Eight rats were trained to make one response if a signal was shorter than a criterion duration and a different response if the signal was longer than the criterion. When exposed to intermediate durations, the rats bisected the interval at the geometric mean and the difference limen divided by the geometric mean was a constant. The rats learned new temporal discriminations more easily when the response maintained its relative, rather than its absolute, meaning. These data were interpreted in terms of a model of an internal clock that included a clock, a criterion, and a response rule.

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Rats' responses on two levers were reinforced according to independent random-interval 1.5-min food schedules. In addition, both lever presses were intermittently punished according to several concurrent random-interval random-interval shock schedules. For the left, the scheduled rate of punishment was kept constant according to a random-interval 6-min schedule. For the right, the rate of punishment varied. As the frequency of punishment for the right lever press increased, its rate decreased. The rate of the left punished lever press increased, however, even though its scheduled reinforcement rate and punishment rate remained unchanged.Key words: choice, concurrent schedules, punishment, matching law, lever press, ratsAccording to Herrnstein's input-output model of operant behavior (Herrnstein, 1970(Herrnstein, , 1971(Herrnstein, , 1974, the absolute rate of a response is directly proportional to its relative rate of reinforcement. This is expressed mathematically as:where R1 is the rate of one response, r1 is the obtained rather than the scheduled reinforcement rate associated with that response, k is a constant, and ro + r1 + r2 + . . . rn represents the sum of the reinforcement rates for all responses. In this equation, ro refers to those reinforcers that are not specified by the experimenter, but which are, nevertheless, present in the organism's milieu. The parameter k represents the asymptotic rate of response 1.The literature on the effects of reinforcement upon singly and concurrently scheduled responses may be accounted for in terms of Equation 1 and its corollaries (see Baum, 1974;Herrnstein, 1970, for ance with Equation 1, if the rate of reinforcement for a pigeon's key pecking remains constant, then increases in the rate of reinforcement from other sources decrease responding (Catania, 1963(Catania, , 1969Rachlin and Baum, 1972). For example, in one such study (Catania, 1963), pigeons' pecking was maintained by two independent variable-interval (VI) schedules operating concurrently for two keys. The value of one VI schedule was varied; the value of the second VI schedule was kept constant. As the rate of reinforcement for pecking the first key increased, responding increased. At the same time, however, the rate of responding on the second key decreased, even though its scheduled reinforcement rate remained constant.When responding is maintained, moreover, by concurrent (conc) VI VI schedules of reinforcement, organisms typically match obtained relative rates of reinforcement to relative rates of responding (Herrnstein, 1961(Herrnstein, , 1970(Herrnstein, , 1974

In the presence and absence of white noise, response-independent aversive events were delivered to rats according to several variable-time electric-shock schedules. The animals could switch from the noise component to the no-noise component and vice versa by making a single lever-press response. If the schedule in one component was not in operation when the animal was in the other component, the proportion of time allocated to one component equalled or matched the proportion of obtained punishers in the other component. If both schedules were always in operation, minimizing tended to occur: the animals allocated almost all of their time to the component having the lower shock rate. An analysis of these results, in terms of the expected time until an aversive event, is presented.

Rats' responding was maintained on a random-interval I-min food schedule. In addition, noncontingent pellets were delivered, independently of the animals' behavior, at either fixed intervals (Experiment 1) or at random intervals (Experiment 2). As the rate of delivery of the periodic and aperiodic free reinforcers increased, the rate of responding decreased. But these free reinforcers, in addition to having this inhibitory effect, had also a local excitatory effect upon responding: leverpressing increased to a level above its mean rate following the delivery of a free food pellet. The time course of this behavioral aftereffect of free reinforcers, for both fixed and random intervals, was dependent upon the proportion of the interval between successive free food deliveries. The relation of these results to those obtained with response-contingent reinforcement, free punishment, and in schedule-induced phenomena is discussed.If the rate of reinforcement for a pigeon's keypecking remains constant, then increases in the rate of other sources of reinforcement result in descreases in responding (Catania, 1963;Rachlin & Baum, 1972). Such interactions have typically been examined in a choice situation wherein responding on two keys is maintained on concurrent schedules of reinforcement (cf. Catania, 1963(cf. Catania, , 1966. But such interactions may also be studied where there is only a single key or lever. Rachlin and Baum (1972) reinforced keypecks according to a variable-interval schedule. In one phase of their experiment, additional food was delivered, independently of responding, according to various variable-time (cf. Zeiler, 1968) schedules. The rate of keypecking decreased with increases in the rate of these response-independent reinforcers (see also Nevin, 1974).The purpose of the present study was to examine the time course of the behavioral aftereffect of free reinforcers. In both experiments, responding was maintained on a random-interval (RI) schedule. In addition, extra pellets were delivered, independently of the animal's responding, at either periodic intervals (Experiment 1) or at aperiodic intervals (Experiment 2). EXPERIMENT 1Method .Subjects. Four male, naive albino Norway rats of the Charles River CD strain served as subjects. Each rat was given a daily ration of 14 g of ground Purina Chow mixed with 25 cc of water. Water was always available in each rat's cage.Apparatus. Four chambers each had inside dimensions of This research was supported by NSF Grant GB-43208 to Russell M. Church. The author wishes to thank Dr. Church for his aid and criticism. Requests for reprints should be sent to Marvin Deluty, Department of Psychology, Brown University, Providence, Rhode Island 02912. 23.2 x 20.3 x 21.9 em. The floors consisted of 16 stainless steel bars, adjacent bars being 1.5 cm apart. The front and back were made of aluminum; the two sides and the top were made of transparent acrylic. A stainless steel lever (5.1 cm wide, 1.3 ern thick, and 5.1 cm above the floor) was located in the center of the fr...