The Reward System

Rats pressing levers thousands of times, wanting versus liking, and dopamine's role in motivation: how the brain's reward circuitry drives us — and how drugs, food and screens can hijack it.

Discovery: the rat that wouldn't stop

In 1954 James Olds and Peter Milner, at McGill University, implanted electrodes in rats' brains and let the rats press a lever to deliver brief electrical pulses. When electrodes were in certain areas, notably around the septum and along the medial forebrain bundle, rats pressed the lever over and over — in later experiments, thousands of times an hour — sometimes ignoring food. They had found brain regions whose stimulation is powerfully rewarding.

The core circuit

The central pathway is the mesolimbic dopamine pathway: dopamine neurons in the ventral tegmental area (VTA) of the midbrain project to the nucleus accumbens in the ventral striatum. They work with the prefrontal cortex, orbitofrontal cortex, amygdala, hippocampus and ventral pallidum to assign value to things, motivate pursuit and learn what predicts reward.

Wanting versus liking

Dopamine was long called the "pleasure chemical". The neuroscientist Kent Berridge showed this was wrong. Rats with dopamine almost entirely depleted still show "liking" facial reactions to sweet tastes (similar to a human baby licking its lips) but won't work to get them. Conversely, boosting dopamine increases wanting — motivation and pursuit — without increasing liking. Pleasure itself depends on small "hedonic hotspots" using opioids and endocannabinoids in the nucleus accumbens and ventral pallidum.

Learning

Dopamine neurons also signal reward prediction errors (Wolfram Schultz's discovery), teaching the brain which cues predict rewards — so cues themselves become motivating.

Hijacking the system

Addictive drugs raise dopamine in the nucleus accumbens directly — often more powerfully and quickly than natural rewards — and repeated use changes the circuit. Berridge and Terry Robinson's incentive-sensitisation theory proposes that in addiction, "wanting" becomes sensitised and driven by cues even as "liking" fades. Highly palatable foods, gambling and some app designs also engage reward circuitry, though whether that amounts to "addiction" is debated case by case.