The insula is a part of the brain that’s deeply involved in consciousness and everyday experience. Problems in the insula are associated with disorders such as anxiety, depression and schizophrenia, but in humans the insula is very hard to study.
A new review paper in Nature Neuroscience summarizes what researchers have learned about how the insula works in animals and how that might translate to its function in humans. These insights may open up the potential for the development of new treatments and therapies that change people’s lives.
“We need more specific language to describe the insula’s structure and its function,” said Eliza Bliss-Moreau, a corresponding author on the paper and Chancellor's Leadership Professor in psychology in the College of Letters and Science. “Our comparative analysis of insula structure and function shows that we are just beginning to learn the scope of its role in our daily lives.”
What is the brain’s insula?
The insula is a widely connected structure in the brain that has been associated with our ability to sense physical sensations and recognize our emotions. It’s also been associated with broader experiences, including our sense of consciousness.
Starting in the 1800s, researchers began to document the rough shape of the human insula, and much more recent studies used brain imaging. In one study, researchers directly stimulated different parts of the insula and found that they triggered different sensations like scent and taste, as well as belching and vomiting.
However, even between people, insulas can be very different. Of the six divisions identified in the insula, the limited number of studies with human cadavers found brains with some parts of the insula poorly developed and other brains with parts of the insula completely missing.
Brain imaging can be a limited way to study the insula. With existing technology it’s impossible to tell the function of the insula’s individual parts since they are so deeply connected with each other. For example, a fMRI, which measures blood flow in the brain, may show multiple parts of the insula as active when not all are responsible for whatever the participant is experiencing.
What can animal studies tell us about the human insula?
Researchers who study monkeys, rats and mice have recently expanded their study of the insula. A study in the 1980s compared this structure between the human brain and the brain of rhesus macaques. That study found that ours was larger, but otherwise very similar to the monkey insula.
Brain imaging in living human participants, despite its limitations, has shown how different parts of the insula connect to each other and to other parts of the brain. Imaging studies have shown some of the similarities between the insulas of humans and monkeys, as well as similarities and differences across multiple rodent species, including rats, squirrels and mice.
Studying the insula in different species can offer insights that would be impossible to gain through human brain imaging.
“Even though our goal is to understand human insula structure and function, we cannot do a lot of the necessary experiments in people,” said Bliss-Moreau.
Research with animals can involve direct manipulation of brain circuits, use of genetic tools and large-scale recordings that provide information about the functioning of cells. For the information gleaned from animals to be useful for understanding the human insula, the animals studied must possess a brain area that is similar in structure, its connections and its function.
“What is clear from the evidence is that not all animals are ideal models for human insula and scientists need to carefully choose the animals they study if they actually want to translate the findings to humans,” said Bliss-Moreau. “Understanding insula’s function is critical for basic and translational science and a there is significant research that will need to be carried out with animals if we ultimately want to improve human health.”
Co-authors on the paper include co-corresponding author Joey A. Charbonneau, Sarah B. Carp, Jeffrey L. Bennett, Savannah M. Maw and Gilda Moadab from UC Davis, John P. Christianson from Boston College and Mark G. Baxter from the Wake Forest University School of Medicine.
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