When presentation of a retractable lever always preceded food delivery, rats licked or gnawed the lever. They also approached but seldom orally contacted a lever signaling brain-stimulation reinforcement; instead, subjects sniffed, pawed, or "explored" the lever. Therefore, a Pavlovian conditioned stimulus evoked directed skeletal responses whose specific form depended on the forthcoming unconditioned stimulus.

In a discrete-trial two-choice conditional discrimination task, pigeons which received food for a correct choice following the presentation of one cue and water for a correct choice following another cue performed better than pigeons which received food and water equally often in both cases when delays of several seconds intervened between the conditional cue and choice stimuli presentations. These results suggest that feedback properties of reinforcer-specific. expectancies cB:n be impor~nt in conditional discrimination learning in pigeons. An additional findmg was that wlld-caught pigeons regularly exhibited a higher percentage of correct choices than domestic subjects.

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Despite the seminal studies of response differentiation by the method of successive approximation detailed in chapter 8 of The Behavior of Organisms (1938), B. F. Skinner never actually shaped an operant response by hand until a memorable incident of startling serendipity on the top floor of a flour mill in Minneapolis in 1943. That occasion appears to have been a genuine eureka experience for Skinner, causing him to appreciate as never before the significance of reinforcement mediated by biological connections with the animate social environment, as opposed to purely mechanical connections with the inanimate physical environment. This insight stimulated him to coin a new term (shaping), and also led directly to a shift in his perspective on verbal behavior from an emphasis on antecedents and molecular topographical details to an emphasis on consequences and more molar, functional properties in which the social dyad inherent to the shaping process became the definitive property of verbal behavior. Moreover, the insight seems to have emboldened Skinner to explore the greater implications of his behaviorism for human behavior writ large, an enterprise that characterized the bulk of his post-World War II scholarship.