Insular cortex
The insular cortex is a portion of the cerebral cortex folded deep within the lateral sulcus within each hemisphere of the mammalian brain.
The insulae are believed to be involved in consciousness and play a role in diverse functions usually linked to emotion, interoception, or the regulation of the body's homeostasis. These functions include compassion, empathy, taste, perception, motor control, self-awareness, cognitive functioning, interpersonal relationships, and awareness of homeostatic emotions such as hunger, pain and fatigue. In relation to these, it is involved in psychopathology.
The insular cortex is divided by the central sulcus of the insula, into two parts: the anterior insula and the posterior insula in which more than a dozen field areas have been identified. The cortical area overlying the insula toward the lateral surface of the brain is the operculum. The opercula are formed from parts of the enclosing frontal, temporal, and parietal lobes.
Structure
The insula is divided into an anterior and a posterior part by the central sulcus of the insula.Connections
The anterior part of the insula is subdivided by shallow sulci into three or four short gyri.The anterior insula receives a direct projection from the basal part of the ventral medial nucleus of the thalamus and a particularly large input from the central nucleus of the amygdala. In addition, the anterior insula itself projects to the amygdala.
One study on rhesus monkeys revealed widespread reciprocal connections between the insular cortex and almost all subnuclei of the amygdaloid complex. The posterior insula projects predominantly to the dorsal aspect of the lateral and to the central amygdaloid nuclei. In contrast, the anterior insula projects to the anterior amygdaloid area as well as the medial, the cortical, the accessory basal magnocellular, the medial basal, and the lateral amygdaloid nuclei.
The posterior part of the insula is formed by a long gyrus.
The posterior insula connects reciprocally with the secondary somatosensory cortex and receives input from spinothalamically activated ventral posterior lateral thalamic nuclei. It has also been shown that this region receives inputs from the ventromedial nucleus of the thalamus that are highly specialized to convey homeostatic information such as pain, temperature, itch, local oxygen status, and sensual touch.
A human neuroimaging study using diffusion tensor imaging revealed that the anterior insula is interconnected to regions in the temporal and occipital lobe, opercular and orbitofrontal cortex, triangular and opercular parts of the inferior frontal gyrus. The same study revealed differences in the anatomical connection patterns between the left and right hemisphere.
The circular sulcus of insula is a semicircular sulcus or fissure that separates the insula from the neighboring gyri of the operculum in the front, above, and
behind.
Cytoarchitecture
The insular cortex has regions of variable cell structure or cytoarchitecture, changing from granular in the posterior portion to agranular in the anterior portion. The insula also receives differential cortical and thalamic input along its length. The anterior insular cortex contains a population of spindle neurons, identified as characterising a distinctive subregion as the agranular frontal insula.Development
The insular cortex is considered a separate lobe of the telencephalon by some authorities. Other sources see the insula as a part of the temporal lobe. It is also sometimes grouped with limbic structures deep in the brain into a limbic lobe. As a paralimbic cortex, the insular cortex is considered to be a relatively old structure.Function
Multi-modal sensory processing, sensory binding
Functional imaging studies show activation of the insula during audio-visual integration tasks.Taste
The anterior insula is part of the primary gustatory cortex. Research in rhesus monkeys has also reported that apart from numerous taste-sensitive neurons, the insular cortex also responds to non-taste properties of oral stimuli related to the texture or temperature of food.Speech
The sensory speech region, Wernicke's area, and the motor speech region, Broca's area, are interconnected by a large axonal fiber system known as the arcuate fasciculus which passes directly beneath the insular cortex. On account of this anatomical architecture, ischemic strokes in the insular region can disrupt the arcuate fasciculus. Functional imaging studies on the cerebral correlates of language production also suggest that the anterior insula forms part of the brain network of speech motor control. Moreover, electrical stimulation of the posterior insular can evoke speech disturbances such as speech arrest and reduced voice intensity.Lesion of the pre-central gyrus of the insula can also cause "pure speech apraxia". This demonstrates that the insular cortex forms part of a critical circuit for the coordination of complex articulatory movements prior to and during the execution of the motor speech plans. Importantly, this specific cortical circuit is different from those that relate to the cognitive aspects of language production. Subvocal, or silent, speech has also been shown to activate right insular cortex, further supporting the theory that the motor control of speech proceeds from the insula.
Interoceptive awareness
There is evidence that, in addition to its base functions, the insula may play a role in certain higher-level functions that operate only in humans and other great apes. The spindle neurons found at a higher density in the right frontal insular cortex are also found in the anterior cingulate cortex, which is another region that has reached a high level of specialization in great apes. It has been speculated that these neurons are involved in cognitive-emotional processes that are specific to primates including great apes, such as empathy and metacognitive emotional feelings. This is supported by functional imaging results showing that the structure and function of the right frontal insula is correlated with the ability to feel one's own heartbeat, or to empathize with the pain of others. It is thought that these functions are not distinct from the lower-level functions of the insula but rather arise as a consequence of the role of the insula in conveying homeostatic information to consciousness. The right anterior insula is engaged in interoceptive awareness of homeostatic emotions such as thirst, pain and fatigue, and the ability to time one's own heartbeat. Moreover, greater right anterior insular gray matter volume correlates with increased accuracy in this subjective sense of the inner body, and with negative emotional experience. It is also involved in the control of blood pressure, in particular during and after exercise, and its activity varies with the amount of effort a person believes he/she is exerting.The insular cortex also is where the sensation of pain is judged as to its degree. Lesion of the insula is associated with dramatic loss of pain perception and isolated insular infarction can lead to contralateral elimination of pinprick perception. Further, the insula is where a person imagines pain when looking at images of painful events while thinking about their happening to one's own body. Those with irritable bowel syndrome have abnormal processing of visceral pain in the insular cortex related to dysfunctional inhibition of pain within the brain.
Physiological studies in rhesus monkeys have shown that neurons in the insula respond to skin stimulation. PET studies have also revealed that the human insula can also be activated by vibrational stimulation to the skin.
Another perception of the right anterior insula is the degree of nonpainful warmth or nonpainful coldness of a skin sensation. Other internal sensations processed by the insula include stomach or abdominal distension. A full bladder also activates the insular cortex.
One brain imaging study suggests that the unpleasantness of subjectively perceived dyspnea is processed in the right human anterior insula and amygdala.
The cerebral cortex processing vestibular sensations extends into the insula, with small lesions in the anterior insular cortex being able to cause loss of balance and vertigo.
Other noninteroceptive perceptions include passive listening to music, laughter and crying, empathy and compassion, and language.
Motor control
In motor control, it contributes to hand-and-eye motor movement, swallowing, gastric motility, and speech articulation. It has been identified as a "central command" centre that ensures that heart rate and blood pressure increase at the onset of exercise. Research upon conversation links it to the capacity for long and complex spoken sentences. It is also involved in motor learning and has been identified as playing a role in the motor recovery from stroke.Homeostasis
It plays a role in a variety of homeostatic functions related to basic survival needs, such as taste, visceral sensation, and autonomic control. The insula controls autonomic functions through the regulation of the sympathetic and parasympathetic systems. It has a role in regulating the immune system.Self
The insula has been identified as playing a role in the experience of bodily self-awareness, sense of agency, and sense of body ownership.Social emotions
The anterior insula processes a person's sense of disgust both to smells and to the sight of contamination and mutilation — even when just imagining the experience. This associates with a mirror neuron-like link between external and internal experiences.In social experience, it is involved in the processing of norm violations, emotional processing, empathy, and orgasms.
The insula is active during social decision making. Tiziana Quarto et al. measured emotional intelligence of sixty-three healthy subjects. Using fMRI EI was measured in correlation with left insular activity. The subjects were shown various pictures of facial expressions and tasked with deciding to approach or avoid the person in the picture. The results of the social decision task yielded that individuals with high EI scores had left insular activation when processing fearful faces. Individuals with low EI scores had left insular activation when processing angry faces.