Animal uaming & Behavior 1991. 19 (4). 317-325 Extinction of operant behavior: An analysis based on foraging considerations ROGER L. MELLGREN University of Texas, Arlington, Texas and TIMOTHY F. ELSMORE Walter Reed Army Institute ofResearch, Washington, D.C. In two experiments, the frequency of food reinforcement provided by variable interval(VI)sched­ ules prior to extinction was varied. In the fIrst experiment, two-component multiple schedules resulted in a greater number of responses in extinction in the presence of the stimulus previ­ ously associated with the richer of the two component schedules than that previously associated with the leaner schedule. In the second experiment, different groups of animals were trained on different VI schedules. Responding in extinction was analyzed into bouts of responding show­ ing that the number of response bouts increased and the number of responses per bout decreased with decreasing frequency of reinforcement during training. These data are compatible with an analysis ofoperant behavior based on an analogy to processes that presumably occur in naturalistic foraging situations. According to this analogy, behavior associated with search for a food source (i.e., number of response bouts) and that of procurement offood from a source (i.e., responses per bout) represent aspects of behavior that are differentially strengthened by different VI sched­ ules. Extinction serves to reveal this differential strengthening. It has been suggested that performance in the Skinner vironrnent (Batson, Best, Phillips, Patel, & Gilleland, & box is a useful analogue to aspects of foraging as it oc­ 1986; Dersich, Mazmanian, & Roberts, 1988; Mellgren curs in nature (e.g., Baum, 1983; Collier & Rovee­ & Olson, 1983; Roberts & Dersich, 1989). "Search," Collier, 1981; Fantino & Abarca, 1985; Kamil & Sar­ as defined in the laboratory context, is a behavior that gent, 1981; Killeen, Smith, & Hanson, 1981; Lea, 1979, brings the subject into contact with a food source, or 1981; Mellgren & Olson, 1983; Shettleworth, 1988, patch, and "contact with a patch" means being close 1989). It may be argued that an operant chamber is analo­ enough to the response manipulandum in a Skinner box gous to a patch, or clump of food, that occurs in a natural to be able to press it. Obviously, search involves move­ environment and the problem the forager/subject faces is ment through space, but contact with a patch (procure­ primarily one ofprocurement ofthe food available in the ment) does not require movement through space. Rather, patch. An operant chamber is a very strange environment, it involves repeated motor actions in contact with (or in however, because there is only one patch available and close proximity to) the response manipulandum. There­ the forager is confined to a single location. fore, we argue that as long as the rat maintains contact Foraging also involves several factors other than and/or close proximity to the response manipulandum in procurement of food from a patch. Among these other the operant box it is "procuring." When the rat breaks factors, search for and location of a patch are of obvious contact with the manipulandum, it recontacts the lever importance. Some of the maze procedures used in the only when it reinstates a "searching" process. As long laboratories of experimental psychologists may be analo­ as the schedule of reinforcement remains in force, the sub­ gous to searching and locating a patch in the natural en- ject will tend to stay in contact with the patch and be procuring. Essentially, the schedule of reinforcement dominates and controls the subject's behavior. The domi­ nation of the schedule is probably enhanced by the fact In conducting the research described in this report, the investigators that standard schedules used in operant studies maintain adhered to the Guide for the Care and Use ofLaboratory Animals, as a constant payoff over time, but in nature most patches promulgated by theCommittee on Care and Use of Laboratoty Animals of the Institute of Laboratory Animal Resources, National Research deplete with continued procurement and therefore result Council. The views ofthe authors do not purport to reflect the position in the subject's breaking contact with the patch. The ex­ of the Department of the Army or the Depanment of Defense (Para. 4-3, tinction schedule, however, is a condition in which con­ AR 360-5). Address correspondence to either Roger L. Mellgren, tact with the patch will not be continuous because it Department of Psychology, University ofTexas at Arlington, Box 19528, Arlington, TX 76019, or Timothy F. Elsmore, Department of Mcdical represents a patch in which total depletion has occurred. Neurosciences, Walter Reed Army Institute of Research, Washington, Thus, the extinction schedule is analogous to a patch that DC 20307. has been totally depleted of resources. 317 Copyright 1991 Psychonomic Society, Inc. 318 MELLGREN AND ELSMORE In laboratory studies of animal learning, extinction has Previous research on the effect of relative density of often been used as a method for assessing the nature of scheduled reinforcers on resistance to extinction is con­ learning that has preceded the extinction phase. Hearst sistent with the conclusion that search (general search) (1986), for example, argues that extinction is a useful and procurement (focal search) represent different procedure for revealing those things that were learned dur­ processes. The resistance to extinction shown by subjects ing acquisition, but not evident in the behavior of the sub­ in runways increases as percentage of reinforcement ject during the acquisition phase. This point has also been decreases (Robbins, 1971). In the operant situation, the made by researchers interested in the partial-reinforcement subject is already in the potential food source andthe most effect (Amsel, 1967; Capaldi, 1967), a phenomenon that critical process is that of procurement by repeated con­ depends on transfer from different acquisition schedules tact with the response manipulandum. Studies that vary to an extinction schedule for its definition. the density of scheduled reinforcers on both interval and Although there is a voluminous literature using extinc­ ratio schedules in the operant box show that resistance tion performance in runways as the main dependent vari­ to extinction increases as the density of reinforcement in­ able to evaluate the effects ofdifferent schedules of rein­ creases: the opposite result of the runway studies (Nevin, forcement, very few of the data have come from the 1979). operant-box situation (e.g., Robbins, 1971). Exceptions We take the differences between runway and operant to this generalization involve attempts to use operant studies to reflect the relative importance in each proce­ methods in a manner analogous to runway studies (e.g., dure of search and procurement processes, that is, the Bitgood & Platt, 1971; Haddad, Walkenbach, & Goed­ components of foraging behavior as it occurs in nature. del, 1980; Overmann & Denny, 1974) or studies done On the other hand, there are several differences between in the operant box in which there was no analogy to run­ the two procedures that might also contribute to differ­ way procedures, but the results were interpreted with ences in results. The runway data are based on between­ respect to theories developed using runway data as their subjects comparisons, but the operant data are based on base (e.g., Pavlik & Carlton, 1965; Pavlik & Collier, within-subject comparisons. We consider both types of 1973). In addition, there have been some operant studies comparisons in the present studies. The dependent varia­ by Nevin and his colleagues (Nevin, 1974, 1979; Nevin, ble in runway studies is the time to respond (or its recipro­ Mandell, & Atak, 1983; Nevin, Mandell, & Yarensky, cal, speed), but rate ofresponse is used in operant studies. 1981) that were independent of both method and theory The experiments in this report were designed and ana­ developed in the runway, and Nevin (1988) has addressed lyzed in order to test the hypothesis that search and some ofthe apparent contradictions between operant and procurement are differentially affected by schedules of runway data with respect to the partial-reinforcement ef­ reinforcement as measured during extinction, with the ob­ fect. Other relevant operant studies are primarily of histor­ jective of reconciling the apparent differences between ical interest (e.g., Jenkins, McFann, & Clayton, 1950; runway and operant data. Experiment 1 utilizes multiple Perin, 1942; Skinner, 1938; Williams, 1938). schedules for within-subject comparisons, and Experi­ In the experiments described in this article, extinction ment 2 uses between-subjects schedule comparisons, of responding following variable interval schedules of analogous to the typical runway methodology. food reinforcement was viewed as reflecting processes analogous to those that might occur in a foraging situa­ tion. In particular, the processes we hypothesized to be EXPERIMENT 1 operating are those of search and procurement. "Search" was operationally defined as a period of noncontact with Previous work on "response strength" (Nevin, 1979) the response manipulandum terminated by contact with has documented that following multiple variable interval (mult Vl) schedules in which different VIs are associated the manipulandum. We do not mean that the subject is with different cues, there is greater response strength in exclusively engaged in searching