Behavioural Processes 40 (1997) 61 – 73
Behavioral contrast with changes in duration and rate of reinforcement Jeffrey N. Weatherly a,b,*, Cam L. Melville a, Samantha Swindell b a
Department of Psychology, McNeese State Uni6ersity, Lake Charles, LA 70609, USA b Washington State Uni6ersity, Pullman, WA, USA
Received 21 August 1996; received in revised form 15 November 1996; accepted 19 November 1996
Abstract Behavioral contrast can be defined as an inverse relation between rate of responding in one component of a multiple schedule and conditions of reinforcement in the other components. The present study was an attempt to produce contrast with changes in duration of reinforcement. Pigeons pecked a key for mixed grain delivered by a multiple variable-interval variable-interval schedule. In Experiment 1, the reinforcer duration in the second component was decreased or increased, for different subjects, by a factor of two, four, and six from that delivered during baseline. These changes usually produced contrast. In Experiment 2, rate, duration, or rate and duration of reinforcement varied in the second component. Contrast was usually observed when only one variable was manipulated. When both variables were manipulated in the same condition, but in opposite directions, responding in the constant component usually changed inversely with the change in rate of reinforcement, not the change in reinforcer duration. The results demonstrate that changes in duration of reinforcement can produce contrast. They also increase the empirical base for which a successful theoretical account of contrast must encompass and the generality of contrast, which increases its potential practical implications. © 1997 Elsevier Science B.V. Keywords: Behavioral contrast; Duration of reinforcement; Pigeon
1. Introduction Behavioral contrast can be defined as an inverse relation between rate of responding in one component of a multiple schedule and conditions of * Corresponding author. Tel.: +1 318 4755435; fax: + 1 318 4755467.
reinforcement in the other component(s) (e.g. McSweeney, 1978). Positive contrast is an increase in the rate of responding during one component of a multiple schedule (constant component) that occurs when the conditions of reinforcement in another component (changing component) are made worse. Negative contrast is a decrease in responding that occurs during the constant component
0376-6357/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 3 7 6 - 6 3 5 7 ( 9 6 ) 0 0 7 7 2 - 3
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when the conditions of reinforcement in the changing component are enriched. Since its original description (Reynolds, 1961), behavioral contrast has been the focus of much empirical research and theoretical formulation (Williams, 1983; for a review). Despite such efforts, however, an universally accepted theoretical account of contrast does not exist. Before an account of contrast can be accepted, it is necessary to know exactly what factors do (or do not) produce contrast. However, to date, the vast majority of studies on contrast have focused on changes in the rate of reinforcement (Williams, 1983). Relatively few studies have focused on other factors (e.g. amount or quality of reinforcement). Several studies have attempted to produce behavioral contrast by altering the amount or duration of reinforcement in the changing component. Some of these attempts have been successful (e.g. Gonzalez and Champlin, 1974; Ettinger et al., 1981; Hamilton and Silberberg, 1978; Kramer and Rilling, 1969) whereas others have not (e.g. Jensen and Fallon, 1973; Mackintosh et al., 1972; Shettleworth and Nevin, 1965). In the studies that have observed contrast with changes in the amount or duration of reinforcement, the results are equivocal or limited. For example, Gonzalez and Champlin (1974) and Kramer and Rilling (1969) altered both the amount and rate of reinforcement, making conclusions about the contribution of amount of reinforcement impossible. Ettinger et al. (1981) demonstrated both positive and negative contrast with changes in duration of reinforcement. However, reinforcement duration was varied by only one value below or above that delivered during baseline. Determining whether behavioral contrast can be produced by changes in duration of reinforcement is also important for determining the generality of contrast. If contrast occurs only under limited conditions (e.g. changes in rate of reinforcement of an identical reinforcer), then the frequency of contrast effects may be low. However, if contrast occurs under many conditions (e.g. changes in rate or amount of reinforcement of an identical or a qualitatively different reinforcer), then contrast effects may be prevalent in
any organisms behavior. If the latter is true, then contrast may have extensive practical implications (e.g. Gross and Drabman, 1981). The initial purpose of the present study was to determine whether behavioral contrast could be produced by changing the duration of reinforcement per reinforcer delivered in the changing component. Experiment 1 determined this. The second purpose was to compare contrast produced by changes in duration of reinforcement to that by changes in rate of reinforcement, a factor known to produce contrast. In Experiment 2, the contrast produced by changes in the duration, rate, or duration and rate of reinforcement in the changing component were examined.
2. Experiment 1 Two groups of four pigeons pecked a key for mixed-grain reinforcers delivered on a multiple variable-interval (VI) VI schedule. For the positive contrast group, the duration of reinforcement per reinforcer (i.e. the number of seconds of access to mixed grain) delivered in the changing component was decreased by a factor of two, four, and six, in different conditions, from that delivered during baseline. For the negative contrast group, the reinforcer duration was increased by a factor of two, four, or six.
3. Method
3.1. Subjects Eight experimentally experienced pigeons, obtained from the Johnson Tower Vivarium at Washington State University, served as subjects. Subjects were maintained at approximately 85% of their free-feeding weight, by post-session feedings when necessary, throughout the course of the experiment. Each subject was housed individually, with water and grit freely available in the home cage. Subjects experienced a 12/12-h light/dark cycle.
J.N. Weatherly et al. / Beha6ioural Processes 40 (1997) 61–73
3.2. Apparatus The apparatus was a single-key experimental enclosure for pigeons, measuring 25.5 × 28 × 29 cm. The response key was a 2 cm diameter Plexiglas panel, located 7.5 cm below the ceiling and 3.5 cm from the left wall. It was operated by applying approximately 0.25 N of force to its center. A 5.5 ×4.5 cm opening could allow access to a solenoid-operated magazine containing mixed grain. This opening was centered, 10 cm from each wall and was 4.5 cm above the floor. The magazine was large enough so that it could not be emptied during a single reinforcer presentation at any duration used in the present study. A 3× 2.5 cm Plexiglas panel served as the houselight. It was located 10 cm above the opening to the grain hopper. The apparatus was enclosed in a sound attenuating chamber. A ventilating fan masked noise from outside the chamber. Experimental events were presented and data were recorded by an IBM-compatible 486 computer connected to a MED Associates interface running MedState software. It was located in an adjacent room.
3.3. Procedure As the subjects were experimentally experienced, they were placed directly on the experimental procedure. Four subjects received the positive contrast procedure. The other four subjects received the negative contrast procedure. Subjects responded on a multiple VI VI schedule. Components alternated every 30 s and were signalled by the key light being illuminated either red or white. For two subjects in each group, the key light was red in the first component. The key light was white in the first component for the other subjects. Sessions lasted 30 min and the houselight was illuminated white throughout the session. Reinforcers were scheduled by independent 25 interval series, constructed as suggested by Fleshler and Hoffman (1962), that were sampled without replacement. The key light was extinguished during reinforcement. Neither the session timer nor the inter-reinforcer interval advanced during reinforcement. Sessions were conducted daily, 6 to 7 days per week.
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The first condition served as baseline. Subjects in the positive contrast group responded on a multiple VI 120 s VI 120 s schedule. Reinforcers in each component were 12 s access to mixed grain. In the subsequent conditions, reinforcers during the second component were 2, 6, and 3 s long, conducted in that order. The final condition was a recovery of baseline. Reinforcers were again 12 s in length during each component. Each condition was conducted for a total of 20 sessions. Conducting 20 sessions has precedence in the study of contrast (e.g. McLean and Morritt, 1994). Furthermore, as the data help to illustrate, 20 sessions were sufficient to achieve stable responding. The negative contrast group responded on a multiple VI 30 s VI 30 s schedule. During baseline, reinforcers in each component were 3 s access to mixed grain. The baseline reinforcer duration differed from that experienced by the positive contrast group to accommodate the increases that would occur in the contrast conditions. The rate of reinforcement was changed so that the total access to reinforcement during the constant component would be equal between groups. In the subsequent conditions, reinforcers in the second component were 18, 6, and 12 s in length, conducted in that order. The final condition was a recovery of baseline in which reinforcers were again 3 s in both components. Each condition was conducted for a total of 20 sessions.
4. Results and discussion Table 1 presents the mean rates of responding for individual subjects and for the mean of all subjects in both groups during the first (constant) and second (changing) component of each condition. Response rates were calculated by dividing the total number of responses emitted during a component by the total number of minutes, excluding reinforcement time, that the component was available. The S.E.s of the means and the mean number of obtained reinforcers for individual subjects and for the mean of all subjects are also presented. Data in Table 1 were calculated
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Table 1 Rates of responding (responses per min; R) in each component of each condition in Experiment 1 for individual subjects and for the mean of all subjects serving in the positive and negative contrast groups Subject
30 R S.E.M OR 29 R S.E.M OR 103 R S.E.M OR 592 R S.E.M OR Mean R S.E.M OR
Positive contrast 12 s
12 s
12 s
6s
12 s
3s
12 s
2s
12 s
12 s
54.3 5.6 7.6 30.1 1.9 7.6 67.7 5.2 7.2 75.4 9.4 9.4 56.9 9.9 8.0
48.0 3.5 7.8 35.4 1.9 8.8 87.9 4.8 7.6 86.4 6.0 8.2 64.4 13.4 8.1
80.7 5.7 6.6 43.1 0.7 6.8 73.6 5.5 5.8 81.6 5.1 9.6 69.8 9.1 7.2
49.0 5.8 6.8 30.5 1.2 6.4 76.0 6.7 6.4 67.1 6.1 8.2 55.7 10.1 7.0
64.3 1.9 7.4 47.1 1.6 7.0 65.7 9.2 7.6 105.3 5.3 6.6 70.6 12.3 7.2
44.4 0.6 7.8 27.8 1.2 8.0 57.8 10.7 5.4 69.0 3.0 6.8 49.8 8.9 7.0
72.8 2.2 8.6 43.8 1.5 8.8 68.6 1.8 6.4 107.5 3.2 8.2 73.2 13.1 8.0
43.4 3.8 6.4 24.3 1.2 6.4 69.2 0.9 8.8 85.8 3.2 8.2 55.7 13.6 7.5
53.9 2.5 7.6 43.3 1.4 7.2 84.7 1.3 5.6 92.2 3.8 7.6 68.5 11.8 7.0
44.5 1.8 9.0 34.4 1.0 7.8 86.9 2.1 8.0 46.8 2.8 7.6 53.1 11.6 8.1
3s
12 s
3s
18 s
3s
3s
41.8 5.3 13.2 42.1 2.1 12.8 68.0 6.7 12.8 52.2 3.9 11.6 51.0 6.2 12.6
59.9 8.1 13.8 62.0 4.8 13.8 75.6 4.2 13.6 83.3 1.3 13.2 70.2 5.6 13.6
63.5 8.1 14.2 67.3 1.5 12.6 82.4 2.9 12.6 79.7 2.7 13.6 73.2 4.6 13.3
Negative contrast
1093 R S.E.M OR 1972 R S.E.M OR 1993 R S.E.M OR 1994 R S.E.M OR Mean R S.E.M OR
3s
3s
3s
6s
68.4 2.0 13.0 98.5 3.4 13.6 68.1 3.3 15.8 96.0 1.7 16.0 82.8 8.4 14.6
57.0 1.5 14.2 93.1 3.3 11.8 65.3 4.2 15.0 75.1 0.8 13.2 72.6 7.8 13.6
55.5 7.4 12.4 75.3 6.7 13.0 60.7 5.7 12.2 79.3 2.5 13.4 67.7 5.7 12.8
70.8 6.8 12.4 64.2 5.2 15.0 69.3 6.2 13.2 88.0 2.4 13.0 73.1 5.2 13.4
21.1 6.8 10.0 16.0 3.7 9.2 42.4 2.8 9.8 33.2 3.1 10.4 28.2 6.0 9.9
37.1 7.6 12.6 29.1 6.8 12.8 59.4 3.8 11.6 45.0 5.6 10.4 42.7 6.5 11.9
36.8 4.5 14.8 36.5 5.0 13.0 58.1 3.5 13.2 49.5 4.1 10.8 45.2 5.3 13.0
The standard errors of the means (S.E.M) and the mean number of obtained reinforcers per component (OR) are also presented. The SEM for the mean of all subjects represents the standard error between subjects. Data represent responding during the final five sessions that each condition was conducted.
using the final five sessions for which each condition was conducted. Data from both baselines are presented because baseline responding was not always recovered. The failure of response rates to recover to baseline levels is not uncommon in the study of behavioral contrast (e.g. McSweeney et al., 1986; Spealman and Gollub, 1974) and has been noted as a potential pitfall of using a be-
tween-session design to study contrast (McSweeney and Melville, 1988). The present failure to recover baseline rates of responding was not due to responding being in transition. The small S.E.s in Table 1 demonstrate that responding was relatively stable. Furthermore, visual inspection of data over the final five sessions of each condition indicated that linear trends in responding were
J.N. Weatherly et al. / Beha6ioural Processes 40 (1997) 61–73
not present, further indicating that response rates were not in a state of transition. Table 1 shows that the rate of responding in the changing component sometimes, but not always, varied directly with changes in reinforcer duration in that component. In the positive contrast group, responding for three of the four subjects generally decreased with decreases in reinforcer duration. The exception was subject 592, whose responding increased with decreases in reinforcer duration. In the negative contrast group, responding generally increased when the reinforcer duration was increased to 6 s, but decreased at longer reinforcer durations. Finding that rate of responding in the changing component did not always vary directly with changes in reinforcer duration in that component is not unprecedented. Both increases and decreases in the rate of responding with changes in the magnitude of reinforcement have previously been reported (e.g. Reed, 1991). Table 1 also shows that responding in the constant component usually varied inversely with changes in reinforcer duration in the changing component. These contrast effects are presented in Fig. 1. It represents the percentage change in the rate of responding during the constant component of each contrast condition from the mean rate of responding during the constant component of the baselines. The upper graph represents the results for the positive contrast group; the lower graph, the results for the negative contrast group. Each bar represents the results for an individual subject. Percentages were calculated by dividing the rate of responding during the constant component in a contrast condition by the mean rate of responding during the constant component in the baselines, subtracting one from the quotient and then multiplying by 100%. Data for the calculations were taken from Table 1. As can be seen in Fig. 1, subjects in the positive contrast group usually, but not always, displayed positive contrast with decreases in reinforcer duration in the changing component. The most notable exception was subject 103, who never displayed contrast. In all, positive contrast was observed in eight of the 12 potential instances. Of the three subjects that displayed positive contrast, each different subject displayed its largest contrast
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effect at a different reinforcer duration. Fig. 1 also shows that negative contrast was always observed for subjects in the negative contrast group. Each subject displayed the largest contrast effect when the reinforcer duration in the changing component was 12 s. Although both groups of subjects displayed contrast with changes in duration of reinforcement, it is possible that these contrast effects were not directly produced by changes in reinforcer
Fig. 1. The percentage change in the rate of responding during the constant component of each contrast condition of Experiment 1 from the mean rate of responding during the constant component of the baselines. The upper graph represents the results for the positive contrast group; the lower graph the results for the negative contrast group. Each bar represents the results for an individual subject.
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duration, but rather were produced by changes in rate of reinforcement. That is, changing the duration of reinforcement may have led to changes in the obtained rate of reinforcement which, in turn, produced the contrast. To determine whether this was the case, the number of obtained reinforcers in each component were analyzed for both groups. One-way repeated measures analyses of variance (ANOVAs) conducted on the obtained number of reinforcers for individual subjects in the positive contrast group (taken from Table 1) showed that the obtained number of reinforcers did not differ across conditions for either component (F(4,12)= 1.06, constant component; F(4,12)=1.25, changing component). Similar analyses for the negative contrast group indicated that the obtained number of reinforcers did not differ across conditions for the changing component (F(4,12) = 1.71), but did differ significantly across conditions for the constant component (F(4,12)=9.13, PB.001). This difference occurred because subjects received fewer reinforcers in the constant component of the 12 s reinforcer duration condition. A follow-up ANOVA that excluded the data from the 12 s reinforcer condition indicated that the obtained number of reinforcers in the constant component did not differ across the other conditions (F(3, 9)= 1.57). Therefore, the contrast observed in the 12 s reinforcer duration condition for the negative contrast group was confounded with a decrease in the rate of reinforcement during the constant component. This difference may be, at least partially, responsible for why each subject displayed its largest contrast effect during the 12 s reinforcer duration condition. However, none of the contrast effects, for either group, can be explained by changes in rate of reinforcement during the changing component. Experiment 1 demonstrates that positive and negative behavioral contrast can be produced by changes in duration of reinforcement. In all, contrast was observed in 20 of 24 potential instances. Experiment 2 was designed to compare the contrast effects produced by changes in duration of reinforcement to those produced by changes in rate of reinforcement. Furthermore, both factors were varied simultaneously in some conditions to
determine how changes in duration and rate of reinforcement would interact.
5. Experiment 2 Four pigeons pecked a key for mixed-grain reinforcers delivered on a multiple VI 60 s VI 60 s schedule during baseline. In contrast conditions, rate of reinforcement or reinforcer duration in the changing component was increased or decreased. In other conditions, both rate of reinforcement and reinforcer duration in the changing component were varied during the same condition, but in opposite directions.
6. Method
6.1. Subjects Four experimentally experienced pigeons served as subjects. The subjects were not those used in Experiment 1. However, they were obtained, housed, and maintained in the same manner, as those in Experiment 1.
6.2. Apparatus The apparatus was a 2-key, 2-treadle experimental enclosure for pigeons, measuring 27× 30×29.5 cm. The two keys were 2.5 cm diameter Plexiglas panels located 4 cm below the ceiling and 12.5 cm from each other. The left key was located 6 cm from the left wall and could be illuminated by red light. The right key was 6.5 cm from the right wall and could be illuminated by blue light. Each key was operated when a force of approximately 0.25 N was applied to its center. A treadle was located 15.5 cm below each key. The treadles were not used in this experiment and will not be described. A 5 × 4 cm opening could allow access to a solenoid-operated magazine which contained mixed grain. The magazine was again large enough to provide grain throughout the largest reinforcer duration. The opening was 12.5 cm from the right wall and 3 cm above the floor. A 1 cm diameter houselight, located 0.5 cm from
J.N. Weatherly et al. / Beha6ioural Processes 40 (1997) 61–73
the left wall and 0.5 cm from the ceiling, illuminated the chamber. The enclosure was housed, the experimental events were programmed, and the data were recorded as in Experiment 1.
Table 2 The rate of reinforcement and the reinforcer duration during the first (constant) and second (changing) component for each condition in Experiment 2 Condition
6.3. Procedure As the subjects were experimentally experienced, they were placed directly on the experimental procedure. The first condition served as baseline. Subjects responded on a multiple VI 60 s VI 60 s schedule. Components alternated every 30 s. Two subjects responded on the left key (red) during the first component and on the right key (blue) during the second component. Components were reversed for the other 2 subjects. During baseline, reinforcers in both components were 6 s access to mixed grain. Reinforcers were scheduled by independent 25 interval series, constructed as suggested by Fleshler and Hoffman (1962), that were sampled without replacement. Sessions lasted 30 min, not including reinforcement time. The houselight was illuminated throughout the session. Conditions were conducted for a total of 20 sessions, with sessions being conducted daily, 6 to 7 days per week. In subsequent conditions, the rate of reinforcement, the reinforcer duration, or both the rate of reinforcement and the reinforcer duration varied in the second component. When both rate and duration of reinforcement varied in the same condition, they varied in the opposite direction. As a result, the constant and changing components both scheduled the same total access to reinforcement. Therefore, these conditions cannot accurately be called contrast conditions. These conditions were conducted so that the ability of these factors to affect responding in the constant component could be assessed. The final condition was baseline recovery. Table 2 presents the conditions in the order in which they were conducted. All other aspects of the procedure remained as in baseline.
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1 2 3 4 5 6 7 8
First component
Second component
Rate
SR duration
Rate
SR duration
6 6 6 6 6 6 6 6
VI VI VI VI VI VI VI VI
6 12 3 3 6 12 6 6
VI VI VI VI VI VI VI VI
60 60 60 60 60 60 60 60
s s s s s s s s
s s s s s s s s
60 s 60 s 30 s 60 s 30 s 120 s 120 s 60 s
s s s s s s s s
Conditions are presented in the order in which they were conducted.
7. Results and discussion Table 3 presents the mean rates of responding for individual subjects and for the mean of all subjects during the constant and changing component of each condition. The S.E.s of the means and the mean number of obtained reinforcers per component for individual subjects and for the mean of all subjects are also presented. Data in Table 3 were calculated as in Table 1. Data from both baselines are presented because, as in Experiment 1, baseline responding was not always recovered. Also as in Experiment 1, the small S.E.s as well as a visual inspection of the data indicated that this failure did not occur because responding was in a state of transition. Table 3 shows that the rate of responding in the changing component for individual subjects did not vary systematically when either rate or duration of reinforcement varied in that component. Compared with responding in both baselines, responding in the changing component of these conditions varied directly with the change in the conditions of reinforcement in seven of 16 potential instances. Conditions in which rate and duration of reinforcement were both varied, but in opposite directions, both components scheduled the same total access to reinforcement per session. Subjects’ rates of responding during the changing
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Table 3 The rates of responding (responses per min; R) in the first (constant) and second (changing) component of each condition in Experiment 2 for individual subjects and for the mean of all subjects Subject
54 R S.E.M. OR 161 R S.E.M. OR 162 R S.E.M. OR 173 R S.E.M. OR Mean R S.E.M. OR
54 R S.E.M. OR 161 R S.E.M. OR 162 R S.E.M. OR 173 R S.E.M. OR Mean R S.E.M. OR
Condition (SR rate and SR duration) VI60 6s
VI60 6s
VI60 6s
VI60 12 s
VI60 6s
VI30 3s
VI60 6s
VI60 3s
85.8 1.9 14.0 70.3 2.3 14.6 49.2 3.2 14.4 125.7 4.0 15.2 82.8 16.2 14.6
73.1 2.0 13.8 75.3 1.8 13.2 53.1 2.2 15.0 97.6 6.0 14.4 74.8 9.1 14.1
83.6 1.2 14.4 58.6 1.6 15.8 56.5 4.4 16.6 62.1 7.8 13.0 65.2 6.2 15.0
92.1 2.1 12.8 104.2 4.1 16.0 45.0 1.4 14.4 66.7 3.0 13.4 77.0 13.2 14.2
91.3 3.0 13.4 55.2 1.7 14.0 34.4 2.8 14.8 114.2 2.8 14.2 73.8 17.9 14.1
94.7 5.0 28.2 101.3 5.0 30.0 43.1 2.5 28.6 89.2 3.9 28.0 82.1 13.2 28.7
118.2 5.2 13.8 64.3 4.3 14.6 55.1 9.3 14.4 82.7 3.3 14.8 80.1 13.9 14.4
72.7 3.9 14.8 59.5 2.6 13.8 46.9 4.8 16.8 62.1 3.5 12.2 60.3 5.3 14.4
VI60 6s
VI30 6s
VI60 6s
VI120 12 s
VI60 6s
VI120 6s
VI60 6s
VI60 6s
88.7 3.2 16.4 44.9 3.0 14.2 22.1 1.1 12.2 69.8 5.8 14.4 56.4 14.5 14.3
103.9 2.3 30.8 67.1 7.6 28.0 63.9 1.7 29.4 79.9 6.9 30.2 78.7 9.1 29.6
105.4 1.9 13.2 82.2 3.4 15.0 97.4 2.7 14.0 88.1 8.7 13.6 93.3 5.1 14.0
88.2 2.2 7.0 79.2 4.4 6.6 60.5 3.8 7.4 80.7 3.1 6.8 77.2 5.9 6.8
109.5 2.4 13.6 91.5 3.8 15.0 108.8 2.5 13.8 63.1 2.2 13.8 93.2 10.9 14.1
84.1 2.7 6.4 77.9 3.7 7.2 38.7 2.6 7.2 85.2 2.4 8.0 71.5 11.0 7.2
97.7 1.4 15.6 77.1 2.5 14.6 98.3 7.9 15.4 68.8 2.4 14.0 85.5 7.4 14.9
92.0 3.5 13.8 89.3 3.9 13.6 30.3 4.2 12.6 78.4 3.8 13.6 72.5 14.4 13.4
The standard errors of the means (S.E.M) and the mean number of obtained reinforcers per component (OR) are also given. The S.E.Ms for the mean of all subjects represent the standard error between subjects. Conditions are presented in the order that they were conducted (see Table 2). Data are taken from the final five sessions that each condition was conducted.
component of these conditions fell between those observed during the first and the second baseline in six of eight instances. Fig. 2 shows that behavioral contrast was usually observed when either duration or rate of reinforcement was decreased or increased in the changing component. It represents the percentage
change in the rate of responding during the constant component when either duration or rate of reinforcement was varied in the changing component from the mean rate of responding during the constant component of the baselines. The upper graph presents results from conditions in which either duration (left panel) or rate of reinforce-
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ment (right panel) was decreased in the changing component. The lower graph presents results from conditions in which either duration (left panel) or rate of reinforcement (right panel) was increased in the changing component. Each bar represents the results for an individual subject. Percentages were calculated and are presented as in Fig. 1 using the data presented in Table 3.
Fig. 2. The percentage change in the rate of responding during the constant component of the conditions in Experiment 2 in which either duration or rate of reinforcement varied in the changing component from the mean rate of responding during the constant component of the baselines. The upper graph represents the results for the positive contrast conditions; the lower graph the results for the negative contrast conditions. Each bar represents the results for an individual subject.
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The upper graph of Fig. 2 shows that positive contrast was observed with decreases in rate of reinforcement, but was not usually observed with decreases in duration of reinforcement. Three of four subjects displayed positive contrast when rate of reinforcement in the changing component was decreased to a VI 120 s schedule. Only one of four subjects displayed positive contrast when duration of reinforcement in the changing component was decreased to 3 s. The lower graph shows that negative contrast was always observed. All four subjects displayed negative contrast when the duration of reinforcement in the changing component was increased to 12 s and when the rate of reinforcement in the changing component was increased to a VI 30 s schedule. As in Experiment 1, it was necessary to ensure that the contrast produced by changes in duration of reinforcement was not actually produced by changes in rate of reinforcement. To do so, an one-way repeated measures ANOVA was conducted on the obtained number of reinforcers in the changing component for individual subjects (taken from Table 3) for the baseline conditions and the conditions in which only reinforcer duration was varied. Results showed that the obtained number of reinforcers did not differ across the changing component of these conditions (F(3,9)= 0.41). Furthermore, an one-way repeated measures ANOVA, conducted on the obtained number of reinforcers for individual subjects in the constant component of each condition, showed that obtained reinforcers did not vary across conditions (F(7,21)= 0.55). Therefore, the contrast observed with changes in duration of reinforcement were not by-products of changes in rate of reinforcement. Furthermore, the contrast effects were not confounded with changes in rate of reinforcement during the constant component. Fig. 3 demonstrates that, when both rate and duration of reinforcement varied in the changing component, responding in the constant component was more sensitive to changes in rate of reinforcement. It represents the percentage change in the rate of responding during the constant component in the conditions in which either rate of reinforcement (left panels) or rate and duration
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were calculated and are presented as in Fig. 1 using the data presented in Table 3. Fig. 3 again shows that contrast was observed in seven of eight instances when only rate of reinforcement was varied in the changing component. The right panels of Fig. 3 show that, when rate and duration of reinforcement varied in the changing component, responding in the constant component varied inversely with the change in rate of reinforcement in six of eight instances (although the change for subject 54 in the lower right panel was negligible). However, varying duration of reinforcement was not without effect. The right panels show that the percentage change in responding during the constant component was smaller for each subject when both rate and duration of reinforcement varied in the changing component than when only rate of reinforcement varied in the changing component. This effect was most notable for subject 173, whose responding varied inversely with the change of duration of reinforcement when rate of reinforcement was increased and duration of reinforcement was decreased (see lower right panel).
8. General discussion Fig. 3. The percentage change in the rate of responding during the constant component of the conditions in Experiment 2 in which rate of reinforcement (left panels) or rate and duration of reinforcement (right panels) varied in the changing component from the mean rate of responding during the constant component of the baselines. The upper graph represents the conditions in which rate of reinforcement in the changing component was decreased; the lower graph the conditions in which rate of reinforcement was increased. Each bar presents the results for an individual subject.
of reinforcement (right panels) was/were varied in the changing component from the mean rate of responding during the constant component of the baselines. The upper graph represents the results for the conditions in which rate of reinforcement was decreased in the changing component. The lower graph represents the results for the conditions in which rate of reinforcement was increased in the changing component. Each bar represents the results for an individual subject. Percentages
The present study supports previous research that suggested that behavioral contrast can be produced by changes in amount or duration of reinforcement (e.g. Gonzalez and Champlin, 1974; Ettinger et al., 1981; Hamilton and Silberberg, 1978; Kramer and Rilling, 1969). Experiment 1 showed that positive and negative contrast could be produced by decreasing and increasing, respectively, the reinforcer duration in the changing component. Response rates in the constant component of Experiment 1 changed in the direction predicted by contrast in 20 of 24 potential instances. Experiment 2 demonstrated that, although contrast could be produced by changes in duration of reinforcement, responding in the constant component was more sensitive to changes in rate of reinforcement in the changing component. When only one factor was varied in the changing component, behavioral contrast was observed in five
J.N. Weatherly et al. / Beha6ioural Processes 40 (1997) 61–73
of eight instances with changes in duration of reinforcement and in seven of eight instances with changes in rate of reinforcement. Furthermore, when both duration and rate of reinforcement were varied in the changing component, but in the opposite direction, responding in the constant component usually varied inversely with the change in rate of reinforcement. However, the change in responding in the constant component was always smaller in these conditions than in the conditions in which only rate of reinforcement varied in the changing component. These results indicate that subjects were sensitive to the change in duration of reinforcement, but that the change in reinforcer duration did not offset the change in reinforcement rate. Several theoretical accounts of behavioral contrast suggest that reinforcer ‘value’ governs responding during multiple schedules (e.g. McSweeney, 1987; Rachlin, 1973). The results of Experiment 2 suggest that reinforcer ‘value’ cannot be measured by total access to reinforcement. When both duration and rate of reinforcement varied in the changing component, total access to reinforcement was equal between the two components. However, responding in the constant component did not remain constant, rather it usually varied inversely with the change in rate of reinforcement. The failure of the change in duration of reinforcement to cancel out the change in rate of reinforcement may reflect the difference between duration and amount of reinforcement. That is, doubling the duration of reinforcement does not necessarily mean that the amount of reinforcement was doubled. On the other hand, these results may suggest that changes in rate of reinforcement are more influential in producing contrast than are changes in amount/duration of reinforcement. Finding that responding was more sensitive to changes in rate than to changes in duration of reinforcement and that total access to reinforcement may not determine reinforcer ‘value’, is consistent with several other areas of research. For example, Boldero et al. (1985) found that the responding of pigeons on a signal detection task was more biased by changes in reinforcer rate than by changes in reinforcer duration. Schneider
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(1973) reinforced pigeons for pecking two concurrently available response keys using a single VI schedule. Schneider found that smaller reinforcers delivered more frequently to one key sustained a higher rate of responding than did larger reinforcers delivered less frequently to the other key despite both keys providing the same total access to reinforcement. Finally, Weatherly et al. (1995) studied the within-session changes in responding for pigeons pecking a key on a multiple VI VI schedule. The value of the VI schedules were equal within a condition but were varied across conditions. Weatherly et al. found larger withinsession changes in responding when reinforcers were short in duration but delivered frequently than when reinforcers were long in duration but delivered infrequently. These results were observed even though total access to reinforcement was held constant across conditions. Although finding that changes in reinforcer duration can produce behavioral contrast does not condemn any single theory of contrast, the present results may pose problems for several theories. For instance, finding that responding in the constant component was more sensitive to changes in rate of reinforcement than to changes in duration of reinforcement is consistent with behavioral competition (i.e. the reallocation of behavior; Hinson and Staddon, 1978). However, this theory is left having to explain how contrast occurs without a systematic change in the rate of operant responding in the changing component. Additive theory (i.e. changes in stimulus-reinforcer relations; Rachlin, 1973) could potentially explain the differential effects of changes in duration and rate of reinforcement by postulating differences in stimulus intensity and stimulus frequency. However, this theory is also left having to explain why these processes did not also control behavior in the changing component. The present study should help to serve as a springboard for the study of other factors of reinforcement that may produce contrast and that have either been overlooked or have received little research attention. For example, changing the quality of one reinforcer has been shown to produce behavioral contrast (Ettinger et al., 1981). However, little research on contrast has been con-
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ducted when two different reinforcers are provided in the different components of a multiple schedule. As a result, it is not yet known whether changes in the rate and/or duration of reinforcement of a qualitatively different reinforcer will produce contrast effects similar to those observed when the same reinforcer is delivered in each component of a multiple schedule. Such empirical research increases in importance in the absence of an universally accepted theoretical account of behavioral contrast (Williams, 1983). As stated above, only by determining the conditions under which contrast will and will not occur does it become possible to assess and accept a comprehensive theoretical account of contrast. As other factors of reinforcement are experimentally tested, behavioral contrast is also bound to become a more interesting topic from a practical standpoint. If organisms only displayed behavioral contrast when the same amount of the same reinforcer was delivered at a different rate, then the implications of contrast would be limited. However, if changes in the amount, quality, and/or rate of reinforcement can produce behavioral contrast, then contrast effects are likely to pervade much of an organisms behavior. If the latter alternative is true, then behavioral contrast may have profound practical implications (e.g. Gross and Drabman, 1981), especially since it has been shown that humans and nonhumans can display contrast when responding under similar circumstances (Weatherly et al., 1996). Indeed, the ultimate importance of behavioral contrast may lie in its practical implications.
Acknowledgements The authors thank Frances K. McSweeney for her comments on an earlier version of this manuscript. Requests for reprints can be sent to Jeffrey N. Weatherly, Department of Psychology, McNeese State University, Lake Charles, LA 70609.
References Boldero, J., Davison, M. and McCarthy, D., 1985. Reinforcer frequency and reinforcer duration as biasers of signal detection. Behav. Process., 10: 131 – 143. Ettinger, R.H., McSweeney, F.K. and Norman, W.D., 1981. Contrast and undermatching as a function of reinforcer duration and quality during multiple schedules. J. Exp. Anal. Behav., 35: 271 – 282. Fleshler, M. and Hoffman, H.S., 1962. A progression for generating variable-interval schedules. J. Exp. Anal. Behav., 5: 529 – 530. Gonzalez, R.C. and Champlin, G., 1974. Positive behavioral contrast, negative simultaneous contrast, and their relation to frustration in pigeons. J. Comp. Physiol. Psychol., 87: 173 – 187. Gross, A.M. and Drabman, R.S., 1981. Behavioral contrast and behavior therapy. Behav. Therapy, 12: 231 – 246. Hamilton, B.E. and Silberberg, A., 1978. Contrast and autoshaping in multiple schedules varying reinforcer rate and duration. J. Exp. Anal. Behav., 30: 107 – 122. Hinson, J.M. and Staddon, J.E.R., 1978. Behavioral competition: A mechanism for schedule interactions. Science, 202: 432 – 434. Jensen, C. and Fallon, D., 1973. Behavioral aftereffects of reinforcement and its omission as a function of reinforcer magnitude. J. Exp. Anal. Behav., 19: 459 – 468. Kramer, T.J. and Rilling, M., 1969. Effects of lowering the magnitude of reinforcement on the response rate from a baseline during a successive discrimination. Psychon. Sci., 16: 249 – 250. Mackintosh, N.L., Little, L. and Lord, J., 1972. Some determinants of behavioral contrast in pigeons and rats. Learn. Motiv., 3: 148 – 161. McLean, A.P. and Morritt, C.F., 1994. Contrast and undermatching with regular or irregular alternation of components. J. Exp. Anal. Behav., 61: 407 – 416. McSweeney, F.K., 1978. Negative behavioral contrast on multiple treadle-press schedules. J. Exp. Anal. Behav., 29: 463 – 473. McSweeney, F.K., 1987. Suppression by reinforcement, a model for multiple-schedule behavioral contrast. Behav. Process., 15: 191 – 209. McSweeney, F.K., Dougan, J.D., Higa, J. and Farmer, V.A., 1986. Behavioral contrast as a function of component duration and baseline rate of reinforcement. Anim. Learn. Behav., 14: 173 – 183. McSweeney, F.K. and Melville, C.L., 1988. Positive contrast as a function of component duration using a within-session procedure. Behav. Process., 16: 21 – 41. Rachlin, H.C., 1973. Contrast and matching. Psychol. Rev., 11: 217 – 234. Reed, P., 1991. Multiple determinants of the effects of reinforcement magnitude on free-operant response rates. J. Exp. Anal. Behav., 55: 109 – 123. Reynolds, G.S., 1961. Behavioral contrast. J. Exp. Anal. Behav., 4: 57 – 71.
J.N. Weatherly et al. / Beha6ioural Processes 40 (1997) 61–73 Schneider, J.W., 1973. Reinforcer effectiveness as a function of reinforcer rate and magnitude: A comparison of concurrent performances. J. Exp. Anal. Behav., 20: 461–471. Shettleworth, S. and Nevin, J.A., 1965. Relative rate of response and relative magnitude of reinforcement in multiple schedules. J. Exp. Anal. Behav., 8: 199–202. Spealman, R.D. and Gollub, L.R., 1974. Behavioral interactions in multiple variable-interval schedules. J. Exp. Anal. Behav., 22: 471 –481.
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Weatherly, J.N., McSweeney, F.K. and Swindell, S., 1995. On the contributions of responding and reinforcement to within-session patterns of responding. Learn. Motiv., 26: 421 – 432. Weatherly, J.N., Melville, C.L. and McSweeney, F.K., 1996. Picking, pecking, and pressing: A cross-species demonstration of behavioral contrast. Psychol. Rec., 46: 351 – 372. Williams, B.A., 1983. Another look at contrast in multiple schedules. J. Exp. Anal. Behav., 39: 345 – 384.
Behavioral contrast with changes in duration and rate of reinforcement Jeffrey N. Weatherly a,b,*, Cam L. Melville a, Samantha Swindell b a
Department of Psychology, McNeese State Uni6ersity, Lake Charles, LA 70609, USA b Washington State Uni6ersity, Pullman, WA, USA
Received 21 August 1996; received in revised form 15 November 1996; accepted 19 November 1996
Abstract Behavioral contrast can be defined as an inverse relation between rate of responding in one component of a multiple schedule and conditions of reinforcement in the other components. The present study was an attempt to produce contrast with changes in duration of reinforcement. Pigeons pecked a key for mixed grain delivered by a multiple variable-interval variable-interval schedule. In Experiment 1, the reinforcer duration in the second component was decreased or increased, for different subjects, by a factor of two, four, and six from that delivered during baseline. These changes usually produced contrast. In Experiment 2, rate, duration, or rate and duration of reinforcement varied in the second component. Contrast was usually observed when only one variable was manipulated. When both variables were manipulated in the same condition, but in opposite directions, responding in the constant component usually changed inversely with the change in rate of reinforcement, not the change in reinforcer duration. The results demonstrate that changes in duration of reinforcement can produce contrast. They also increase the empirical base for which a successful theoretical account of contrast must encompass and the generality of contrast, which increases its potential practical implications. © 1997 Elsevier Science B.V. Keywords: Behavioral contrast; Duration of reinforcement; Pigeon
1. Introduction Behavioral contrast can be defined as an inverse relation between rate of responding in one component of a multiple schedule and conditions of * Corresponding author. Tel.: +1 318 4755435; fax: + 1 318 4755467.
reinforcement in the other component(s) (e.g. McSweeney, 1978). Positive contrast is an increase in the rate of responding during one component of a multiple schedule (constant component) that occurs when the conditions of reinforcement in another component (changing component) are made worse. Negative contrast is a decrease in responding that occurs during the constant component
0376-6357/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 3 7 6 - 6 3 5 7 ( 9 6 ) 0 0 7 7 2 - 3
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when the conditions of reinforcement in the changing component are enriched. Since its original description (Reynolds, 1961), behavioral contrast has been the focus of much empirical research and theoretical formulation (Williams, 1983; for a review). Despite such efforts, however, an universally accepted theoretical account of contrast does not exist. Before an account of contrast can be accepted, it is necessary to know exactly what factors do (or do not) produce contrast. However, to date, the vast majority of studies on contrast have focused on changes in the rate of reinforcement (Williams, 1983). Relatively few studies have focused on other factors (e.g. amount or quality of reinforcement). Several studies have attempted to produce behavioral contrast by altering the amount or duration of reinforcement in the changing component. Some of these attempts have been successful (e.g. Gonzalez and Champlin, 1974; Ettinger et al., 1981; Hamilton and Silberberg, 1978; Kramer and Rilling, 1969) whereas others have not (e.g. Jensen and Fallon, 1973; Mackintosh et al., 1972; Shettleworth and Nevin, 1965). In the studies that have observed contrast with changes in the amount or duration of reinforcement, the results are equivocal or limited. For example, Gonzalez and Champlin (1974) and Kramer and Rilling (1969) altered both the amount and rate of reinforcement, making conclusions about the contribution of amount of reinforcement impossible. Ettinger et al. (1981) demonstrated both positive and negative contrast with changes in duration of reinforcement. However, reinforcement duration was varied by only one value below or above that delivered during baseline. Determining whether behavioral contrast can be produced by changes in duration of reinforcement is also important for determining the generality of contrast. If contrast occurs only under limited conditions (e.g. changes in rate of reinforcement of an identical reinforcer), then the frequency of contrast effects may be low. However, if contrast occurs under many conditions (e.g. changes in rate or amount of reinforcement of an identical or a qualitatively different reinforcer), then contrast effects may be prevalent in
any organisms behavior. If the latter is true, then contrast may have extensive practical implications (e.g. Gross and Drabman, 1981). The initial purpose of the present study was to determine whether behavioral contrast could be produced by changing the duration of reinforcement per reinforcer delivered in the changing component. Experiment 1 determined this. The second purpose was to compare contrast produced by changes in duration of reinforcement to that by changes in rate of reinforcement, a factor known to produce contrast. In Experiment 2, the contrast produced by changes in the duration, rate, or duration and rate of reinforcement in the changing component were examined.
2. Experiment 1 Two groups of four pigeons pecked a key for mixed-grain reinforcers delivered on a multiple variable-interval (VI) VI schedule. For the positive contrast group, the duration of reinforcement per reinforcer (i.e. the number of seconds of access to mixed grain) delivered in the changing component was decreased by a factor of two, four, and six, in different conditions, from that delivered during baseline. For the negative contrast group, the reinforcer duration was increased by a factor of two, four, or six.
3. Method
3.1. Subjects Eight experimentally experienced pigeons, obtained from the Johnson Tower Vivarium at Washington State University, served as subjects. Subjects were maintained at approximately 85% of their free-feeding weight, by post-session feedings when necessary, throughout the course of the experiment. Each subject was housed individually, with water and grit freely available in the home cage. Subjects experienced a 12/12-h light/dark cycle.
J.N. Weatherly et al. / Beha6ioural Processes 40 (1997) 61–73
3.2. Apparatus The apparatus was a single-key experimental enclosure for pigeons, measuring 25.5 × 28 × 29 cm. The response key was a 2 cm diameter Plexiglas panel, located 7.5 cm below the ceiling and 3.5 cm from the left wall. It was operated by applying approximately 0.25 N of force to its center. A 5.5 ×4.5 cm opening could allow access to a solenoid-operated magazine containing mixed grain. This opening was centered, 10 cm from each wall and was 4.5 cm above the floor. The magazine was large enough so that it could not be emptied during a single reinforcer presentation at any duration used in the present study. A 3× 2.5 cm Plexiglas panel served as the houselight. It was located 10 cm above the opening to the grain hopper. The apparatus was enclosed in a sound attenuating chamber. A ventilating fan masked noise from outside the chamber. Experimental events were presented and data were recorded by an IBM-compatible 486 computer connected to a MED Associates interface running MedState software. It was located in an adjacent room.
3.3. Procedure As the subjects were experimentally experienced, they were placed directly on the experimental procedure. Four subjects received the positive contrast procedure. The other four subjects received the negative contrast procedure. Subjects responded on a multiple VI VI schedule. Components alternated every 30 s and were signalled by the key light being illuminated either red or white. For two subjects in each group, the key light was red in the first component. The key light was white in the first component for the other subjects. Sessions lasted 30 min and the houselight was illuminated white throughout the session. Reinforcers were scheduled by independent 25 interval series, constructed as suggested by Fleshler and Hoffman (1962), that were sampled without replacement. The key light was extinguished during reinforcement. Neither the session timer nor the inter-reinforcer interval advanced during reinforcement. Sessions were conducted daily, 6 to 7 days per week.
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The first condition served as baseline. Subjects in the positive contrast group responded on a multiple VI 120 s VI 120 s schedule. Reinforcers in each component were 12 s access to mixed grain. In the subsequent conditions, reinforcers during the second component were 2, 6, and 3 s long, conducted in that order. The final condition was a recovery of baseline. Reinforcers were again 12 s in length during each component. Each condition was conducted for a total of 20 sessions. Conducting 20 sessions has precedence in the study of contrast (e.g. McLean and Morritt, 1994). Furthermore, as the data help to illustrate, 20 sessions were sufficient to achieve stable responding. The negative contrast group responded on a multiple VI 30 s VI 30 s schedule. During baseline, reinforcers in each component were 3 s access to mixed grain. The baseline reinforcer duration differed from that experienced by the positive contrast group to accommodate the increases that would occur in the contrast conditions. The rate of reinforcement was changed so that the total access to reinforcement during the constant component would be equal between groups. In the subsequent conditions, reinforcers in the second component were 18, 6, and 12 s in length, conducted in that order. The final condition was a recovery of baseline in which reinforcers were again 3 s in both components. Each condition was conducted for a total of 20 sessions.
4. Results and discussion Table 1 presents the mean rates of responding for individual subjects and for the mean of all subjects in both groups during the first (constant) and second (changing) component of each condition. Response rates were calculated by dividing the total number of responses emitted during a component by the total number of minutes, excluding reinforcement time, that the component was available. The S.E.s of the means and the mean number of obtained reinforcers for individual subjects and for the mean of all subjects are also presented. Data in Table 1 were calculated
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Table 1 Rates of responding (responses per min; R) in each component of each condition in Experiment 1 for individual subjects and for the mean of all subjects serving in the positive and negative contrast groups Subject
30 R S.E.M OR 29 R S.E.M OR 103 R S.E.M OR 592 R S.E.M OR Mean R S.E.M OR
Positive contrast 12 s
12 s
12 s
6s
12 s
3s
12 s
2s
12 s
12 s
54.3 5.6 7.6 30.1 1.9 7.6 67.7 5.2 7.2 75.4 9.4 9.4 56.9 9.9 8.0
48.0 3.5 7.8 35.4 1.9 8.8 87.9 4.8 7.6 86.4 6.0 8.2 64.4 13.4 8.1
80.7 5.7 6.6 43.1 0.7 6.8 73.6 5.5 5.8 81.6 5.1 9.6 69.8 9.1 7.2
49.0 5.8 6.8 30.5 1.2 6.4 76.0 6.7 6.4 67.1 6.1 8.2 55.7 10.1 7.0
64.3 1.9 7.4 47.1 1.6 7.0 65.7 9.2 7.6 105.3 5.3 6.6 70.6 12.3 7.2
44.4 0.6 7.8 27.8 1.2 8.0 57.8 10.7 5.4 69.0 3.0 6.8 49.8 8.9 7.0
72.8 2.2 8.6 43.8 1.5 8.8 68.6 1.8 6.4 107.5 3.2 8.2 73.2 13.1 8.0
43.4 3.8 6.4 24.3 1.2 6.4 69.2 0.9 8.8 85.8 3.2 8.2 55.7 13.6 7.5
53.9 2.5 7.6 43.3 1.4 7.2 84.7 1.3 5.6 92.2 3.8 7.6 68.5 11.8 7.0
44.5 1.8 9.0 34.4 1.0 7.8 86.9 2.1 8.0 46.8 2.8 7.6 53.1 11.6 8.1
3s
12 s
3s
18 s
3s
3s
41.8 5.3 13.2 42.1 2.1 12.8 68.0 6.7 12.8 52.2 3.9 11.6 51.0 6.2 12.6
59.9 8.1 13.8 62.0 4.8 13.8 75.6 4.2 13.6 83.3 1.3 13.2 70.2 5.6 13.6
63.5 8.1 14.2 67.3 1.5 12.6 82.4 2.9 12.6 79.7 2.7 13.6 73.2 4.6 13.3
Negative contrast
1093 R S.E.M OR 1972 R S.E.M OR 1993 R S.E.M OR 1994 R S.E.M OR Mean R S.E.M OR
3s
3s
3s
6s
68.4 2.0 13.0 98.5 3.4 13.6 68.1 3.3 15.8 96.0 1.7 16.0 82.8 8.4 14.6
57.0 1.5 14.2 93.1 3.3 11.8 65.3 4.2 15.0 75.1 0.8 13.2 72.6 7.8 13.6
55.5 7.4 12.4 75.3 6.7 13.0 60.7 5.7 12.2 79.3 2.5 13.4 67.7 5.7 12.8
70.8 6.8 12.4 64.2 5.2 15.0 69.3 6.2 13.2 88.0 2.4 13.0 73.1 5.2 13.4
21.1 6.8 10.0 16.0 3.7 9.2 42.4 2.8 9.8 33.2 3.1 10.4 28.2 6.0 9.9
37.1 7.6 12.6 29.1 6.8 12.8 59.4 3.8 11.6 45.0 5.6 10.4 42.7 6.5 11.9
36.8 4.5 14.8 36.5 5.0 13.0 58.1 3.5 13.2 49.5 4.1 10.8 45.2 5.3 13.0
The standard errors of the means (S.E.M) and the mean number of obtained reinforcers per component (OR) are also presented. The SEM for the mean of all subjects represents the standard error between subjects. Data represent responding during the final five sessions that each condition was conducted.
using the final five sessions for which each condition was conducted. Data from both baselines are presented because baseline responding was not always recovered. The failure of response rates to recover to baseline levels is not uncommon in the study of behavioral contrast (e.g. McSweeney et al., 1986; Spealman and Gollub, 1974) and has been noted as a potential pitfall of using a be-
tween-session design to study contrast (McSweeney and Melville, 1988). The present failure to recover baseline rates of responding was not due to responding being in transition. The small S.E.s in Table 1 demonstrate that responding was relatively stable. Furthermore, visual inspection of data over the final five sessions of each condition indicated that linear trends in responding were
J.N. Weatherly et al. / Beha6ioural Processes 40 (1997) 61–73
not present, further indicating that response rates were not in a state of transition. Table 1 shows that the rate of responding in the changing component sometimes, but not always, varied directly with changes in reinforcer duration in that component. In the positive contrast group, responding for three of the four subjects generally decreased with decreases in reinforcer duration. The exception was subject 592, whose responding increased with decreases in reinforcer duration. In the negative contrast group, responding generally increased when the reinforcer duration was increased to 6 s, but decreased at longer reinforcer durations. Finding that rate of responding in the changing component did not always vary directly with changes in reinforcer duration in that component is not unprecedented. Both increases and decreases in the rate of responding with changes in the magnitude of reinforcement have previously been reported (e.g. Reed, 1991). Table 1 also shows that responding in the constant component usually varied inversely with changes in reinforcer duration in the changing component. These contrast effects are presented in Fig. 1. It represents the percentage change in the rate of responding during the constant component of each contrast condition from the mean rate of responding during the constant component of the baselines. The upper graph represents the results for the positive contrast group; the lower graph, the results for the negative contrast group. Each bar represents the results for an individual subject. Percentages were calculated by dividing the rate of responding during the constant component in a contrast condition by the mean rate of responding during the constant component in the baselines, subtracting one from the quotient and then multiplying by 100%. Data for the calculations were taken from Table 1. As can be seen in Fig. 1, subjects in the positive contrast group usually, but not always, displayed positive contrast with decreases in reinforcer duration in the changing component. The most notable exception was subject 103, who never displayed contrast. In all, positive contrast was observed in eight of the 12 potential instances. Of the three subjects that displayed positive contrast, each different subject displayed its largest contrast
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effect at a different reinforcer duration. Fig. 1 also shows that negative contrast was always observed for subjects in the negative contrast group. Each subject displayed the largest contrast effect when the reinforcer duration in the changing component was 12 s. Although both groups of subjects displayed contrast with changes in duration of reinforcement, it is possible that these contrast effects were not directly produced by changes in reinforcer
Fig. 1. The percentage change in the rate of responding during the constant component of each contrast condition of Experiment 1 from the mean rate of responding during the constant component of the baselines. The upper graph represents the results for the positive contrast group; the lower graph the results for the negative contrast group. Each bar represents the results for an individual subject.
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duration, but rather were produced by changes in rate of reinforcement. That is, changing the duration of reinforcement may have led to changes in the obtained rate of reinforcement which, in turn, produced the contrast. To determine whether this was the case, the number of obtained reinforcers in each component were analyzed for both groups. One-way repeated measures analyses of variance (ANOVAs) conducted on the obtained number of reinforcers for individual subjects in the positive contrast group (taken from Table 1) showed that the obtained number of reinforcers did not differ across conditions for either component (F(4,12)= 1.06, constant component; F(4,12)=1.25, changing component). Similar analyses for the negative contrast group indicated that the obtained number of reinforcers did not differ across conditions for the changing component (F(4,12) = 1.71), but did differ significantly across conditions for the constant component (F(4,12)=9.13, PB.001). This difference occurred because subjects received fewer reinforcers in the constant component of the 12 s reinforcer duration condition. A follow-up ANOVA that excluded the data from the 12 s reinforcer condition indicated that the obtained number of reinforcers in the constant component did not differ across the other conditions (F(3, 9)= 1.57). Therefore, the contrast observed in the 12 s reinforcer duration condition for the negative contrast group was confounded with a decrease in the rate of reinforcement during the constant component. This difference may be, at least partially, responsible for why each subject displayed its largest contrast effect during the 12 s reinforcer duration condition. However, none of the contrast effects, for either group, can be explained by changes in rate of reinforcement during the changing component. Experiment 1 demonstrates that positive and negative behavioral contrast can be produced by changes in duration of reinforcement. In all, contrast was observed in 20 of 24 potential instances. Experiment 2 was designed to compare the contrast effects produced by changes in duration of reinforcement to those produced by changes in rate of reinforcement. Furthermore, both factors were varied simultaneously in some conditions to
determine how changes in duration and rate of reinforcement would interact.
5. Experiment 2 Four pigeons pecked a key for mixed-grain reinforcers delivered on a multiple VI 60 s VI 60 s schedule during baseline. In contrast conditions, rate of reinforcement or reinforcer duration in the changing component was increased or decreased. In other conditions, both rate of reinforcement and reinforcer duration in the changing component were varied during the same condition, but in opposite directions.
6. Method
6.1. Subjects Four experimentally experienced pigeons served as subjects. The subjects were not those used in Experiment 1. However, they were obtained, housed, and maintained in the same manner, as those in Experiment 1.
6.2. Apparatus The apparatus was a 2-key, 2-treadle experimental enclosure for pigeons, measuring 27× 30×29.5 cm. The two keys were 2.5 cm diameter Plexiglas panels located 4 cm below the ceiling and 12.5 cm from each other. The left key was located 6 cm from the left wall and could be illuminated by red light. The right key was 6.5 cm from the right wall and could be illuminated by blue light. Each key was operated when a force of approximately 0.25 N was applied to its center. A treadle was located 15.5 cm below each key. The treadles were not used in this experiment and will not be described. A 5 × 4 cm opening could allow access to a solenoid-operated magazine which contained mixed grain. The magazine was again large enough to provide grain throughout the largest reinforcer duration. The opening was 12.5 cm from the right wall and 3 cm above the floor. A 1 cm diameter houselight, located 0.5 cm from
J.N. Weatherly et al. / Beha6ioural Processes 40 (1997) 61–73
the left wall and 0.5 cm from the ceiling, illuminated the chamber. The enclosure was housed, the experimental events were programmed, and the data were recorded as in Experiment 1.
Table 2 The rate of reinforcement and the reinforcer duration during the first (constant) and second (changing) component for each condition in Experiment 2 Condition
6.3. Procedure As the subjects were experimentally experienced, they were placed directly on the experimental procedure. The first condition served as baseline. Subjects responded on a multiple VI 60 s VI 60 s schedule. Components alternated every 30 s. Two subjects responded on the left key (red) during the first component and on the right key (blue) during the second component. Components were reversed for the other 2 subjects. During baseline, reinforcers in both components were 6 s access to mixed grain. Reinforcers were scheduled by independent 25 interval series, constructed as suggested by Fleshler and Hoffman (1962), that were sampled without replacement. Sessions lasted 30 min, not including reinforcement time. The houselight was illuminated throughout the session. Conditions were conducted for a total of 20 sessions, with sessions being conducted daily, 6 to 7 days per week. In subsequent conditions, the rate of reinforcement, the reinforcer duration, or both the rate of reinforcement and the reinforcer duration varied in the second component. When both rate and duration of reinforcement varied in the same condition, they varied in the opposite direction. As a result, the constant and changing components both scheduled the same total access to reinforcement. Therefore, these conditions cannot accurately be called contrast conditions. These conditions were conducted so that the ability of these factors to affect responding in the constant component could be assessed. The final condition was baseline recovery. Table 2 presents the conditions in the order in which they were conducted. All other aspects of the procedure remained as in baseline.
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1 2 3 4 5 6 7 8
First component
Second component
Rate
SR duration
Rate
SR duration
6 6 6 6 6 6 6 6
VI VI VI VI VI VI VI VI
6 12 3 3 6 12 6 6
VI VI VI VI VI VI VI VI
60 60 60 60 60 60 60 60
s s s s s s s s
s s s s s s s s
60 s 60 s 30 s 60 s 30 s 120 s 120 s 60 s
s s s s s s s s
Conditions are presented in the order in which they were conducted.
7. Results and discussion Table 3 presents the mean rates of responding for individual subjects and for the mean of all subjects during the constant and changing component of each condition. The S.E.s of the means and the mean number of obtained reinforcers per component for individual subjects and for the mean of all subjects are also presented. Data in Table 3 were calculated as in Table 1. Data from both baselines are presented because, as in Experiment 1, baseline responding was not always recovered. Also as in Experiment 1, the small S.E.s as well as a visual inspection of the data indicated that this failure did not occur because responding was in a state of transition. Table 3 shows that the rate of responding in the changing component for individual subjects did not vary systematically when either rate or duration of reinforcement varied in that component. Compared with responding in both baselines, responding in the changing component of these conditions varied directly with the change in the conditions of reinforcement in seven of 16 potential instances. Conditions in which rate and duration of reinforcement were both varied, but in opposite directions, both components scheduled the same total access to reinforcement per session. Subjects’ rates of responding during the changing
J.N. Weatherly et al. / Beha6ioural Processes 40 (1997) 61–73
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Table 3 The rates of responding (responses per min; R) in the first (constant) and second (changing) component of each condition in Experiment 2 for individual subjects and for the mean of all subjects Subject
54 R S.E.M. OR 161 R S.E.M. OR 162 R S.E.M. OR 173 R S.E.M. OR Mean R S.E.M. OR
54 R S.E.M. OR 161 R S.E.M. OR 162 R S.E.M. OR 173 R S.E.M. OR Mean R S.E.M. OR
Condition (SR rate and SR duration) VI60 6s
VI60 6s
VI60 6s
VI60 12 s
VI60 6s
VI30 3s
VI60 6s
VI60 3s
85.8 1.9 14.0 70.3 2.3 14.6 49.2 3.2 14.4 125.7 4.0 15.2 82.8 16.2 14.6
73.1 2.0 13.8 75.3 1.8 13.2 53.1 2.2 15.0 97.6 6.0 14.4 74.8 9.1 14.1
83.6 1.2 14.4 58.6 1.6 15.8 56.5 4.4 16.6 62.1 7.8 13.0 65.2 6.2 15.0
92.1 2.1 12.8 104.2 4.1 16.0 45.0 1.4 14.4 66.7 3.0 13.4 77.0 13.2 14.2
91.3 3.0 13.4 55.2 1.7 14.0 34.4 2.8 14.8 114.2 2.8 14.2 73.8 17.9 14.1
94.7 5.0 28.2 101.3 5.0 30.0 43.1 2.5 28.6 89.2 3.9 28.0 82.1 13.2 28.7
118.2 5.2 13.8 64.3 4.3 14.6 55.1 9.3 14.4 82.7 3.3 14.8 80.1 13.9 14.4
72.7 3.9 14.8 59.5 2.6 13.8 46.9 4.8 16.8 62.1 3.5 12.2 60.3 5.3 14.4
VI60 6s
VI30 6s
VI60 6s
VI120 12 s
VI60 6s
VI120 6s
VI60 6s
VI60 6s
88.7 3.2 16.4 44.9 3.0 14.2 22.1 1.1 12.2 69.8 5.8 14.4 56.4 14.5 14.3
103.9 2.3 30.8 67.1 7.6 28.0 63.9 1.7 29.4 79.9 6.9 30.2 78.7 9.1 29.6
105.4 1.9 13.2 82.2 3.4 15.0 97.4 2.7 14.0 88.1 8.7 13.6 93.3 5.1 14.0
88.2 2.2 7.0 79.2 4.4 6.6 60.5 3.8 7.4 80.7 3.1 6.8 77.2 5.9 6.8
109.5 2.4 13.6 91.5 3.8 15.0 108.8 2.5 13.8 63.1 2.2 13.8 93.2 10.9 14.1
84.1 2.7 6.4 77.9 3.7 7.2 38.7 2.6 7.2 85.2 2.4 8.0 71.5 11.0 7.2
97.7 1.4 15.6 77.1 2.5 14.6 98.3 7.9 15.4 68.8 2.4 14.0 85.5 7.4 14.9
92.0 3.5 13.8 89.3 3.9 13.6 30.3 4.2 12.6 78.4 3.8 13.6 72.5 14.4 13.4
The standard errors of the means (S.E.M) and the mean number of obtained reinforcers per component (OR) are also given. The S.E.Ms for the mean of all subjects represent the standard error between subjects. Conditions are presented in the order that they were conducted (see Table 2). Data are taken from the final five sessions that each condition was conducted.
component of these conditions fell between those observed during the first and the second baseline in six of eight instances. Fig. 2 shows that behavioral contrast was usually observed when either duration or rate of reinforcement was decreased or increased in the changing component. It represents the percentage
change in the rate of responding during the constant component when either duration or rate of reinforcement was varied in the changing component from the mean rate of responding during the constant component of the baselines. The upper graph presents results from conditions in which either duration (left panel) or rate of reinforce-
J.N. Weatherly et al. / Beha6ioural Processes 40 (1997) 61–73
ment (right panel) was decreased in the changing component. The lower graph presents results from conditions in which either duration (left panel) or rate of reinforcement (right panel) was increased in the changing component. Each bar represents the results for an individual subject. Percentages were calculated and are presented as in Fig. 1 using the data presented in Table 3.
Fig. 2. The percentage change in the rate of responding during the constant component of the conditions in Experiment 2 in which either duration or rate of reinforcement varied in the changing component from the mean rate of responding during the constant component of the baselines. The upper graph represents the results for the positive contrast conditions; the lower graph the results for the negative contrast conditions. Each bar represents the results for an individual subject.
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The upper graph of Fig. 2 shows that positive contrast was observed with decreases in rate of reinforcement, but was not usually observed with decreases in duration of reinforcement. Three of four subjects displayed positive contrast when rate of reinforcement in the changing component was decreased to a VI 120 s schedule. Only one of four subjects displayed positive contrast when duration of reinforcement in the changing component was decreased to 3 s. The lower graph shows that negative contrast was always observed. All four subjects displayed negative contrast when the duration of reinforcement in the changing component was increased to 12 s and when the rate of reinforcement in the changing component was increased to a VI 30 s schedule. As in Experiment 1, it was necessary to ensure that the contrast produced by changes in duration of reinforcement was not actually produced by changes in rate of reinforcement. To do so, an one-way repeated measures ANOVA was conducted on the obtained number of reinforcers in the changing component for individual subjects (taken from Table 3) for the baseline conditions and the conditions in which only reinforcer duration was varied. Results showed that the obtained number of reinforcers did not differ across the changing component of these conditions (F(3,9)= 0.41). Furthermore, an one-way repeated measures ANOVA, conducted on the obtained number of reinforcers for individual subjects in the constant component of each condition, showed that obtained reinforcers did not vary across conditions (F(7,21)= 0.55). Therefore, the contrast observed with changes in duration of reinforcement were not by-products of changes in rate of reinforcement. Furthermore, the contrast effects were not confounded with changes in rate of reinforcement during the constant component. Fig. 3 demonstrates that, when both rate and duration of reinforcement varied in the changing component, responding in the constant component was more sensitive to changes in rate of reinforcement. It represents the percentage change in the rate of responding during the constant component in the conditions in which either rate of reinforcement (left panels) or rate and duration
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were calculated and are presented as in Fig. 1 using the data presented in Table 3. Fig. 3 again shows that contrast was observed in seven of eight instances when only rate of reinforcement was varied in the changing component. The right panels of Fig. 3 show that, when rate and duration of reinforcement varied in the changing component, responding in the constant component varied inversely with the change in rate of reinforcement in six of eight instances (although the change for subject 54 in the lower right panel was negligible). However, varying duration of reinforcement was not without effect. The right panels show that the percentage change in responding during the constant component was smaller for each subject when both rate and duration of reinforcement varied in the changing component than when only rate of reinforcement varied in the changing component. This effect was most notable for subject 173, whose responding varied inversely with the change of duration of reinforcement when rate of reinforcement was increased and duration of reinforcement was decreased (see lower right panel).
8. General discussion Fig. 3. The percentage change in the rate of responding during the constant component of the conditions in Experiment 2 in which rate of reinforcement (left panels) or rate and duration of reinforcement (right panels) varied in the changing component from the mean rate of responding during the constant component of the baselines. The upper graph represents the conditions in which rate of reinforcement in the changing component was decreased; the lower graph the conditions in which rate of reinforcement was increased. Each bar presents the results for an individual subject.
of reinforcement (right panels) was/were varied in the changing component from the mean rate of responding during the constant component of the baselines. The upper graph represents the results for the conditions in which rate of reinforcement was decreased in the changing component. The lower graph represents the results for the conditions in which rate of reinforcement was increased in the changing component. Each bar represents the results for an individual subject. Percentages
The present study supports previous research that suggested that behavioral contrast can be produced by changes in amount or duration of reinforcement (e.g. Gonzalez and Champlin, 1974; Ettinger et al., 1981; Hamilton and Silberberg, 1978; Kramer and Rilling, 1969). Experiment 1 showed that positive and negative contrast could be produced by decreasing and increasing, respectively, the reinforcer duration in the changing component. Response rates in the constant component of Experiment 1 changed in the direction predicted by contrast in 20 of 24 potential instances. Experiment 2 demonstrated that, although contrast could be produced by changes in duration of reinforcement, responding in the constant component was more sensitive to changes in rate of reinforcement in the changing component. When only one factor was varied in the changing component, behavioral contrast was observed in five
J.N. Weatherly et al. / Beha6ioural Processes 40 (1997) 61–73
of eight instances with changes in duration of reinforcement and in seven of eight instances with changes in rate of reinforcement. Furthermore, when both duration and rate of reinforcement were varied in the changing component, but in the opposite direction, responding in the constant component usually varied inversely with the change in rate of reinforcement. However, the change in responding in the constant component was always smaller in these conditions than in the conditions in which only rate of reinforcement varied in the changing component. These results indicate that subjects were sensitive to the change in duration of reinforcement, but that the change in reinforcer duration did not offset the change in reinforcement rate. Several theoretical accounts of behavioral contrast suggest that reinforcer ‘value’ governs responding during multiple schedules (e.g. McSweeney, 1987; Rachlin, 1973). The results of Experiment 2 suggest that reinforcer ‘value’ cannot be measured by total access to reinforcement. When both duration and rate of reinforcement varied in the changing component, total access to reinforcement was equal between the two components. However, responding in the constant component did not remain constant, rather it usually varied inversely with the change in rate of reinforcement. The failure of the change in duration of reinforcement to cancel out the change in rate of reinforcement may reflect the difference between duration and amount of reinforcement. That is, doubling the duration of reinforcement does not necessarily mean that the amount of reinforcement was doubled. On the other hand, these results may suggest that changes in rate of reinforcement are more influential in producing contrast than are changes in amount/duration of reinforcement. Finding that responding was more sensitive to changes in rate than to changes in duration of reinforcement and that total access to reinforcement may not determine reinforcer ‘value’, is consistent with several other areas of research. For example, Boldero et al. (1985) found that the responding of pigeons on a signal detection task was more biased by changes in reinforcer rate than by changes in reinforcer duration. Schneider
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(1973) reinforced pigeons for pecking two concurrently available response keys using a single VI schedule. Schneider found that smaller reinforcers delivered more frequently to one key sustained a higher rate of responding than did larger reinforcers delivered less frequently to the other key despite both keys providing the same total access to reinforcement. Finally, Weatherly et al. (1995) studied the within-session changes in responding for pigeons pecking a key on a multiple VI VI schedule. The value of the VI schedules were equal within a condition but were varied across conditions. Weatherly et al. found larger withinsession changes in responding when reinforcers were short in duration but delivered frequently than when reinforcers were long in duration but delivered infrequently. These results were observed even though total access to reinforcement was held constant across conditions. Although finding that changes in reinforcer duration can produce behavioral contrast does not condemn any single theory of contrast, the present results may pose problems for several theories. For instance, finding that responding in the constant component was more sensitive to changes in rate of reinforcement than to changes in duration of reinforcement is consistent with behavioral competition (i.e. the reallocation of behavior; Hinson and Staddon, 1978). However, this theory is left having to explain how contrast occurs without a systematic change in the rate of operant responding in the changing component. Additive theory (i.e. changes in stimulus-reinforcer relations; Rachlin, 1973) could potentially explain the differential effects of changes in duration and rate of reinforcement by postulating differences in stimulus intensity and stimulus frequency. However, this theory is also left having to explain why these processes did not also control behavior in the changing component. The present study should help to serve as a springboard for the study of other factors of reinforcement that may produce contrast and that have either been overlooked or have received little research attention. For example, changing the quality of one reinforcer has been shown to produce behavioral contrast (Ettinger et al., 1981). However, little research on contrast has been con-
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ducted when two different reinforcers are provided in the different components of a multiple schedule. As a result, it is not yet known whether changes in the rate and/or duration of reinforcement of a qualitatively different reinforcer will produce contrast effects similar to those observed when the same reinforcer is delivered in each component of a multiple schedule. Such empirical research increases in importance in the absence of an universally accepted theoretical account of behavioral contrast (Williams, 1983). As stated above, only by determining the conditions under which contrast will and will not occur does it become possible to assess and accept a comprehensive theoretical account of contrast. As other factors of reinforcement are experimentally tested, behavioral contrast is also bound to become a more interesting topic from a practical standpoint. If organisms only displayed behavioral contrast when the same amount of the same reinforcer was delivered at a different rate, then the implications of contrast would be limited. However, if changes in the amount, quality, and/or rate of reinforcement can produce behavioral contrast, then contrast effects are likely to pervade much of an organisms behavior. If the latter alternative is true, then behavioral contrast may have profound practical implications (e.g. Gross and Drabman, 1981), especially since it has been shown that humans and nonhumans can display contrast when responding under similar circumstances (Weatherly et al., 1996). Indeed, the ultimate importance of behavioral contrast may lie in its practical implications.
Acknowledgements The authors thank Frances K. McSweeney for her comments on an earlier version of this manuscript. Requests for reprints can be sent to Jeffrey N. Weatherly, Department of Psychology, McNeese State University, Lake Charles, LA 70609.
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