Ever since I first read Antonio Damasio's Descartes' Error: Emotion, Reason, and the Human Brain (1994), I have been impressed by his "somatic marker hypothesis" for explaining the importance of emotion in decision-making and consciousness. Building on an idea proposed by William James and Carl Lange, Damasio believes that emotions arise from physiological states of the body, so that, for example, the emotion of fear arises from the physiological disturbance of the body associated with some fearful event. Emotions help us to make decisions by assigning emotional valence to our choices. Through imaginative projection, we can foresee the emotional outcome of a choice by anticipating how we will feel--our somatic markers--if we make that choice, and thus we might avoid a choice with fearful associations. Ultimately, this emotional decision-making mechanism can be explained as an evolutionary adaptation to secure the survival and well-being of the body.
I introduced some of Damasio's research into my chapter on psychopathy in Darwinian Natural Right, because it's likely that the brains of psychopaths are abnormal in their failure to process social emotions such as fear, guilt, and shame, so that they don't feel the moral sentiments that guide the behavior of normal people.
This line of thought has been extended by A. D. (Bud) Craig, a functional neuroanatomist at the Barrow Neurological Foundation. He has traced out the fundamental neuroanatomical basis for all human emotions, and he has argued that this shows how the neural substrates for human self-awareness or consciousness are based on the neural representation of the physiological state (the homeostasis) of one's body. This manifests the embodiment of emotional self-consciousness. In particular, he argues that there is a phylogenetically novel sensory pathway in primates, most fully developed in human beings, that provides for a self-conscious integration of the physiological condition of the body (the material "self") with one's sensory environment, with one's motivational condition, and with one's social situation in the anterior insular cortex (AIC).
In imaging studies of emotion, the AIC is jointly activated with the anterior cingulate cortex (ACC). The AIC seems to be the primary site for self-awareness based on representations of the feelings from the body, while the ACC seems to be the site for the initiation of behavior, which thus provides volitional agency.
Wikipedia articles provide some good diagrams and analysis of the AIC and ACC here and here.
My special interest in all of this is in how it might explain the evolutionary neurophysiological basis for Thomas Aquinas's account of natural law or John Locke's account of natural rights. Reasoning about natural law or natural rights ultimately depends on discerning natural human inclinations, such as self-preservation, property, social attachment, practical judgment, and intellectual understanding, which correspond to what I have identified as the twenty natural desires. Evolutionary neuroscience explains how the human nervous system has evolved to serve those natural inclinations or desires. The concrete expression of those natural inclinations will vary according to individual temperament, individual life history, and cultural circumstances. But there will be a universal human pattern that manifests the evolved natural needs of human beings as the very smart social mammals that they are.
The evolution of mammalian social behavior depends on the evolution of pain or "negative affect," which includes pain, fear, panic, and anxiety. In all vertebrates, fear and pain are represented in the brainstem and hypothalamus as signals to elicit self-preserving behavior. In mammalian evolution, these neural mechanisms are modified so that animals care for their offspring as well as themselves. This includes modifying the cortex of the mammalian brain to elaborate the representation of pain to include anxiety tied to separation from or threat to loved ones.
Craig's research clarifies this neural evolution of pain by classifying pain as a homeostatic emotion rather than as a sensation of touch. Pain belongs to "interoception"--the sense of the physiological condition of the body--and it is therefore part of the evolved mechanisms for self-preservation.
The insular cortex receives signals from all the tissues of the body, and these signals are integrated with physical and social stimuli from outside the body and with the memory of past experiences as well as imaginative projections of future experiences. This supports a general awareness of the body's condition in space and time. The ACC can then be activated to motivate behavior to correct whatever is wrong. This neural processing mechanism seems to be unique to primates, but it's more highly developed in human beings.
Both the insular cortex and the ACC respond not only to physical pain from bodily injury but also social pain from social injury. It seems that in mammalian evolution, the neural circuitry for physical pain was appropriated for registering social pain in animals adapted for social attachment. Mammals must care for the survival and well-being not only of themselves but of others to whom they are attached. Extending the neural mechanisms originally evolved for individual self-preservation to include the welfare of offspring and social partners secures mammalian social order. The uniquely human evolution of the neocortex elaborates this mammalian development to sustain human love and concern for others.
When we use the language of physical pain to describe our social pain ("a broken heart"), we recognize the embodiment of our natural social consciousness, in which our mind, our brain, our body, and our social life are inseparably intertwined.
If this is true, then the Platonic idea of the disembodied immortality of the soul is nonsensical.
REFERENCES
Patricia Churchland, Braintrust: What Neuroscience Tells Us about Morality (Princeton: Princeton University Press, 2011)
A. D. (Bud) Craig, "How Do You Feel? Interoception: The Sense of the Physiological Condition of the Body," Nature Reviews Neuroscience 3 (August 2002): 655-666
Craig, "A New View of Pain as a Homeostatic Emotion," Trends in Neurosciences 26 (June 2003): 303-307
Craig, "Interoception and Emotion: A Neuroanatomical Perspective," In Handbook of Emotions, 3rd ed., edited by Michael Lewis, Jeannette Haviland-Jones, and Lisa Barrett, 272-288. New York: Guilford, 2008.
Craig, "How Do You Feel--Now? The Anterior Insula and Human Awareness," Nature Reviews Neuroscience 10 (January 2009): 59-70.
Naomi I. Eisenberger and Matthew D. Lieberman, "Why Rejection Hurts: A Common Neural Alarm System for Physical and Social Pain," Trends in Cognitive Sciences 8 (July 2004): 294-300.
A few of the many posts on related themes can be found here, here, here, and here.
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