The Concept of the Limbic System

Paul MacLean introduced the term limbic system in 1952 (see MacLean 1949,1990). MacLean, who was influenced by psychoanalytic theory, attempted to separate the emotional brain from other components and to place those structures within an evolutionary context. The phrase limbic system represents a synthetic concept in which an old term introduced by Pierre Paul Broca, in the nineteenth century, was placed in a new perspective. Broca described a ring of tissue on the medial cerebral hemisphere, which he called le grande lobe limbique. Limbique means border or fringe. Later this ring came to be called the old brain or the rhinencephelon—the nose brain. But in 1930 James Papez showed that Broca's limbic lobe, the rhinencephelon with its olfactory inputs, was primarily part of a circuitry that was functionally related to emotion.1 Papez advanced the theory that "the hypothalamus, the anterior thalmic nuclei, the cingulate gyrus, the hippocampus, and their connections constitute a harmonious mechanism which may elaborate the functions of central emotion as well as participate in emotional expression" (MacLean 1949). MacLean revitalized Papez's theory as well as Broca's old term by calling Papez's ensemble of structures "the limbic system."

There is, however, no agreement among authorities as to what structures constitute the limbic system and no agreement as to its phylogenetic significance. Most neuroscien-tists, such as the neuroanatomists Nauta and Feirtag (1986), would include in the limbic system the hypothalamus, the hippocampus, the amygdala, and those cortical areas that Broca described, as well as the entorhinal cortex adjacent to the hippocampus. Panksepp (1998a) would also include the midbrain, specifically, the mesencephalic reticular forma tion and the periaqueductal gray (PAG). As an example of the difference of opinion regarding phylogeny, MacLean (1990) claims that the limbic system coincides with the old mammalian brain, whereas Freeman (1999b) believes that the limbic system coincides with the salamander's forebrain. It should also be added that there is some current debate regarding the legitimacy of the limbic system as a concept. LeDoux believes that the neural circuits that the concept describes are imprecise and that the concept has "unwarranted functional (emotional) implications and should be discarded" (1996, p. 101). Panksepp (1998a), on the other hand, believes LeDoux's rejection of the limbic system as a concept to be totally misguided. My own view is that the limbic system is a heuristically necessary part of the "conceptual" nervous system, as it allows us to think of the evolution of emotion as a separate and distinct function of the mind/brain.

There is recent evidence that a limbic system can be found in protovertebrates. A rudimentary limbic system has been observed in the lancelet, a primitive protovertebrate and marine organism that is a relative of the lamprey (Zimmer 2000). The evolution of the limbic system may be related to the organism's need to monitor its internal states as preparation for motor action in the environment. The appearance of the limbic system in the lancelet seems to have coincided with the animal's transition from passive feeding to predatory behavior. For unlike the passive feeder, the predatory animal must be prepared to react to internal signals that prime the animal to either fight or flee (Zimmer 2000). It would appear that insects are programmed to fight or flee in a more robotic manner that precludes the need for a limbic system or, for that matter, consciousness.

This would suggest that the origin of emotions may be related to the need for the individual animal to mobilize its bodily responses as a whole. This holistic response presages the development of self and consciousness. The need for the psychological coherence of the self may reflect this ancient physiological requirement. Panksepp described this holistic response as "primal state spaces" generated by emotional value systems (2000). The limbic system monitors and responds to the requirements for homeostasis; emotions provide the appropriate internal signals that mobilize the individual to either fight or flee.

Paul MacLean (1990) speculated that the limbic system might be designed to amplify or lower the intensity of feelings involved in guiding the behaviors required for self-preservation. I have adopted the convention (suggested in Damasio 1999) of differentiating the terms feeling and emotion: only conscious individuals feel, whereas it is highly probable that emotions existed before the evolution of consciousness. Emotions can monitor and transmit information concerning the individual's vital needs without consciousness. As emotions are keyed to the regulation of homeostasis, they transmit and inform one of value. So it is a reasonable assumption that unconscious emotions, functioning as internal automatic signals, were present long before consciousness itself evolved. As an inheritance from this older biological self, the psychological self demonstrates a similar need for coherence.

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