Limbic system

A disputed concept, referring to an interconnected collection of cortical and subcortical structures in the medial parts of the mammalian brain that are implicated in autonomic, emotional, and cognitive functions.

Limbus is rim or border in Latin. Already in 1664, Willis described the brain area that surrounds the brainstem as 'the limbus' (cited in Witter et al. 1989). He was followed by Broca, who termed more or less the same part of the brain as the 'great limbic lobe' (Broca 1878; see also Schiller 1992). Later, MacLean (1952) referred to the limbic lobe together with subcortical structures interconnected to it as 'the limbic system', or 'visceral brain'. MacLean further suggested that this visceral brain processes emotions and guides some 'primitive' types of behaviour essential for the preservation of the individual and the species (MacLean 1970). With time, multiple lines of evidence have led to the notion that the limbic system plays a central part in learning and memory. Interestingly, the idea that 'limbic' is associated with memory echoes Dante; Limbo was the first circle in Inferno, and according to an hermeneutic analogy made between the circles and the human body, Limbo was the site of memory (Dante 1314/1996).

The definition of the limbic system given above is rather vague. This is intentionally so. Almost from the outset, no two authorities agreed on the precise anatomy and function of the limbic system (Swanson 1987; LeDoux 1991; Kotter and Meyer 1992). A popular, "classical morphological description of the limbic system portrayed it as being composed of two interconnected circuits (Livingston and Escobar 1971). One circuit is centred on the "hippocampal formation, and is called the medial or 'Papez circuit' (Papez 1937). In this circuit, information flows from the entorhinal cortex to the hippocampal formation, from there through the fornix to the anterior thalamus (directly or via the mammillary bodies), from the anterior thalamus to the cingulate gyrus, and from there back to the entorhinal "cortex via the cingulum bundle. The other major limbic circuit, called the basolateral or 'Yakovlev circuit' (Yakovlev 1948), is centred on the "amygdala. It includes the orbitofrontal, insular, and anterior temporal cortical areas, together with their interconnections to the amygdala and the dorsomedial nucleus of the thalamus.

Over the years, two major trends have characterized the research on the limbic system. The first involved the attribution of a growing number of physiological and behavioural deficits to limbic dysfunction. This was done on the basis of the effect of circumscribed brain damage resulting from disease, injuries, and lesions. The identified deficits were related to a large spectrum of normal functions. At first the emphasis was on emotion and sociopathology (Papez 1937; Kluver and Bucy 1938). These limbic functions are now attributed mostly to the "amygdala (LeDoux 1991). Later the discussion came to stress also visceral functions and sensory integration (MacLean 1952, 1970), as well as learning and memory (Scoville and Milner 1957). In the context of learning and memory, certain limbic lesions were specifically related to "amnesia, and limbic functions to both non"declarative (Pavlovian "fear conditioning) and declarative memory (e.g. see Fernandez et al. 1999 for a recent study of real-time tracking of declarative memory formation in limbic circuits). One possibility raised was that limbic circuits contribute to the evaluation of saliency and importance of neocorti-cal input, and instruct neocortical circuits whether to form lasting "internal representations of that input (Dudai 1989).

A second trend, a natural outcome of the first, involved the expansion of the limbic system concept to include more and more brain regions interconnected with the 'original' limbic components, up to a stage in which very substantial portions of the brain were included (LeDoux 1991; Kotter and Meyer 1992). This growing scope of the limbic system concept, combined with the generality and fuzziness of the functions attributed to the system, have contributed to a growing concern whether the limbic system represents a natural structure or rather is an artificial concept ("taxonomy). Furthermore, the mere usefulness of the concept was questioned (e.g. Brodal 1981; LeDoux 1991, 1996; Kotter and Meyer 1992; Blessing 1997). Judging by our current knowledge of the functional anatomy of the widespread so-called 'limbic' structures and circuits, and of their multifaceted role in physiology and behaviour, the conclusion might indeed be reached that the 'limbic system' does not exist in reality as such. As Kotter and Meyer (1992) put it, '(it) is a non-empirical explanatory concept for poorly understood brain functions'. This is, however, a case in which a concept, in spite of being at some odds with reality, succeeded in stimulating tremendously influential research. The use of the modifier 'limbic' in the scientific literature is still prominent (ibid.). It even found its way long ago into the popular press.

So what is it that still keeps the limbic system concept going? Some suggest that whatever the real limbic system is, it is useful to think in terms of 'limbic-ness' (Isaacson 1992), which refers to basic and 'primitive' brain functions in the domain of emotions, "homeosta-tic behaviour, and phylogenetically ancient drives such as hunger and sex (e.g. MacLean 1970). This 'limbic-ness' is different from 'neocortical-ness', which involves high-order processing and retention of sensory and cognitive representations. Others may claim that such a division is artificial, simplistic, and misleading.

For example, the "hippocampus is a classical limbic structure, but is involved in functions that are characterized by more 'neocortical-ness' than 'limbic-ness'; so are 'limbic'cortici (e.g. Suzuki et al. 1993).

After so many viable years it is unlikely that the limbic system notion will suddenly disappear. It will probably give way to more sophisticated classifications of brain structure and function. But in that respect it does not differ from some other concepts of brain organization and function, that may reflect an artificial entity rather than a natural one (e.g. Kirkpatrick 1996).

Selected associations: Amygdala, Amnesia, Fear conditioning, Hippocampus

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