in the anatomical connection patterns between the left and right hemisphere. The 'circular sulcus of insula' (or sulcus of Reil) is a semi-circular 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 Multimodal 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. 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