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The question of the nature of subjective emotional experience, or affect, remains a central issue in both psychology and philosophy, intimately related to the question of the nature of consciousness. My previous post reviewed evidence challenging theories stemming from William James' notion that emotional experiences involve the feeling of bodily changes Specifically, these theories cannot account for the sparing of emotional experience in spinal cord injuries, or for the speed and complexity of emotional experience. What, then, IS the source of emotional experience: the directly-known and self-evident qualia of feelings and desires? Is there a physiological process that is associated with such experience? A possible answer is surprising and controversial: a physiological process hypothesized to be closely identified with many emotional experiences involves specifiable neurochemical systems associated with some of the most ancient molecules in the body: the peptides. The peptide hypothesis can be stated succinctly: where there is a mood, there is a molecule.
Peptides are molecules formed of chains of amino acids strung together like beads on a string, ranging from a few in number to hundreds. Longer chains (e.g., 200 units) are conventionally known as proteins, so peptides are basically relatively short proteins. Like proteins, peptides are direct products of genes: genes physically construct the amino acid chains. The sequencing of amino acids in the chain determines the shape of the molecule and therefore its physiological effects. The peptides function by fitting into receptor sites on cell membranes like keys into locks. Peptides composed of shorter chains function like neurotransmitters, in that they have relatively rapid but transient effects. Peptides composed of longer chain function like hormones, in that their effects are slower but longer lasting. Because of this dual identity peptides are sometimes referred to as neurohormones.
Peptides are ancient substances, indeed primordial. Many peptides functional in human beings can be found in the simplest microbes, indicating that the genes responsible for their construction derive from our last common ancestor: many millions and indeed billions of years in the past. For example, the peptide Gonadotropin Hormone Releasing Hormone (GnRH), which is associated with sexual functioning and possibly erotic feelings in human beings, is a sexual pheromone in yeast (Loumaye and colleagues, 1982). Other peptides including insulin, beta endorphin, and ACTH are found in single celled microbes. Neuroscientist Candace Pert noted In her book Molecules of Emotion that Charles Darwin predicted that the physiological basis for emotions would be conserved throughout evolution, and that appears to be the case with many peptides.
Pert also outlined several lines of evidence consistent with the peptide hypothesis. Perhaps most compellingly, many psychoactive drugs known to influence specific moods have endogenous analogs: that is, analogs in the form of substances found naturally in the body. Many of these are peptides. For example, the affective aspects of primary drives such as hunger, thirst, sex, and pain have been associated with specific peptides: angiotensin, gherelin and insulin, GnRH, and substance P, respectively. Also, manipulation of the actions of many peptide neurohormones by agonists (which support their functioning) or antagonists (which oppose it) can have predictable effects on moods. For example, specific drugs thought to alter peptides are known to influence feelings of elation, depression, anxiety, panic, trust, nurturance, and pleasure. Moreover, receptor sites sensitive to these putative mood altering neurohormones are located in limbic system brain structures classically identified with emotion.
Although we cannot have access to subjective experiences of others, there are ways to study the subjective effects of drugs indirectly: even animals can provide a sort of "self-report" of their feelings. Rewarding or punishing effects of drugs can be demonstrated in rats in studies of conditioned place preferences: if a certain spatial position in a n enclosure is associated with a positive drug, the animal will show a preference for that place; if the drug effects are negative, it will avoid that place. Similarly, drug discrimination techniques can provide detailed evidence relating to the strength and quality of the subjective impact of specific drugs. Results of such studies provide powerful converging evidence that emotion is subjectively experienced in nonhuman animals.
The discrete emotions emerging from studies of the peptide neurochemical systems are different from, but compatible with, the primary affects associated with universal facial expressions such as happiness, sadness, fear, and anger. For example, the primary affect of fear is a response to danger in the environment that requires communication at a distance, and the face is well suited to display at a distance. Fear may involve a number of neurochemical systems: e.g., panic associated with cholecystokinin (CCK), stress associated with corticotropin releasing hormone (CRH), and/or anxiety associated with the "anxiety peptide," diazepam binding inhibitor (DBI). However, the facial expression of fear may be equally relevant in situations of panic, stress, and anxiety. Therefore, "fear" emerges as an ecological reality associated with universal facial display that may not be associated with any one brain system but rather be potentially composed of a number of fear-related "modules" at the physiological level.
Other discrete emotions are displayed and communicated more intimately. Feelings of bonding, nurturance, and erotic arousal are communicated via physical immediacy, eye contact, touch, pheromones, and the sense of radiant heat. Facial expression is relatively unimportant and ineffective as display at such intimate distances.
Portions taken from R. Buck, Human Motivation and Emotion, 2nd Edition, Wiley 1988; and R. Buck (1999). The biological affects: A typology. Psychological Review. 106, 301-336.
Loumaye, E., Thorner, J., & Catt, K. J. (1982). Yeast mating pheromone activates mammalian gonadotropins: evolutionary conservation of a reproductive hormone? Science, 218, 1323-1325.
