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Abstract

The insula exerts a crucial role in processing interoceptive and emotional states, and in interdependently engendering subjective feelings of self-awareness and empathy. One major problem in understanding how this occurs is that little is known about the insula's anatomical organization, including its architecture and hodology. We re-examined the architecture of the macaque insula and found that each of its three classical “sectors” contains several smaller, reproducibly distinct and sharply delimited areas including 8 agranular areas, 4 dysgranular areas and 4 granular areas. Each area appears to be subdivided into smaller sub-areas organized in a series of anteroposterior stripes. Using intra- and extra-insular injections of neuronal tracers, (1) we confirmed the existence of smaller areas and of a stripe-like hodological pattern and (2) we observed that each area harbours a complex but highly organized topography. On the basis of this and other works, we proposed a new working model of the primate insular organization that resonates to some extent with models recently proposed on the basis of human imaging studies. We are currently completing the analysis of our tract-tracing to test and refine our model. Finally, using careful anatomical examination, we found that one specific area of the macaque insula contains von Economo neurons (VENs), an atypical form of projection neurons previously thought to occur only in humans and great apes among primates and known to be particularly vulnerable in the behavioural variant of frontotemporal lobe dementia. In connexion with our other studies, this finding offers new and much needed opportunities to investigate the primal connections and physiology of a neuron and brain area that could be crucial for human self- awareness, social cognition, and related neuropsychiatric disorders.