Psychoactivity is a broad term for the action of the many chemicals that affect the function of the brain. There are many classes of these substances, such as stimulants, anesthetics, sedatives, narcotics, depressants, antidepressants -- and also psychedelics. The mechanism of action of such drugs always involves psychoneurological systems. Medically valuable psychoactive drugs are most often discovered in animal behavior experiments, and finding out how the drugs work frequently calls upon sophisticated research using appropriately radio-labeled synthetic samples.

But for the past four decades, I have studied psychoactive drugs at the far end of the spectrum: those that affect the mind. These substances are usually discovered by people experimenting on humans. Rats have brains, and we can remove them, cut them into slices, and see where experimental drugs have gone -- but I am not sure rats have what most people think of as minds.