In both responses, Dr. Volkow said, the gasping for oxygen and the wolfing down of something you would ordinarily spurn, the dopamine pathways of the brain are at full throttle. “The whole brain is of one mindset,” she said. “The intense drive to get you out of a state of deprivation and keep you alive.”

Image Credit... Serge Bloch

Dopamine is also part of the brain’s salience filter, its get-a-load-of-this device. “You can’t pay attention to everything, but you want to be adept as an organism at recognizing things that are novel,” Dr. Volkow said. “You might not notice a fly in the room, but if that fly was fluorescent, your dopamine cells would fire.”

In addition, our dopamine-driven salience detector will focus on familiar objects that we have imbued with high value, both positive and negative: objects we want and objects we fear. If we love chocolate, our dopamine neurons will most likely start to fire at the sight of a pert little chocolate bean lying on the counter. But if we fear cockroaches, those same neurons may fire even harder when we notice that the “bean” has six legs. The pleasurable taste of chocolate per se, however, or the anxiety of cockroach phobia, may well be the handiwork of other signaling molecules, like opiates or stress hormones. Dopamine simply makes a relevant object almost impossible to ignore.

Should the brain want to ignore what it might otherwise notice, dopamine must be muzzled. Reporting recently in Nature Neuroscience, Regina M. Sullivan of New York University Medical Center, Gordon A. Barr of Children’s Hospital of Philadelphia and their colleagues found that, whereas rats older than 12 days would quickly develop an aversion to any odors that were paired with a mild electric shock, young rats would perversely show a preference for such odors if their mothers were nearby when the tutorial jolt was delivered. The researchers traced that infantile Candide spirit to a suppression of dopamine activity in the amygdala, where fear memories are born. Infant rats know their mother by smell, Dr. Sullivan explained, and they must not learn to avoid her, for even an abusive caretaker is better than none.

Large as its impact may be, dopamine is a compact molecule, built of 22 atoms, with the characteristic nitrogenous amine knob at one end. (Dopamine, by the way, takes its name from its chemical composition, and has nothing to do with the word dope  as in heroin or other recreational drugs  which is thought to derive from the Dutch term for stew.)

The dopamine production corps is tiny as well. Fewer than 1 percent of all neurons generate the neurotransmitter, most of them in midbrain structures like the substantia nigra, which helps control movement; it is the degradation of this population of dopamine cells that results in the tremors and other symptoms of Parkinson’s disease.

There is also dopamine activity higher up, in the prefrontal cortex parked right behind the forehead, that great executive brain where storylines are written, impulses controlled and excuses contrived. An impoverishment of prefrontal dopamine is thought to contribute to schizophrenia.