Contingency discriminability, matching, and bias in the concurrent-schedule responding of possums (Trichosurus vulpecula)

Six possums (Trichosurus vulpecula) responded under dependent concurrent variable-interval variable- interval schedules of reinforcement. Over 15 conditions, barley–carob was one reinforcer with the other reinforcer consisting of Coco Popst, coconut, or a barley–carob mixture with 0%, 2%, 4%, or 6% salt added to the barley. The schedules were both variable-interval 40 s. As has been found with other species, behavior on the concurrent schedules was biased by the type of feed, with the 6% salt and the coconut giving the greatest biases towards the barley–carob mixture. The schedules were varied over 17 conditions using the barley–carob mixture alone or the barley–carob mixture versus the mixture with 4% or 6% salt. Both the contingency-discriminability model (Davison & Jenkins, 1985) and the generalized matching law described the data from the three sets of conditions equally well. Both gave similar measures of bias; however, some of the parameter values found with the contingency discriminability model were uninterpretable. Thus, any argument for this model based on the interpretability of the parameter values becomes weak. It is worth retaining the generalized matching law as a descriptor of such data