Feelings and the insula. Interoceptive information mapped in the brainstem is projected rostrally to the subcortical basal forebrain and to the cortical telencephalon, where it is remapped in the insula and somatosensory cortices SI and SII.
Contemporary neuroscience has identified the insula as the main cortical target for signals from the interoceptive system, and functional neuroimaging studies consistently implicate the human insula in both interoceptive and emotional feelings.
Recently, it has been proposed that the insula is not merely involved in human feelings but is their sole platform and, by extension, the critical provider of human awareness. Several findings suggest that this hypothesis is problematic. First, given that several topographically organized nuclei of the upper brainstem, which are obligatory relay stations for most signals conveyed from the body to the insula, can produce elaborate representations of multiple parameters of body states, these regions should not be excluded a priori as platforms for feelings. Second, children born without cerebral cortex exhibit behaviours compatible with feeling states. Third, bilateral insular damage does not abolish all feelings. Specifically, complete bilateral destruction of the insula as a result of herpes simplex encephalitis does not abolish either body or emotional feelings, including pain, pleasure, itch, tickle, happiness, sadness, apprehension, irritation, caring and compassion, in addition to hunger, thirst, and bladder and colon distension. In fact, feelings seem to dominate the mental landscape of patients with bilateral insular damage. Immediate comfort appears to be their main concern, fairly unbridled by cognitive constraints.
These observations do not support a view of the insula as a necessary substrate for feeling states. Thus, the generation of feelings must also rely on the brainstem and possibly on the SI and SII somatosensory cortices of the parietal lobe, which are spared in some patients that lack the insular cortices but remain fully capable of feeling. Indeed, damage to the posterior half of the upper brainstem is associated with coma or vegetative state — two conditions in which feelings and sentience are abolished.After reviewing data on how feelings persist after lesions to other cortical regions suggested central to feelings, Damasio suggests that subcortical regions such as the upper brainstem and hypothalamus are most central in the generation of feelings, and that this has resounding evolutionary implications:
...the fundamental elements of body state mapping, sentience and feelings imbued with valence are likely to be far older than our species, and probably even older than the advent of cerebral cortices. There is good reason to believe that the primate brain inherited the neural instruments for feeling from its ancestors and elaborated upon them.