The Interoceptive Pathway to the Insular Cortex
This chapter looks at the experiments that demonstrated in monkeys and humans the unforeseen lamina I pathway to the thalamus and its subsequent projection to the interoceptive cortex. The ascending interoceptive thalamocortical pathway is phylogenetically unique to primates; it most likely arose in conjunction with the enormous encephalization associated with the emergence of the primate lineage. The existence of this pathway was a surprise to most investigators in the field of somatosensory neurobiology. As mentioned in chapter 1, a sensory representation of general feelings from the body had been envisioned by the German natural philosophers of the nineteenth century. However, that concept was superseded by the heuristic codification of nociception and the assignment of pain and temperature sensations to the somatosensory cortex. The chapter's findings rectify that misconception and substantiate the fundamental neurobiological distinction between interoception and exteroception at the thalamocortical level in the monkey and human.