the test, only 18 and 24 month old infants showed a preference for novelty. This shows that babies’ memory is extremely specific. Small changes in the central stimulus or context can lead to reduced resilience.
The neurodevelopment of the hippocampus
To understand the neurodevelopment of the hippocampus and relate it to the behavioral events we have discussed, we need to understand a number of processes related to neural maturation that are common in all areas of the brain.
First, we have the bias of thinking that “neurogenesis”, or the birth of new neurons, is about the development of the brain. It is a mistake. Maturation also involves “cell migration”, whereby neurons reach their appropriate final position. When they have reached their position, the neurons send their axons to the target regions which will innervate and, later, these axons will be myelinated. When the cell and this operation begin the processes of “dendritic arborization” of the cell body and the axon. In this way, we will obtain a large number of synapses – “Synaptogenesis” – which will be largely eliminated during childhood according to our experiences. In this way, the brain makes sure to leave alone these synapses which participate in the functioning circuits. In more adult stages, “apoptosis” will also play a very important role in the elimination of neurons which, like synapses, do not play an important role in neural circuits. Therefore, maturation in our brain is not about adding, but rather about subtracting. The brain is a spectacular organ and is always in search of efficiency. The maturation is similar to the work done by Michelangelo to carve a block of marble of his David. The only difference is that we are sculpted by our experiences, parents, loved ones, etc., to give birth to our phenotype.
By this speech I meant something very simple that we will now quickly understand. If we look at the neuroanatomy of the hippocampus, we will be surprised to know that most of the structures related to it (entorhinal cortex, subiculum, Ammonis horn …) can already be differentiated at week 10 of gestation. , and at week 14-15. they are already differentiated at the cellular level. Cell migration is also very rapid and already resembles that of an adult in the first trimester. So why if the hippocampus is already formed and functioning within three months of the baby’s birth, do we see so much difference in our experiences between 6 and 12 months, for example? Well, for the same reason that I already pointed out in other articles: the hippocampus is not everything and neither is neurogenesis. The dentate gyrus – a structure related to the hippocampus – requires a much longer period of development than the hippocampus and the authors claim that its granular cell layers mature at 11 months and would adopt a morphology similar to that of the adult to a year. . On the other hand, in the hippocampus we find different groups of GABAergic cells – small inhibitory interneurons – which 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 we have even seen that GABA plays opposite roles depending on the age we observe. These cells mature between 2 and 8 years. Thus, much of the mnemonic 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 inhibitory circuits.
The thing doesn’t end there …
As we have seen, declarative memory is medial temporal lobe (MTL) dependent, and the maturation of the dentate gyrus tells us a lot about the differences we see in infants 1 month to 2 years old. But is that all? There is a question that we have not yet answered. Why does infantile amnesia occur? Or why don’t we remember anything for 3 years? Once again the question is answered if we leave the hippocampus alone for a while.
The maturation of connections between LTM and regions of the prefrontal cortex has been associated with a large number of mnemonic strategies in adult children. Declarative memory constantly evolves during childhood and improves through coding, retention and retrieval strategies. Neuroimaging studies have shown that if the memory capacity of a story is linked to TML in children aged 7 to 8 years; in children aged 10 to 18, it is linked to both LTM and the prefrontal cortex. Therefore, one of the main hypotheses explaining infantile amnesia is the poor functional connections between the prefrontal cortex and the hippocampus and the LTM. so I there is no definitive conclusion to this question and other molecular hypotheses in this regard are also of interest. But these are points that we will address on another occasion.
When we are born, the brain makes up 10% of our body weight – when we are adults it is 2% – and we spend 20% body oxygen and 25% glucose – more or less the same as ‘an adult. In turn, we are dependent beings in need of parental care. No baby can survive alone. We are an easy target in any natural environment. The rationale for this “neuro-decompensation” is that the fetus and baby have a considerable amount of learning mechanisms – some of which have not been cited here, such as the ability to “prime”. There is something all grandmothers say and it’s true: babies and children are sponges. But they are because our evolution has required it. And this not only in humans, but in other mammals.
So, declarative or explicit memory exists in infants, but in an immature way. To mature satisfactorily requires experience and education of the social environment in which we find ourselves enveloped as gregarious mammals. But why study all this?
In a society that has put clinical emphasis on cancer and Alzheimer’s disease, more minority diseases such as childhood paralysis, autism, various learning disabilities, ADHD – which exists gentlemen – are being forgotten. , If it exists, epilepsy in children and for a long time. on (sorry if I leave a lot more unnamed minority); affecting our children. They cause delays in the development of their school. They also cause them backwardness and social rejection. And we’re not talking about people who have completed their lifecycle. We are talking about children whose integration into society may be at stake.
Understanding normal neurological development is essential to be able to understand pathological development. And understanding the biological substrate of a pathology is essential to search for pharmacological targets, for effective non-pharmacological therapies and to seek means of early diagnosis and prevention. And for this we need to investigate not only memory, but all cognitive faculties affected by the mentioned pathologies: language, normal psychomotor development, attention, executive functions, etc. Understanding this is essential.
Text edited and edited by Frederic Muniente Peix
- Barr R, Dowden A, Hayne H. Changes in the development of delayed imitation for infants 6 to 24 months. Child behavior and development 1996; 19: 159-170.
- Chiu P, Schmithorst V, Douglas Brown R, Holland S, Dunn S. Making memories: a cross-sectional investigation of the coding of episodic memory in childhood using fMRI. Developmental Neuropsychology 2006; 29: 321-340.
- Hayne H. Development of childhood memory: implications for infantile amnesia. Development review 2004; 24: 33-73.
- McKee R, Squire L. On the development of declarative memory. Journal of Experimental Psychology: Learning, Memory, and Cognition 1993; 19: 397-404
- Nelson C. The ontogeny of human memory: a cognitive neuroscience perspective. Developmental Psychology 1995; 31: 723-738.
- Nelson, C .; de Haan, M .; Thomas, K. Neural Bases of Cognitive Development. A: Damon, W .; Lerner, R .; Kuhn, D .; Siegler, R., editors. Manual of child psychology. 6th ed. Flight. 2: Cognitive, perception and language. New Jersey: John Wiley and Sons, Inc .; 2006. p. 3-57.
- Nemanic S, Alvarado M, Bachevalier J. The hippocampal / parahippocampal regions and memory of memory: insights into the visual comparison of parell versus delayed object mismatch in monkeys. Journal of Neuroscience 2004; 24: 2013-2026.
- Richmong J, Nelson CA (2007). Accounting for change in declarative memory: a cognitive neuroscience perspective. Dev. Revelation 27: 349-373.
- Robinson A, Pascalis O. Development of flexible visual recognition memory in human infants. Development Science 2004; 7: 527-533.
- Rose S, Gottfried A, Melloy-Carminar P, Bridger W. Familiarity preferences and novelty in children’s recognition memory: implications for information processing. Developmental Psychology 1982; 18: 704-713.
- Seress L, Abraham H, Tornoczky T, Kosztolanyi G. Cellular formation in the formation of the human hippocampus from mid-gestation to the end of the postnatal period. Neurosciences 2001; 105: 831-843.
- Zola S, Squire L, Teng E, Stefanacci L, Buffalo E, Clark R. Deterioration of recognition memory in monkeys after damage limited to the hippocampal region. Journal of Neuroscience 2000; 20: 451-463.
- Shaffer RS, Kipp K (2007). Developmental psychology. Childhood and adolescence (7ªed). Mexico: Thomson editors SA