The scientific approach to self-help suggests that a better understanding of who we are can help us achieve happiness and other goals. Most centrally, it will be helpful to understand our brains, because it is our brains that generate happiness and goals.

In particular, I'd like to explore the neuroscience of pleasure and desire. Today's post covers the neuroscience of pleasure; the next post will cover the neuroscience of desire. After each post I'll consider some of the implications for self-help. In a later post, I'll consider how this research can inform the pursuit of Friendly AI.

Introducing Affective Neuroscience

The last decade has seen the arrival of affective neuroscience: the study of the neural mechanisms behind emotion, including pleasure and desire.¹ Most questions remain unanswered, and experts disagree on many specifics,² but there are some things we can state with confidence. We begin with the reward system in the brain.

The reward system consists of three major components (image)³:

Liking: The 'hedonic impact' of reward, comprised of (1) neural processes⁴ that may or may not be conscious and (2) the conscious experience of pleasure.
Wanting: Motivation for reward, comprised of (1) processes of 'incentive salience' that may or may not be conscious and (2) conscious desires.
Learning: Associations, representations, and predictions about future rewards, comprised of (1) explicit predictions and (2) implicit knowledge and associative conditioning (e.g. Pavlovian associations).

Unfortunately, the interaction between these components is extraordinarily complex, and many puzzles remain.⁵

I'll share two examples of our ignorance. First: pleasure electrodes. For decades, it was thought that electrical stimulation of certain structures caused pleasure, because rats and humans would self-administer this stimulation hundreds or thousands of times each hour if allowed to do so.⁶ But a careful reading of the transcripts reveals the causation of wanting, not liking. Sometimes, the cause was even an unpleasant nervousness.⁷ Though, there are a few exceptions where liking was produced.⁸

Second: dopamine. For decades, experts thought dopamine was 'the pleasure chemical'.⁹ But this is probably false.¹⁰ Lack of dopamine does not impair 'liking' reactions.¹¹ And in humans, perceived pleasure is not reduced by loss of dopamine.¹² Dopamine has a big role in wanting, instead.¹³

Today we focus on 'liking' or pleasure.

Pleasure

Thoughts and sensations are not intrinsically pleasurable. Rather, the reward system must paint them with a 'hedonic gloss' to make them 'liked'.¹⁴ Food and sex are reliably pleasurable for obvious Darwinian reasons.¹⁵ These 'fundamental pleasures' use the same brain structures as 'higher-order pleasures' like social pleasures, art, money, and altruism.¹⁶

The basic structures which produce pleasure are shared by other mammals (for Darwinian reasons), and other mammals exhibit liking processes in the brain.¹⁷ It's not clear how similar their subjective experience of pleasure is to our own, though. Humans do have unique cognitive, representational and 'savoring' capacities,¹⁸ among others.

Much research concerns 'hedonic hotspots' in the brain. A hedonic hotspot might be a necessary cause of pleasure (pleasure doesn't occur without it; e.g. the ventral pallidum), a sufficient cause of pleasure (if activated, pleasure occurs; e.g. the nucleus accumbens), or it may code for pleasure (its activation correlates with pleasure, but might be either a cause or effect of pleasure, or both; e.g. the orbitofrontal cortex and ventral pallidum). A substrate may play multiple roles, or it may code without causing, or it may be a sufficient cause without being a necessary cause.¹⁹

We know thousands of specific things about pleasure and the reward system, but we don't yet understand pleasure on a systems level. We don't know how to integrate these thousands of bits of information into a cohesive theory of how pleasure works.

Many have tried, of course. Here is Tim Schroeder's attractively simple theory of pleasure:

To be pleased is (at least) to represent a net increase in desire satisfaction relative to expectation; to be displeased is to represent a net decrease in desire satisfaction relative to expectation. Intensity of pleasure or displeasure represents degrees of change in desire satisfaction relative to expectations.²⁰

But this is too simple. Though pleasure often results from experiencing more satisfaction than expected, there are many other sources of pleasure,²¹ and pleasure can occur in animals that lack representational capacities or expectations, for example anecephalic infants and animals with most of their brains removed.²²

Among neuroscientists, there are many overlapping theories of pleasure, which are often not mutually exclusive.²³ Rather than survey them all, let me jump to the self-help advice: what can brain science teach us about pleasure and how to get more of it?

Self-Help Implications

The neuroscience of pleasure is most useful when designing new drugs or performing neurosurgery, but there are a few self-help recommendations we can draw from the field:

Wanting and liking are different signals, though they sometimes share use of the same neurons, like phone and internet data traveling along the same wire.²⁴ As a result, we usually like what we want and want what we like, but sometimes we don't want what we like or don't like what we want.²⁵ Understanding why this happens may relieve the confusion that often results when it occurs (not to mention philosophical confusion).
Contrary to some theories,²⁶ pleasure is not merely the perception of certain bodily sensations (e.g. blood rising, organs warming). Instead, sensations and thoughts are painted with a hedonic gloss that makes them pleasurable. Moreover, the selection of sensations and thoughts that are painted with a hedonic gloss can be changed.²⁷ This is why we can change (to some degree) what we like and dislike via classical and operant conditioning, and other methods.
Anticipation matters. Anticipating future pain is itself painful,²⁸ and anticipating pleasure is itself pleasant.²⁹ Spend more time reliving happy memories and anticipating future pleasures, and spend less time anticipating future pains. But beware: trying not to think of the event will only bring it to your mind again. Instead, get 'lost' in a challenging task that matches your level of ability: that is, get in flow.
If you experience a continuing lack of pleasure in life (anhedonia), your problem might be neurological, not situational. A doctor may be able to solve the problem with drugs or deep brain stimulation.³⁰

This is a short list, but additional self-help recommendations can be gleaned after we build on this knowledge to discuss the neuroscience of desire, which we will cover next.

Notes

¹ LeDoux & Phelps (2000); Berridge (2003a); Davidson et al. (2003); Damasio (2004); Rolls (2005); Feldman & Wager (2006); Kringelbach (2005); Kringelbach & Berridge (2009a, 2009b).

² Kringelbach et al. (2009).

³ Berridge & Kringelbach (2008); Berridge et al. (2009); Berridge (1996); Berridge & Robinson (1998); Pecina et al. (2003); Camerer (2006); Kringelbach & Berridge (2010b). Some experts will talk as if there is no such thing as unconscious pleasure (Kringelbach et al. 2009), but this is only a manner of speaking adopted to respect the folk understanding of conscious pleasure, for they acknowledge the same unconscious hedonic processes that other researchers do (Winkielman & Berridge 2004; Winkielman et al. 2005; Schooler & Mauss 2009). Likewise, though ever researcher may not use the liking / wanting / learning scheme, I haven't found any major expert who disagrees with the minimal factual claims presupposed by that scheme. Experts agree that pleasure, motivation, and learning are distinct components of the reward system in the brain.

⁴ The neural processes that apply the 'hedonic gloss' to sensations and cognitions seem to be a particular pattern of excitation of neurons in a hedonic hotspot in the ventral pallidum (Tindell et al. 2005; Aldridge & Berridge 2009), though other brain structures may play a role as well, in particular the nucleus accumbens (Smith et al. 2001), the orbitofrontal cortex (Kringelbach 2009), and perhaps the perigenual anterior cingulate cortex (Schroeder 2004, ch. 3). Commonly measured 'surface' pleasure reactions are (1) subjective report in humans, and (2) particular facial expressions in human adults, human infants, chimpanzees, and rats (Smith et al. 2005). See videos here.

⁵ Baldo & Kelley (2007); Balleine & Kilcross (2006); Balleine et al. (2007); Beaver et al. (2006); Burke et al. (2009), Di Chiara & Bassareo (2007); Evans et al. (2006); Everitt & Robbins (2005); Izard (2007); Kuhn & Koob (2010); Panksepp (2007); Salamone et al. (2007); Schultz (2006); Stoeckel et al. (2008); Van Leijenhorst et al. (2010); Volkow et al. (2006); Voon et al. (2010); Wise (2006); Kringelbach & Berridge (2010a).

⁶ Delgado (1969); Heath (1972); Sem-Jacobsen (1976).

⁷ Berridge (2003b); Pecina et al. (2006); Smith et al. (2009).

⁸ Morgan et al. (2006).

⁹ Hoebel et al. (1999); Shizgal (1999); Wise & Bozarth (1985).

¹⁰ Smith et al. (2009); Berridge (2007); Robinson & Berridge (2003).

¹¹ Berridge & Robinson (1998); Berridge et al. (1989); Robinson et al. (2005); Pecina et al. (1997).

¹² Sienkiewicz-Jarosz et al. (2005); Brauer et al. (2001); Leyton et al. (2005); Leyton (2009).

¹³ Berridge (2007).

¹⁴ Frijda (2006, 2009); Aldridge & Berridge (2009); Ryle (1954).

¹⁵ Cabanac (2009); Kringelbach (2005, 2009); Rolls (2005); Schulkin (2004).

¹⁶ Kringelbach (2005, 2009); Pecina et al. (2006); Small et al. (2001); Gottfried (2009); Kahneman et al. (2004).

¹⁷ Steiner et al. (2001); Berridge (2000); Kringelbach (2008, 2009); Smith et al. (2009); Calder et al. (2007).

¹⁸ Barrett et al. (2007); Frijda (2006); Frijda & Sundarajan (2007); Gilbert & Wilson (2007).

¹⁹ Berridge & Kringelbach (2008); Smith et al. (2009).

²⁰ Schroeder (2004), p. 94.

²¹ Smith et al. (2009); Kringelbach & Berridge (2009b).

²² Steiner (1973); Steiner et al. (2001); Goltz (1892); Miller & Sherrington (1915); Grill & Norgren (1978). It must be noted that my summary is a bit unfair to Schroeder: Schroeder is offering a theory only of conscious pleasure, which may indeed require the representational capacities unneeded for unconscious 'liking.' Still, it remains the case that there seem to be many other causes and implementations of pleasure than representations of a positive difference between actual desire satisfaction and expected desire satisfaction. But Schroeder's theory is not, as far as I can tell, decisively falsified by the data.

²³ See Dickinson & Balleine (2009) for a comparison of theories. Some leading theories are: the somantic marker hypothesis by Damasio (1996), the hedonic interface theory by Dickinson & Balleine (2009), the common currency theory of Cabanac (1992), and the multiple-components theories of Berridge, Kringelbach, and others: see Berridge & Kringelbach (2008); Kringelbach (2008).

²⁴ Smith et al. (2009); Tindell et al. (2005); Smith et al. (2007); Berridge et al. (2009).

²⁵ Smith et al. (2009); Dickinson & Balleine (2009).

²⁶ For example, Damasio (1994), p. 263.

²⁷ Burke et al. (2009); Aldridge & Berridge (2009).

²⁸ Berns et al. (2006).

²⁹ Gard et al. (2006).

³⁰ Green et al. (2009).

References

Aldridge & Berridge (2009). Neural coding of pleasure: 'rose-tinted glasses' of the ventral pallidum. In Kringelbach & Berridge (eds.), Pleasures of the brain (pp. 62-73). Oxford University Press.

Baldo & Kelley (2007). Discrete neurochemical coding of distinguishable motivational processes: insights from nucleus accumbens control of feeding. Psychopharmacology, 191: 439-459.

Balleine & Kilcross (2006). Parallel incentive processing: an integrated view of amygdala function. Trends in Neuroscience, 29: 272-279.

Balleine, Delgado, & Hikosaka (2007). The role of the dorsal striatum in reward and decision-making. The Journal of Neuroscience, 27: 8161-8165.

Barrett, Mesquita, Ochsner, & Gross (2007). The experience of emotion. Annual Review of Psychology, 58: 373-403.

Beaver, Lawrence, van Ditzhuijzen, David, Woods, & Calder (2006). Individual differences in reward drive predict neural responses to images of food. Journal of Neuroscience, 26: 5160-5166.

Berns, Chappelow, Cekic, Zink, Pagnoni, & Martin-Shurski (2006). Neurobiological substrates of dread. Science, 312: 754-758.

Berridge (1996). Food reward: Brain substrates of wanting and liking. Neuroscience and Biobehavioral Reviews, 20: 1-25.

Berridge (2000). Measuring hedonic impact in animals and infants: Microstructure of affective taste reactivity patterns. Neuroscience and Biobehavioral Reviews, 24: 173-198.

Berridge (2003a). Comparing the emotional brain of humans to other animals. In Davidson, Goldsmith, & Scherer (eds.), Handbook of affective sciences. Oxford University Press.

Berridge (2003b). Pleasures of the brain. Brain and Cognition, 52: 106-128.

Berridge (2007). The debate over dopamine's role in reward: the case for incentive salience. Psychopharmacology, 191: 391-431.

Berridge (2009). ‘Liking’ and ‘wanting’ food rewards: Brain substrates and roles in eating disorders. Physiology & Behavior, 97: 537-550.

Berridge & Robinson (1998). What is the role of dopamine in reward: Hedonic impact, reward learning, or incentive salience? Brain Research Reviews, 28: 309-369.

Berridge & Kringelbach (2008). Affective neuroscience of pleasure: Reward in humans and other animals. Psychopharmacology 199, 457-80.

Berridge, Venier, & Robinson (1989). Taste reactivity analysis of 6-hydroxydopamine-induced aphagia: Implications for arousal and anhedonia hypotheses of dopamine function. Behavioral Neuroscience, 103: 36-45.

Berridge, Robinson, & Aldridge (2009). Dissecting components of reward: ‘liking’, ‘wanting’, and learning. Current Opinion in Pharmacology, 9: 65–67.

Brauer, Cramblett, Paxton, & Rose (2001). Haloperidol reduces smoking of both nicotine-containing and denicotinized cigarettes. Psychopharmacology, 159: 31-37.

Burke, Franz, Miller, & Schoenbaum (2009). Conditioned reinforcement and the specialized role of corticolimbic circuits in the pursuit of happiness and other more specific rewards. In Kringelbach & Berridge (eds.), Pleasures of the brain (pp. 50-61). Oxford University Press.

Cabanac (1992). Pleasure: the common currency. Journal of Theoretical Biology, 155: 173-200.

Cabanac (2009). The dialectics of pleasure. In Kringelbach & Berridge (eds.), Pleasures of the brain (pp. 113-124). Oxford University Press.

Calder, Beaver, Davis, van Ditzhuijzen, Keane, & Lawrence (2007). Disgust sensitivity predicts the insula and pallidal response to pictures of disgusting foods. European Journal of Neuroscience, 25: 3422-3428.

Camerer (2006). Wanting, liking, and learning: Neuroscience and paternalism. The University of Chicago Law Review, 73: 87-110.

Damasio (1994). Descartes’ error: Emotion, reason, and the human brain. Putnam’s.

Damasio (1996). The somatic marker hypothesis and the possible functions of the prefrontal cortex. Philosophical Transactions of the Royal Society B, 351: 1413-1420.

Damasio (2004). Emotions and feelings: a neurobiological perspective. In Manstead, Frijda, & Fischer (eds.), Feelings and emotions: the Amsterdam symposium (pp. 49-57). Cambridge University Press.

Davidson, Scherer, & Goldsmith, eds. (2003). Handbook of affective sciences. Oxford University Press.

Delgado (1969). Physical Control of the Mind: Toward a Psychocivilized Society. Harper and Row.

Di Chiara & Bassareo (2007). Reward system and addiction: what dopamine does and doesn't do. Current Opinion in Pharmacology, 7: 69-76.

Dickinson & Balleine (2009). Hedonics: the cognitive-motivational interface. In Kringelbach & Berridge (eds.), Pleasures of the brain (pp. 74-84). Oxford University Press.

Evans, Pavese, Lawrence, Tai, Appel, Doder, Brooks, Lees, & Piccini (2006). Compulsive drug use linked to sensitized ventral striatal dopamine transmission. Annals of Neurology, 59: 852-858.

Everitt & Robbins (2005). Neural systems of reinforcement for drug addiction: from actions to habits to compulsion. Nature Neuroscience, 8: 1481-1489.

Feldman & Wager (2006). The structure of emotion: evidence from neuroimaging studies. Current Directions in Psychological Science, 15(2): 79-83.

Frijda (2006). The laws of emotion. Psychology Press.

Frijda (2009). On the nature and function of pleasure. In Kringelbach & Berridge (eds.), Pleasures of the brain (pp. 99-112). Oxford University Press.

Frijda & Sundararajan (2007). Emotion refinement: a theory inspired by Chinese poetics. Perspectives on Psychological Science, 2: 227–241.

Gard, Gard, Kring, & John (2006). Anticipatory and consummatory components of the experience of pleasure: a scale development study. Journal of Research in Personality, 40: 1086-1102.

Gilbert & Wilson (2007). Prospection: experiencing the future. Science 317: 1351–1354.

Goltz (1892). Der hund ohne grosshirn. Pflügers Archiv European Journal of Physiology, 51: 570.

Gottfried (2009). Olfaction and its pleasures: human neuroimaging perspectives. In Kringelbach & Berridge (eds.), Pleasures of the brain (pp. 125-145). Oxford University Press.

Green, Pereira, & Aziz (2009). Deep brain stimulation and pleasure. In Kringelbach & Berridge (eds.), Pleasures of the brain (pp. 302-319). Oxford University Press.

Grill & Norgren (1978). The taste reactivity test II: Mimetic responses to gustatory stimuli in chronic thalamic and chronic decerebrate rats. Brain Research, 143: 263-279.

Heath (1972). Pleasure and brain activity in man: Deep and surface electroencephalograms during orgasm. Journal of Nervous and Mental Disease, 154: 3-18.

Hoebel, Rada, Mark, & Pathos (1999). Neural systems for reinforcement and inhibition of behavior: Relevance to eating, addiction, and depression. In Kahneman, Diener, & Schwarz (eds.), Well-being: the foundations of hedonic psychology (pp. 558-572). Russell Sage Foundation.

Izard (2007). Basic emotions, natural kinds, emotion schemas, and a new paradigm. Perspectives on Psychological Science, 2: 260-280.

Kahneman, Krueger, Schkade, Schwarz, & Stone (2004). A survey method for characterizing daily life experience: the day reconstruction method. Science, 306: 1776–1780.

Kringelbach (2005). The human orbitofrontal cortex: linking reward to hedonic experience. Nature Reviews Neuroscience, 6: 691-702.

Kringelbach (2008). The pleasure center: Trust your animal instincts. Oxford University Press.

Kringelbach (2009). The hedonic brain: a functional neuroanatomy of human pleasure. In Kringelbach & Berridge (eds.), Pleasures of the brain (pp. 202-221). Oxford University Press.

Kringelbach & Berridge (2009a). Towards a functional neuroanatomy of pleasure and happiness. Trends in Cognitive Sciences, 13(11): 479-487.

Kringelbach & Berridge, eds. (2009b). Pleasures of the Brain. Oxford University Press.

Kringelbach & Berridge (2010a). The functional neuroanatomy of pleasure and happiness. Discovery Medicine, 9: 579-587.

Kringelbach & Berridge (2010b). The neuroscience of happiness and pleasure. Social Research, 77(2): 659-678.

Kringelbach, Berridge, Cabanac, Aldridge, Frijda, Leknes, Dickinson, Shizgal, Gottfried, Komisaruk, Petrovic, Green, Schoenbaum, & Small (2009). Fundamental pleasure questions. In Kringelbach & Berridge (eds.), Pleasures of the brain (pp. 7-23). Oxford University Press.

Kuhn & Koob (2010). Advances in the Neuroscience of Addiction. CRC Press.

LeDoux & Phelps (2000). Emotional networks in the brain. In Lewis & Haviland-Jones (eds.), Handbook of emotions (pp. 157-172). Guilford.

Leyton, Casey, Delaney, Kolivakis, & Benkelfat (2005). Cocaine craving, euphoria, and self-administration: a preliminary study of the effect of catecholamine precursor depletion. Behavioral Neuroscience, 119: 1619-1627.

Leyton (2009). The neurobiology of desire: Dopamine and the regulation of mood and motivational states in humans. In Kringelbach & Berridge (eds.), Pleasures of the brain (pp. 222-243). Oxford University Press.

Miller & Sherrington (1915). Some observations on the bucco-pharyngeal stage of reflex deglutition in the cat. Quarterly Journal of Experimental Physiology, 9: 147-186.

Morgan, di Donato, Iyer, Jenkins, Smith, & Sethi (2006). Self-stimulatory behavior associated with deep brain stimulation in Parkinson's disease. Movement Disorders, 21: 283-285.

Panksepp (2007). Neurologizing the psychology of affects: how appraisal-based constructivism and basic emotion theory can coexist. Perspectives on Psychological Science, 2: 281-296.

Pecina, Berridge, & Parker (1997). Pimozide does not shift palatibility: Separation of anhedonia from sensorimotor suppression by taste reactivity. Pharmacology Biochemistry and Behavior, 58: 801-811.

Pecina, Cagniard, Berridge, Aldridge, & Zhuang (2003). Hyperdopaminergic mutant mice have higher 'wanting' but not 'liking' for sweet rewards. The Journal of Neuroscience, 23: 9395-9402.

Pecina, Smith, & Berridge (2006). Hedonic hot spots in the brain. Neuroscientist, 12: 500–511.

Robinson & Berridge (2003). Addiction. Annual Review of Psychology, 54: 25-53.

Robinson, Sandstrom, Denenberg, & Palmiter (2005). Distinguishing whether dopamine regulates liking, wanting, and/or learning about rewards. Behavioral Neuroscience, 119: 5-15.

Rolls (2005). Emotion explained. Oxford University Press.

Ryle G (1954). Pleasure. Proceedings of the Aristotelian Society, 28: 135–146.

Salamone, Correa, Farrar, & Mingote (2007). Effort-related functions of nucleus accumbens dopamine and associated forebrain circuits. Psychopharmacology, 191: 461-482.

Schooler & Mauss (2009). To be happy and to know it: the experience and meta-awareness of pleasure. In Kringelbach & Berridge (eds.), Pleasures of the brain (pp. 244-254). Oxford University Press.

Schroeder (2004). Three faces of desire. Oxford University Press.

Schulkin (2004). Allostasis, homeostasis, and the costs of physiological adaptation. Cambridge University Press.

Schultz (2006). Behavioral theories and the neurophysiology of reward. Annual Review of Psychology, 57: 87-115.

Sem-Jacobsen (1976). Electrical stimulation and self-stimulation with chronic implanted electrodes: Interpretation and pitfalls of results. In Wauquier & Rolls (eds.), Brain-Stimulation Reward (pp. 505-520). Elsevier.

Shizgall (1999). On the neural computation of utility: Implications from the studies of brain stimulation reward. In Kahneman, Diener, & Schwarz (eds.), Well-being: the foundations of hedonic psychology (pp. 500-524). Russell Sage Foundation.

Sienkiewicz-Jarosz, Scinska, Kuran, Ryglewicz, Rogowski, Wrobel, Korkosz, Kukwa, Kostowski, & Bienkowski (2005). Taste responses in patients with Parkinson's disease. Journal of Neurology, Neurosurgery, & Psychiatry, 76: 40-46.

Small, Zatorre, Dagher, Evans, & Jones-Gotman (2001). Changes in brain activity related to eating chocolate: From pleasure to aversion. Brain, 124: 1720–1733.

Smith, Berridge, & Aldridge (2007). Ventral pallidal neurons distinguish 'liking' and 'wanting' elevations caused by opioids versus dopamine in nucleus acumbens. Program No. 310.5, 2007 Neuroscience Meeting Planner. San Diego, CA: Society for Neuroscience.

Smith, Mahler, Pecina, & Berridge (2009). Hedonic hotspots: generating sensory pleasure in the brain. In Kringelbach & Berridge (eds.), Pleasures of the brain (pp. 27-49). Oxford University Press.

Steiner (1973). The gustofacial response: Observation on normal and anecephalic newborn infants. Symposium on Oral Sensation and Perception, 4: 254-278.

Steiner, Glaser, Hawillo, & Berridge (2001). Comparative expression of hedonic impact: affective reactions to taste by human infants and other primates. Neuroscience and Biobehavioral Reviews, 25: 53-74.

Stoeckel, Weller, Cook, Twieg, Knowlton, & Cox (2008). Widespread reward-system activation in obese women in response to pictures of high-calorie foods. Neuroimage, 41(2): 636-647.

Tindell, Berridge, Zhang, Pecina, & Aldridge (2005). Ventral pallidal neurons code incentive motivation: Amplification by mesolimbic sensitization and amphetamine. European Journal of Neuroscience, 22: 2617-2634.

Van Leijenhorst, Zanolie, Van Meel, Westenberg, Rombouts, & Crone (2010). What motivates the adolescent? Brain regions mediating reward sensitivity across adolescence. Cerebral Cortex 20, 61-69.

Veldhuizen, Rudenga, & Small (2009). The pleasure of taste, flavor, and food. In Kringelbach & Berridge (eds.), Pleasures of the brain (pp. 146-168). Oxford University Press.

Volkow, Wang, Telang, Fowler, Logan, Childress, Jayne, Ma, & Wong (2006). Cocaine cues and dopamine in dorsal striatum: mechanism of craving in cocaine addiction. Journal of Neuroscience, 26: 6583-6588.

Voon, Pessiglione, Brezing, Gallea, Fernandez, Dolan, & Hallett (2010). Mechanisms underlying dopamine-mediated reward bias in compulsive behaviors. Neuron, 65: 135.

Winkielman & Berridge (2004). Unconscious emotion. Current Directions in Psychological Science, 13: 120-123.

Winkielman, Berridge, & Wilbarger (2005). Unconscious affective reactions to masked happy versus angry faces influence consumption behavior and judgments of value. Personality and Social Psychology Bulletin, 31: 121-135.

Wise (2006). Role of brain dopamine in food reward and reinforcement. Philosophical Transactions of the Royal Society B, 361: 1149-1158.

Wise & Bozarth (1985). Brain mechanisms of drug reward and euphoria. Psychiatric Medicine, 3: 445-460.