Key pecking by pigeons was maintained on chained schedules of food reinforcement. Within each session, two chained schedules with identical initial links but different terminal-link conditions of reinforcement alternated irregularly on two keys. Across successive experimental conditions, the duration of access to food (Experiment 1) or the length of the terminal link and the duration of access to food (Experiment 2) were varied systematically. Asymptotic response rates were determined in each condition, and resistance to change was assessed by arranging alternative reinforcement on a third key or by prefeeding. Response rates were lower and responding was always less resistant to change in the initial links than in the terminal links. Response rate and resistance to change in both links of a chain increased when the duration of access to food increased or the length of the terminal link decreased. The results of both experiments suggested that relative asymptotic response rate and relative resistance to change in both links of the chains depended on a single variable, relative total rate of access to food per unit of terminal-link time.This research was supported in part by Grants BNS 74-11849 and BNS 76-11028 from the National Science Foundation to the Univeristy of New Hampshire. Jean Atak and Peter Jenkins helped to conduct the experiments and contributed valuable insights. Portions of the data reported here were presented by the first two authors at the meeting of the Eastern Psychological Association, Boston, April 1977. We are indebted to Carol Eckerman (Reference Note 1) for making her data available to us.

Pigeons were trained to discriminate temporal stimuli in a discrete-trial signal-detection procedure. Pecks to one side key were reinforced intermittently after exposure to one duration, and pecks to the other side key were reinforced intermittently after exposure to a different duration. In Experiment I, the allocation of reinforcers was varied systematically for correct responses and for errors, using a procedure that controlled the obtained numbers of reinforcers. When reinforcers were allocated symmetrically, the level of discrimination decreased as the proportion of reinforcers for errors increased. When reinforcers were allocated asymmetrically, the decrease in discrimination was less systematic. Bias toward one or the other side key roughly matched the ratio of reinforcers obtained by pecks at those keys, independent of the level of discrimination. In Experiment II, the overall rate of reinforcement for correct responses was varied both within and between experimental conditions. The level of discrimination was positively related to the overall rate of reinforcement. The discrimination data of both experiments were interpreted in relation to the contingencies of reinforcement and nonreinforcement, characterized by the average difference in reinforcement probability for correct responses and errors.

Reinforcement was introduced for responses normally treated as errors in signal-detection procedures. The first experiment used a standard two-response discrete-trial procedure with no reinforcement for errors. Results showed that rats altered their response biases but maintained constant sensitivity to visual signals when reinforcement probabilities varied, and that their sensitivity depended on the physical difference between signals, in accordance with the predictions of signal-detection theory. Experiment II, with rats, and Experiment III, with pigeons, demonstrated that sensitivity decreased in this procedure when reinforcement was scheduled for errors with the signals held constant, despite independence of overall number of reinforcers and sensitivity. Experiment IV, with rats, replicated the decrease in sensitivity in a continuous procedure employing only one response. The decrements in sensitivity were similar across Experiments II, III, and IV, and accorded well with earlier research. Thus, contrary to a fundamental assumption of signal-detection theory, estimates of sensitivity are not always invariant with respect to the outcomes of responding, but depend on relative reinforcement of correct responses.

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