In a discrete-trial conditional discrimination procedure, 4 pigeons obtained food reinforcers by pecking a key with a short latency on trials signaled by one stimulus and by pecking the same key with a long latency on trials signaled by a second stimulus. The physical difference between the two stimuli and the temporal separation between the latency values required for reinforcement were varied factorially over four sets of conditions, and the ratio of reinforcer rates for short and long latencies was varied within each set of conditions. Stimulus discrimination varied directly with both stimulus and response differences and was unaffected by the reinforcer ratio. Sensitivity to reinforcement, estimated by generalized-matching-law fits to the data within each set of conditions, varied directly with the response difference but inversely with the stimulus difference arranged between sets of conditions. Because variations in stimulus differences, response differences, and reinforcer differences did not have equivalent effects, these findings question the functional equivalence of the three terms of the discriminated operant: antecedent stimuli, behavior, and consequences.

Preferences of hungry pigeons among 10 grains and pellets were analyzed using a Thurstone scaling procedure. The recovered scales were positively correlated with size of the feed. The correlations improved when the Thurstonian assumption of equal-sized discriminal dispersions (Case V) was replaced with the assumption of proportional-sized dispersions (Case VI), as entailed by Weber's law. The correlations weakened when the experiments were conducted with the pigeons close to their freefeeding weights, where the probability of sampling alternative grains increased. In the final experiment, exposure to a large pellet shifted the preferences between two smaller pellets.Key words: reinforcers, grain, preference, Weber's law, Thurstone, Fechner, scaling, magnitude of reinforcement, quality of reinforcement, foraging, food choice, pigeons Discriminating nuances in the quality of reinforcers is a major preoccupation of humans. Everything is rated, critics outnumber creators, and books-of-lists top the lists of best selling books. For luxury items such as wines and stereos, marginal utility seems never to decrease, with large premiums paid for differences in quality that require years of training to discriminate.Despite the importance of quality in human economy, little attention has been paid to it in the learning literature. Many texts merely refer to Elliot's (1928) classic study in which it was found that rats perform better for wet bran than for sunflower seeds, or to Young's (1928) demonstration of rats' preferential discrimination among types of grain. Thereafter, discussion of quality is usually embedded in that of quantity. Many of the extant studies of quality of reinforcers employ sucrose, whose concentration can be conveniently varied over a large range (Young, 1961

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