Experimental investigations of conditioned reinforcement tended to use one of two basic procedures, sometimes described as the 'new response' and 'extinction' methods. Chain schedules of reinforcement programme a sequence of reinforcement schedules, each associated with its own discriminative stimulus, only the schedule of the sequence providing reinforcement. In particular, the information hypothesis obtains support from experiments in which an animal emits a response which merely informs him of the schedule of reinforcement in operation. This observing response method has developed from the pioneering work of L. B. Wyckoff, and is illustrated by an experiment by J. Steiner. The alternative theory came from F. Skinner, and was developed into a clearly stated hypothesis by F. S. Keller and W. Schoenfeld, who suggested that only discriminative stimuli become conditioned reinforcers. A discriminative stimulus is something in the environment which sets the occasion for an operant response to be followed by a reinforcer.