APA Dictionary of Psychology

the ability of an organism to distinguish between the internal states produced by different drugs (or by a particular drug and saline). In a typical experimental procedure, a nonhuman animal is injected with one drug, and a certain response (e.g., pressing the left-hand lever in a two-lever apparatus) is reinforced. When injected with a different drug (or with saline), a different response (e.g., pressing the right-hand lever) is reinforced. Thus, the animal must discriminate between the internal cues produced by the drugs in order to make the correct response.