Pleasure arises within the brain. Sweetness or other natural pleasures are mere sensations as they enter the brain, and brain systems must actively paint the pleasure onto sensation to generate a 'liking' reaction -- as a sort of pleasure gloss or varnish. Our lab has discovered brain generators of sensory pleasure, in the form of anatomical 'hedonic hotspots' in the brain, which use neurochemcal signals paint intense pleasure on sensation, embedded within larger hedonic circuits. It is important to identify such pleasure-causing brain hedonic hotspots, neurochemicals and circuits, in order to identify true mechanisms of pleasure. The need to find true pleasure generators is especially pressing because hedonic circuit dysfunctions may underlie mood disorders and related clinical disorders, and because several other brain candidates once thought to mediate pleasure are now increasingly recognized to not cause pleasure after all (e.g., dopamine, electrical brain stimulation). Therefore we aim to find true causes and mechanisms in the brain for pleasure. [General review papers on pleasure 'liking': ]
The hedonic hotspots that generate pleasure 'liking' are each about a cubic millimeter in size (in rats; perhaps a cubic centimeter in you), and contained in limbic forebrain structures such as the nucleus accumbens and the ventral pallidum. In hotspots the hedonic gloss is painted by brain chemicals such as mu opioids and endocannabinoids, which are natural brain versions of heroin and marijuana that amplify a sensory pleasure. If we activate those neurochemical receptors (via painless microinjection of tiny droplets of drug directly into a hedonic hotspot) we increase the 'liking' reactions elicited by sweetness. [Research examples]
Using microinjections and lasers to activate the brain. For drug microinjections, we have developed a Fos plume mapping technique to more precisely map the hedonic hotspots (and chemical pleasure paints) when drug microinjections amplify 'liking'. Similarly we are now using laser pulses of light to activate hotspots via optogenetics techniques in which a desired neural system is targeted by a virus microinjection that makes it develop photoreceptors and become activated by light (photo of brain illuminated by optic fiber that transmits laser).
We have also studied how pleasure is coded by the firing patterns of neurons within those hotspots, similar to a brain Morse code for 'liking', in collaboration with the Aldridge lab . Our goal is to better understand how brain hedonic hotspots act together in an integrated hedonic circuit to mediate 'liking' for sensory pleasures.
What is 'liking'? 'Liking' is an objective process of positive hedonic reaction that underlies subjective sensory pleasure. We rely on a useful natural window into 'liking' reactions, facial affective expressions of taste pleasure ['liking' expressions that are homologous in human infant, non-human primates, and even rodents[Infant/primate sample; Expression overview)]. Combining that window into 'liking' with painless neuroscience techniques, we map the brain hedonic hotspots that paint a pleasure gloss onto sensation [Pleasure overviews].
Side view of hedonic hotspot in nucleus accumbens where opioids amplify sweetness 'liking' (red/yellow) based on Peciña & Berridge (2005).
What's a ventral pallidum? The limbic ventral pallidum is relatively new on the affective neuroscience scene, having been named by anatomists only a decade or so ago. It lies at the base of the forebrain, in front of the hypothalamus, and as chief target of nucleus accumbens is the output channel through which most mesocorticolimbic circuits must work. . We have found a special hedonic hotspot that is crucial for reward ‘liking’ and ‘wanting’ (and codes reward learning too). The opioid hedonic hotspot is shown in red above. It works together with another hedonic hotspot in the more famous nucleus accumbens to generate pleasure 'liking'.
'Wanting' - Incentive Salience
Desire versus dread in in
limbic nucleus accumbens
Why is drug addiction so compulsive and long lasting? The distinction of 'wanting' from 'liking' has important implications found in the Incentive-Sensitization theory of addiction . Addictive drugs can cause permanent neural sensitization in brain mesolimbic systems of incentive salience. Sensitized incentive salience means addicts have compulsive 'wanting' to take drugs, which can last months or years. Much evidence has emerged to favor this idea in the decade since Terry Robinson and Berridge first proposed it, and it continues to influence current neuroscience research . We stress Incentive-Sensitization theory does not provide a cure for addiction, but it does help pinpoint a crucial aspect of what goes wrong.
Other Human Applications
Incentive salience 'wanting' mechanisms have implications for other forms of human irrational desire (Irrational choices). We have also explored how basic 'liking' / 'wanting' systems may relate to conscious and unconscious emotion processes in normal people (Unconscious emotion; see also Piotr Winkielman's web page).