The Isle of Craig: Neuroanatomical and Functional Evidence for a Role of the Insular Cortex in Subjective Feelings

At the start of the twenty-first century, Arthur D. (Bud) Craig brought back to the fore the Island of Reil (insular cortex or insula). He did so by following, step by step, with rigor and tenacity, the afferent sensory pathway that informs the forebrain about the ongoing physiological status of the organs and tissues of the body. Along with his demonstration of the existence of a primate-specific ascending interoceptive pathway and his subsequent re-interpretation of Sherrington's concept of interoception, Bud Craig's seminal experiments and profound interpretations led him to make the groundbreaking proposals that the dorsal posterior insular cortex provides an ideal substrate for James's concept of emotional embodiment, that the insular cortex contextualizes interoception across a posterior-to-mid-to-anterior integration with multimodal activities, and that the anterior insular cortex has a crucial role in the evolutionary emergence of the awareness of subjective feelings in humans, for the purpose of optimizing metabolic energy usage. Bud Craig's unique work paves the path for further elucidation of the role of the insula and other brain regions in subjective feelings. His discoveries and proposals rest on implacable attention to neuroanatomical and neurophysiological details and a serendipitous quest for the fundamental evolutionary Logic of Life. This chapter provides a detailed description of the ascending interoceptive pathway and the functional and comparative neuroanatomy of the insular cortex in primates. Building on Bud Craig's work, our recent findings suggest that the primary interoceptive cortex serves as a representation of the spino-solitary-parabrachial neuraxis, merging with posterior-to-mid-anterior and dorsal-to-ventral processing streams that form a latticework integration pattern. At the ventral anterior tip of this integration, the von Economo neuron area closes the corticofugal interoceptive-autonomic loop of the sensory-motor homeostatic system through projections to all brainstem nuclei integrating interoceptive afferences.