History Module: The Quest for the “Emotional Brain”

For more than a century, scientists have been ardently attempting to determine what brain structures might be responsible for human emotions.

This quest began in the 19th century with Franz Josef Gall’s efforts to find the locations of the brain’s various functions. This quest continued with the pseudo-science of phrenology, whose proponents argued that the greater or lesser development of various personality traits could be correlated with the bumps in the human skull.

Not long afterward, neurologists and psychologists began to apply the experimental method to the hunt for the “emotional brain”. The two main techniques available at the time were to stimulate certain regions of the brain or simply to remove them; both approaches are still in use.

Walter Cannon, best known for his attacks on William James’s theory of the emotions, also conducted several experiments with Philip Bard to find a physical substrate for emotions in the brain.his neuron are strengthened for a prolonged period. In these experiments, Cannon and Bard made a series of lesions at strategic locations and concluded that the hypothalamus was the central structure of the “emotional brain”. This conclusion made a lot of sense, given that the hypothalamus lies between the evolutionarily advanced cortex and the more primitive cortical sub-structures and was already known to play a role in regulating the autonomic nervous system.

Then, in 1937, the anatomist James Papez proposed what became one of the most influential theories on the cerebral substrate of the emotions. Judson Herrick, a specialist in evolution, who had established a distinction between the lateral and the medial parts of the cortex. According to Herrick, in evolutionary terms, the medial cortex was far older than the lateral cortex. Papez, ingeniously combining Herrick’s ideas with past research on brain lesions and the hypothalamus, developed a theory that attributed the subjective feeling of emotions to the passage of information through a circuit connecting the hypothalamus to the medial cortex. This is still known as Papez’s circuit.

In contrast to Cannon, who treated the hypothalamus as a homogeneous structure, Papez distinguished a subregion consisting of what he called the mammillary bodies (because they protrude like breasts). According to Papez, it is this specific subregion that receives sensory information from the thalamus and relays it to the cingulate cortex via the anterior thalamic nucleus. Papez also identified the hippocampus as the structure that received the outputs from the cingulate cortex and thus closed the loop by sending information to the hypothalamus. Papez suggested that emotions could be generated through this circuit in two ways. First, by sensory stimuli entering via the thalamus and successively passing through the various other structures of the circuit just described. Second, by thoughts coming from the cortex and entering the circuit via the cingulate cortex.

Papez’s circuit was a brilliant attempt to explain the anatomical pathways of the emotions. Few methods for examining brain pathways were available in Papez’s time. But since that time, practically all of the pathways that he predicted have been shown to actually exist. Unfortunately for Papez, however, very few of these pathways are now recognized as being involved in emotions. But the historical importance of Papez’s circuit was undeniable, because it was to become the origin of the theory of the limbic system.

Research on the cerebral bases of the emotions was slowed by World War II but made an important advance in 1949, when Paul MacLean revived discussion of Papez’s circuit in a more complex form. According to MacLean, the rhinencephalon, an older part of the cortex involved in the sense of smell, also played a central role in the emotions. This theory was supported by the fact that the rhinencephalon had substantial connections with the hypothalamus and was still relatively large in human beings, even though the human sense of smell has largely atrophied compared with that of other species.

Having found that stimulating the rhinencephalon produced visceral responses similar to those caused by emotions, MacLean identified it as the "centre" of the emotions. MacLean attributed special importance to the hippocampus, whose well aligned pyramidal neurons he compared to an “emotional keyboard” that received information from both the outside world and internal sources. For MacLean, the combination of these two types of sensations was the foundation of the emotional experience.

In 1952, MacLean introduced the term “limbic system” to designate the visceral brain of the rhinencephalon. The word “limbic” was originally introduced by French anatomist Paul Pierre Broca to refer to the ring-like shape of the medial part of the cortex. (The word comes from the Latin limbus, meaning “edge”.) MacLean regarded the limbic system as including not only Papez’s circuit, but also such regions as the amygdala, the septum, and the prefrontal cortex. According to MacLean, all of these structures form an integrated, phylogenetically ancient system that ensures the individual’s survival through adaptive physiological and affective responses.

In 1970, MacLean introduced his theory of the “triune” or “three-in-one” brain, according to which the brain has gone through three stages of evolution over time: the reptilian, the paleomammalian, and the neomammalian. In human beings and higher mammals, these three brains all coexist. The most primitive mammals have only the paleomammalian and reptilian brains, while birds, reptiles, amphibians and fish have only the reptilian brain. It is the paleomammalian brain, present in all mammals, that basically corresponds to the limbic system.

Very few theories in the history of the neurosciences have had so large and broad an influence as MacLean’s. His intuitions that the cerebral bases of the emotions must be examined from an evolutionary perspective, and that the circuits for the emotions may use pathways independent of those for cognition are still accepted. Even so, scientists today generally reject the idea that the limbic system as described by MacLean is the “emotional brain”.

In this regard, numerous experiments have shown that the limbic system taken as a whole is not very closely involved in emotional responses, whereas other parts of the brain (such as some regions of the brainstem) are very much involved in regulating the body’s reactions. Moreover, certain structures in the limbic system, such as the hippocampus, are far more involved in cognition than in emotions.

If the limbic system has survived as a concept up until today and is still described in many textbooks as the “emotional brain”, one reason may be that it is a practical way of describing a set of brain structures located in a sort of "no man's land " between the hypothalamus and the neocortex. But another reason may be that certain limbic structures are in fact involved in emotional functions.

MacLean’s main error was simply that he wanted to place the “emotional brain” in a single, unique system. The emotions are in fact functions related to our survival. But because there are many survival-related functions–such as defending oneself from danger, finding food or a sexual partner, and taking care of one’s children– there may be not just one “emotional brain”, but several.

 


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