Memory and emotion influence each other. On the one hand, the emotion It is a behavioral output and as such serves to express memories; the consequences of the memory, when we remember something, they often involve emotional expression (eg, with the inflection of language, the sweat of the hands). On the other hand, the events that occurred around emotional experiences are usually remembered better or in a very vivid way.
- 1 The amygdala
- 2 The emotional memory
- 3 Studies with animals of emotional memory
- 4 Human studies of emotional memory
- 5 Memory modulation
- 6 Animal studies on memory modulation
- 7 Human studies on memory modulation
The amygdala or tonsil complex It is located inside the temporal lobe, just ahead of hippocampus.
If we looked inside the human brain, we would see the tonsil in the depths of the temporal lobe, in an anterior position in the hippocampus.
The amygdala receives multimodal information (visceral, specific thalamic inputs with sensory information, information on areas of association of the cortex). The intrinsic connectivity of the amygdala combines these inputs and orchestra a wide range of behavioral influences. For example, project at thalamic areas and cortical from which it receives information, sends influences to other systems related to other forms of memory (striatum, hippocampus), and also has outputs to the autonomous systems, endocrine and motor, which generate the bodily responses of emotional expression.
The amygdala is in a central position between the processing of cortical information, the circuitry of the limbic system and the hypothalamic outputs that intervene in the response mechanisms mediated by the brain stem.
These connections and the Recent research with injuries and records indicate that the amygdala appears to be a key structure for learning and emotional memory; It also seems important for memory modulation.
Experience can change the way we feel what is processed. The way we evaluate information (eg, if we add positive or negative feelings to a stimulus, our preferences and aversions) is a product, unconscious, of learning.
We feel in a certain way a type of food, a place, or a supposedly neutral stimulus, such as a tone, due to the experiences that have been associated with certain foods, places, tones.
A demonstration of unconscious learning about likes and dislikes is found in the study of the effect of "mere exposure." In one experiment, photos of geometric shapes were presented with a very fast exposure time (1 millisecond per shape). In a subsequent memory test, the subjects did not recognize any of the figures they had seen as familiar. Instead, they showed preferences for the shapes they had seen when compared to totally new ones. Thus, the subjects had developed positive judgments about the material they had seen, although they were not aware of having seen it before.
It seems that learning that involves emotions can be done independently of conscious cognition.
Studies with animals of emotional memory
The biology of emotional learning has been studied in a task of classical conditioning called "conditioned fear." In this study the rat or mouse is placed in a box that has a ground that can be electrified providing a slight shock (unconditioned stimulus, IE) to the animal's legs. After a couple of minutes there is a tone (conditioned stimulus, EC) followed by shock. After one or two times of matching the EC and the EI, the animal responds to the tone, presented in any environment, as if it were afraid or faced with a threat or danger: it stays still, it lifts its hair, increases its heart rate ...
The fear response learned is eliminated with the bilateral lesion of the tonsil.
The circuit that seems important for learning conditioned fear is based on the fact that the information of the EC and the EI converge in the amygdala. The tonsil sends information to different structures making possible the expression of fear.
The tone information seems to reach the amygdala (the basolateral nucleus) from sensory areas of the thalamus that first process the stimulus and from the peririnic cortex and insular. The central nucleus of the amygdala is critical for communicating the state of fear to the large number of systems that act together to express the organism's response to fear.
There are studies of registration of neuronal activity that show changes in the neural activity of the central nucleus of the tonsil parallel to the emergence of CR. Other experiments demonstrate plasticity in the receptor fields of the thalamus, auditory cortex and basolateral tonsil neurons.
Another task used to study the neural bases of emotional learning is the potentiation of the startle response. Many species, including humans, are more frightened at a loud noise if they were already in a state of fear or activation. The task is to match a stimulus in neutral principle (eg, a light) with a shock. Then, another stimulus (loud noise) is presented alone or in the presence of light. The startle reflex (a jump) is greater when noise is presented along with light than when it is presented alone.
The tonsil is not necessary for the startle response, but it is for the potentiation of the response through fear; The tonsil, then, has a modulatory influence on the startle reflex circuit.
The amygdala is not only necessary for the fear learned or to enhance the scare response, but also participates in the basic ability to express the fear.
With a tonsil lesion, a syndrome that is characterized by a decrease in the response to affective stimuli occurs; the animals they become calmer and show no signs of fear.
Tonsil stimulation can produce a complex pattern of behavior and changes in autonomic responses that resemble fear.
Human studies of emotional memory
The tonsil also has an important role in learning fear in humans.
When a neutral tone (EC) is associated with a loud noise (EI), after several pairings, the subjects exhibit signs of emotional activation when the tone occurs. One of the signs of activation consists of changes in sweating, such as an increase in skin conductance.
Patients with tonsil lesions do not develop an emotional reaction to the EC, although they can explain that a tone (EC) was normally followed by a loud noise (EI).
In patients with lesion of the entire medial temporal lobe including the tonsil, they are able to withstand harsh or unpleasant conditions without complaining, they do not even generate a normal galvanic response of the skin. Nor are they able to identify any stimulus as painful, although their perception does not have to be altered.
Selective lesions of the amygdala also produce deficits in the recognition of facial expressions of emotions, without affecting the language, perception or memory of faces.
There are electrophysiological data in monkeys and humans that show that the tonsil neurons respond to faces. In addition, a recent study with RMF (functional magnetic resonance imaging) showed that the tonsil is preferentially activated in response to the vision of faces that express fear than to the vision of neutral faces.
In other neuroimaging studies, changes in the activity of the amygdala have been detected when the subjects saw scenes that produced fear or when psychiatric patients remembered traumatic events of the past.
All these data in humans and animals indicate the following:
The amygdala is an "execution system" for analyzing affective information and for expressing emotional response..
There are many data that indicate that memories associated with strong emotions are more vivid, precise and stable than the memories of more ordinary or neutral events. This makes adaptive sense since in this way organisms better remember important events.
An example of those detailed and intense memories that were stored once and that can last a lifetime are the events that were surprising and full of emotion. An example may be the memory we have of what we were doing, where we were and who we were with when we learned that the two passenger planes had crashed into the twin towers of New York.
There are specific neural mechanisms that modulate (facilitate or deteriorate) the strength of the memories that have just been formed. It is thought that the amygdala could be a key structure in the modulation of memory in two ways:
- Tonsil activation modulates memory by increasing arousal and attention during learning experiences.
- The tonsil modulates memory consolidation due to the effect of hormones that are released in situations of moderate stress or activation.
Stress hormones (catecholamines, glucocorticoids, opioids) act as endogenous modulators for the memory of the events that caused their release.
The amygdala is the brain region most clearly involved in the modulating effects on the memory of drugs and hormones. Direct stimulation of the tonsil can modulate memory, and the effects of stimulation of the amygdala on memory depend on the integrity of the adrenal glands.
The modulating mechanisms of memory are based, at least in part, on the effects of emotional activation on memory, through the amygdala.
Animal studies on memory modulation
In laboratory animals it has been observed that slightly stimulating experiences produce the release of a variety of hormones in the blood and brain. When these same hormones are injected into the animals shortly after they have been trained in a learning task, the animals retain the training better.
Stress hormones act through the amygdala, since the lesions of the tonsil or the terminal stria (its most important aference-eference pathway) block the modulation of the memory of many drugs and hormones.
When the tonsil is activated it can cause the cerebral cortex to activate and facilitate the processing of the stimuli present; also the anatomical connections between the amygdala and the hippocampus could directly influence declarative memory.
Human studies on memory modulation
There are experiments that demonstrate the role of the amygdala in facilitating memory in humans. Volunteer subjects watched some slides while listening to a story. The story and the slides explained that a boy was hit by a car and was taken to the hospital for an emergency operation.
- The volunteer subjects experienced a great emotional activation during the central part of the story (the one that told about the accident and the surgery).
- They also remembered this part of the story better than the initial and final parts (which told relatively neutral events).
- The central part of the story was remembered better for these subjects than by other people who saw the same images but listened to a story that interpreted the slides in a non-emotional way (the boy had seen some shattered cars and also witnessed an emergency drill in a hospital).
- Patients with tonsil-restricted lesions remembered the non-emotional parts of the story as well as healthy volunteer subjects, but did not have the normal tendency to remember the emotional part of the story better than the other two parts.
- Subjects receiving adrenergic antagonists (catecholamine antagonists) showed no facilitation of declarative memory by the emotional component of the story. The administration of adrenergic agonists enhances the memory of the emotional part of the story.
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