The insular cortex (also insula and insular lobe) is a portion of the cerebral cortex folded deep within the lateral sulcus (the fissure separating the temporal lobe from the parietal and frontal lobes) 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 (meaning lid). 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 shallow sulci divided 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 (posterior part) 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 (or sulcus of Reil) 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 (also called von Economo 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 (viscosity, grittiness) 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" (i.e. the inability to speak with no apparent aphasic or orofacial motor impairments). 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 (e.g., Broca's area on the inferior frontal gyrus). 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
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