Once an island, now the focus of attention.

The insula (originally called the ‘‘island of Reil’’) is emerging from its hiding place inside of the human brain. It is easy to find articles and textbooks which show the lateral aspect of the brain but barely mention the insula, if at all (http://www.ninds.nih.gov/disorders/brain_basics/ know_your_brain.htm) or treat it as a deep brain structure, like the amygdala (http://www.scientificamerican.com/ article.cfm?id=faulty-circuits). In fact, in Brodmann’s famous map of cortical cytoarchitectonic areas, it was not even worthy of a number! [See Kurth et al. 2009, PMID: 19822572; Brodmann (1909) described only a posterior granular and an anterior agranular region in the human insular cortex.] Older neuroscientists remember the insula as a portion of the visceral brain, based on prominent writings by Penfield, Mesulam, Saper, and others (Penfield and Faulk 1955; Mesulam and Mufson 1982; Saper 2002); some investigators simply call it a multi-modal region and cite the brief reviews by Augustine (1985, 1996); but for many new investigators who find it unexpectedly activated in their functional imaging study, it is simply an enigma. I came upon the insula by following a neural pathway for pain and temperature using functional anatomical methods (Craig et al. 1994). Eventually, I realized that the cortical terminus of this pathway in the posterior insula of primates provides a homeostatic representation of the physiological condition of the body (Craig 2002). When my collaborators and I performed an imaging study designed to document this interoceptive pathway in humans, we found that an extension of this pathway to the anterior insula correlated with subjective feelings from the body, consonant with the well-known predictions of the James-Lange theory of emotion and the ‘‘somatic marker’’ hypothesis (Craig et al. 2000). At that time, functional imaging was still emerging, and few studies had observed insular activation. Today, a PubMed search for ‘‘insula AND imaging’’ limited to English and Humans pulls up 30,415 references! In order to provide an overview of this vast literature for a 2009 opinion article (Craig 2009), I compiled reports from disparate and unfamiliar branches of neuroscience. To my mind, this burgeoning literature compelled the hypothesis that the anterior insula engenders human awareness, yet only one article had directly addressed this possibility (Klein et al. 2007), and an astonishing number of authors had reported strong activation of the insula without comment. The immediate need for an anthology became obvious, in which leading primary investigators from these disparate fields could re-appraise the role of the insula in light of the new perspective provided by this extraordinary convergence of evidence. I shared this idea with Antoine Bechara, Hugo Critchley, and Steve Petersen, who graciously agreed to serve as Co-Editors. We elected to generate a Special Issue in a leading journal, to enable rapid on-line publication and broad accessibility. Our excitement was validated when every investigator queried agreed to contribute to this Special Issue on the insula. The first six articles of this set are contributed by clinical investigators, and while each focuses on particular aspects of human behavior, each one also emphasizes the multiplicity of functions attributable to the insula (Ibanez et al. 2010; Karnath and Baier 2010; Ackermann and Riecker 2010; Naqvi and Bechara 2010; Paulus and Stein 2010; Seeley 2010). The next three articles address neuroanatomical issues that are significant for our appreciation of how special the human insula really is, and these articles underscore fertile opportunities for neurogenetic and functional analyses (Butti and Hof 2010; Allman et al. 2010; Kurth et al. A. D. Craig (&) Barrow Neurological Institute, Phoenix, AZ, USA e-mail: Bud.Craig@CHW.edu