Previous experiments have shown that positively reinforced operant responding is suppressed during a conditioned stimulus terminated with an electric shock (conditioned suppression). In the present experiment, the conditioned stimulus was terminated with a positive unconditioned stimulus, and it was found that the duration of the conditioned stimulus was a key factor in determining whether response suppression or response enhancement was observed during the stimulus. The lever-pressing responses of rats were maintained by a variable-interval schedule of food reinforcement. While the rats were pressing the lever, a light was occasionally turned on, its offset coincident with a brief period of access to a sucrose solution. In consecutive blocks of sessions, the light duration was 40 sec, 12 sec, or 120 sec. Results showed that the rate of lever pressing was substantially suppressed during the 12-sec stimulus, slightly suppressed during the 40-sec stimulus, and enhanced during the 120-sec stimulus.One characteristic that differentiates operant and respondent conditioning is the experimenter's criterion for delivering reinforcement. In operant conditioning, the delivery of reinforcement is response-contingent, but in respondent conditioning, the delivery of the unconditioned stimulus (UCS) is dependent only on the presentation of the conditioned stimulus (CS). Rather than studying operant and respondent conditioning procedures separately, a number of investigators have designed experiments to explore the interaction of the two procedures. Typically, the purpose of these experiments has been to observe any change in the rate of operant responding during a concurrently presented CS.For example, experiments have demonstrated that established, food-reinforced operant responding is suppressed during a CS terminated with electric shock (e.g., Estes and Skinner, 1941;Kamin, 1965). Stein, Sidman, and Brady (1958) showed that one of the parameters affecting the degree of response suppression was the duration of the CS. At long CS durations, response suppression was relatively slight, but when CS duration was short, response suppression was almost complete. The experimenters said that at least one reason for such a relationship was that the subject would have missed more reinforcements 'Reprints may be obtained from Donald Meltzer, Department of Psychology, Southern Illinois University, Carbondale, Illinois 62901.by suppressing during a long CS than during a short CS.Other investigators have studied a directly comparable paradigm in which a CS terminated with a positive UCS was presented while a subject was responding for contingent positive reinforcement. Herrnstein and Morse (1957) reported one such experiment in which pigeons were differentially reinforced for responding at low rates. The rate of food-reinforced operant responding was enhanced in four out of six pigeons during presentations of a CS that preceded non-contingent food delivery. Azrin and Hake (1969) suggested that the enhancement of operant responding during...

Two groups (N = 5) of hybrid-strain rats were conditioned to press a lever on a fIXed-interval 3 min schedule of food reinforcement. For Although quantity or quality of reinforcement has often been varied in studies of free operant behavior (e.g., Collier & Meyers, 1961;Guttman, 1954), relatively few experimenters have studied the effects of these variables on responding maintained by a fIXed-interval reinforcement schedule. Hutt (1954), in a study of fIXed-interval responding, showed that mean response rate increased with increases in both the quality and the quantity of a liquid reinforcer. Similarly, Stebbins, Mead, & Martin (1959), using a within-S procedure, reported that the mean response rate on a fIXed-interval reinforcement schedule increased as the concentration of the sucrose reinforcer increased.Stebbins et al also reported that the distribution of responses within an interreinforcement interval depended on the sucrose concentration of the reinforcer. As sucrose concentration increased from 5% to 32%, the percentage of responses made early in the interreinforcement interval increased, and the percentage made late in the interval decreased. Since the mean response rate increased as well with greater sucrose concentration, the implication is that the increase in response rate occurred at least partly because the Ss made more responses early in the interreinforcement interval at the higher sucrose concentration.In the present experiment, a between-S procedure was used to study the effects of two quantities of reinforcement on fixedinterval responding. We were specifically interested in examining differences in the interreinforcement response distributions associated with the two reinforcement quantities. METHOD Subjects Ten male, hooded rats of a hybrid strain bred in our laboratory were the Ss. Each S was 90-100 days old at the beginning of the experiment and was placed on a 22-h food deprivation schedule one week before the first experimental session. Water was always available to an S except for the time it was in the experimental chamber. ApparatusThe experiment was run in a Skinner box (Lehigh Valley No. 1316) with interior dimensions of 7-1/2 in. x 12 in. x 8 in. A single response lever was located 1-3/16 in. above the grid floor and 1-1/2 in. from the right side of the box. The food hopper was located midway along the base of the front wall, and 45 mg Noyes pellets were used as reinforcement.Masking white noise was always present in the room in which the experimental chamber was located. All the programming and recording equipment was in an adjacent room. Procedure Each S was given one to three I-h sessions of continuous reinforcement in which each lever press produced one food pellet. The reinforcement schedule was then changed to fixed-interval 3 min. Two groups were randomly formed with five Ss in each. For the l-pellet group the reinforcement was one 45 mg pellet delivered immediately after the lever press. For the 3-pellet group the reinforcement consisted of three 45 mg pellets. One pell...

reinforcement on those days was three pellets and one pellet, respectively.Sessions were scheduled daily, and a session was terminated by the first reinforcement following 1 h of session time. Each S had a total of 50 sessions during the experiment, 25 sessions in which it received 3-pellet reinforcements and 25 sessions in which it received I-pellet reinforcements. The data from the last 10 sessions-five 3-pellet sessions and five I-pellet sessions-were used in the statistical analysis. Meltzer & Brahlek (1968) recently described the effects of different reinforcement quantities on the fixed-interval (FI) performance of two groups of rats. The Ss in both groups were reinforced on an FI 3-min schedule. blrt Ss in the first group received a I-pellet reinforcement while Ss in the second group received a 3-pellet reinforcement. Response rates of the two groups were compared in successive quarters of the 3-min interval. and it was found that differences in rate increased as the interval progressed. However. when the mean percent of responses emitted in successive quarters of the FI was calculated. the two groups were shown to have almost identical response distributions. The mean response rate of the 3-pellet group during the last three-quarters of the interval could be approximated by multiplying the mean response rate of the I-pellet group by 1.6. DONALD MELTZER and JAMESThe data conflicted with the only other description of the effects of reinforcement quantity on response di stri bu tion during FI performance. Stebbins, Mead,

The conditioned suppression of a previously trained lever pressing response was observed in hooded rats. One group of rats received pairings of one 60-sec stimulus followed by shock. The remaining two groups received two consecutive 3D-sec stimuli with the second stimulus followed by shock. It was found that the one-stimulus group suppressed lever pressing strongly during every presentation of the stimulus. Both of the two-stimuli groups suppressed lever pressing in both stimuli on early trials. On later trials. both groups showed less suppression of lever pressing in the first stimulus than in the second stimulus. indicating the development of a discrimination of the time of shock delivery.When pairings of a stimulus, followed by brief electric shock, are presented several times to a rat emitting operant responses, the operant response rate decreases during the stimulus and returns to the pre-stimulus rate after the shack is presented and the stimulus is withdrawn. The decrease in operant response rate during the stimulus is a demonstration of conditioned suppression.A large number of experiments (e.g., Brady, 1955;Annau & Kamin, 1961) have studied the conditioned suppression of operant behavior in a single stimulus that preceded shock. In the present experiment, which is similar to an experiment reported briefly by Kamin (1965), the suppression of operant behavior was obselVed when two stimuli preceded shock. A control group of rats received delayed conditioning trials composed of a single 6D-sec stimulus that ended with shock. Two experimental groups of rats received trials composed of two 3Q-sec stimuli in succession, and the second stimulus ended with shock. One of the experimental groups received the trials according to a trace conditioning procedure, and the other group received the trials according to a compound (successive cue) conditioning procedure. The two experimental groups provided a comparison of response suppression in two pre-shock stimuli when the second stimulus and the intertrial stimulus were either the same (trace procedure) or different (successive cue procedure). METHOD SubjectsThe Ss were 15 male hooded rats, 13S days old at the beginning of the experiment. Before training the lever pressing response, Ss were allowed free access to Purina Lab Chow pellets and water. Each S was tested at SO% of its ad lib weight, which was maintained during the experiment by feeding an appropriate amount of Purina Lab .Chow after the daily experimental session. ApparatusThe Ss were tested during daily 6O-min sessions in a standard Lehigh Valley Electronics (LVE) operant conditioning chamber (Model 1417 c). Reinforcement for lever pressing was a 4S mg Noyes rat pellet. A 1.0 mA shock was presented through the grid floor of the chamber by a LVE shock supply"(ModeI153l) and a scrambler (Model BliSS). A light stimulus was provided by the illumination of two 7-W lamps mounted in a wall-of the chamber, and a tone (2000 .cps, 80 dB) was delivered to the . .chamber by a Hewlett-Packard oscillator (Model 200cD). ...

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