Monday, March 04, 2013

Is the insula necessary for our feelings? If not, where are they?

I have been fairly uncritical in passing on simplifications like “The insula is the sensory cortex for our internal visceral feelings” and so was grateful to see, in Damasio’s recent review article “The nature of feelings: evolutionary and neurobiological origins” a critique of this idea:
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.


  1. How does the enteric nervous system play into feelings and emotions? Or, for that matter, sensory input?

    A recent study in tadpoles showed that by transplanting an ectopic eye with no direct connection to the brain, the animals were still responsive to light cues.

    "This has never been shown before," says Levin. "No one would have guessed that eyes on the flank of a tadpole could see, especially when wired only to the spinal cord and not the brain."

    Blackiston, B. J. and Levin, M. Ectopic eyes outside the head in Xenopus tadpoles provide sensory data for light-mediated learning. J. Exp. Biol., 2013; 216, 1031-1040

    Then there's this blind dog, whose eyes were removed during puppyhood, but still learned to play fetch.

    I'm curious as to what your thoughts are on these two cases.

  2. That blind dog has to have photoreceptors somewhere that can respond to light, maybe melanopsin containing neurons in the brain? In both this case and the tadpole, the finding that spinal cord or brain fashion a response appropriate to an whatever sensory input it receives fits with a long line of research on brain plasticity. For example he brain can learn to recognize an array of light to pressure transducers on the arm or back as a crude visual field. (Paul Bach-y-Rita and other's work).