The insula brain, or insular lobe/cortex, tends to be forgotten when looking at simple brain anatomy. The reason for this is perhaps it sits deeper into the cortex and cannot be seen from the outside. However, the insula brain has many vital functions and connections to other structures in the brain.
The insula brain is located underneath the cerebral cortex. It is a part of the brain responsible for sensory processing and plays an important role in emotions and empathy. The insula also has motor functions and is associated with decision-making abilities and consciousness.
The insula brain is integral to brain structure and function, working with other parts of the brain to facilitate various tasks. If the insula is damaged, specific symptoms may appear. Insular epilepsy and Frontotemporal Dementia are two disorders that stem from damage to the insular.
Location Of The Insula Brain
The location of the insula brain sparked its unique name. Insula means island in Latin. It is also known as the “Island of Reil, ” discovered by Johann Christian Reil in 1809. Another term for the insula brain is the hidden fifth lobe, as it is not visible from the brain’s surface.
The insula brain, perhaps more commonly known as the insular cortex, makes up part of the cerebral cortex. It is found deep in the lateral sulcus of the brain, making it invisible on the surface level. The lateral sulcus is a great landmark in the brain as it divides the frontal and parietal lobes from the temporal lobe.
Both hemispheres house an insular cortex. Because it is located deep to the lateral sulcus, the insular cortex lies behind parts of the three different lobes – frontal, parietal, and temporal. Imagine the brain, and pulling back the operculum (meaning ‘little lid’) of the temporal, parietal and frontal lobes, you’ll find the grey matter of the insular lobe.
Anatomy Of The Insula Brain
The insula makes up about 2% of the brain’s cortical surface. It is shaped like an irregular pyramid. The circular sulcus constrains the base of this pyramid structure. The pyramid’s apex goes laterally into the lateral sulcus (also known as the Sylvian fissure).
The insula brain is divided into two main parts: the anterior and posterior lobules. Between these lobules is the central sulcus of the insula. Different gyri form part of the two lobules. The anterior part contains the anterior, middle, and posterior short insular gyri, while the posterior part contains the anterior and posterior long insular gyri.
The anterior lobule has a point that shows the anteroinferior-most positions of the insula. Considering its three different short gyri (anterior, middle, and posterior), this lobule can look like the letter “W”. The posterior lobule can resemble a “Y” or “V” due to the structure of the two long gyri.
The insula brain consists of 3 areas: granular, dysgranular, and agranular subdivisions. These areas are part of the cytoarchitecture of the insula.
Connections Of The Insula Brain
The insula brain is connected to various other structures. These structures include the neocortex, basal ganglia, thalamus, and the limbic system. Because of these different connections, the insular cortex has some functions connected to these structures’ functions.
For instance, due to the limbic system connection, the insula plays a role in emotional responses and regulation. More specific connections include the anterior insula connected to the anterior cingulate as well as the frontal, orbitofrontal, and anterior temporal areas.
The posterior section of the insula brain is connected to the posterior temporal, parietal, and sensorimotor areas. The middle region of the insula has also been shown to communicate with the anterior and posterior insular cortices, acting as a transitional area.
Cytoarchitecture Of The Insula Brain
The cytoarchitecture shows how the cells are arranged in the tissues of an area in the brain. Specifically, it describes the structure and organization of the neurons which make up part of the central nervous system.
Three areas within the insula brain system are identified as relevant to the cytoarchitecture. There are two granular areas and one dysgranular area. More specifically, the central agranular cortex, the intermediate dysgranular cortex, and the outermost granular cortex.
The anterior lobule is made up of the dysgranular cortex. In layer 5 of the dysgranular cortex, there are large pyramidal neurons. Moving on to the central sulcus of the insula brain, there is a gradual shift to a granular cortex. The two long gyri of the posterior lobule are made up of this granular cortex.
The granular cortex gets information and signals from sensory pathways like pain, temperature, and vestibular pathways. These signals are detailed as they move to the dysgranular cortex and then finally conveyed to the most anterior part of the insula, which is agranular.
Further processing includes interaction between the sensory signals and emotional and cognitive control. In this way, the cytoarchitecture of the insula brain is an interface between sensation in the body and emotional processing.
Function Of The Insula Brain
The insula brain has several essential functions in the brain—these range from sensory processing to motor functions. The insula is also associated with other brain systems like the limbic system, which is related to emotions. Although insula brain functioning has been difficult to research, some functions have been associated with it.
Insula Brain Plays Role In Emotions
The insular cortex plays a role in different emotions, such as sadness, happiness, anger, and fear. The anterior insular cortex mainly plays a role in emotions like love, disbelief, and trust. Related to these emotions, empathy is another feeling generated with the insula’s help.
Empathy and social cognition go hand in hand, and both are functions of the insula brain. The anterior insular cortex is specifically responsible for one’s response to seeing others in pain and other emotional expressions.
There is also a difference in the right and left anterior insula functioning and empathy. The right anterior insula was linked to a specific form of empathy called affective-perceptual empathy. The left side was also linked to affective-perceptual empathy and cognitive-evaluative forms of empathy.
The insula brain not only helps emotions to be generated from ourselves, like subjective feelings but also in response to external stimuli and awareness of our different emotions. Brain imaging studies have found that the insula is responsible for emotions.
Insula Helps With Sensory Processing
The insula brain is responsible for processing information via our senses, like taste, smell, sound, sight, and touch. It helps the brain to interpret the incoming information obtained by our senses with the help of thalamic and cortical afferents in the body.
The ability to detect various bodily sensations is called interoceptive processing. This process plays a role in detecting, identifying, and making a person aware of certain body sensations, like hunger and pain.
Neuroimaging studies showed more activity in the insula when the participants were made aware of hunger, thirst, and even their heartbeat. The insula brain also plays a vital role in somatosensory functions like tingling sensation or a shiver.
Researchers also found painful sensations obtained after stimulating the insula brain, such as a burning sensation or a pinch.
In terms of autonomic functions, homeostasis plays a role in the insula. The insula is responsible for physiological stability, including the control and regulation of the immune system. The insula makes you aware of when you’re cold, full from eating or have a full bladder.
Auditory Processing And The Insula
Due to the insula brain’s connections with the primary auditory, auditory association, and post-auditory cortices, it also plays a role in auditory processing. The insular cortex activates when participants detect sound and processing of non-verbal stimuli.
Vestibular processing also plays a role in the insula. Specifically, involvement of the posterior insula was observed with vestibular functions (for instance, balance). Vestibular functions occur because the parietal lobe operculum is involved with the insular brain, so vestibular processing comes into play.
Vestibular processing and the insula brain require much more research, and so far, only one person has shown symptoms such as vertigo and imbalance after only insular cortex damage.
Motor Functions And The Insula
Additional to sensory processing, the insula brain helps with certain motor control functions such as hand-eye coordination, speech, and the motor aspect of the gastrointestinal tract. During the start of physical activity, the insular cortex controls blood pressure and heart rate.
Consciousness And Decision-Making Abilities
The insula brain plays a role in conscious desires and how certain emotional states can influence and change the body. Interestingly, the insula plays a part in sensory processing related to bodily conditions; it, therefore, is responsible for when we have cravings.
Somewhat related to cravings, the insula brain has also been associated with decision-making abilities. When you crave something, you’ll have to decide for or against it. The decision-making function was discovered when the insula was damaged, and brain imaging studies showed a link between the insula and our capacity to weigh different choices to make a decision. This functioning even extends to what happens in the brain when making a ‘right’ decision based on the values choices.
What Happens When the Insula Brain Is Damaged?
The location of the insula has made it challenging to study scientifically. It isn’t easy to reach the insula as it is situated underneath the cerebral cortex, hidden from view. However, some studies have shown what happens when there is damage or lesions on the insula brain.
Problems With Sensory Processing
Due to the significant function that the insular cortex plays in interpreting sensory information from all our senses, damage to it can cause various sensory issues. If the insula is not working correctly, there may not be a problem with experiencing sensation but rather having an appropriate response.
For example, there may not be an appropriate response to pain or taste. Our body tells us when something tastes bad or even when eating something can be harmful. This ability may be affected by a damaged insula.
Because of the insula brain connection with interoception, damage to the insula can delay awareness of sensations like being full from eating or any heart sensations.
Insular Epilepsy
Neuroscience has shown that insular epilepsy is seizures similar to temporal epilepsy but originates from dysfunction in the insula. Insular epilepsy is still an ongoing field in research, as it is an example of the challenges associated with studying the insula brain. EEG research is required, but due to the deep location of the insula, it is difficult to get accurate readings.
Symptoms of insular epilepsy include consciousness during a seizure and movement on the same or opposite sides of the face, upper limbs and head, and eyes. Nausea, vomiting, and abdominal elongation can also be present during insular epilepsy.
Frontotemporal Dementia From Insula Damage
Frontotemporal Dementia (FTD) is dementia that causes someone to lose empathy and have poor judgment. Other symptoms include speech changes, repetitive and compulsive behaviors, and inappropriate social actions.
Many of these functions are due to the insula brain. FTD can be caused by degeneration of parts of the insula and other structures like the anterior cingulate cortex. People who do not suffer from FTD but show an abnormal lack of judgment could also have damaged insulas.
An example may be an extreme expression of trust in those that clearly do not deserve it. Risk judgment is equally important and can manifest as a symptom of insula damage.
Issues With Processing Emotions
One of the insula brain’s primary functions is to help process emotions and feelings. Studies show that damage to the insula can change a person’s emotional experience. Recognition of emotions can also be impaired; in some cases, people will find that their emotional responses are diminished.
Conclusion
The insula brain, or insular cortex, is tucked away under the lateral sulcus under the frontal, parietal and temporal lobes. It is a small yet essential structure in the brain. Its functions include sensory processing, emotional expression, and regulation, as well as some motor functions in the body.
References
https://www.sciencedirect.com/science/article/pii/S0960982217305468
https://n.neurology.org/content/93/21/932
https://www.masterclass.com/articles/insular-cortex-explained#4-dysfunctions-of-the-insular-cortexhttps://radiopaedia.org/articles/insular-cortex
