Choice, contingency discrimination, and foraging theory

Four pigeons were trained on eight or nine pairs of independent concurrent variable-interval schedules, The range of reinforcement ratios included ext reme ratios (up to 532 to 1). Large samples of stable performance were gath ered. Contrary to the findings of Davison and Jones (1995), the generalized matching law described choice more accurately than a contingency-discrimin ability model. Taking small samples (5 to 10 sessions) and applying a more liberal stability criterion used by Davison and Jones only increased the un systematic variance in the data and in estimates of generalized-matching-la w sensitivity. Because changing to dependent scheduling and inserting a cha ngeover delay had no systematic effect, the deviations from generalized mat ching reported by Davison and Jones probably arose from imperfectly discrim inated stimuli. Analysis of visits revealed that visits to the nonpreferred alternative were brief and approximately constant. When choice between the preferred (rich) and nonpreferred (lean) alternatives, regardless of posit ion, was analyzed according to the generalized matching law, sensitivities approximated 1.0, with bias in favor of the lean alternative. This bias, wh ich arose from an excessive frequency of visits to the lean alternative, ex plains undermatching as the result of fitting one line to a choice relation that consists of two displaced lines, both with a slope of 1.0. The patter n of deviation from the generalized matching line confirmed this account. T he findings suggest an alternative analysis of choice that focuses on proba bility of visiting the lean alternative as the dependent variable. This pro bability was directly proportional to ratio of reinforcement. Matching, und ermatching, and overmatching may all be explained by a view of concurrent p erformance based on foraging theory, in which responding occurs primarily a t the rich alternative and is occasionally interrupted by brief visits to t he lean alternative.