Disturbed BEHAVIOUR of animals in an EXPERIMENT when they are faced with an impossible problem. This PHENOMENON was discovered by PAVLOV when he rewarded a dog with food for responding (by salivating) to a circle and then withheld the reward when the stimulus was an ellipse. Pavlov gradually made the two stimuli more and more alike. When the point was reached at which the dog could no longer discriminate the circle from the ellipse the animal lashed out in a frenzy of wild and random behaviour. Whether this behaviour can be equated with human NEUROSIS is a matter of long-standing dispute among psychologists.