Insular cortex

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Brain: Insular cortex
The insula of the left side, exposed by
removing the opercula.
Coronal section of brain immediately in
front of pons. (Insula labeled at upper right.)
Latin lobus insularis
Gray's subject #189 825
Artery Middle cerebral
NeuroNames hier-93
NeuroLex ID birnlex_1117

The insular cortex (often called insula, insulary cortex) is a cerebral cortex structure deep within the lateral sulcus between the temporal lobe and the parietal lobe. The overlying cortical areas are the opercula (meaning "lids") and these are formed from parts of the enclosing frontal, temporal and parietal lobes.

The insular is divided into two parts: the larger anterior insula and the smaller posterior insular in which more than a dozen field areas have been identified.

The insula plays a role in diverse functions usually linked to emotion or the regulation of the body's homeostasis. These functions include perception, motor control, self-awareness, cognitive functioning, and interpersonal experience. Related to these it is involved in psychopathology.

The insular was first described by Johann Christian Reil while describing cranial and spinal nerves and plexi.[1] Henry Gray in Gray's Anatomy is responsible for it being known as the Island of Reil.[1]

Contents

[edit] Functions

[edit] Interoceptive awareness

The right anterior insular aids interoceptive awareness of body states such as the ability to time ones own heart beat. Moreover, its greater gray matter volume links to greater accuracy in this greater subjective sense of awareness of the inner body, and negative emotional experience.[2] It is also involved in the control of blood pressure[3] particularly during and after exercise.[3] It is also activated with greater perceived exertion.[4][5]

The insular cortex also is where the sensation of pain is judged as to its degree.[6] The insula is also we imagine pain if looking at images of painful events while thinking about them happening to our body.[7] Those with irritable bowel syndrome have abnormal processing of visceral pain in the insular cortex related to dysfunctional inhibition of pain within the brain.[8]

Another perception of the right anterior insula is the degree of nonpainful warmth[9] or nonpainful coldness[10] of a skin sensation. Other internal sensations processed by the insula include stomach or gastric distension[11] distention of the descending colon,[12] A full bladder activates the insular cortex[13]

The cerebral cortex processing vestibular sensations extends into the insula[14] with small lesions in the anterior insular cortex being able to cause loss of balance and vertigo.[15]

Other noninteroceptive perceptions includes passive listening to music,[16] laughter and crying[17] and language [18]

[edit] Motor control

In motor control it contributes to hand and eye motor movement,[19][20] swallowing,[21] gastric motility,[22]speech articulation.[23][24] Research upon conversation links it to the capacity for long and complex spoken sentences.[25] It is involved in motor learning.[26] It has also related to this been identified as playing a role in the motor recovery from stroke.[27]

[edit] Homeostasis

In homeostasis, it controls autonomic functions through the regulation of the sympathetic and parasympathetic systems.[28][29] It has a role in regulating the immune system.[30][31][32]

[edit] Self

It has been identified as play a role in the experience of bodily self-awareness, [33][34] sense of agency[35] and sense body ownership.[36]

[edit] Social emotions

The anterior insular processes a person's sense of disgust both to smells[37] but also the sight of contamination and mutilation[38] -- even when just imagining the experience.[39] This associates with a mirror neuron like link between external and internal experience: as it is activated both when a person smells odorants that cause a feeling of disgust but also when people see people make facial expression of disgusts.[37][39]

In social experience it is involved in the processing of norm violations,[40] emotional processing,[41] empathy, [42] and orgasms.[43]

[edit] Emotions

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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.[44]

[edit] Neuroanatomy

[edit] Connections

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.

[edit] 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.

[edit] Origins

The insular cortex is considered a separate lobe of the telencephalon by some authorities.[45] Other sources see the insula as a part of the temporal lobe.[46] It is also sometimes grouped with limbic structures deep in the brain into a limbic lobe.[citation needed]

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.

[edit] Clinical

[edit] Progressive non-fluent aphasia

Progressive non-fluent aphasia is the deterioration of normal language function which causes individuals to lose the ability to communicate fluently while being still being able to comprehend single words and intact other non-linguistic cognition. It is found in a variety of degenerative neurological conditions including Pick's disease, motor neuron disease, corticobasal degeneration, frontotemporal dementia and Alzheimer’s disease. It is associated with hypomeabolism[47] and atrophy of the left anterior insular cortex.[48]

[edit] 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." [49] 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 these findings, the insula has been ignored within the drug addiciton literature, perhaps because it is not known to be a direct target of the mesotelencephalic dopamine system which is central to current dopamine reward theories of addiction. Recent research [50] has shown that cigarette smokers who suffer damage to the insular cortex, from a stroke for instance, have their addiction to cigarettes practically eliminated. However, the study was conducted on average eight years after the strokes, and the author admits recall bias could affect the results[51]

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 possible target for novel anti-addiction medication. In addition, this finding suggests that functions mediated by the insula, especially conscious feelings, may be particularly important for maintaining drug addiction, although this view is not represented in any modern research or reviews of the subject.[52]

A recent study in rats by Contreras et al. [53] 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. (see above) 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, according to this study.

[edit] Other clinical conditions

The insular cortex has been suggested to have a role in anxiety disorders,[54] and emotion dysregulation.[55]

[edit] Gallery

Coronal section of brain through intermediate mass of third ventricle.

Coronal section through anterior cornua of lateral ventricles.

Coronal section of brain through anterior commissure.

Orbital surface of left frontal lobe.

Section of brain showing upper surface of temporal lobe.

Horizontal section of left cerebral hemisphere.

Human brain frontal (coronal) section

Human brain view on transverse temporal and insular gyri

[edit] References

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[edit] External links

[edit] See also

[edit] External links

v • d • e
Brain: telencephalon (cerebrum · cerebral cortex · cerebral hemispheres)
Primary four
surface lobes
Somatosensory cortex (Primary (1 · 2 · 3 · 43) · Secondary (5)) · Precuneus (7m) · Parietal operculum

Parietal lobules (Superior (7l) · Inferior (40)) · Angular gyrus (39)

Intraparietal sulcus · Marginal sulcus
Cingulate cortex/gyrus
White matter tracts
Other
Some categorizations are approximations, and some Brodmann areas span gyri.
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