The featured paper is ‘Insular Cortex and Neuropsychiatric Disorders: A review of recent literature’ by M Nagai and colleagues.
The authors state their aim was to summarise the relationship between the insular cortex and ‘neuropsychiatric disorders’.
The methodology section is brief and the authors have identified relevant literature using the keyword ‘insular’ together with others such as ‘mood disorder’ in the Pub-Med database. There was no reference to other databases or handsearching. The number of articles selected was not stated nor was the method used to select the final articles reviewed in the paper.
The discussion of the relationship between the Insula and psychiatric conditions is clearly organised into sections on schizophrenia, normal emotions, pathological emotions, OCD, panic disorder and PTSD. Each of these sections is further subdivided according to relevant neuroimaging data studies – fMRI, MRI and PET as well as electrical stimulation studies where appropriate.
Penfield’s electrical stimulation study is cited in which the stimulation of the anterior insula produced a sensation of fear. There is also a Penfield case report about a man with epilepsy who after resection of the insula experienced gastrointestinal symptoms and signs. The difficulty with interpreting studies of this kind however are that symptoms or signs may arise as a result of interruption of pathways rather than due directly to a lack of insular function.
In terms of emotions, recall related sadness produced an increase in regional cerebral blood flow in the anterior insular cortex whilst guilt produced an increase in the left anterior insular cortex and a decrease in the left posterior insular cortex. The authors also examine other studies concluding that rCBF in the insula is related to changes in insulin levels and that the insula can be considered as part of the gustatory cortex although perhaps this might considered to fall under the general rubric of perceptions.
In terms of depression research was cited finding that right insular lesions was associated with anergia and tiredness although this was a case-series which merits confirmation with larger studies. Other studies are noted in which glucose metabolism is decreased in the insular cortex in depressed patients who have responded to medication. An interesting finding in one study that was cited was that the response to carbamazapine in cycling mood disorders was associated with decreasing metabolism in the left insular cortex.
The authors cited a study showing reduced benzodiazepine binding in the insular cortex in panic disorder which is similar to a recently reviewed paper on this blog. There were also findings of increased rCBF in the insular cortex during anxiety in PTSD, OCD and phobia. A 1992 insular cortex stimulation study is also quoted which found that bradycardia resulted from stimulation of the left insula and tachycardia from the right. There have been suggestions elsewhere of reciprocal roles for the right and left insula cortices. Benarroch’s theory of a central autonomic network is also discussed in which it is suggested that a circuit which includes the insula controls a number of predominantly autonomic functions.
The authors then finish with an examination of the literature on schizophrenia which showed reduced insular activity during visual, speech and olfactory tasks. However, schizophrenia is divided into a number of subtypes and even within paranoid schizophrenia for instance, the phenomenology of one person’s experiences are distinct from another. Thus in drawing inferences the generic label may be less useful. Nevertheless a consistent finding in the volumetric studies was a reduction in grey matter volume in the insula relative to controls. This may be more indicative of wider cortical involvement rather than being specific to the insula.
There is a final section on integrating models of the Insula. The authors look at some models of integration and also propose their own model in which the anterior insula mediates a balance between emotional and cardiovascular functions whereas the posterior insula regulates visual-somatosensory functions and may be involved in the aetiology of hallucinations. In this regards, the theory might compete with Crow’s theory or could perhaps be integrated with it, after it is more fully developed.
The authors display technical virtuosity in this paper which summarises findings from a vast array of different types of studies relating to the neuropsychiatric function of the insula. This is a useful addition to the literature particularly for those with an interest in the neuranatomy/physiology of psychiatric conditions.
M Nagai, K Kishi and S Kato. Insular cortex and neuropsychiatric disorders: A review of recent literature. European Psychiatry. 22. 2007. 387-394.
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