PIGEONS CHOICES BETWEEN FIXED-RATIO AND GEOMETRICALLY ESCALATING SCHEDULES

When pigeons choose between situations that provide access to food rei nforcers after a delay, choice is better predicted by computations bas ed upon sums-of-reciprocals distances from the point of choice to each of the next three or four reinforcers in series than by computations of optimality based upon mean rates of reinforcement. The present expe riments were designed to examine the generality of this finding. Pigeo ns were exposed to concurrent-chains schedules in which one brief init ial link led to a fixed-ratio schedule (either 15, 30, or 60, dependin g on the condition), and the other link led to a geometrically increas ing progressive-ratio schedule whose rate of escalation was systematic ally varied across conditions. Each combination of fixed-ratio size an d escalation rate of the progressive schedule was assessed at two diff erent levels of deprivation (75% and 80% of free-feeding weights). Com putations based upon the sums-of-reciprocals principle, treating ratio schedule sizes as proportional to delays, predicted and described the pigeons' median switch points better than those based on arithmetic m eans. Neither the distance to the next reinforcer (as implied by some molecular analyses) nor molar optimization (as described by arithmetic means) were as successful at accounting for patterns of choice in the se situations. Hence, it appears that the birds' choices were most inf luenced by the relative proximity of a choice to several reinforcers i n a series of reinforcers, with each of the less proximal reinforcers having relatively less influence over the current choice.