Replication and extension of Skinner's "superstition" experiment showed the development of two kinds of behavior at asymptote: interim activities (related to adjunctive behavior) occurred just after food delivery; the terminal response (a discriminated operant) occurred toward the end of the interval and continued until food delivery. These data suggest a view of operant conditioning (the terminal response) in terms of two sets of principles: principles of behavioral variation that describe the origins of behavior "appropriate" to a situation, in advance of reinforcement; and principles of reinforcement that describe the selective elimination of behavior so produced. This approach was supported by (a) an account of the parallels between the Law of Effect and evolution by means of natural selection, (fc) its ability to shed light on persistent problems in learning (e.g., continuity vs. noncontinuity, variability associated with extinction, the relationship between classical and instrumental conditioning, the controversy between behaviorist and cognitive approaches to learning), and (c) its ability to deal with a number of recent anomalies in the learning literature ("instinctive drift," auto-shaping, and auto-maintenance). The interim activities were interpreted in terms of interactions among motivational systems, and this view was supported by a review of the literature on adjunctive behavior and by comparison with similar phenomena in ethology (displacement, redirection, and "vacuum" activities). The proposed theoretical scheme represents a shift away from hypothetical "laws of learning" toward an interpretation of behavioral change in terms of interaction and competition among tendencies to action according to principles evolved in phylogeny.The field of learning has undergone in-At times, one senses a widespread feeling of discreasing fractionation in recent years. In-couragement about the prospects of ever getting , !.•«••,. i »> j * clear on the fundamentals of conditioning. Attempts terest in miniature systems and exact to arrive at firm dedsions abou f alternat f ve theories of local effects has grown _ to the formulations rarely produce incisive results. Every detriment of any attempt at overall integra-finding seems capable of many explanations. Issues tion. Consequently, as one perceptive ob-become old, shopworn, and disappear without a server has noted:proper burial [Jenkins ' 197 °-™ 107 -108 1

Pigeons were trained with 4 pairs of visual stimuli in a 5-term series-A+ B-, B+ C-, C+ D-. and D+ E-(in which plus[+] denotes reward and minus(-] denotes nonreward)-before the unreinforced test pair B D was presented. All pigeons chose Item 8, demonstrating inferential choice. A novel theory (value transfer theory), based on reinforcement mechanisms, is proposed. In Experiment 2, the series was extended to 7 terms. Performance on test pairs was transitive, and performance on training pairs accorded with the theory. The 7-term series was closed in Experiment 3 by training the flrst and last items together. In accordance with the theory, the Ss could not solve the circular series. The authors suggest that primates, including humans, also solve these problems using the value transfer mechanism, without resorting to the symbolic processes usually assumed. 1. Edith is fairer than Suzanne. 2. Edith is darker than Lili. 3. Who is the darkest, Edith, Suzanne, or Lili? Here the competent subject concludes that Suzanne is the darkest, although no direct information about the relationship between Lili and Suzanne was given. In this purely linguistic Juan D. Delius and Lorenzo von Fe~n were supported by the Deutsche Forschungsgemeinschaft, Bonn; C. D. L. Wynne was SUJ>ported by the Science and Engineering Research Council, London, and the Alexander von Humboldt.Stiftung, Bonn; and J. E. R. Staddon was supported by the National Science Foundation and the Alexander von Humboldt-Stiftung. We thank W. Mathaus for initial motivation, F. von Mi.inchow-Pohl for assistance in programming, and A. Lohmann for help in running the experiments. We are grateful to A. Elepfandt, B. Mc-Gonigle, J. Pearce, H. Terrace, and T. Trabasso for helpful comments on drafts of this article.

A popular view of interval timing in animals is that it is driven by a discrete pacemaker-accumulator mechanism that yields a linear scale for encoded time. But these mechanisms are fundamentally at odds with the Weber law property of interval timing, and experiments that support linear encoded time can be interpreted in other ways. We argue that the dominant pacemaker-accumulator theory, scalar expectancy theory (SET), fails to explain some basic properties of operant behavior on interval-timing procedures and can only accommodate a number of discrepancies by modifications and elaborations that raise questions about the entire theory. We propose an alternative that is based on principles of memory dynamics derived from the multiple-time-scale (MTS) model of habituation. The MTS timing model can account for data from a wide variety of time-related experiments: proportional and Weber law temporal discrimination, transient as well as persistent effects of reinforcement omission and reinforcement magnitude, bisection, the discrimination of relative as well as absolute duration, and the choose-short effect and its analogue in number-discrimination experiments. Resemblances between timing and counting are an automatic consequence of the model. We also argue that the transient and persistent effects of drugs on time estimates can be interpreted as well within MTS theory as in SET. Recent real-time physiological data conform in surprising detail to the assumptions of the MTS habituation model. Comparisons between the two views suggest a number of novel experiments.

Reinforcement schedules restrict an organism's access to one activity (the contingent response, or reinforcer) by requiring it to engage in a second activity (the instrumental response) for access to the first. Behavior is also constrained by limitations of time, so that an increase in one activity entails a decrease in some others. If an organism's repertoire consists of N independent, mutually exclusive and exhaustive activities, these two constraints can be represented as surfaces in an Af-dimensional space whose axes are the levels of the N activities. The distribution of activities under free conditions is represented by a point (the freebehavior point) in such a space. Functional relations between the equilibrium levels of the instrumental and contingent responses under different schedules of reinforcement can be generated by a homeostatic assumption: that organisms act to minimize the distance between the point representing their distribution of activities under schedule conditions and the free-behavior point.This simplified approach predicts the form of the functional relations obtained on ratio and interval (and several other) schedules, as well as the differences between them. It also accounts in a natural way for failures to regulate food intake following physiological insult. The approach incorporates into a single framework the work of Premack and his successors on the relativity of the reinforcement relation and more traditional studies of operant behavior using "strong" reinforcers. Past optimality analyses of operant behavior may have failed because they considered only the level of the contingent response.Theories of operant behavior are of two mechanistic model, which explains behavior general types. The traditional ideal is the in terms of antecedent causal relations, in an input-output fashion. The theories of Versions of the approach in this article were Hul1 ( 1943 )' S P ence ( 1960 )> Bush and presented as the author's contribution to the Sym-Mosteller (1955), and more recently, Kilposium on Response Strength, chaired by J. A. leen (1975), are of this kind. A second type Nevin at the American Psychological Association m j g ht be termed equilibrium theories (Stad-S^f a ti2^S^£5c S CS; L 97 Iho a n d Jon, 1972Jon, , 1973. In the equilibrium theories, Term Behavioral Regulation," as part of the Sym-f ew mechanistic details are given; instead, posium on Multiple-Response Baselines, chaired by behavior is explained by reference to a set of W. Timberlake at the Midwestern Psychological conditions that it must satisfy. The simplest Association Meeting in Chicago May 1977. examples are the conservation theory of This research was supported by grants from the A «• / AH-•»«••.< o -nr National Science Foundation to Duke University. Allison (e.g., Allison, Miller, & Wozny, I thank Peter Killeen and John Vaughn for com-1979) and Herrnstein's (1970) matching ments on the manuscript, and Susan Motheral for law. In conservation theory, the weighted comments as well as help with data analysis and sum of ...

Operant behavior is behavior "controlled" by its consequences. In practice, operant conditioning is the study of reversible behavior maintained by reinforcement schedules. We review empirical studies and theoretical approaches to two large classes of operant behavior: interval timing and choice. We discuss cognitive versus behavioral approaches to timing, the "gap" experiment and its implications, proportional timing and Weber's law, temporal dynamics and linear waiting, and the problem of simple chain-interval schedules. We review the long history of research on operant choice: the matching law, its extensions and problems, concurrent chain schedules, and self-control. We point out how linear waiting may be involved in timing, choice, and reinforcement schedules generally. There are prospects for a unified approach to all these areas.