CONTINGENCY DISCRIMINABILITY AND PEAK SHIFT IN CONCURRENT SCHEDULES CHRISTIAN U. KRA ¨ GELOH,DOUGLAS M. ELLIFFE, AND MICHAEL DAVISON UNIVERSITY OF AUCKLAND, NEW ZEALAND We investigated the effects of discriminative stimuli on choice in a highly variable environment using a procedure in which multiple two-key concurrent VI VI components changed every 10 reinforcers and were signaled by differential flashes of red and yellow keylights. Across conditions, five pigeons were exposed to a number of different combinations of the following component reinforcer ratios: 27:1, 9:1, 3:1, 1:1, 1:3, 1:9, 1:27. Overall, there was clear control by the component signals in that preference, early in components and particularly before any reinforcers had been delivered, was ordinally related to the signaled reinforcer ratios. In conditions in which only two components arranged unequal reinforcer ratios (e.g., 27:1 and 1:27) with the remaining components arranging 1:1 reinforcer ratios, preference before the first reinforcer in a component showed peak shift in that the most extreme preference did not occur in the unequal reinforcer-ratio components, but in 1:1 components further towards the ends of the stimulus dimension. The contingency-discriminability model (Davison & Nevin, 1999) was fitted to the data and provided an excellent description of the interactions between stimulus and reinforcer effects in a highly variable environment. Key words: choice, stimulus control, contingency discriminability, peak shift, multiple-concurrent schedules, key peck, pigeons _______________________________________________________________________________ The generalized matching law (Baum, 1974) proposes a linear relation between log re- sponse ratios and log obtained reinforcer ratios in concurrent schedules: log B 1 B 2 ~ a log R 1 R 2 z log c , ð1Þ where a refers to sensitivity to reinforcer ratio (Lobb & Davison, 1975) and log c to a constant preference (i.e., bias) for one response alter- native over the other. Typical sensitivities in standard concurrent variable-interval (VI) VI schedules are around 0.8 to 0.9 (Baum, 1979; Taylor & Davison, 1983; Wearden & Burgess, 1982). When the stimuli signalling the concurrent alternatives are not easily discriminable, sensi- tivity decreases. Miller, Saunders, and Bour- land (1980) arranged concurrent schedules in which a switching key alternated stimuli and schedules on a main key (Findley, 1958) and varied the discriminative stimuli associated with the two concurrent alternatives across conditions. When the stimuli were line orien- tations that differed by 45u, pigeons strictly matched response ratios to the reinforcer ratios (a 5 1.0), but when the line orientations differed by only 15u, sensitivity decreased to 0.28 and 0.37. When identical stimuli signaled both alternatives, sensitivity was 0.17 for both pigeons. This effect also was confirmed by Bourland and Miller (1981). Alsop and Davi- son (1991) investigated whether the fact that some control by reinforcer ratios remained when the discriminative stimuli were identical could indicate a win/stay lose/shift strategy. To test this, pecks on the switching key did not strictly alternate the operative schedule and stimulus, but initiated one or the other concurrent alternative randomly. The sched- ule and stimulus in effect after a reinforcer was also determined randomly. When the discrimi- native stimuli were identical for both concur- rent alternatives, there was no control by the reinforcer ratio and sensitivity to reinforcer ratio was zero. A quantitative model of choice that is particularly suitable when the concurrent alternatives are less than perfectly discrimina- This research was carried out by the first author in partial fulfillment of his doctoral degree at The University of Auckland. Reprints may be obtained from any author, Department of Psychology, The University of Auckland City Campus, Private Bag 92019, Auckland, New Zealand. Email ad- dresses: c.krageloh@auckland.ac.nz, d.elliffe@auckland. ac.nz, m.davison@auckland.ac.nz. We would like to thank the members of the Experi- mental Analysis of Behaviour Research Unit for their help in conducting this experiment, and Mick Sibley for looking after the pigeons. doi: 10:1901/jeab.2006.11-05 JOURNAL OF THE EXPERIMENTAL ANALYSIS OF BEHAVIOR 2006, 86, 11–30 NUMBER 1( JULY) 11