Insular cortex

The insular cortex (abbrev. insula) is a structure of the human brain. It lies deep to the brain's lateral surface, within the lateral sulcus which separates the temporal lobe and inferior parietal cortex. These overlying cortical areas are known as opercula (meaning "lids"), and parts of the frontal, temporal and parietal lobes form opercula over the insula.

The insular cortex is also known by the name Island of Reil, named after Johann Christian Reil. Its Latin name is lobus insularis.

The insular cortex is considered a separate lobe of the telencephalon by some authorities.^[1] It is also sometimes grouped with limbic structures deep in the brain into a limbic lobe.

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Contrasting anterior and posterior architecture

It 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 insula receives a direct projection from the basal part of the ventral medial nucleus (VMb) of the thalamus and a particularly large input from the central nucleus of the amygdala. Additionally, the anterior insula itself projects to the amygdala. The posterior insula connects reciprocally with the secondary primary sensory cortex (S2) and receives input from spinothalamically activated ventral posterior inferior (VPI) thalamic nuclei. More recent work by Bud Craig and his colleagues has shown that this region receives inputs from the ventromedial nucleus (posterior part) of the thalamus that are highly specialized to convey emotional/homeostatic information such as pain, temperature, itch, local oxygen status and sensual touch.

Role in emotions and feelings (relationship to the "limbic system")

The insular cortex, in particular its most anterior portion, is considered a limbic-related cortex. The insula has increasingly become the focus of attention for its role in body representation and subjective emotional experience. In particular, Antonio Damasio has proposed that this region plays a role in mapping visceral states that are associated with emotional experience, giving rise to conscious feelings. This is in essence a neurobiological formulation of the ideas of William James, who first proposed that subjective emotional experience (i.e. feelings) arise from our brain's interpretation of bodily states that are elicited by emotional events. This is an example of embodied cognition.

Functionally speaking, the insula is believed to process convergent information to produce an emotionally relevant context for sensory experience. More specifically, the anterior insula is related more to olfactory, gustatory, vicero-autonomic, and limbic function, while the posterior insula is related more to auditory-somesthetic-skeletomotor function. Functional imaging experiments have revealed that the insula has an important role in pain experience and the experience of a number of basic emotions, including anger, fear, disgust, happiness and sadness.

Functional imaging studies have also implicated the insula in conscious desires, such as food craving and drug craving. What is common to all of these emotional states is that they each change the body in some way and are associated with highly salient subjective qualities. The insula is well situated for the integration of information relating to bodily states into higher-order cognitive and emotional processes. The insula receives information from "homeostatic afferent" sensory pathways via the thalamus and sends output to a number of other limbic-related structures, such as the amygdala, the ventral striatum and the orbitofrontal cortex.

A single study using magnetic resonance imaging has found that the right anterior insula was significantly thicker in people who meditate.^[2]

Role in Addiction

"The insula also reads body states like hunger and craving and helps push people into reaching for the next sandwich, cigarette or line of cocaine." ^[3] A number of functional brain imaging studies have shown that the insular cortex is activated when drug abusers are exposed to environmental cues that trigger cravings. This has been shown for a variety of drugs of abuse, including cocaine, alcohol, opiates and nicotine. Despite this consistent finding, the insula has been largely ignored within the drug addiciton literature, largely because it was not known to be a direct target of the mesotelencephalic dopamine system. Recent research led by Nasir Naqvi ^[4] at the University of Iowa has shown that cigarette smokers who suffer damage to the insular cortex, from a stroke for instance, have their addiction to cigarettes practically eliminated. These individuals were found to be up to 136 times more likely to undergo a disruption of smoking addiction than smokers with damage in other areas. Disruption of addiction was evidenced by self-reported behavior changes such as quitting smoking less than one day after the brain injury, quitting smoking with great ease, not smoking again after quitting, and having no urge to resume smoking since quitting. This suggests a significant role for the insular cortex in the neurological mechanisms underlying addiction to nicotine and other drugs, and would make this area of the brain a promising target for novel anti-addiction medications (or possibly transcranial magnetic stimulation). In addition, this finding suggests that functions mediated by the insula, especially conscious feelings, may be particularly important for maintaining drug addiction. Such processes may not be addressed by traditional animal models of drug addiction, which focus strictly upon externally observable behavior. Furthermore, the results suggest that certain bodily/interoceptive/visceral effects of drug use may be important for addiction.

A recent study in rats by Contreras et al. [1] corroborates these findings by showing that reversible inactivation of the insula disrupts amphetamine conditioned place preference, an animal model of cue-induced drug craving. In this study, insula inactivation also disrupted "malaise" responses to lithium chloride injection, suggesting that the representation of negative interoceptive states by the insula plays a role in addiction. However, in this same study, the conditioned place preference took place immediately after the injection of amphetamine, suggesting that it was the immediate, pleasurable interoceptive effects of amphetamine administration, rather than the delayed, aversive effects of amphetamine withdrawal that are represented within the insula.

A model proposed by Naqvi et al. ^[5] is that the insula stores a representation of the pleasurable interoceptive effects of drug use (e.g. the airway sensory effects of nicotine, the cardiovascular effects of amphetamine), and that this representation is activated by exposure to cues that have previously been associated with drug use. A number of functional imaging studies have shown the insula to be activated during the administration of drugs of abuse. Several functional imaging studies have also shown that the insula is activated when drug users are exposed to drug cues, and that this activity is correlated with subjective urges. In the cue-exposure studies, insula activity is elicited when there is no actual change in the level of drug in the body. Therefore, rather than merely representing the interoceptive effects of drug use as it occurs, the insula may play a role in memory for the pleasurable interoceptive effects of past drug use, anticipation of these effects in the future, or both. Such a representation may give rise to conscious urges that feel as if they arise from within the body. This may make addicts feel as if their bodies need to use a drug, and may result in persons with lesions in the insula reporting that their bodies have forgotten the urge to use, as reported by Naqvi et al.

Phylogenetic considerations

As a paralimbic cortex, the insular cortex is considered to be a relatively old structure. It plays a role in a variety of highly conserved functions that are related to basic survival needs, such as taste, visceral sensation and autonomic control (so-called homeostatic functions). There is evidence that in addition to its more conserved functions, the insula may play a role in certain "higher" functions that operate only in humans and other great apes. John Allman and his colleagues have shown that the anterior insular cortex contains a population of neurons, called spindle neurons, that are specific to great apes. These neurons are also found in the anterior cingulate cortex, which is another region that has reached a high level of specialization in great apes. Spindle neurons are found at a higher density in the right insular cortex. It has been speculated that these neurons are involved in cognitive-emotional processes that are specific to great apes, such as empathy and self-aware emotional feelings. This is supported by functional imaging results showing that the structure and function of the right anterior insula are 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" functions of the insula, but rather arise as a consequence of the role of the insula in conveying homeostatic information to consciousness.

Additional images

References

1. ^ Brain, MSN Encarta.
2. ^ Sara W. Lazar, Catherine E. Kerr, Rachel H. Wasserman, Jeremy R. Gray, Douglas N. Greve, Michael T. Treadway, Metta McGarvey, Brian T. Quinn, Jeffery A. Dusek, Herbert Benson, Scott L. Rauch, Christopher I. Moore, and Bruce Fischl (November 2005). "Meditation experience is associated with increased cortical thickness". NeuroReport 16 (17): 1893-1897.
3. ^ BLAKESLEE, SANDRA. "A Small Part of the Brain, and Its Profound Effects", The New York Times, February 6, 2007. 
4. ^ Nasir H. Naqvi, David Rudrauf, Hanna Damasio, Antoine Bechara. (2007). "Damage to the Insula Disrupts Addiction to Cigarette Smoking". Science 315 (5811): 531-534. doi:10.1126/science.1135926.
5. ^ Nasir H. Naqvi, David Rudrauf, Hanna Damasio, Antoine Bechara. (2007). "Damage to the Insula Disrupts Addiction to Cigarette Smoking". Science 315 (5811): 531-534. doi:10.1126/science.1135926.

See also

• List of regions in the human brain