The limbic system has been commonly referred
to in scientific literature as the emotional brain. But we know now with more
information and advanced technologies that the limbic system is an ambiguous
concept, it is what LeDoux says is just a “useful anatomical shorthand”.
One evidence that challenges this theory is
that areas that are considered unique to mammals (therefore termed the “mammalian
brain”) is also present in ‘primitive’ creatures such as reptiles and amphibians.
For example, a study conducted by Gilles Laurent and colleagues in 2018 using
single cell RNA-sequencing technology showed that the neurons of the pallium of
reptiles had similar genetic makeup to that of the hippocampus and amygdala of
mouse and humans! These areas are what MacLean included in the limbic system
theory, yet they are present in these ‘lower’ animals. Moreover, the same study
showed that the anterior dorsal cortex of the pallium in reptiles was similar
to the human neocortex, which is a layer that covers the limbic system and
thought to have only existed in ‘higher’ mammals. This goes to show that since
animals have different selective pressures, no single group can be thought of
as representing the ancestral form of another.
Another piece of evidence is that there are
other areas of the brain involved in what we consider as emotion. We see in recent
emotion studies that emotion is not localized and is multifaceted. For example,
in the famous patient S.M., we see that bilateral damage to her amygdala
(limbic area) caused an isolated fear deficit, without involvement in
other emotions. Moreover, although S.M. did not recognize fearful faces unless
she was told to adjust her gaze to look at the eyes of the subject. Eye gaze is
thought to involve various areas such as the inferior temporal lobe and
superior temporal sulcus, areas not involved in the limbic system.
Other brain areas have connections to the
limbic system and are involved in emotion. These include the hypothalamus that
regulates physiological responses and the insular cortex for interoception, a
hot topic for emotion researchers in studying the regulation of mood and
overall body state.
Moreover, there are areas included in the limbic
system that are not specific to emotional reaction. Although the hippocampus is
involved in emotional associative learning, it is also involved in declarative
memory, explicit memory that does not necessarily involve emotion.
Therefore, it could be suggested that calling
the limbic system the ‘emotional brain’ would be misinterpreting the function
of these brain regions, as well as oversimplifying the complexity of emotional
reactions. As Lisa Feldman Barrett states in her book How Emotion is Made, emotion
is a whole-brain process and localizing it to specific regions such as the
limbic system is limiting our full understanding of these complex processes.
LeDoux – The Emotional Brain
Gazzaniga, Ivry, Mangun- Cognitive
Neuroscience (5th Ed.)
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