The limbic system, also known as the paleomammalian cortex, is a set of brain structures located on both sides of the thalamus, immediately beneath the medial temporal lobe of the cerebrum primarily in the forebrain.

[3] This processed information is often relayed to a collection of structures from the telencephalon, diencephalon, and mesencephalon, including the prefrontal cortex, cingulate gyrus, limbic thalamus, hippocampus including the parahippocampal gyrus and subiculum, nucleus accumbens (limbic striatum), anterior hypothalamus, ventral tegmental area, midbrain raphe nuclei, habenular commissure, entorhinal cortex, and olfactory bulbs.

[3][4][5] The limbic lobe was originally defined by Paul Broca in 1878, as a series of cortical structures surrounding the boundary between the cerebral hemispheres and the brainstem.

[7] Further studies began to associate these areas with emotional and motivational processes and linked them to subcortical components that were then grouped into the limbic system.

In 1954, Olds and Milner found that rats with metal electrodes implanted into their nucleus accumbens, as well as their septal nuclei, repeatedly pressed a lever activating this region.

These interactions are closely linked to olfaction, emotions, drives, autonomic regulation, memory, and pathologically to encephalopathy, epilepsy, psychotic symptoms, cognitive defects.

The dorsal hippocampus was found to be an important component for the generation of new neurons, called adult-born granules (GC), in adolescence and adulthood.

This is thought to integrate spatial and episodic memories with the limbic system via a feedback loop that provides emotional context of a particular sensory input.

Eichenbaum[19] and his team found, when studying the hippocampal lesions in rats, that the left hippocampus is "critical for effectively combining the 'what', 'when', and 'where' qualities of each experience to compose the retrieved memory".

This researcher and his team employed many different types of mental and physical training on their subjects, and found that the hippocampus is highly responsive to these latter tasks.

Thus, they discovered an upsurge of new neurons and neural circuits in the hippocampus as a result of the training, causing an overall improvement in the learning of the task.

[23] In an attempt to curtail life-threatening epileptic seizures, 27-year-old Henry Gustav Molaison underwent bilateral removal of almost all of his hippocampus in 1953.

Semantic and episodic events faded within minutes, having never reached his long-term memory, yet emotions, unconnected from the details of causation, were often retained.

These cells not only were a crucial part of neurogenesis and the strengthening of spatial memory and learning in the hippocampus but also appear to be an essential component to the function of the amygdala.

A deficit of these cells, as Pessoa (2009) predicted in his studies, would result in low emotional functioning, leading to high retention rate of mental diseases, such as anxiety disorders.

After the study, Todorov concluded from his fMRI results that the amygdala did indeed play a key role in the general evaluation of faces.

Paul D. MacLean, as part of his triune brain theory (which is now considered outdated [citation needed][31][32]), hypothesized that the limbic system is older than other parts of the forebrain, and that it developed to manage circuitry attributed to the fight or flight first identified by Hans Selye[33] in his report of the General Adaptation Syndrome in 1936.

However, while the categorization into structures is reasonable, the recent studies of the limbic system of tetrapods, both living and extinct, have challenged several aspects of this hypothesis, notably the accuracy of the terms "reptilian" and "old mammalian".

The common ancestors of reptiles and mammals had a well-developed limbic system in which the basic subdivisions and connections of the amygdalar nuclei were established.

[38] The first evidence that the limbic system was responsible for the cortical representation of emotions was discovered in 1939, by Heinrich Kluver and Paul Bucy.

Kluver and Bucy, after much research, demonstrated that the bilateral removal of the temporal lobes in monkeys created an extreme behavioral syndrome.

[36] MacLean developed the theory of the "triune brain" to explain its evolution and to try to reconcile rational human behavior with its more "primal" and "violent" side.

Developing observations made by Papez, he hypothesized that the limbic system had evolved in early mammals to control fight-or-flight responses and react to both emotionally pleasurable and painful sensations.

[citation needed][41] Additionally, MacLean said that the idea of the limbic system leads to a recognition that its presence "represents the history of the evolution of mammals and their distinctive family way of life.

In addition to identifying the limbic system, he hypothesized a supposedly more primitive brain called the R-complex, related to reptiles, which controls basic functions like muscle movement and breathing.