By Kent Berridge, PhD
Pleasure is one of the simplest phenomena in psychology. It is a basic aspect of mental life, and an important feature of positive emotions. But pleasure is not entirely simple. New findings in hedonic psychology and affective neuroscience are revealing intriguing complexities.
Even mere sensory pleasures can give insight into hedonic psychology. For example, sweetness tastes nice. It is one of the sensations most reliably able to cause pleasure in people. The pleasure of sweetness lies not in the intrinsic sensation itself, but in something done to it. Sweets are not necessarily nice - there are nasty sweet tastes in this world too. For instance, we can easily acquire a learned taste aversion for particular sweet flavors (such as a novel sweet flavor that is associatively paired with visceral illness). Sweet tastes for which we have learned aversions remain sweet afterwards - but their sweetness becomes nasty, instead of nice.
The Pleasure Gloss
In other words, pleasure is a sort of gloss on sensation, a value added. The pleasure gloss is actively painted onto mere sensory representations by limbic brain circuits. The pleasure gloss and our desire for it involve many complexities, both neurobiological and psychological.
Which Brain Systems Paint the Pleasure Gloss?
First it's interesting to ask how the brain paints the pleasure gloss. Pleasures activate brain cerebral cortex (especially medial prefrontal cortex), amygdala, and deep brain structures such as nucleus accumbens and the midbrain dopamine neurons that project to it, the ventral pallidum which accumbens projects to in turn, and even some hindbrain structures. All these can be activated by pleasures. But not all need actually cause pleasure. Instead many brain co-activations are pleasure consequences, not pleasure causes (causing other psychological functions instead) . So which brain events actually paint the pleasure gloss onto sensation?
Psychologists and neuroscientists are interested in the causation of all pleasures, of course, but in practice we must study them one at a time. To identify how the brain paints the pleasure gloss we have studied taste pleasure in our laboratory at the University of Michigan. Sweet tastes elicits 'liking' facial expressions that are homologous in human infants and many animals (e.g., tongue protrusions), whereas nasty bitter tastes elicit 'disliking' expressions (e.g., gapes). We have used those expressions in affective neuroscience studies of rats and mice to map brain systems that cause pleasure. In these studies, we gently tweak a brain system to see if it causes changes in a taste's pleasure gloss (for example, by making a painless microinjection of a tiny drug droplet into a brain structure).
In this way, we've identified several types of brain activation that cause a pleasure gloss on sweet sensation. For instance, we've found that triggering activation of opioid circuits in the nucleus accumbens (e.g., by microinjecting morphine there) causes increased pleasure 'liking'. This is an initial link in a neural chain of pleasure causation. The chain continues in structures that receive signals from accumbens, such as ventral pallidum, forming together a limbic circuit that paints the pleasure gloss.
False 'Liking': Dopamine and Electrical Brain Stimulation
We have also turned up surprising brain failures to cause pleasure. These brain systems were once thought to cause sensory pleasure, but turn out not to. For example, brain dopamine, although often called a pleasure neurotransmitter, fails after all to live up to its pleasure label. To make a long story short, dopamine systems seem unable to cause a pleasure gloss. We've tried both activating and suppressing dopamine in several ways, but it never alters the pleasure gloss. 'Liking' reactions to sweetness simply persist unchanged and normal, no matter what brain dopamine systems are doing.
So if dopamine is a faux-pleasure, what is its real psychological role? We've suggested that pleasure 'wanting', rather than 'liking', best captures what dopamine does. Usually 'liking' and 'wanting' go together for pleasant incentives, as two sides of the same psychological coin. But our findings indicate 'wanting' may be separable in the brain from 'liking', and that mesolimbic dopamine systems mediate only 'wanting'. My colleagues and I coined the phrase incentive salience for the particular psychological form of 'wanting' we think is mediated by brain dopamine systems.
False Pleasure Electrodes
Another surprising case of false 'liking' may be so-called brain 'pleasure electrodes'. In our animal studies, such electrodes appear to function similarly to dopamine, causing pleasure 'wanting' without 'liking'. In humans, famous cases of intense 'pleasure electrodes' are cited by many textbooks. But if we scrutinize these cases more closely, we may be forced to a surprising conclusion that they did not cause much sensory pleasure after all. For example, a well-known case is "B-19", a young man implanted with stimulation electrodes by Heath and colleagues in the 1960s. B-19 voraciously self-stimulated his electrode, and protested when the stimulation button was taken away. In addition, his electrode caused "feelings of pleasure, alertness, and warmth (goodwill); he had feelings of sexual arousal and described a compulsion to masturbate" (p. 6, Heath, 1972).
But did his electrode really cause a pleasure sensation? Perhaps not. B-19 never was quoted as saying it did; not even an exclamation or anything like "Oh -- that feels nice!". Instead B19's electrode stimulation evoked desire to stimulate again and strong sexual arousal - while never producing sexual orgasm or clear evidence of actual pleasure sensation. Clearly the stimulation did not serve as a substitute for sexual acts. What it did instead was to make him want to do sexual acts. Similarly, a woman patient, implanted with an electrode decades later, compulsively stimulated her electrode at home. "At its most frequent, the patient self-stimulated throughout the day, neglecting personal hygiene and family commitments" (p. 279, Portenoy et al., 1986).
When her electrode was stimulated in the clinic, it produced a strong desire to drink liquids, and some erotic feelings, as well as a continuing desire to stimulate again. However, "Though sexual arousal was prominent, no orgasm occurred" (p. 279, Portenoy et al., 1986). Doesn't this seem similar to B-19? "She described erotic sensations often intermixed with an undercurrent of anxiety. She also noted extreme thirst, drinking copiously during the session, and alternating generalized hot and cold sensations" (p. 282, Portenoy et al., 1986). Clearly this woman felt a mixture of subjective feelings, but the description's emphasis is on aversive thirst and anxiety -- without evidence of distinct pleasure sensations.
What could these electrodes be doing, if not pleasure? Among other things, they might be activating incentive salience attribution to surroundings and perceived stimuli, especially the act of stimulating the electrode. If the electrodes caused 'wanting', a person might well describe a sudden feeling that life was suddenly more attractive, desirable, and compelling to pursue. They might well 'want' to activate their electrode again, even if it produced no pleasure sensation. That would be mere incentive salience 'wanting' -- without hedonic 'liking'.
The psychology of incentive salience creates the possibility for irrational desire. Defined as a want for something you neither like nor expect to like, strongly irrational desire is rare but may exist (the electrode cases above might be examples). In animal experiments in my laboratory, we can create irrational 'wanting' by tweaking the brain dopamine system into over-activation. My colleague Terry Robinson and I believe something similar may occur in some human drug addicts. In drug addiction, the cause may be a nearly-permanent brain change known as neural sensitization, produced by addictive drugs. Sensitization makes dopamine-related brain systems over-react subsequently to drugs and cues for them. Sensitization can persist years after drug use ends. Sensitized incentive salience may keep drug addicts vulnerable to relapse, via compulsive cue-triggered 'wanting' to take drugs again. This might happen even for drugs that don't give much pleasure, and even after symptoms of withdrawal are long gone.
Unconscious 'Liking' and 'Wanting' for Pleasures
Strongly irrational desire, and dissociations between 'liking' and 'wanting', may seem counter-intuitive. If these occur, why are we not more aware of them? The reason may be precisely because we do not have direct conscious access to core psychological processes that occur within pleasure, such as 'liking' or 'wanting'. For example, in experiments led by my colleague Piotr Winkielman, unconscious 'liking' and 'wanting' has been produced in ordinary people. Their consumption behavior was altered by subliminal exposure to happy/angry facial expressions, which changed their desire to drink a subsequently-encountered beverage even though they felt no conscious emotional reactions at all at the moment the subliminal faces occurred. Such dissociation of emotional reaction from conscious feelings suggests that unconscious dissociations among underlying pleasure 'liking' and 'wanting' components might also occur without being felt.
Simple pleasures are not so simple. Both psychological and neurobiological complexities exist within even the simplest sensory pleasure. Recent surprising insights into the hedonic psychology and affective neuroscience of pleasure have been gained, and new advances seem likely to continue. That might make any psychologist feel pleased.
Acknowledgement: I thank colleagues who have participated in our lab's pleasure studies: Terry Robinson, Elliot Valenstein, J. Wayne Aldridge, Susana Peciña, H. Casey Cromwell, Piotr Winkielman, Cindy Wyvell, Sheila Reynolds, Amy Tindell, Kyle Smith, Stephen Mahler, Linda Parker, Xiaoxi Zhuang, Barbara Cagniard, Julie Wilbarger.
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About the Author
Kent Berridge is a professor at the University of Michigan in the Department of Psychology (and a member of its Biopsychology program). He studies the psychology and affective neuroscience of reward liking and wanting in motivation and emotion. Berridge and colleagues seek answers to questions such as: How is pleasure produced in the brain? What causes addiction? Can emotions be unconscious? How do brain mechanisms of reward and desire interact with those of stress and fear? More information is available on the University of Michigan Psychology Department (Biopsychology Department) website.