Limbic system

The limbic system is a “classical” neuroscience concept, starting with the work of Paul Broca (1878), regarding the function of ventral / medial cortical areas largely overlapping with the cingulate cortex, termed the limbic lobe. These areas are interconnected with subcortical areas that are generally the ventral and medial elements of each anatomically defined brain region. Papez (1937) significantly advanced this concept, identifying the Papez Circuit of interconnected areas and hypothesizing that they are critical for emotion and motivation. MacLean (1952) further advanced the limbic system concept, including more subcortical systems.

In the current neuroscience literature, the limbic system has a controversial status. On the positive side, there is clearly evidence in favor of a collection of interconnected brain areas that are involved in emotional and motivational processes. On the other hand, the limbic system is not very well defined and has morphed in various ways over the years (LeDoux, 2000). Therefore, it is unclear if there is anything more to limbic system concept other than “the collection of brain areas most strongly involved in emotional processing”.

The Rubicon model can be seen as a current update to these classical ideas, focusing on the activation and maintenance of goal states, guided by emotional and motivational signals. The explicit computational basis of the Rubicon model provides a more rigorous foundation for the specific contributions of the different brain areas.

Neural systems that have been strongly shaped by evolution over the millenia are exceptionally complex and have multiple distinct functions intertwined within the same areas, circuits, and even neurons, which can release multiple different neurotransmitters. Thus, the computational approach is unlikely to provide a full account of these neural complexities. Nevertheless, hopefully it captures some critical aspects of the functions of these systems.

One notable aspect of the classical limbic system is the central role of the hippocampus in this circuit, although the precise function of this brain system was unclear at the time (the discussion in Papez, 1937 is quite speculative). With the benefit of significant further research, we now know that the hippocampus is critical for episodic memory, which might otherwise seem at odds with a central role in emotion and motivation. However, these two functions may indeed by synergistic, to the extent that we use our memory to guide future planning and goal-driven behavior, which is now a widely accepted idea, and is central to the function of the Rubicon model.

Furthermore,the mammillary bodies and associated pathways including the fornix are central players in the classical limbic system, but they are much more involved in processing space and time than emotion, although there may be some aspects of this functionality also embedded in these evolutionarily ancient pathways.

From an evolutionary and developmental perspective, the mesocortical ring provides a more concrete anatomical substrate that corresponds to many aspects of the original limbic lobe construct, as discussed in evolution of neocortex.