HOW DOES THE HUMAN BRAIN LEARN? - NEUROPSYCHOLOGY - 2024

Our brain learns from experiences: facing our environment alters our behavior through the modification of our nervous system (Carlson, 2010). Despite the fact that we are still far from knowing exactly and at all levels each of the neurochemical and physical mechanisms that participate in this process, the different experimental evidences have accumulated quite extensive knowledge about the mechanisms involved in the learning process.

The brain changes throughout our lives. The neurons that compose it can be modified as a consequence of different causes: development, suffering from some type of brain injury, exposure to environmental stimulation and fundamentally, as a consequence of learning (BNA, 2003).

Basic characteristics of brain learning

Learning is an essential process that, together with memory, is the main means that living beings have of adapting to the recurring modifications of our environment.

We use the term learning to refer to the fact that the experience produces changes in our nervous system (NS), which can be long-lasting and involve a modification at the behavioral level (Morgado, 2005).

The experiences themselves change the way in which our body perceives, acts, thinks or plans, through the modification of the NS, altering the circuits that participate in these processes (Carlson, 2010).

In this way, at the same time that our organism interacts with the environment, the synaptic connections of our brain will undergo changes, new connections may be established, those that are useful in our behavioral repertoire strengthened or others that are not useful or efficient disappear (BNA, 2003).

Therefore, if learning has to do with the changes that occur in our nervous system as a result of our experiences, when these changes are consolidated we can speak of memories. (Carlson, 2010). Memory is a phenomenon inferred from those changes that occur in the NS and gives a sense of continuity to our lives (Morgado, 2005).

Due to the multiple forms of learning and memory systems, it is currently thought that the learning process and the formation of new memories depend on synaptic plasticity, a phenomenon through which neurons alter their ability to communicate with each other (BNA, 2003).

Types of brain learning

Before describing the brain mechanisms involved in the learning process, it will be necessary to characterize the different forms of learning, within which we can differentiate at least two basic types of learning: non-associative learning and associative learning.

-Non-associative learning

Non-associative learning refers to the change in the functional response that occurs in response to the presentation of a single stimulus. Non-associative learning in turn can be of two types: habituation or sensitization (Bear et al., 2008).

Habituation

The repeated presentation of a stimulus produces a decrease in the intensity of the response to it (Bear et al., 2008).

Example: if you lived in a house with only one phone. When it rings, she runs to answer the call, however every time she does, the call is for someone else. As this event occurs repeatedly, they will stop reacting to the phone and may even stop hearing it (Bear et al., 2008).

Sensitization

The presentation of a new or intense stimulus produces a response of increased magnitude to all subsequent stimuli.

Example: Suppose you are walking on a sidewalk in a well-lit street at night, and a blackout suddenly occurs. Any new or strange stimulus that appears, such as hearing footsteps or seeing the headlights of an approaching car, will upset it. The sensitive stimulus (blackout) led to sensitization, which intensifies their response to all subsequent stimuli (Bear et al., 2008).

-Associative learning

This type of learning is based on the establishment of associations between different stimuli or events. Within associative learning we can distinguish two subtypes: classical conditioning and instrumental conditioning (Bear et al., 2008).

Classical conditioning

In this type of learning there will be an association between a stimulus that causes a response (unconditioned response or unconditioned response, RNC / RI), unconditioned or unconditioned stimulus (ENC / EI), and another stimulus that does not normally provoke the response, conditioned stimulus (CS), and that will require training.

The paired presentation of the CS and the US will involve the presentation of the learned response (conditioned response, CR) to the trained stimulus. Conditioning will only occur if the stimuli are presented simultaneously or if the CS precedes the ENC in a very short time interval (Bear et al., 2008).

Example: an ENC / EC stimulus, in the case of dogs, can be a piece of meat. Upon viewing the meat, the dogs will emit a salivation response (RNC / RI). However, if a dog is presented as a stimulus by the sound of a bell, it will not present any particular response. If we present both stimuli simultaneously or first the sound of the bell (CE) and then the meat, after repeated training. The sound will be able to provoke the salivation response, without the meat being present. There has been an association between food and meat. Sound (EC) is capable of provoking a conditioned response (CR), salivation.

Instrumental conditioning

In this type of learning, you learn to associate a response (motor act) with a significant stimulus (a reward). For instrumental conditioning to occur, it is necessary that the stimulus or reward occurs after the individual's response.

Furthermore, motivation will also be an important factor. On the other hand, an instrumental type of conditioning will also occur if instead of a reward, the individual obtains a disappearance of an aversive valence stimulus (Bear et al., 2008).

Example: if we introduce a hungry rat into a box with a lever that will provide food, when exploring the box the rat will press the lever (motor act) and observe that food appears (reward). After you do this more times, the rat will associate pressing the lever with getting food. Therefore, you will press the lever until you are satisfied (Bear et al., 2008).

Neurochemistry of brain learning

Empowerment and depression

As we referenced earlier, learning and memory are thought to depend on synaptic plasticity processes.

Thus, different studies have shown that learning processes (among which are those described above) and memory, give rise to changes in synaptic connectivity that alter the strength and communication capacity between neurons.

These changes in connectivity would be the result of molecular and cellular mechanisms that regulate this activity as a consequence of neuronal excitation and inhibition that regulates structural plasticity.

Thus, one of the main characteristics of excitatory and inhibitory synapses is the high level of variability in their morphology and stability that occurs as a consequence of their activity and the passage of time (Caroni et al., 2012).

Scientists specialized in this area are specifically interested in long-term changes in synaptic strength, as a consequence of long-term potentiation (PLP) - and long-term depression (DLP) processes.

• Long-term potentiation: there is an increase in synaptic strength as a consequence of the stimulation or repeated activation of the synaptic connection. Therefore, a consistent response will appear in the presence of the stimulus, as in the case of sensitization.
• Long-term depression (DLP): there is an increase in synaptic strength as a consequence of the absence of repeated activation of the synaptic connection. Therefore, the magnitude of the response to the stimulus will be less or even zero. We could say that a process of habituation occurs.

Habituation and awareness

The first experimental studies interested in identifying the neural changes that underlie learning and memory, used simple forms of learning such as habituation, sensitization or classical conditioning.

Against this background, the American scientist Eric Kandel focused his studies on the gill retraction reflex of the Aplysia Califórnica, starting from the premise that neural structures are analogous between these and higher systems.

These studies provided first evidence that memory and learning are mediated by the plasticity of synaptic connections between neurons involved in behavior, revealing that learning leads to profound structural changes that accompany memory storage (Mayford et al., 2012).

Kandel, like Ramón y Cajal, concludes that synaptic connections are not immutable and that structural and / or anatomical changes constitute the basis of memory storage (Mayford et al., 2012).

In the context of neurochemical mechanisms of learning, different events will take place for both habituation and sensitization.

Habituation

As we mentioned earlier, habituation consists of a decrease in the intensity of the response, a consequence of the repeated presentation of a stimulus. When a stimulus is perceived by the sensory neuron, an excitatory potential is generated that allows an effective response.

As the stimulus is repeated, the excitatory potential progressively decreases, until it finally fails to exceed the minimum discharge threshold necessary to generate a postsynaptic action potential, which makes muscle contraction possible.

The reason why this excitatory potential decreases is due to the fact that, as the stimulus is continuously repeated, there is an increasing output of potassium ions (K ⁺), which in turn causes the closure of calcium channels (Ca ²⁺), which prevents the entry of calcium ions. Therefore, this process is caused by a decrease in the release of glutamate (Mayford et al, 2012).

Sensitization

Sensitization is a more complex form of learning than habituation, in which an intense stimulus produces an exaggerated response to all subsequent stimuli, even those that previously elicited little or no response.

Despite being a basic form of learning, it has different stages, short and long term. While short-term sensitization would imply rapid and dynamic synaptic changes, long-term sensitization would lead to long-lasting and stable changes, the consequence of profound structural changes.

In this sense, in the presence of the sensitizing stimulus (intense or new), a release of glutamate will occur, when the amount released by the presynaptic terminal is excessive, it will activate the postsynaptic AMPA receptors.

This fact will allow the entry of Na2 + into the postsynaptic neuron, allowing its depolarization as well as the release of NMDA receptors, which until now were blocked by Mg2 + ions, both events will allow a massive entry of Ca2 + into the postsynaptic neuron.

If the sensitizing stimulus is presented continuously, it will cause a persistent increase in Ca2 + input, which will activate different kinases, leading to the early expression of genetic factors and protein synthesis. All of this will lead to long-term structural modifications.

Therefore, the fundamental difference between the two processes is found in protein synthesis. In the first of them, in the short-term sensitization, its action is not necessary for it to occur.

For its part, in long-term sensitization, it is essential that protein synthesis takes place so that lasting and stable changes take place that aim at the formation and maintenance of new learning.

Consolidation of learning in the brain

Learning and memory are the result of structural changes that occur as a consequence of synaptic plasticity. For these structural changes to take place, it is necessary that a long-term enhancement process, or consolidation of synaptic strength, takes place.

As in the induction of long-term sensitization, both protein synthesis and expression of genetic factors that will lead to structural changes are necessary. For these events to occur, a series of molecular factors must take place:

• The persistent increase in the entry of Ca2 + in the terminal will activate different kinases, leading to the initiation of the early expression of genetic factors and the synthesis of proteins that will lead to the induction of new AMPA receptors that will be inserted into the membrane and will maintain PLP.

These molecular events will result in the alteration of the dendritic size and shape, with the possibility of increasing or decreasing the number of dendritic spines in certain areas.

In addition to these localized changes, current research has shown that changes also occur on a global level, as the brain acts as a unified system.

Therefore, these structural changes are the basis of learning, in addition, when these changes tend to last over time, we will be talking about memory.

References

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3. Caroni, P., Donato, F., & Muller, D. (2012). Structural plasticity upon learning: regulation and fuctions. Nature, 13, 478-490.
4. Fundamentals of behavioral physiology. (2010). In N. Carlson. Madrid: Pearson.
5. Mayford, M., Siegelbaum, SA, & Kandel, ER (nd). Synapses and Memory Storage.
6. Morgado, L. (2005). Psychobiology of learning and memory: fundamentals and recent advances. Rev Neurol, 40 (5), 258-297.