Neuroscientists still argue over which parts make up the limbic system
In 1954 rats with electrodes in two small brain regions kept pressing a lever to stimulate them, over and over. Those regions belong to the limbic system, the brain's emotional and motivational core. Yet experts no longer treat it as a single emotion centre, and its membership list is disputed.
The limbic system is a cluster of structures on either side of the thalamus, just below the medial temporal lobe, sometimes called the paleomammalian cortex. The French anatomist Paul Broca defined a limbic lobe in 1878 as a ring of cortex at the boundary between the cerebral hemispheres and the brainstem, taking the name from limbus, Latin for border. Later work tied these areas to emotion and motivation and bundled them with deeper structures into a system.
Commonly listed parts include the amygdala, deep in the temporal lobes and linked to emotion; the hippocampus, central to laying down new memories; the nucleus accumbens, involved in reward, pleasure and addiction; the septal nuclei, dubbed a pleasure zone; and the hypothalamus, a hub regulating many automatic body processes. The system works partly by steering the endocrine and autonomic nervous systems, and it trades signals with the basal ganglia and prefrontal cortex. Because it is now seen as one of many regions managing visceral processes rather than a stand-alone emotion unit, which structures belong to it remains contested.
Some of the clearest evidence came from damage. In 1953 Henry Molaison, aged 27, had most of his hippocampus removed on both sides to stop dangerous seizures. For the next fifty years he took part in thousands of studies: new facts and events vanished within minutes, yet feelings sometimes lingered even when their cause was forgotten. Suzanne Corkin, who worked with him for 46 years, described the case in a 2013 book. Severing limbic links to the prefrontal cortex, the operation known as prefrontal lobotomy, left many patients passive and unmotivated.
Animal studies add detail. Rats stressed by shock or restraint, or given anxiety-inducing drugs, performed worse on hippocampus-dependent maze tasks, and long exposure to stress hormones can disrupt explicit memory. Training, by contrast, spurred new neurons in the hippocampus that improved learning. Hans Markowitsch's research suggests the amygdala tags memories with emotional significance so they can be found and reactivated later.
Source: Limbic system
