Synaptic Self by Joseph LeDoux

Author:Joseph LeDoux [LeDoux, Joseph]
Language: eng
Format: epub
Publisher: Penguin Publishing Group
Published: 2003-01-28T00:00:00+00:00

A QUICK FIX?

One way to begin to reassemble the mental trilogy might be to put all the newly acquired information about the thinking brain that came from the cognitive revolution together with the compelling view of the emotional brain provided long ago by the limbic system concept. Perhaps the notion of the limbic system simply needs to be modernized by treating it as an emotional-processing network rather than as the seat of conscious feelings. However, while the limbic system remains the predominant explanation (both in neuroscience and popular culture) of how the brain makes emotions, it is a flawed and inadequate theory of the emotional brain. I made this point very forcefully in The Emotional Brain, but those criticisms bear repeating.

The limbic system concept, brainchild of the pioneering neuroscientist Paul MacLean, was put forth in the context of an evolutionary explanation of mind and behavior. 27 It built upon the view, promoted by comparative anatomists earlier in the twentieth century, that the neocortex is a mammalian specialization—other vertebrates have primordial cortex, but only mammals were believed to have neocortex (chap. 3). And since thinking, reasoning, memory, and problem-solving are especially well developed in mammals, and particularly in humans and other primates that have relatively more neocortical tissue, these cognitive processes were believed to be mediated by the neocortex and not by the old cortex or other brain areas. In contrast, the old cortex and related subcortical regions form the limbic system, which was said to mediate the evolutionarily older aspects of mental life and behavior, our emotions. In this way, cognition came to be thought of as the business of the neocortex, and emotions of the limbic system.

The limbic system theory began to run into trouble almost immediately when it was discovered, in the mid-1950s, that damage to the hippocampus, an old cortical area and the centerpiece of the limbic system, led to severe deficits in a distinctly cognitive function, long-term memory. 28 This finding was incompatible with the original idea that the primitive architecture of the limbic system, and especially of the hippocampus, was poorly suited to participate in cognitive functions. 29 Subsequently, in the late 1960s, it was discovered that the equivalent of mammalian neocortex was present, though in a rudimentary form, in nonmammalian vertebrates (chap. 3). As a result, the old/new cortex distinction broke down, challenging the evolutionary basis of the assignment of emotion to the old cortex (limbic system) and cognition to the neocortex. 30

The limbic system itself has been a moving target. Within a few years after its inception, the definition expanded from the original notion of the old cortex and related subcortical forebrain nuclei to include some areas of the midbrain, 31 and even some regions of the neocortex. 32 Several attempts have been made to salvage the limbic system by defining it more precisely. 33 Nevertheless, after half a century of debate and discussion, there are still no generally accepted criteria for stipulating which areas of the brain belong to the limbic system. Some scientists have suggested that the limbic system be abandoned.

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