The amygdala is a part of the brain associated with emotions. It got its name from the Greek word in this sense, as its shape resembles an almond. The medial temporal lobe is also located just in front of the hippocampus. Like the hippocampus , the amygdala has two, one in each hemisphere of the brain. The amygdala is part of the neural network associated with emotions and memory, called the limbic system . Although it has historically been associated with deterrent and disturbing emotions such as fear, it is also known to be effective in rewarding positive emotions.
The amygdala contains a group of nuclei (nuclei) formed by neurons. Located in the outer and middle parts of the amygdala, the basolateral complex is the largest of these neuron clusters and consists of lateral, basal, and accessory-basal nuclei. The lateral nucleus receives data from the sensory cortices that process all types of sensory input. In mice, it has been shown that the amygdala receives auditory stimuli directly from the subcortical level, from the medial geniculate nucleus.
The cortical and medial nuclei are called the cortico-medial group. The sense of smell comes directly from the olfactory bulb and piriform cortex to the cortico-medial amygdala. Inhibitory neurons that control the flow of information from the basolateral complex to the central nucleus are located in the intercalated mass.
Apart from sensory inputs, the amygdala also receives signals from other cortical and subcortical brain areas. Intense data comes from the anterior cingulate and orbitofrontal cortices in the prefrontal cortex. The insula, hippocampus, and rhinal cortices are important input sources for the amygdala. Subcortical information comes to the amygdala from almost every neuromodulatory system.
Signals from the amygdala are directed to both subcortical and cortical brain structures. The central core regulates various autonomic, physiological and behavioral responses associated with the emotional state. The basal and accessory-basal nuclei are the main parts that send signals to the cerebral cortex. These signaling pathways enable the amygdala to regulate cognitive processes such as decision making, attention, and memory.
Emotional learning and many aspects of behavior have important functions in regulation. There are many emotions such as joy, sadness, disgust, excitement, regret, satisfaction. Many emotions have a value, such as positive or negative, and a low to high intensity. Animal experiments that examine the neural basis of emotion use physiological and behavioral measures that reflect the value and intensity of emotional experience. At the beginning of the twentieth century, Heinrich Klüver and Paul C. Bucy’s studies on monkeys showed that the amygdala has an important regulatory effect on nutrition, emotion and sexual behavior.
Pheromones and instinctive attractive and deterrent stimuli (specific smells, tastes, or sexual images) can produce certain physiological and behavioral expressions of emotional states. For olfactory stimuli, the cortico-medial amygdala is known to regulate instinctive emotional behaviors. In other self-sustaining instinctual stimuli, such as drug abuse, circuits in the basolateral cortex contribute to emotional responses.
Emotional learning is most commonly studied in animals and humans using Pavlovian conditioning. In this conditioning, a normally neutral stimulus is paired with an instinctive unconditioned aversive stimulus. This paradigm is called fear conditioning and can reinforce learning. Neuromodulatory inputs can also contribute to this learning. As the animal learns, the response of the amygdala neurons to the conditioned stimulus changes. Activation of neurons in the basolateral amygdala can also trigger learning. After learning, signals from the basolateral complex of the amygdala to its central nucleus trigger a series of physiological and behavioral responses associated with the emotional state. Fear conditioning is measured by parameters such as cessation of movement (freezing), an increased response in skin conductivity, or an increase in blood pressure.
Although studies of the amygdala are mostly done using aversive stimuli, there is ample evidence that it also plays a role in rewarding stimuli and appetitive learning. Activation of neurons in the amygdala in response to rewarding stimuli can trigger both Pavlovian and instrumental (where behavior is affected by outcomes) learning. A pathway from the amygdala to the ventral striatum may influence learned behavior in the reward process of addiction. In contrast, damage to the amygdala generally does not impair appetite learning. Therefore, it is thought that this learning style is supported by parallel neural pathways that do not involve the amygdala.
Regulation of Emotions
Emotional responses to sensory stimuli do not arise simply from instinct or learning. It can also be modified by extinction and cognitive control mechanisms. These mechanisms are based on connections between the prefrontal cortex and the amygdala. Extinction is also a learning process. It occurs when the conditioned stimulus is repeated in the absence of the previously associated unconditioned stimulus. Thus, the previously revealed answer is lost. Neural pathways from the prefrontal cortex to the amygdala regulate the extinction phenomenon. Complex circuits in the central nucleus, basolateral complex, and intersecting masses are involved in modifying responses to preconditioned stimuli.
Cognitive control of emotions is important in normal adaptive emotion behavior. Human studies using functional magnetic resonance imaging have demonstrated the interaction of the prefrontal cortex with the amygdala. It is still poorly understood, as it is difficult to make an animal model of this process.
Emotions affect mental processes such as attention, memory formation and decision making, which have important roles in social behavior. The amygdala has a key role in these functions. Their connections with the prefrontal cortex, sensory cortices, hippocampus, rhinal cortices and subcortical neuromodulatory systems are important in this respect. The basolateral amygdala regulates the formation of memories associated with emotional events. The framing effect during economic decisions may also be regulated by the amygdala.
Damage to the amygdala or to the pathways in which the amygdala connects to other parts of the brain can contribute to a variety of neuropsychiatric disorders. The connections between the amygdala and the prefrontal cortex necessary for normal adaptive emotional behavior do not develop until early adulthood. During this period, many neuropsychiatric diseases may occur. Amygdala dysfunction may play a role in diseases such as anxiety disorder, addiction, and autism.