ABSTRACT
Most learning is the result of encounters with a limited number of environmental circumstances. In laboratory experiments the environment is usually held as constant as possible, or else just one or two selected features of the environment may be manipulated. For example, a pigeon may be fed if it pecks at a yellow spot on the wall of an experimental chamber but not fed if it pecks elsewhere. The pigeon may have been indifferent to the spot originally, that is, it may have been no more likely to peck at the spot than to peck on other parts of the wall, but as a result of the training procedure it becomes more likely to peck, and the pecks come to be predominantly on the yellow spot.
