Five pigeons were trained in a delayed matching-to-sample task with red and green stimuli. The retention interval between sample-stimulus presentation and the availability of the choice stimuli was varied between 0.01 s and 12 s within each session. The probability of food produced by correct-red and correct-green responses was varied across conditions. Sample-stimulus discriminability and response bias were measured at four different retention intervals. The results of these analyses showed an interaction between the discriminability of the sample stimuli and the control exerted by differential reinforcement. At longer retention intervals, sample discriminability decreased and sensitivity of choice behavior to changes in the red/green reinforcer ratio increased. An analogous relation has been reported in conditional discriminations in which the physical disparity of stimuli has been varied. This correspondence suggests that increasing the delay between presentation of one of two stimuli and an opportunity to respond discriminatively to it may be functionally similar to increasing the physical similarity of the two stimuli.

Six homing pigeons were trained on a variety of concurrent variable-interval schedules in a switchingkey procedure. Unlike previous work, reinforcer ratios of up to 160 to 1 and concurrent extinction variable-interval schedules were arranged in order to investigate choice when reinforcer-frequency outcomes were extremely different. The data obtained over 11 conditions were initially analyzed according to the generalized matching law, which fitted the data well. The generalized matching law was then fitted only to conditions in which the reinforcer ratios were between 1 to 10 and 10 to 1. The deviations of choice measures from the other four more extreme reinforcer-ratio conditions were significantly more towards equal choice than predicted by this second generalized matching fit. A contingency-discriminability model, which predicts such deviations, described the data more effectively than did the generalized matching law, and also correctly predicted the maintenance of responding on both alternatives when one was associated with extinction. (1987) showed that the value of a was not independent of overall reinforcer rate, but increased with increasing reinforcer rates. Thus, it is not simply the ratio of obtained reinforcers that controls behavior allocation. Second, Logue and Chavarro also reported that sensitivity to reinforcer magnitude and reinforcer delay in the concatenated version of the generalized matching law was not independent of the absolute values of these independent variables. Third, Davison (1988) showed that sensitivity to reinforcer magnitude was affected by overall reinforcer rate. Fourth, Davison and Hogsden (1984) 147 1995, 64,[147][148][149][150][151][152][153][154][155][156][157][158][159][160][161][162] NUMBER 2 (SEPrEMBER)

Five pigeons were trained to perform a delayed matching-to-sample task in which red-and greencolored keys were presented as sample and choice stimuli, and the duration of a delay interval varied across trials. Experiment 1 investigated the effects on delayed-matching accuracy of signaling different durations offood access for the two correct responses (the differential-outcomes effect), and of signaling nondifferential but larger durations for both responses (the signaled-magnitudes effect). In Condition 1, a vertical bar on either sample signaled different rewards (or different outcomes, DOs) for correct red and correct green responses (0.5 and 3.5 sec, respectively), and a horizontal bar signaled equal durations of food access (or same outcomes, SOs) for these responses (1.5 sec). In Condition 2, the horizontal bar signaled equally large rewards for the two correct responses (3.5 sec), and the vertical bar signaled equally small rewards (0.5 sec). Delayed-matching accuracies were higher on DO trials than on SO trials, and they were higher on large-reward trials than on smallreward trials. However, analyses of discriminability estimates as a function of delay-interval duration revealed differences between the forgetting functions reflecting these two effects. Signaling DOs increased the initial level of the function and reduced its slope relative to signaling SOs, whereas signaling larger rewards increased the initial level of the function but did not affect its slope relative to signaling smaller rewards. Experiment 2 investigated whether the difference between the initial levels of DO and SO functions in Condition 1 resulted from overall longer food access on the former trials. However, varying the food-access times on SO trials across three conditions (0.5, 3.5, and 1.5 sec) failed to produce systematic effects consistent with this hypothesis. The results are discussed with respect to the mechanisms that could be responsible for the two effects.

Five pigeons were trained on concurrent variable‐interval schedules in which two intensities of yellow light served as discriminative stimuli in a switching‐key procedure. A conditional discrimination involving a simultaneous choice between red and green keys followed every reinforcer obtained from both alternatives. A response to the red side key was occasionally reinforced if the prior reinforcer had been obtained from the bright alternative, and a response to the green side key was occasionally reinforced if the prior reinforcer had been obtained from the dim alternative. Measures of the discriminability between the concurrent‐schedule alternatives were obtained by varying the reinforcer ratio for correct red and correct green responses across conditions in two parts. Part 1 arranged equal rates of reinforcement in the concurrent schedule, and Part 2 provided a 9:1 concurrent‐schedule reinforcer ratio. Part 3 arranged a 1:9 reinforcer ratio in the conditional discrimination, and the concurrent‐schedule reinforcer ratio was varied across conditions. Varying the conditional discrimination reinforcer ratio did not affect response allocation in the concurrent schedule, but varying the concurrent‐schedule reinforcer ratio did affect conditional discrimination performance. These effects were incompatible with a contingency‐discriminability model of concurrent‐schedule performance (Davison & Jenkins, 1985), which implies a constant discriminability parameter that is independent of the obtained reinforcer ratio. However, a more detailed analysis of conditional discrimination performance showed that the discriminability between the concurrent‐schedule alternatives decreased with time since changing over to an alternative. This effect, combined with aspects of the temporal distribution of reinforcers obtained in the concurrent schedules, qualitatively predicted the molar results and identified the conditions that operate whenever contingency discriminability remains constant.

The differential-outcomes effect is manifest as more accurate performance of a delayed conditional discrimination when alternative choice responses are followed by different reinforcers than when they are followed by the same reinforcer. In Experiment 1, a differential-outcomes effect was demonstrated within sessions by signaling the duration of food access for correct responses with stimuli appearing in conjunction with the sample stimuli. The delayed matching-to-sample performance of 5 pigeons was more accurate when green choice responses (matching a green sample) were followed by 3.5-s food access and red choice responses (matching a red sample) were followed by 0.5-s food access (different-outcome trials) than when the correct choice responses were both followed by 1.5-s reinforcers (same-outcome trials). In Experiment 2, the acquisition of this differential-outcomes effect was characterized by a progressive decrease in rate of forgetting on different-outcome trials and no change in rate of forgetting on same-outcome trials. In addition, accuracy at the shortest delay intervals for both different-outcome and same-outcome trials increased over acquisition, but to a greater extent for different-outcome trials. These data suggest that both memorial and attentional (time-dependent and time-independent) factors contribute to the differential-outcomes effect.Key words: differential-outcomes effect, delayed matching to sample, initial discriminability, rate of forgetting, key peck, pigeonThe differential-outcomes effect (DOE) refers to more accurate remembering when the outcomes of correct choice responses are different than when the outcomes are the same (Brodigan & Peterson, 1976). The DOE is often observed in discrete-trial delayed conditional discriminations in which the correct choice of either of two comparison stimuli is conditional on the sample stimulus presented earlier in the trial. For example, if red and green are the choice stimuli, delayed matching is more accurate when correct choices of red are followed by a large amount of food and correct choices of green are followed by a small amount of food than when correct choices are followed by the same amount of food. The DOE has been demonstrated with a range of outcomes in delayed conditional discriminations. For example, matching accuracy is higher when the correct choice responses are followed by different probabilities of food presentation