In 1937, James Papez proposed that the circuit connecting the hypothalamus to the limbic lobe was the basis for emotional experiences.

MacLean redefined the circuit as the "visceral brain" which consisted of the limbic lobe and its major connections in the forebrain – hypothalamus, amygdala, and septum.

Over time, the concept of a forebrain circuit for the control of emotional expression has been modified to include the prefrontal cortex.

Animal behavioral studies show that electrical stimulation of the anterior cerebellum can cause arousal, predatory attack, and feeding responses, all of which are thought to be expressions of emotion.

[5] In recent years, multiple additional limbic fiber connectivity has been revealed using Diffusion MRI-weighted imaging MRI techniques.

[6] Based on Papez's experiment with aggression in rats and other studies, it was initially believed that the circuit was involved with emotion.

As a result of these adverse effects on episodic memory, damage to the Papez circuit can not only indicate or predict amnesia but also Alzheimer's in a patient.

[15] Semantic dementia is a rare degenerative disorder that exhibits defects in all semantic memory functions, including naming, single word comprehension and impoverished general knowledge, with relative preservation of other components of speech, perceptual and nonverbal problem-solving skills, and episodic memory.

As a result, the ability to learn new information or to retrieve recently acquired memories is significantly reduced or lost.

Damage to the hippocampus and other medial lobe structures in the Papez circuit is thought to be the cause of reduced or lost episodic memory.

By date, he held the "... first interpretation of the limbic or ‘internal’ brain as a viscero-emotional mechanism..." He traced the structures of the system based on degeneration experiments in apes and dogs as well as neurodegenerative diseases in humans.

[18] Theoretically, this made sense to Papez who asserted that the hippocampus is responsible for the expression of emotion because of its connection to the autonomic nervous system.

Papez's paper on the emotional circuit which involved the connection between the hypothalamus and the limbic lobe set MacLean on a journey to learn more.

MacLean believed that including the visceral brain in the limbic system accounted for the external sensory information associated with subjective emotional experiences.

Since the limbic system consists of evolutionary primitive structures which prevents its interpretation verbally, the visceral brain accounts for the visual, auditory, olfactory, and other external sensory inputs that are associated with emotions.

The limbic system, the modified version of the circuit proposed by MacLean, is thought to be involved in the same functions as well as behavior, motivation, and olfaction.