Prediction and preparation: Pavlovian implications of research animals discriminating among humans.

A growing body of evidence suggests that animals of various species can discriminate among the humans with whom they have regular contact. This discriminative ability has considerable implications for research. Because animal life is hedonistic, there is a strong incentive for animal subjects to predict the events that bring them pleasure and pain. Many research settings attempt to deliver hedonic stimuli under strictly regulated conditions without formal warning. Nevertheless, the possibility remains that the presence of a particular human may signal delivery of an important event, thus allowing the animal to prepare for its occurrence. In Pavlovian terms, humans become walking conditioned stimuli, eliciting measurable conditioned responses from animal subjects. These preparatory responses may take behavioral, physiological, and/or motivational forms and modulate the effects of the variables under study. The discussion addresses practical implications of knowing that discrimination among humans by animal subjects may affect one's research agenda.