Pleasures of the heart

It is a key component of good emotions and a fundamental aspect of mental life. However, pleasure is not fully straightforward. New research from the fields of hedonic psychology and affective neuroscience is uncovering some fascinating complexities.

Even simple sensory pleasures can reveal hedonistic psychology. Sweetness, for example, is enjoyable. It is one of the sensations that can reliably make people happy. The pleasure of sweetness comes from something done to it rather than from the sensation itself. Sweets aren't always pleasant; there are some unpleasant sweet flavors out there. For example, we might readily develop a learned aversion to certain sweet flavors (such as a novel sweet flavor that is associatively paired with visceral illness). Delicious tastes for which we have developed aversions remain sweet after they have been learnt aversions, but their sweetness becomes unpleasant rather than pleasant.

The Gloss of Pleasure

In other words, enjoyment is a kind of additional value to feeling. Pleasure gloss is actively painted by limbic brain circuitry upon plain sensory perceptions. The pleasure gloss and our desire for it are both neurobiological and psychologically complicated.

Which Brain Systems are Responsible for the Gloss of Pleasure?

To begin, it's fascinating to consider how the brain creates the pleasure gloss. Pleasures engage the cerebral cortex (particularly the medial prefrontal cortex), amygdala, and deep brain regions such as the nucleus accumbens and the midbrain dopamine neurons that project to it, the ventral pallidum, and even some hindbrain areas. Pleasures can trigger all of these. However, not all of them must be pleasurable. Rather, many brain co-activations are the result of pleasure rather than the cause of pleasure (producing other psychological functions instead). So, which brain activities are responsible for the pleasure gloss that is applied to sensation?

Of course, psychologists and neuroscientists are interested in the causes of all joys, but we must study them one at a time in practice. We researched taste pleasure in our lab at the University of Michigan to figure out how the brain creates the pleasure gloss. Sweet flavors generate analogous 'liking' facial expressions in human infants and many animals (e.g., tongue protrusions), whereas bitter tastes elicit 'disliking' emotions (e.g., gapes). We've used those expressions to map pleasure-producing brain circuits in rats and mice in affective neuroscience investigations. In these experiments, we slightly tinker with a brain system to see if it affects the pleasure gloss of a taste (for example, by making a painless microinjection of a tiny drug droplet into a brain structure).

We've discovered numerous sorts of brain activity that create a pleasure gloss on sweet sensations in this way. For example, we discovered that stimulating opioid circuit activation in the nucleus accumbens (for example, by microinjecting morphine there) leads to enhanced pleasure 'liking.' This is the first step in a neuronal chain that leads to pleasure. The chain continues in regions such as the ventral pallidum that receive impulses from the accumbens, establishing a limbic circuit that paints the pleasure gloss.