limbic system

limbic system The term ‘limbic system’ (from Latin limbus: edge) was first used by MacLean in 1952 to describe a set of structurally and functionally related structures of the brain bordering the midline, inner surface of each cerebral hemisphere. These structures were considered to be evolutionarily ancient. MacLean called them the ‘visceral brain’ and suggested they mediate behaviourally ‘primitive’ functions inherited from lower mammals, particularly emotion and motivational behaviour. Although such phylogenetic arguments (based on comparison between species) are now commonly rejected, the concept of the limbic system survives and has since grown to be highly influential yet controversial.

First, there is no consensus over exactly which structures comprise the limbic system. Most schemes, however, consider it to consist of various parts of the cerebral cortex forming a set of ‘rings’ on the inner surface of each hemisphere, linked to a central core of structures lying below the cerebral cortex. The cortical areas include the cingulate cortex, hippocampus, parahippocampal cortex, and rhinal cortex. The various subcortical areas included in the limbic system extend down through the core of the brain to the upper part of the brain stem.

Second, there is considerable debate over what the function of the limbic system is. In addition to early ideas relating the limbic system to emotion and motivation, it has also now been implicated in the processing of sensory (especially olfactory) and cognitive information, learning and memory, sexual function (as part of a reward system serving emotional reactions), and motor functions. Most intriguing is the suggestion that the limbic system is concerned with mental integration of all functions related to personal ‘experience’.

As the number of brain areas said to belong to the limbic system has grown, its proposed functions have, not surprisingly, proliferated. It has been argued that such a heterogeneous collection of structures and functions can no longer be defined by a single general criterion and that the concept of the limbic system has become incoherent, even meaningless. An alternative view is that a quantitative approach (a ‘fuzzy limbic system’), in which different brain regions are described as having a certain degree of ‘limbic-ness’, would avoid the problem of having to define precise boundaries.

Despite controversy, the popularity and universal recognition of the term cannot be denied. This may be due partly to the very vagueness of the concept, which has often been used by authors as a convenience to refer to particularly poorly understood areas of the brain.

Mark J. Buckley

Bibliography

Kotter, R. and and Meyer, N. (1992). The limbic system: a review of its empirical foundation. Behavioural Brain Research, 52, 105–27.

See also brain; cerebral cortex; emotion; memory.