Lockard's (1963) review of studies exploring the conditions controlling light-onset and light-offset reinforcement and theories of light-reinforced bar pressing is discussed. The evidence is reexamined in terms of temporal and response variables and it is shown, contrary to Lockard's conclusions, that the data can then be seen as relatively systematic. Lockard's discussion of theories is criticized on the grounds that in the case of the facilitation hypothesis, the stimulus-change hypothesis, and the discrepancy hypothesis, the position discussed is much more powerful than Lockard suggests. Evidence contrary to Lockard's "preference theory" is cited in the course of the discussion.
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