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Dishabituation produces interactions during multiple schedules
Learning and Motivation 35 (2004) 419–434 www.elsevier.com/locate/l&m
Dishabituation produces interactions during multiple schedules Frances K. McSweeney,¤ Benjamin P. Kowal, Eric S. Murphy, and Duane M. Isava Washington State University, Pullman, WA 99164-4820, USA Received 22 January 2004; received in revised form 18 May 2004 Available online 29 July 2004
Abstract McSweeney and Weatherly (1998) argued that diVerential habituation to the reinforcer contributes to the behavioral interactions observed during multiple schedules. The present experiment conWrmed that introducing dishabituators into one component of a multiple schedule increases response rate in the other, constant, component. During baseline, pigeons and rats responded on multiple variable interval 30-s variable interval 30-s schedules. During experimental conditions, subjects responded on the same schedule except that a dishabituating stimulus (manipulation of a light) was also presented randomly during one of the components. Constant-component responding was faster during the experimental than during the baseline conditions. This diVerence in responding grew larger across the session. The within-session pattern of responding was similar for the two components of each multiple schedule. Qualitatively similar results were observed for rats and pigeons. These results suggest that behavioral interactions sometimes arise from a change in reinforcer eVectiveness between the baseline and experimental phases of the experiment, rather than from an assessment of reinforcer relativity (a comparison of reinforcers delivered during the two components in the experimental phase). Behavioral contrast and induction are sometimes produced by similar factors. 2004 Elsevier Inc. All rights reserved
An “interaction” occurs when changing the conditions of reinforcement in one component of a multiple schedule changes behavior in the other component (Reynolds, 1961). Two types of interactions are often distinguished (Reynolds, 1961). ¤ Corresponding author. Fax: 1-509-335-5043. E-mail address: [email protected] (F.K. McSweeney).
0023-9690/$ - see front matter 2004 Elsevier Inc. All rights reserved doi:10.1016/j.lmot.2004.06.001
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Behavioral contrast occurs when response rates change in the opposite directions in the two components. Induction occurs when response rates change in the same direction. Interactions are usually labeled positive if response rate increases in the unchanged component and negative if response rate decreases. Behavioral interactions are usually studied by comparing the rate of responding during a baseline schedule that provides the same conditions of reinforcement in both components (e.g., a multiple variable interval (VI) £ VI schedule) to responding during an experimental schedule that provides diVerent conditions in the two components (e.g., a multiple VI £ VI y schedule; McSweeney & Norman, 1979). For example, suppose that one component of a baseline, multiple variable interval 60-s (VI 60-s) VI 60-s, schedule is changed to extinction. If rate of responding during the constant, VI 60-s, component increases and rate of responding during the changed component decreases, the results would be labeled “positive contrast.” Suppose, instead, that one component of the baseline, multiple VI 60-s VI 60-s, schedule is changed to a VI 15-s schedule. If responding during the constant, VI 60-s, component decreases and responding during the changed component increases, the results would be labeled “negative contrast.” For convenience, the component that is changed to produce the interaction will be called the “variable” component. The component that is held constant will be the “constant” component. Few studies have examined induction, but behavioral contrast has been studied extensively. In spite of this interest, no one theory of contrast has been generally accepted. Most theories attribute contrast primarily to diVerences in the conditions of reinforcement provided during the two components of the schedule in which contrast is measured (the experimental schedule, e.g., Gamzu & Schwartz, 1973; Hearst & Jenkins, 1974; Hinson & Staddon, 1978; McLean, 1992; McSweeney, 1987; Rachlin, 1973). More recently, McSweeney and Weatherly (1998) argued that multiple-schedule contrast is produced partially by diVerences in habituation to the reinforcer between the baseline and experimental schedules (e.g., McSweeney, Hinson, & Cannon, 1996). According to this idea, reducing the rate of reinforcement in one component of a multiple schedule reduces the amount of habituation that occurs to the reinforcer during the experimental session (e.g., McSweeney, 1992). If the same type of reinforcer is provided in the other, constant, component, then that reinforcer should be more eVective (less habituation) and support a higher rate of responding (positive contrast). Providing more reinforcers in one component increases habituation to the reinforcer, reducing the eVectiveness of constant-component reinforcers. Less eVective reinforcers support a lower rate of responding (negative contrast). The earlier examples illustrate this idea. A multiple VI 60-s VI 60-s, baseline, schedule delivers a maximum of 60 reinforcers per hour on the average. A multiple VI 60-s extinction, experimental, schedule delivers approximately 30 reinforcers per hour. If each delivery of the reinforcer produces some habituation, then less habituation to the reinforcer occurs during the experimental (30 reinforcers) than during the baseline (60 reinforcers) sessions. As a result, reinforcers delivered in the constant, VI 60-s, component support a higher response rate during the experimental than during the baseline phase (positive contrast). A multiple VI 60-s VI 15-s, experimental, schedule delivers a maximum of 150 reinforcers per hour on the average. As a result,
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reinforcers delivered in the constant, VI 60-s, component of this experimental schedule support a lower response rate (more habituation) than they did during baseline (negative contrast). The theory proposed by McSweeney and Weatherly (1998) is parsimonious, relies only on processes that have been demonstrated by independent evidence, and is consistent with much empirical research. (See McSweeney & Weatherly, 1998, for a review.) The theory also makes predictions that diVer from those of other theories. The present experiment tested a fundamental prediction of McSweeney and Weatherly's theory. According to McSweeney and Weatherly (1998), introducing a neutral stimulus into one component of a multiple schedule might produce a behavioral interaction. Introducing a neutral stimulus should produce dishabituation, deWned as a reduction of habituation by the introduction of a strong, diVerent, or extra stimulus (e.g., Groves & Thompson, 1970). Dishabituation is a fundamental characteristic of behavior undergoing habituation (e.g., Thompson & Spencer, 1966). If introducing a neutral stimulus reduces habituation to the reinforcer, then the reinforcer should be more eVective in supporting responding and response rate should rise in the other, constant, component (a behavioral interaction). Some evidence suggests that this is true. Adding a stimulus that signals the availability of reinforcers to one component of a multiple schedule increases response rate in the other, unchanged, component (e.g., Wilkie, 1973, 1977). However, an alternative explanation for these results must be dismissed before they can be attributed to dishabituation. Adding signals for reinforcement may convert the time when the signal is not present to a signal for the absence of reinforcement (S-). Introducing an S- is known to produce contrast. Brownstein and Newsom (1970) and Hughes (1971) provided evidence that questions creation of an S- as an explanation for Wilkie's results. Brownstein and Newsom reported that positive contrast occurred in one component of a multiple Wxed interval (FI) FI schedule when a signal for the availability of reinforcement was added to the other component. An S- is already present during FI schedules because the time that immediately follows the delivery of a reinforcer cannot contain another reinforcer. Therefore, introducing a new stimulus might not substantially alter the existing S-. Nevertheless, Brownstein and Newsom found that the addition of the stimulus produced positive contrast. Hughes (1971) maintained responding on a multiple VI VI schedule. Then, he added a signal for the availability of reinforcers to the variable component and reported that rates of responding increased in the constant component. Finally, Hughes introduced an equal number of stimuli that were presented randomly with respect to reinforcement to the variable component. Response rates increased still further in the constant component. That is, the addition of a neutral stimulus increased response rate in the constant component even when the stimulus was presented randomly with respect to reinforcement. A stimulus that is presented randomly does not create an S-. The present experiment provided further evidence that introducing a dishabituator can produce a behavioral interaction. During baseline, pigeons and rats responded on a multiple VI 30-s VI 30-s schedule. This schedule was used because
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past research showed that it produces robust habituation to the reinforcer (e.g., McSweeney, 1992). During the experimental phase, subjects responded on the same multiple VI 30-s VI 30-s schedule, but a dishabituating event (manipulation of a light) was introduced randomly with respect to reinforcement into one component. Because the dishabituating stimulus was introduced randomly, increases in response rates cannot be attributed to the creation of an S- during the time when the stimulus was absent. The present experiment tested several additional predictions of McSweeney and Weatherly's (1998) theory. First, within-session changes in responding should be similar for the two components of the multiple schedules. Components were presented randomly in this experiment to provide a relatively pure test of McSweeney and Weatherly's (1998) theory. McSweeney and Weatherly argued that habituation is only one of several factors that contribute to contrast. Other studies suggest that factors related to the conditions of reinforcement in the following component also contribute (e.g., Williams, 1991). The present experiment reduced the contribution of the following conditions of reinforcement by making those conditions less predictable through random component presentation. When components are presented randomly, the amount of habituation that has occurred to the reinforcer should be similar for the two components at any time in the session. Similar habituation should yield similar within-session patterns of responding on the assumption that these patterns are produced primarily by habituation to the reinforcer (McSweeney et al., 1996). Second, the within-session response pattern during the constant component should be Xatter during the experimental than during the baseline condition. Greater habituation should produce steeper late-session decreases (McSweeney et al., 1996). If the increase in constant-component response rate results partly from a reduction in habituation between the baseline and experimental schedules, then the late-session decrease in constant-component responding should be steeper (more habituation) during the baseline than during the experimental schedule. As a result of these diVerences in steepness, the increase in constant-component responding from the baseline to the experimental schedule should become larger as the session progresses. Finally, similar results should be found for rats and pigeons. Some theories argue that contrast is not always observed when rats serve as subjects under conditions that produce it for pigeons (e.g., Gamzu & Schwartz, 1973; Hearst & Jenkins, 1974; Rachlin, 1973). However, habituation occurs for most, if not all, species (e.g., Thorpe, 1966). Therefore, if changes in habituation contribute to behavioral interactions, those interactions should be observed in both rats and pigeons.
Method Subjects Nine experimentally experienced pigeons and 10 naïve male rats served as subjects. The rats were bred from Sprague–Dawley stock in the Washington State University Psychology Department Vivarium. They were approximately 120 days old at
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the start of the experiment. Subjects were housed individually and experienced a 12 h light/12 h dark cycle. They had free access to water but were maintained at approximately 85% of their free-feeding weights by post-session feedings given when all subjects had completed the experiment for the day. Apparatus A three-key operant chamber, measuring 30.0 £ 35.5 £ 27.0 cm, was used for pigeons. Response keys were 2.5 cm diameter Plexiglas panels, located 7.0 cm apart and 3.0 cm from the ceiling. The left and right keys were situated 6.5 cm from each side wall. A force of approximately 0.25 N was required to operate each key. A 5.0 £ 4.0 cm opening, located directly below the center key and 8.0 cm above the Xoor, allowed access to the food hopper. A light behind a 4.0 cm diameter panel, 1.0 cm below the ceiling, and 5.0 cm from the right side, served as a houselight. Two treadles were located on the Xoor, directly below the keys. These treadles will not be described because they were not used in this experiment. A two-lever experimental chamber, measuring 20.0 £ 24.5 £ 24.5 cm, was used for rats. A 5.0 £ 5.5 cm opening, located in the center of the instrument panel and 0.5 cm above the Xoor, allowed access to a 0.5 ml dipper. Two retractable 4.0 £ 1.5-cm levers were located 2.5 cm from either side of this opening. Each lever was 5.0 cm above the Xoor and extended 1.5 cm into the enclosure. A 2.0-cm in diameter light was located 2.5 cm above each lever and a third 2.0-cm light was centered, 4.0 cm below the ceiling. A fourth 2.0-cm light that served as a houselight was located at the center of the ceiling. Each apparatus was enclosed in a sound-attenuating chamber. A ventilating fan masked noises from outside the chamber. An IBM compatible computer, running MED Associates software, controlled the experimental events and recorded the data. The computer and printer were located in a diVerent room from the experimental enclosures, and the enclosure for pigeons was located in a diVerent room than the enclosure for rats. Procedure Because the pigeons were experimentally experienced, they were placed directly on the experimental procedure. That is, pigeons began responding on a baseline, multiple VI 30-s VI 30-s, schedule in which pecking the center key produced reinforcers (5s of access to mixed grain). Reinforcers were presented according to two independent 25-interval Fleshler and HoVman (1962) series, one for each component. Each schedule timed only when its component was presented. Red and green lights signaled the components. They were presented randomly every 30 s with the stipulation that a component could occur no more than three times in a row. Baseline was followed by an experimental condition that was identical to baseline except that a stimulus was added to one component of the multiple schedule. The stimulus was a darkening of the response key for 3-s. It was presented randomly with respect to reinforcement, according to a variable time (VT) 30-s schedule. Again, a
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25-interval Fleshler and HoVman (1962) series was used to schedule stimulus deliveries. The schedule only timed during the presentation of the component to which the stimulus was added. The stimulus was added to the component signaled by the red light for 4 subjects and to the component signaled by the green light for the other 5 subjects. Baseline was recovered again after the experimental condition. Each of the three conditions (baseline, experimental, and baseline) was conducted for 30 sessions. Sessions were 60 min long, excluding the time of reinforcer presentation, and were conducted daily, Wve or six times per week. The houselight was illuminated throughout the session. The key was also constantly illuminated with either red or green light except during reinforcement or when it was darkened as a dishabituator. The procedure for rats was similar to that for pigeons with the following exceptions. The rats were trained to press both levers using a shaping by successive approximations procedure. Pressing the left lever yielded reinforcers associated with one component; pressing the right lever yielded reinforcers associated with the other component. The light above the lever was illuminated when its associated component was available. This light was not extinguished during reinforcement. The reinforcer was a 5-s presentation of the dipper that contained 0.5 ml of sweetened condensed milk mixed 1:1 with water. The added stimulus was a Xashing light (5-s in which the light above the lever changed from lighted to unlighted or vice versa every second). This stimulus was added to the component associated with the right lever for 5 rats and to the component associated with the left lever for the other 5 rats. A Xashing light was used because it produced dishabituation for rats in a past study (Aoyama & McSweeney, 2001). Preliminary results showed that turning oV the light, the stimulus used for pigeons, did not produce dishabituation for rats, a species that does not see well.
Results Table 1 presents the rate of responding (responses per min) averaged over the session for each subject (pigeons, top; rats, bottom) and for the mean of all subjects’ responding during each component of the baseline and experimental schedules. Response rates were calculated by dividing the number of responses emitted in a component per session by the time for which that component was available. The time of reinforcer presentation was excluded from all calculations. Here, and throughout this paper, results were averaged over the last 5 sessions for which each schedule was available except for Pigeon 5 during the experimental condition. The obtained rate of reinforcement decreased from the baseline to the experimental phase for Pigeon 5 when data were averaged over the last 5 sessions of each condition (22.2 reinforcers per hour decrease for the constant component; 27.2 reinforcers per hour decrease for the variable component). As a result, a change in constant-component responding from the baseline to the experimental phase could be attributed to a change in obtained rate of reinforcement, rather than to a behavioral interaction. To avoid this problem, the results for Pigeon 5, responding during the experimental phase, were averaged over 5 earlier sessions that provided an obtained rate of reinforcement that was not signiWcantly diVerent from that obtained during the baseline phases.
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Table 1 Rates of responding (responses per min) and an index of the change in responding for each subject and the mean of all subjects on each component of each multiple schedule Subject
Baseline
Experimental
Baseline
Constant
Variable
Constant
Variable
Constant
Pigeons 3 4 5 6 41 91 367 795 5503 Mean
13.9 8.0 21.0 73.8 76.0 39.7 50.0 58.1 46.5 43.0
13.3 6.9 21.0 67.6 79.5 43.2 57.0 50.6 48.4 43.1
17.4 16.1 33.7 100.1 97.9 61.0 56.4 46.6 48.7 53.1
17.9 14.3 29.2 100.6 101.8 73.7 59.5 41.1 46.4 53.8
15.5 11.3 20.1 38.2 85.5 58.5 34.1 42.0 49.3 39.4
Rats 1 2 3 4 5 101 102 103 104 105 Mean
40.8 25.3 15.8 34.1 59.6 8.2 55.2 23.9 18.1 28.5 31.0
47.9 18.9 17.1 25.0 20.2 23.7 60.9 36.8 32.3 29.6 31.2
49.8 42.3 19.4 28.7 105.6 8.4 56.9 27.6 18.9 65.7 42.3
40.6 26.6 16.4 21.4 26.3 20.8 75.6 19.5 16.5 43.4 30.7
42.0 47.8 11.4 20.1 113.2 8.2 50.2 26.4 17.9 67.6 40.5
Index Variable
Constant
Variable
14.4 11.2 17.8 41.7 84.1 70.5 35.9 38.9 47.4 40.2
1.18 1.66 1.64 1.79 1.21 1.24 1.34 0.93 1.02 1.29
1.29 1.57 1.51 1.84 1.24 1.30 1.28 0.92 0.97 1.29
38.2 30.6 14.6 17.3 42.0 24.0 102.8 18.2 26.5 49.1 36.3
1.20 1.16 1.43 1.06 1.22 1.02 1.08 1.10 1.05 1.37 1.18
0.94 1.07 1.03 1.01 0.85 0.87 0.92 0.71 0.56 1.10 0.91
To determine whether results could be averaged over the two baselines, a two-way (baseline £ 5-min interval) analysis of variance (ANOVA) was applied to the rates of responding emitted by individual pigeons and rats responding in each of the components. The ANOVAs showed that neither the absolute rates of responding averaged over the session (main eVect of baseline—F(1, 8) D 0.44, constant component for pigeons; F(1, 8) D 0.30, variable component for pigeons; F(1, 9) D 1.92, constant component for rats; and F(1, 9) D 0.77, variable component for rats), nor the forms of the within-session patterns of responding (interaction term—F(11, 88) D 0.93, constant component for pigeons; F(11, 88) D 0.97, variable component for pigeons; F(11, 99) D 0.33, constant component for rats; and F(11, 99) D 1.29, variable component for rats) diVered signiWcantly between the two baselines. Response rate did change signiWcantly within the session (main eVect of 5-min interval—F(11, 88) D 16.47, constant component for pigeons; F(11, 88) D 14.80, variable component for pigeons; F(11, 99) D 6.14, constant component for rats; and F(11, 99) D 8.19, variable component for rats). Throughout this paper, results will be considered to be signiWcant when p 0 .05. Because neither absolute response rates (main eVect of baseline), nor the forms of the within-session patterns of responding (interaction term), diVered for the two baselines, data from the average baseline were used in further analyses.
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An index of the size of the change in response rate from the baseline to the experimental schedule was computed for each subject and component. The index was calculated by dividing the rate of responding during each component of the experimental schedule by the average baseline rate of responding during the same component. Results appear in Table 1. Ratios greater than 1.0 indicate that responding was faster during the experimental than during the baseline schedules. Ratios reported for the mean are ratios calculated using the mean response rates, not the mean of the ratios. Table 1 shows that responding was usually faster during the constant component of the experimental schedule than during the constant component of the baseline. This was true for 16 of 18 comparisons for pigeons and for 16 of 20 comparisons for rats, although some of the changes in response rates were small for rats. Eight of 9 indices for pigeons and 10 of 10 indices for rats were greater than 1.0. T tests for matched pairs showed that constant-component responding was signiWcantly faster during the experimental than during the averaged baseline schedule for both pigeons (t(8) D 2.56) and rats (t(9) D 3.11). The introduction of the dishabituator increased response rate in the variable component for pigeons, but not for rats. The rate of variable-component responding was greater during the experimental schedule than during baseline in 15 of 18 comparisons for pigeons, but in only 8 of 20 comparisons for rats. Indices were greater than 1.0 for 7 of 9 pigeons, but for only 4 of 10 rats. T tests for matched pairs also showed that the diVerence in response rate between the average baseline and experimental condition was statistically signiWcant for pigeons (t(8) D 2.45), but not for rats (t(9) D 1.94). Fig. 1 presents the within-session patterns of responding for the average baseline (left) and experimental (right) conditions for pigeons (top) and rats (bottom). The functions in each graph represent the proportion of total-session responses emitted for each 5-min interval in the constant (solid) and variable (dashed) components of that schedule. Proportions are presented instead of absolute response rates to control for diVerences in absolute rates of responding during the two components for rats (see Table 1). They were calculated by dividing the rate of responding during a 5-min interval by the sum of the rates of responding during all twelve 5-min intervals in the session. Response rates were, in turn, calculated by dividing the number of responses made during a component in a 5-min interval by the number of min that that component was available during the 5-min interval. The reported proportions are the proportions calculated using mean response rates, not the mean of the proportions. Fig. 1 shows that respones rate changed systematically within the session for both pigeons and rats responding on both schedules. The within-session patterns of responding were also similar in the two components of each multiple schedule. These visual impressions were conWrmed by two-way (component £ 5-min interval) ANOVAs applied to the rates of responding during the average baseline and experimental schedules. The main eVect of 5-min interval was always signiWcant (F(11, 88) D 15.91, average baseline for pigeons; F(11, 88) D 20.34, experimental schedule for pigeons; F(11, 99) D 11.29, average baseline for rats; and F(11, 99) D 5.44, experimental schedule for rats), indicating that responding changed signiWcantly within the session. The
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Fig. 1. Proportion of total-session responses during successive 5-min intervals in the session for the mean of all pigeons (top) and rats (bottom) responding in the components of the average baseline (left) or experimental (right) schedules. The solid line represents the constant component; the dashed line represents the variable component.
interaction terms were never signiWcant (F 0 1.0), indicating that the forms of the within-session patterns of responding did not diVer for the two components of the multiple schedules. ANOVAs were applied to response rate, rather than to the proportions plotted in the Wgure, because proportions are bounded and cannot be assumed to be normally distributed. Although it is somewhat hard to see, Fig. 1 also shows that the within-session decreases in constant-component responding were steeper during the baseline than during the experimental schedule for both pigeons and rats. For example, the diVerence between the proportion of total session responding in the last 5-min interval and the proportion of total responding during the 5-min interval in which responding was fastest (peak-last) was 0.097 for the baseline and 0.078 for the experimental condition for pigeons. Similar numbers for rats were 0.042 and 0.033. Fig. 2 presents clearer evidence for these diVerences in steepness. If responding decreased more steeply during the baseline than during the experimental condition, then the diVerences between response rates on these two schedules should increase across the session. Fig. 2 shows that it does. Fig. 2 presents within-session changes in the size of the increase in response rate from the baseline to the experimental conditions for pigeons (top) and rats (bottom). The ratios that represent these increases
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Fig. 2. Ratio of constant-component responding during the experimental and during the average baseline schedules for the mean of all pigeons (top) and rats (bottom) during successive 5-min intervals in the session.
were calculated by dividing the rate of responding in the constant component of the experimental schedule by the rate of responding in the same component in the average baseline. Response rates were calculated as for Fig. 1. The results are the mean of the ratios calculated for individual subjects. Ratios greater than 1.0 represent an increase in response rate from the baseline to the experimental condition. Although the results presented in Figs. 1 and 2 are related, Fig. 2 cannot be derived directly from Fig. 1. The presentation of proportions in Fig. 1 removes the information about absolute response rates that is necessary to calculate the ratios presented in Fig. 2. Fig. 2 shows that the size of the change in response rate from the baseline to the experimental schedules increased across the session for both rats and pigeons. The ratio increased from 1.26 at the beginning of the session to 1.95 at the end of the session for pigeons. Ratios increased from 1.03 to 1.22 across the session for rats. The linear contrast of a one-way (5-min interval) repeated measures ANOVA showed
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that the linear trend in the data was signiWcant for the pigeons (F(1, 8) D 8.07) but not for rats (F(1, 9) D 3.12, p 1 .05).
Discussion Consistent with the results of several past studies (Brownstein & Newsom, 1970; Hughes, 1971; Wilkie, 1973, 1977), the introduction of a stimulus into one component of a multiple schedule increased response rate during the other, unchanged, component. McSweeney and Weatherly's (1998) theory predicts these results. The theory argues that a change in habituation from the baseline to the experimental schedules contributes to behavioral interactions during multiple schedules. The introduction of neutral stimuli alters habituation by producing dishabituation (e.g., Groves & Thompson, 1970). To the best of our knowledge, no other theory of contrast makes this prediction. Most modern theories argue that contrast occurs when aspects of the reinforcers, not aspects of the environment, are changed (e.g., Williams, 1983). Rate of responding in the variable component also increased from the baseline to the experimental schedule for pigeons, but not for rats. McSweeney and Weatherly's (1998) theory does not make a clear prediction about variable-component responding. Introducing the dishabituator into the variable component should increase the eVectiveness of the variable-component reinforcers by decreasing habituation to those reinforcers, but this eVect will be confounded by the direct eVect of the dishabituator. For example, altering the light may have elicited freezing for rats, but not for pigeons. In that case, variable-component response rates might increase from the baseline to the experimental phase for pigeons (rate increasing eVect of a reduction in habituation to the reinforcer), but decrease or remain unchanged from the baseline to the experimental phase for rats (rate increasing eVect of a reduction in habituation confounded by the rate decreasing eVect of freezing). Therefore, although McSweeney and Weatherly's theory is consistent with the results for the variable component, it does not strongly predict them. Other results are predicted by McSweeney and Weatherly's (1998) theory. First, within-session patterns of responding were similar for the two components of the multiple schedules (Fig. 1). The components were presented randomly in this study. Therefore, the amount of habituation that occurred to food should have been similar for the two components at any time in the session. Fig. 1 joins results reported by McSweeney, Murphy, and Kowal (2004) in conWrming this prediction. Second, as predicted, the late-session decreases in responding were steeper during the baseline than during the experimental phase (Fig. 1). As a result, the diVerence between responding during the baseline and experimental schedules grew larger across the session (Fig. 2). As indicated earlier, more habituation to the reinforcer should produce steeper late-session decreases in responding (McSweeney et al., 1996). If the increase in response rate from the baseline to the experimental schedule results from a reduction in habituation, then the late-session decrease in responding should be steeper during the baseline than during the experimental schedule for the component
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that is held constant across these schedules. Figs. 1 and 2 join the results of several past studies in showing that the size of behavioral interactions may increase within the session and that these interactions may be accompanied by appropriate changes in within-session response patterns (McSweeney et al., 2004; McSweeney, Swindell, Murphy, & Kowal, in press; but see also McSweeney, Murphy, & Kowal, 2003b). The changes in response patterns provide independent evidence for the changes in habituation to which McSweeney and Weatherly attribute some behavioral interactions. Third, consistent with results reported by McSweeney et al. (2004), behavioral interactions were found for both rats and pigeons. Habituation occurs for most, if not all, species (e.g., Thorpe, 1966). Therefore, if changes in habituation contribute to behavioral interactions, then these interactions should be observed for most, if not all, species. Although behavioral interactions were observed for both species, the interactions were smaller for rats than for pigeons (e.g., Table 1). This diVerence may also be consistent with McSweeney and Weatherly's (1998) theory. The strength and speed of habituation varies across species and stimuli (e.g., Hinde, 1970). Before accepting dishabituation as an explanation for the present results, however, three alternative explanations should be considered. First, the results for pigeons might be attributed to a failure to discriminate between the components, rather than to dishabituation. The results for pigeons are induction. That is, response rates changed in the same (induction), not in opposite (contrast), directions in the two components from the baseline to the experimental phases (see Table 1). A failure of discrimination would produce such similar changes in responding in the two components. Failure of discrimination cannot explain all of the present results, however. Failure to discriminate can explain why response rates changed in the same direction for the two components, but it cannot explain why response rate increased rather than decreased or remained constant. It cannot explain why the diVerences between responding on the baseline and experimental schedules increased across sessions or why similar results were found for rats even though responding during the variable component did not consistently increase from the baseline to the experimental phase for these subjects. Explaining all of these results requires postulation of an additional factor such as dishabituation. Second, the dishabituating stimulus may have increased constant-component responding by serving as an S-, not as a dishabituator. As argued earlier, introducing an S- (a stimulus in the presence of which responding is not reinforced) is known to produce positive contrast. However, this idea cannot explain the present results. To begin with, the dishabituating stimuli were not S-s. Responding was reinforced in their presence at the same programmed rate as in their absence. Rats did respond and obtain reinforcers during the dishabituating stimulus. Because reinforcers were presented on VI schedules, similar rates of reinforcement would be obtained in the presence and absence of the dishabituator even if the rats responded at substantially diVerent rates at these times. Pigeons do not peck darkened keys and they did not usually respond during the present dishabituating stimuli. However, constant-component responding should not be raised by the introduction of a stimulus during which animals do not respond (e.g., Terrace, 1963). Positive contrast does not occur unless the stimulus serves as a true S-. That
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is, subjects must respond, and fail to receive reinforcers, in the presence of that stimulus. Therefore, the dishabituating stimuli should not have served as an S- for either pigeons or rats. Third, turning oV the keylight may have increased response rates for pigeons because it served as a conditioned reinforcer, rather than as a dishabituator. The keylight was extinguished during reinforcement for pigeons. If generalization occurred from the 5-s darkening of the key during reinforcement to the 3-s darkening of the key as a dishabituator, then presenting the dishabituator may have increased variable-component responding through conditioned reinforcement. The darkening would also increase constant-component responding if the increased variable-component responding spilled over into the constant component. An explanation in terms of conditioned reinforcement fails to account for many of the present results. First, it provides no explanation for the results for rats. The Xashing-light dishabituator used for rats should not have served as a conditioned reinforcer because no change in illumination accompanied the presentation of food for these subjects. Second, presentation of the dishabituating stimulus was independent of, not contingent on, responding for both pigeons and rats. Presenting responseindependent reinforcers usually decreases, not increases, rate of operant responding (e.g., Herrnstein, 1970; Rachlin & Baum, 1972). Finally, research on discrimination questions the assumption that responding spills over from the variable to the constant component. Table 1 shows that this spillover was virtually complete. The increase in responding from the baseline to the experimental schedule was equally large for the constant and variable components for pigeons (index D 1.29, see Table 1). Such a large spillover is inconsistent with results showing that subjects are relatively accurate in assigning reinforcers to the stimuli that signal their presentation (e.g., Herrnstein, 1970). As argued, the present results follow directly from the idea that within-session changes in responding are produced primarily by habituation to the reinforcer (e.g., McSweeney et al., 1996). The present results are not obviously predicted by the idea that satiation produces within-session changes in responding (Bizo, Bogdanov, & Killeen, 1998; DeMarse, Killeen, & Baker, 1999; Hinson & Tennison, 1999; Killeen, 1995; Palya & Walter, 1997). The introduction of a neutral stimulus (e.g., turning oV a light) should not alter satiety factors other than habituation (e.g., caloric content, stomach distention, and blood glucose levels; Mook, 1996). An explanation in terms of satiation fails to explain many additional results (McSweeney & Murphy, 2000; McSweeney & Roll, 1998). It may seem odd that a theory that was formulated to explain behavioral contrast (McSweeney & Weatherly, 1998) also accounts for induction (see results for pigeons). Traditional thinking attributes contrast and induction to diVerent theoretical mechanisms. For example, induction is often dismissed as a failure of discrimination because it represents a change in response rates in the same direction in the two components of the multiple schedule. Contrast has been attributed to many diVerent factors such as the summation of diVerent types of responses (e.g., Gamzu & Schwartz, 1973; Hearst & Jenkins, 1974; Rachlin, 1973), or competition between responses (e.g., Hinson & Staddon, 1978) or reinforcers (e.g., McLean, 1992).
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McSweeney and Weatherly's (1998) theory questions this traditional thinking. According to their theory, the same mechanism (a change in habituation to the reinforcer) may produce contrast under some circumstances and induction under others. Introducing a manipulation that changes habituation to the reinforcer should change the eVectiveness of that reinforcer in any component in which the reinforcer is presented. As a result, a manipulation that, for example, reduces habituation should raise response rate in the other, unchanged, component. The eVect of the manipulation on responding during the component in which it is introduced is more complicated because the manipulation may also have a direct eVect on responding (e.g., it might elicit freezing). The increase in responding that results from a decrease in habituation to the reinforcer may be either augmented or reduced by this direct eVect. If the manipulation itself increases or does not alter responding, then response rate will rise in the variable component, as it did in the constant component, producing induction (see the results for pigeons). If the direct eVect of the manipulation (e.g., freezing) reduces responding more than the reduction in habituation increases responding, then response rate might decrease in the variable component, but increase in the constant component, producing behavioral contrast (see some of the results for rats). Although the results of the present study and of several others (McSweeney et al., 2003b, in press-a, in press-b; Swindell, McSweeney, & Murphy, 2003) suggest that changes in habituation to the reinforcer contribute to the behavioral interactions observed during multiple schedules, the results do not establish the size or generality of this contribution. Many factors undoubtedly contribute to behavioral interactions (e.g., Williams, 1983). Habituation to the reinforcer is only one of them. A great deal of additional research will be needed to determine the importance of the contribution of habituation.
Acknowledgments The conduct of these experiments and the preparation of the manuscript was partially supported by Grant RO1 MH6170 from the National Institute of Mental Health to F.K.M. Portions of these data were part of a thesis submitted by Duane M. Isava to the Department of Psychology at Washington State University in partial fulWllment of the requirements for the Masters Degree. Portions of these data were also presented at the 26th annual meeting of the Association for Behavior Analysis, May, 2000, in Washington, DC.
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Dishabituation produces interactions during multiple schedules Frances K. McSweeney,¤ Benjamin P. Kowal, Eric S. Murphy, and Duane M. Isava Washington State University, Pullman, WA 99164-4820, USA Received 22 January 2004; received in revised form 18 May 2004 Available online 29 July 2004
Abstract McSweeney and Weatherly (1998) argued that diVerential habituation to the reinforcer contributes to the behavioral interactions observed during multiple schedules. The present experiment conWrmed that introducing dishabituators into one component of a multiple schedule increases response rate in the other, constant, component. During baseline, pigeons and rats responded on multiple variable interval 30-s variable interval 30-s schedules. During experimental conditions, subjects responded on the same schedule except that a dishabituating stimulus (manipulation of a light) was also presented randomly during one of the components. Constant-component responding was faster during the experimental than during the baseline conditions. This diVerence in responding grew larger across the session. The within-session pattern of responding was similar for the two components of each multiple schedule. Qualitatively similar results were observed for rats and pigeons. These results suggest that behavioral interactions sometimes arise from a change in reinforcer eVectiveness between the baseline and experimental phases of the experiment, rather than from an assessment of reinforcer relativity (a comparison of reinforcers delivered during the two components in the experimental phase). Behavioral contrast and induction are sometimes produced by similar factors. 2004 Elsevier Inc. All rights reserved
An “interaction” occurs when changing the conditions of reinforcement in one component of a multiple schedule changes behavior in the other component (Reynolds, 1961). Two types of interactions are often distinguished (Reynolds, 1961). ¤ Corresponding author. Fax: 1-509-335-5043. E-mail address: [email protected] (F.K. McSweeney).
0023-9690/$ - see front matter 2004 Elsevier Inc. All rights reserved doi:10.1016/j.lmot.2004.06.001
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Behavioral contrast occurs when response rates change in the opposite directions in the two components. Induction occurs when response rates change in the same direction. Interactions are usually labeled positive if response rate increases in the unchanged component and negative if response rate decreases. Behavioral interactions are usually studied by comparing the rate of responding during a baseline schedule that provides the same conditions of reinforcement in both components (e.g., a multiple variable interval (VI) £ VI schedule) to responding during an experimental schedule that provides diVerent conditions in the two components (e.g., a multiple VI £ VI y schedule; McSweeney & Norman, 1979). For example, suppose that one component of a baseline, multiple variable interval 60-s (VI 60-s) VI 60-s, schedule is changed to extinction. If rate of responding during the constant, VI 60-s, component increases and rate of responding during the changed component decreases, the results would be labeled “positive contrast.” Suppose, instead, that one component of the baseline, multiple VI 60-s VI 60-s, schedule is changed to a VI 15-s schedule. If responding during the constant, VI 60-s, component decreases and responding during the changed component increases, the results would be labeled “negative contrast.” For convenience, the component that is changed to produce the interaction will be called the “variable” component. The component that is held constant will be the “constant” component. Few studies have examined induction, but behavioral contrast has been studied extensively. In spite of this interest, no one theory of contrast has been generally accepted. Most theories attribute contrast primarily to diVerences in the conditions of reinforcement provided during the two components of the schedule in which contrast is measured (the experimental schedule, e.g., Gamzu & Schwartz, 1973; Hearst & Jenkins, 1974; Hinson & Staddon, 1978; McLean, 1992; McSweeney, 1987; Rachlin, 1973). More recently, McSweeney and Weatherly (1998) argued that multiple-schedule contrast is produced partially by diVerences in habituation to the reinforcer between the baseline and experimental schedules (e.g., McSweeney, Hinson, & Cannon, 1996). According to this idea, reducing the rate of reinforcement in one component of a multiple schedule reduces the amount of habituation that occurs to the reinforcer during the experimental session (e.g., McSweeney, 1992). If the same type of reinforcer is provided in the other, constant, component, then that reinforcer should be more eVective (less habituation) and support a higher rate of responding (positive contrast). Providing more reinforcers in one component increases habituation to the reinforcer, reducing the eVectiveness of constant-component reinforcers. Less eVective reinforcers support a lower rate of responding (negative contrast). The earlier examples illustrate this idea. A multiple VI 60-s VI 60-s, baseline, schedule delivers a maximum of 60 reinforcers per hour on the average. A multiple VI 60-s extinction, experimental, schedule delivers approximately 30 reinforcers per hour. If each delivery of the reinforcer produces some habituation, then less habituation to the reinforcer occurs during the experimental (30 reinforcers) than during the baseline (60 reinforcers) sessions. As a result, reinforcers delivered in the constant, VI 60-s, component support a higher response rate during the experimental than during the baseline phase (positive contrast). A multiple VI 60-s VI 15-s, experimental, schedule delivers a maximum of 150 reinforcers per hour on the average. As a result,
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reinforcers delivered in the constant, VI 60-s, component of this experimental schedule support a lower response rate (more habituation) than they did during baseline (negative contrast). The theory proposed by McSweeney and Weatherly (1998) is parsimonious, relies only on processes that have been demonstrated by independent evidence, and is consistent with much empirical research. (See McSweeney & Weatherly, 1998, for a review.) The theory also makes predictions that diVer from those of other theories. The present experiment tested a fundamental prediction of McSweeney and Weatherly's theory. According to McSweeney and Weatherly (1998), introducing a neutral stimulus into one component of a multiple schedule might produce a behavioral interaction. Introducing a neutral stimulus should produce dishabituation, deWned as a reduction of habituation by the introduction of a strong, diVerent, or extra stimulus (e.g., Groves & Thompson, 1970). Dishabituation is a fundamental characteristic of behavior undergoing habituation (e.g., Thompson & Spencer, 1966). If introducing a neutral stimulus reduces habituation to the reinforcer, then the reinforcer should be more eVective in supporting responding and response rate should rise in the other, constant, component (a behavioral interaction). Some evidence suggests that this is true. Adding a stimulus that signals the availability of reinforcers to one component of a multiple schedule increases response rate in the other, unchanged, component (e.g., Wilkie, 1973, 1977). However, an alternative explanation for these results must be dismissed before they can be attributed to dishabituation. Adding signals for reinforcement may convert the time when the signal is not present to a signal for the absence of reinforcement (S-). Introducing an S- is known to produce contrast. Brownstein and Newsom (1970) and Hughes (1971) provided evidence that questions creation of an S- as an explanation for Wilkie's results. Brownstein and Newsom reported that positive contrast occurred in one component of a multiple Wxed interval (FI) FI schedule when a signal for the availability of reinforcement was added to the other component. An S- is already present during FI schedules because the time that immediately follows the delivery of a reinforcer cannot contain another reinforcer. Therefore, introducing a new stimulus might not substantially alter the existing S-. Nevertheless, Brownstein and Newsom found that the addition of the stimulus produced positive contrast. Hughes (1971) maintained responding on a multiple VI VI schedule. Then, he added a signal for the availability of reinforcers to the variable component and reported that rates of responding increased in the constant component. Finally, Hughes introduced an equal number of stimuli that were presented randomly with respect to reinforcement to the variable component. Response rates increased still further in the constant component. That is, the addition of a neutral stimulus increased response rate in the constant component even when the stimulus was presented randomly with respect to reinforcement. A stimulus that is presented randomly does not create an S-. The present experiment provided further evidence that introducing a dishabituator can produce a behavioral interaction. During baseline, pigeons and rats responded on a multiple VI 30-s VI 30-s schedule. This schedule was used because
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past research showed that it produces robust habituation to the reinforcer (e.g., McSweeney, 1992). During the experimental phase, subjects responded on the same multiple VI 30-s VI 30-s schedule, but a dishabituating event (manipulation of a light) was introduced randomly with respect to reinforcement into one component. Because the dishabituating stimulus was introduced randomly, increases in response rates cannot be attributed to the creation of an S- during the time when the stimulus was absent. The present experiment tested several additional predictions of McSweeney and Weatherly's (1998) theory. First, within-session changes in responding should be similar for the two components of the multiple schedules. Components were presented randomly in this experiment to provide a relatively pure test of McSweeney and Weatherly's (1998) theory. McSweeney and Weatherly argued that habituation is only one of several factors that contribute to contrast. Other studies suggest that factors related to the conditions of reinforcement in the following component also contribute (e.g., Williams, 1991). The present experiment reduced the contribution of the following conditions of reinforcement by making those conditions less predictable through random component presentation. When components are presented randomly, the amount of habituation that has occurred to the reinforcer should be similar for the two components at any time in the session. Similar habituation should yield similar within-session patterns of responding on the assumption that these patterns are produced primarily by habituation to the reinforcer (McSweeney et al., 1996). Second, the within-session response pattern during the constant component should be Xatter during the experimental than during the baseline condition. Greater habituation should produce steeper late-session decreases (McSweeney et al., 1996). If the increase in constant-component response rate results partly from a reduction in habituation between the baseline and experimental schedules, then the late-session decrease in constant-component responding should be steeper (more habituation) during the baseline than during the experimental schedule. As a result of these diVerences in steepness, the increase in constant-component responding from the baseline to the experimental schedule should become larger as the session progresses. Finally, similar results should be found for rats and pigeons. Some theories argue that contrast is not always observed when rats serve as subjects under conditions that produce it for pigeons (e.g., Gamzu & Schwartz, 1973; Hearst & Jenkins, 1974; Rachlin, 1973). However, habituation occurs for most, if not all, species (e.g., Thorpe, 1966). Therefore, if changes in habituation contribute to behavioral interactions, those interactions should be observed in both rats and pigeons.
Method Subjects Nine experimentally experienced pigeons and 10 naïve male rats served as subjects. The rats were bred from Sprague–Dawley stock in the Washington State University Psychology Department Vivarium. They were approximately 120 days old at
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the start of the experiment. Subjects were housed individually and experienced a 12 h light/12 h dark cycle. They had free access to water but were maintained at approximately 85% of their free-feeding weights by post-session feedings given when all subjects had completed the experiment for the day. Apparatus A three-key operant chamber, measuring 30.0 £ 35.5 £ 27.0 cm, was used for pigeons. Response keys were 2.5 cm diameter Plexiglas panels, located 7.0 cm apart and 3.0 cm from the ceiling. The left and right keys were situated 6.5 cm from each side wall. A force of approximately 0.25 N was required to operate each key. A 5.0 £ 4.0 cm opening, located directly below the center key and 8.0 cm above the Xoor, allowed access to the food hopper. A light behind a 4.0 cm diameter panel, 1.0 cm below the ceiling, and 5.0 cm from the right side, served as a houselight. Two treadles were located on the Xoor, directly below the keys. These treadles will not be described because they were not used in this experiment. A two-lever experimental chamber, measuring 20.0 £ 24.5 £ 24.5 cm, was used for rats. A 5.0 £ 5.5 cm opening, located in the center of the instrument panel and 0.5 cm above the Xoor, allowed access to a 0.5 ml dipper. Two retractable 4.0 £ 1.5-cm levers were located 2.5 cm from either side of this opening. Each lever was 5.0 cm above the Xoor and extended 1.5 cm into the enclosure. A 2.0-cm in diameter light was located 2.5 cm above each lever and a third 2.0-cm light was centered, 4.0 cm below the ceiling. A fourth 2.0-cm light that served as a houselight was located at the center of the ceiling. Each apparatus was enclosed in a sound-attenuating chamber. A ventilating fan masked noises from outside the chamber. An IBM compatible computer, running MED Associates software, controlled the experimental events and recorded the data. The computer and printer were located in a diVerent room from the experimental enclosures, and the enclosure for pigeons was located in a diVerent room than the enclosure for rats. Procedure Because the pigeons were experimentally experienced, they were placed directly on the experimental procedure. That is, pigeons began responding on a baseline, multiple VI 30-s VI 30-s, schedule in which pecking the center key produced reinforcers (5s of access to mixed grain). Reinforcers were presented according to two independent 25-interval Fleshler and HoVman (1962) series, one for each component. Each schedule timed only when its component was presented. Red and green lights signaled the components. They were presented randomly every 30 s with the stipulation that a component could occur no more than three times in a row. Baseline was followed by an experimental condition that was identical to baseline except that a stimulus was added to one component of the multiple schedule. The stimulus was a darkening of the response key for 3-s. It was presented randomly with respect to reinforcement, according to a variable time (VT) 30-s schedule. Again, a
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25-interval Fleshler and HoVman (1962) series was used to schedule stimulus deliveries. The schedule only timed during the presentation of the component to which the stimulus was added. The stimulus was added to the component signaled by the red light for 4 subjects and to the component signaled by the green light for the other 5 subjects. Baseline was recovered again after the experimental condition. Each of the three conditions (baseline, experimental, and baseline) was conducted for 30 sessions. Sessions were 60 min long, excluding the time of reinforcer presentation, and were conducted daily, Wve or six times per week. The houselight was illuminated throughout the session. The key was also constantly illuminated with either red or green light except during reinforcement or when it was darkened as a dishabituator. The procedure for rats was similar to that for pigeons with the following exceptions. The rats were trained to press both levers using a shaping by successive approximations procedure. Pressing the left lever yielded reinforcers associated with one component; pressing the right lever yielded reinforcers associated with the other component. The light above the lever was illuminated when its associated component was available. This light was not extinguished during reinforcement. The reinforcer was a 5-s presentation of the dipper that contained 0.5 ml of sweetened condensed milk mixed 1:1 with water. The added stimulus was a Xashing light (5-s in which the light above the lever changed from lighted to unlighted or vice versa every second). This stimulus was added to the component associated with the right lever for 5 rats and to the component associated with the left lever for the other 5 rats. A Xashing light was used because it produced dishabituation for rats in a past study (Aoyama & McSweeney, 2001). Preliminary results showed that turning oV the light, the stimulus used for pigeons, did not produce dishabituation for rats, a species that does not see well.
Results Table 1 presents the rate of responding (responses per min) averaged over the session for each subject (pigeons, top; rats, bottom) and for the mean of all subjects’ responding during each component of the baseline and experimental schedules. Response rates were calculated by dividing the number of responses emitted in a component per session by the time for which that component was available. The time of reinforcer presentation was excluded from all calculations. Here, and throughout this paper, results were averaged over the last 5 sessions for which each schedule was available except for Pigeon 5 during the experimental condition. The obtained rate of reinforcement decreased from the baseline to the experimental phase for Pigeon 5 when data were averaged over the last 5 sessions of each condition (22.2 reinforcers per hour decrease for the constant component; 27.2 reinforcers per hour decrease for the variable component). As a result, a change in constant-component responding from the baseline to the experimental phase could be attributed to a change in obtained rate of reinforcement, rather than to a behavioral interaction. To avoid this problem, the results for Pigeon 5, responding during the experimental phase, were averaged over 5 earlier sessions that provided an obtained rate of reinforcement that was not signiWcantly diVerent from that obtained during the baseline phases.
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Table 1 Rates of responding (responses per min) and an index of the change in responding for each subject and the mean of all subjects on each component of each multiple schedule Subject
Baseline
Experimental
Baseline
Constant
Variable
Constant
Variable
Constant
Pigeons 3 4 5 6 41 91 367 795 5503 Mean
13.9 8.0 21.0 73.8 76.0 39.7 50.0 58.1 46.5 43.0
13.3 6.9 21.0 67.6 79.5 43.2 57.0 50.6 48.4 43.1
17.4 16.1 33.7 100.1 97.9 61.0 56.4 46.6 48.7 53.1
17.9 14.3 29.2 100.6 101.8 73.7 59.5 41.1 46.4 53.8
15.5 11.3 20.1 38.2 85.5 58.5 34.1 42.0 49.3 39.4
Rats 1 2 3 4 5 101 102 103 104 105 Mean
40.8 25.3 15.8 34.1 59.6 8.2 55.2 23.9 18.1 28.5 31.0
47.9 18.9 17.1 25.0 20.2 23.7 60.9 36.8 32.3 29.6 31.2
49.8 42.3 19.4 28.7 105.6 8.4 56.9 27.6 18.9 65.7 42.3
40.6 26.6 16.4 21.4 26.3 20.8 75.6 19.5 16.5 43.4 30.7
42.0 47.8 11.4 20.1 113.2 8.2 50.2 26.4 17.9 67.6 40.5
Index Variable
Constant
Variable
14.4 11.2 17.8 41.7 84.1 70.5 35.9 38.9 47.4 40.2
1.18 1.66 1.64 1.79 1.21 1.24 1.34 0.93 1.02 1.29
1.29 1.57 1.51 1.84 1.24 1.30 1.28 0.92 0.97 1.29
38.2 30.6 14.6 17.3 42.0 24.0 102.8 18.2 26.5 49.1 36.3
1.20 1.16 1.43 1.06 1.22 1.02 1.08 1.10 1.05 1.37 1.18
0.94 1.07 1.03 1.01 0.85 0.87 0.92 0.71 0.56 1.10 0.91
To determine whether results could be averaged over the two baselines, a two-way (baseline £ 5-min interval) analysis of variance (ANOVA) was applied to the rates of responding emitted by individual pigeons and rats responding in each of the components. The ANOVAs showed that neither the absolute rates of responding averaged over the session (main eVect of baseline—F(1, 8) D 0.44, constant component for pigeons; F(1, 8) D 0.30, variable component for pigeons; F(1, 9) D 1.92, constant component for rats; and F(1, 9) D 0.77, variable component for rats), nor the forms of the within-session patterns of responding (interaction term—F(11, 88) D 0.93, constant component for pigeons; F(11, 88) D 0.97, variable component for pigeons; F(11, 99) D 0.33, constant component for rats; and F(11, 99) D 1.29, variable component for rats) diVered signiWcantly between the two baselines. Response rate did change signiWcantly within the session (main eVect of 5-min interval—F(11, 88) D 16.47, constant component for pigeons; F(11, 88) D 14.80, variable component for pigeons; F(11, 99) D 6.14, constant component for rats; and F(11, 99) D 8.19, variable component for rats). Throughout this paper, results will be considered to be signiWcant when p 0 .05. Because neither absolute response rates (main eVect of baseline), nor the forms of the within-session patterns of responding (interaction term), diVered for the two baselines, data from the average baseline were used in further analyses.
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An index of the size of the change in response rate from the baseline to the experimental schedule was computed for each subject and component. The index was calculated by dividing the rate of responding during each component of the experimental schedule by the average baseline rate of responding during the same component. Results appear in Table 1. Ratios greater than 1.0 indicate that responding was faster during the experimental than during the baseline schedules. Ratios reported for the mean are ratios calculated using the mean response rates, not the mean of the ratios. Table 1 shows that responding was usually faster during the constant component of the experimental schedule than during the constant component of the baseline. This was true for 16 of 18 comparisons for pigeons and for 16 of 20 comparisons for rats, although some of the changes in response rates were small for rats. Eight of 9 indices for pigeons and 10 of 10 indices for rats were greater than 1.0. T tests for matched pairs showed that constant-component responding was signiWcantly faster during the experimental than during the averaged baseline schedule for both pigeons (t(8) D 2.56) and rats (t(9) D 3.11). The introduction of the dishabituator increased response rate in the variable component for pigeons, but not for rats. The rate of variable-component responding was greater during the experimental schedule than during baseline in 15 of 18 comparisons for pigeons, but in only 8 of 20 comparisons for rats. Indices were greater than 1.0 for 7 of 9 pigeons, but for only 4 of 10 rats. T tests for matched pairs also showed that the diVerence in response rate between the average baseline and experimental condition was statistically signiWcant for pigeons (t(8) D 2.45), but not for rats (t(9) D 1.94). Fig. 1 presents the within-session patterns of responding for the average baseline (left) and experimental (right) conditions for pigeons (top) and rats (bottom). The functions in each graph represent the proportion of total-session responses emitted for each 5-min interval in the constant (solid) and variable (dashed) components of that schedule. Proportions are presented instead of absolute response rates to control for diVerences in absolute rates of responding during the two components for rats (see Table 1). They were calculated by dividing the rate of responding during a 5-min interval by the sum of the rates of responding during all twelve 5-min intervals in the session. Response rates were, in turn, calculated by dividing the number of responses made during a component in a 5-min interval by the number of min that that component was available during the 5-min interval. The reported proportions are the proportions calculated using mean response rates, not the mean of the proportions. Fig. 1 shows that respones rate changed systematically within the session for both pigeons and rats responding on both schedules. The within-session patterns of responding were also similar in the two components of each multiple schedule. These visual impressions were conWrmed by two-way (component £ 5-min interval) ANOVAs applied to the rates of responding during the average baseline and experimental schedules. The main eVect of 5-min interval was always signiWcant (F(11, 88) D 15.91, average baseline for pigeons; F(11, 88) D 20.34, experimental schedule for pigeons; F(11, 99) D 11.29, average baseline for rats; and F(11, 99) D 5.44, experimental schedule for rats), indicating that responding changed signiWcantly within the session. The
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Fig. 1. Proportion of total-session responses during successive 5-min intervals in the session for the mean of all pigeons (top) and rats (bottom) responding in the components of the average baseline (left) or experimental (right) schedules. The solid line represents the constant component; the dashed line represents the variable component.
interaction terms were never signiWcant (F 0 1.0), indicating that the forms of the within-session patterns of responding did not diVer for the two components of the multiple schedules. ANOVAs were applied to response rate, rather than to the proportions plotted in the Wgure, because proportions are bounded and cannot be assumed to be normally distributed. Although it is somewhat hard to see, Fig. 1 also shows that the within-session decreases in constant-component responding were steeper during the baseline than during the experimental schedule for both pigeons and rats. For example, the diVerence between the proportion of total session responding in the last 5-min interval and the proportion of total responding during the 5-min interval in which responding was fastest (peak-last) was 0.097 for the baseline and 0.078 for the experimental condition for pigeons. Similar numbers for rats were 0.042 and 0.033. Fig. 2 presents clearer evidence for these diVerences in steepness. If responding decreased more steeply during the baseline than during the experimental condition, then the diVerences between response rates on these two schedules should increase across the session. Fig. 2 shows that it does. Fig. 2 presents within-session changes in the size of the increase in response rate from the baseline to the experimental conditions for pigeons (top) and rats (bottom). The ratios that represent these increases
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Fig. 2. Ratio of constant-component responding during the experimental and during the average baseline schedules for the mean of all pigeons (top) and rats (bottom) during successive 5-min intervals in the session.
were calculated by dividing the rate of responding in the constant component of the experimental schedule by the rate of responding in the same component in the average baseline. Response rates were calculated as for Fig. 1. The results are the mean of the ratios calculated for individual subjects. Ratios greater than 1.0 represent an increase in response rate from the baseline to the experimental condition. Although the results presented in Figs. 1 and 2 are related, Fig. 2 cannot be derived directly from Fig. 1. The presentation of proportions in Fig. 1 removes the information about absolute response rates that is necessary to calculate the ratios presented in Fig. 2. Fig. 2 shows that the size of the change in response rate from the baseline to the experimental schedules increased across the session for both rats and pigeons. The ratio increased from 1.26 at the beginning of the session to 1.95 at the end of the session for pigeons. Ratios increased from 1.03 to 1.22 across the session for rats. The linear contrast of a one-way (5-min interval) repeated measures ANOVA showed
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that the linear trend in the data was signiWcant for the pigeons (F(1, 8) D 8.07) but not for rats (F(1, 9) D 3.12, p 1 .05).
Discussion Consistent with the results of several past studies (Brownstein & Newsom, 1970; Hughes, 1971; Wilkie, 1973, 1977), the introduction of a stimulus into one component of a multiple schedule increased response rate during the other, unchanged, component. McSweeney and Weatherly's (1998) theory predicts these results. The theory argues that a change in habituation from the baseline to the experimental schedules contributes to behavioral interactions during multiple schedules. The introduction of neutral stimuli alters habituation by producing dishabituation (e.g., Groves & Thompson, 1970). To the best of our knowledge, no other theory of contrast makes this prediction. Most modern theories argue that contrast occurs when aspects of the reinforcers, not aspects of the environment, are changed (e.g., Williams, 1983). Rate of responding in the variable component also increased from the baseline to the experimental schedule for pigeons, but not for rats. McSweeney and Weatherly's (1998) theory does not make a clear prediction about variable-component responding. Introducing the dishabituator into the variable component should increase the eVectiveness of the variable-component reinforcers by decreasing habituation to those reinforcers, but this eVect will be confounded by the direct eVect of the dishabituator. For example, altering the light may have elicited freezing for rats, but not for pigeons. In that case, variable-component response rates might increase from the baseline to the experimental phase for pigeons (rate increasing eVect of a reduction in habituation to the reinforcer), but decrease or remain unchanged from the baseline to the experimental phase for rats (rate increasing eVect of a reduction in habituation confounded by the rate decreasing eVect of freezing). Therefore, although McSweeney and Weatherly's theory is consistent with the results for the variable component, it does not strongly predict them. Other results are predicted by McSweeney and Weatherly's (1998) theory. First, within-session patterns of responding were similar for the two components of the multiple schedules (Fig. 1). The components were presented randomly in this study. Therefore, the amount of habituation that occurred to food should have been similar for the two components at any time in the session. Fig. 1 joins results reported by McSweeney, Murphy, and Kowal (2004) in conWrming this prediction. Second, as predicted, the late-session decreases in responding were steeper during the baseline than during the experimental phase (Fig. 1). As a result, the diVerence between responding during the baseline and experimental schedules grew larger across the session (Fig. 2). As indicated earlier, more habituation to the reinforcer should produce steeper late-session decreases in responding (McSweeney et al., 1996). If the increase in response rate from the baseline to the experimental schedule results from a reduction in habituation, then the late-session decrease in responding should be steeper during the baseline than during the experimental schedule for the component
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that is held constant across these schedules. Figs. 1 and 2 join the results of several past studies in showing that the size of behavioral interactions may increase within the session and that these interactions may be accompanied by appropriate changes in within-session response patterns (McSweeney et al., 2004; McSweeney, Swindell, Murphy, & Kowal, in press; but see also McSweeney, Murphy, & Kowal, 2003b). The changes in response patterns provide independent evidence for the changes in habituation to which McSweeney and Weatherly attribute some behavioral interactions. Third, consistent with results reported by McSweeney et al. (2004), behavioral interactions were found for both rats and pigeons. Habituation occurs for most, if not all, species (e.g., Thorpe, 1966). Therefore, if changes in habituation contribute to behavioral interactions, then these interactions should be observed for most, if not all, species. Although behavioral interactions were observed for both species, the interactions were smaller for rats than for pigeons (e.g., Table 1). This diVerence may also be consistent with McSweeney and Weatherly's (1998) theory. The strength and speed of habituation varies across species and stimuli (e.g., Hinde, 1970). Before accepting dishabituation as an explanation for the present results, however, three alternative explanations should be considered. First, the results for pigeons might be attributed to a failure to discriminate between the components, rather than to dishabituation. The results for pigeons are induction. That is, response rates changed in the same (induction), not in opposite (contrast), directions in the two components from the baseline to the experimental phases (see Table 1). A failure of discrimination would produce such similar changes in responding in the two components. Failure of discrimination cannot explain all of the present results, however. Failure to discriminate can explain why response rates changed in the same direction for the two components, but it cannot explain why response rate increased rather than decreased or remained constant. It cannot explain why the diVerences between responding on the baseline and experimental schedules increased across sessions or why similar results were found for rats even though responding during the variable component did not consistently increase from the baseline to the experimental phase for these subjects. Explaining all of these results requires postulation of an additional factor such as dishabituation. Second, the dishabituating stimulus may have increased constant-component responding by serving as an S-, not as a dishabituator. As argued earlier, introducing an S- (a stimulus in the presence of which responding is not reinforced) is known to produce positive contrast. However, this idea cannot explain the present results. To begin with, the dishabituating stimuli were not S-s. Responding was reinforced in their presence at the same programmed rate as in their absence. Rats did respond and obtain reinforcers during the dishabituating stimulus. Because reinforcers were presented on VI schedules, similar rates of reinforcement would be obtained in the presence and absence of the dishabituator even if the rats responded at substantially diVerent rates at these times. Pigeons do not peck darkened keys and they did not usually respond during the present dishabituating stimuli. However, constant-component responding should not be raised by the introduction of a stimulus during which animals do not respond (e.g., Terrace, 1963). Positive contrast does not occur unless the stimulus serves as a true S-. That
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is, subjects must respond, and fail to receive reinforcers, in the presence of that stimulus. Therefore, the dishabituating stimuli should not have served as an S- for either pigeons or rats. Third, turning oV the keylight may have increased response rates for pigeons because it served as a conditioned reinforcer, rather than as a dishabituator. The keylight was extinguished during reinforcement for pigeons. If generalization occurred from the 5-s darkening of the key during reinforcement to the 3-s darkening of the key as a dishabituator, then presenting the dishabituator may have increased variable-component responding through conditioned reinforcement. The darkening would also increase constant-component responding if the increased variable-component responding spilled over into the constant component. An explanation in terms of conditioned reinforcement fails to account for many of the present results. First, it provides no explanation for the results for rats. The Xashing-light dishabituator used for rats should not have served as a conditioned reinforcer because no change in illumination accompanied the presentation of food for these subjects. Second, presentation of the dishabituating stimulus was independent of, not contingent on, responding for both pigeons and rats. Presenting responseindependent reinforcers usually decreases, not increases, rate of operant responding (e.g., Herrnstein, 1970; Rachlin & Baum, 1972). Finally, research on discrimination questions the assumption that responding spills over from the variable to the constant component. Table 1 shows that this spillover was virtually complete. The increase in responding from the baseline to the experimental schedule was equally large for the constant and variable components for pigeons (index D 1.29, see Table 1). Such a large spillover is inconsistent with results showing that subjects are relatively accurate in assigning reinforcers to the stimuli that signal their presentation (e.g., Herrnstein, 1970). As argued, the present results follow directly from the idea that within-session changes in responding are produced primarily by habituation to the reinforcer (e.g., McSweeney et al., 1996). The present results are not obviously predicted by the idea that satiation produces within-session changes in responding (Bizo, Bogdanov, & Killeen, 1998; DeMarse, Killeen, & Baker, 1999; Hinson & Tennison, 1999; Killeen, 1995; Palya & Walter, 1997). The introduction of a neutral stimulus (e.g., turning oV a light) should not alter satiety factors other than habituation (e.g., caloric content, stomach distention, and blood glucose levels; Mook, 1996). An explanation in terms of satiation fails to explain many additional results (McSweeney & Murphy, 2000; McSweeney & Roll, 1998). It may seem odd that a theory that was formulated to explain behavioral contrast (McSweeney & Weatherly, 1998) also accounts for induction (see results for pigeons). Traditional thinking attributes contrast and induction to diVerent theoretical mechanisms. For example, induction is often dismissed as a failure of discrimination because it represents a change in response rates in the same direction in the two components of the multiple schedule. Contrast has been attributed to many diVerent factors such as the summation of diVerent types of responses (e.g., Gamzu & Schwartz, 1973; Hearst & Jenkins, 1974; Rachlin, 1973), or competition between responses (e.g., Hinson & Staddon, 1978) or reinforcers (e.g., McLean, 1992).
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McSweeney and Weatherly's (1998) theory questions this traditional thinking. According to their theory, the same mechanism (a change in habituation to the reinforcer) may produce contrast under some circumstances and induction under others. Introducing a manipulation that changes habituation to the reinforcer should change the eVectiveness of that reinforcer in any component in which the reinforcer is presented. As a result, a manipulation that, for example, reduces habituation should raise response rate in the other, unchanged, component. The eVect of the manipulation on responding during the component in which it is introduced is more complicated because the manipulation may also have a direct eVect on responding (e.g., it might elicit freezing). The increase in responding that results from a decrease in habituation to the reinforcer may be either augmented or reduced by this direct eVect. If the manipulation itself increases or does not alter responding, then response rate will rise in the variable component, as it did in the constant component, producing induction (see the results for pigeons). If the direct eVect of the manipulation (e.g., freezing) reduces responding more than the reduction in habituation increases responding, then response rate might decrease in the variable component, but increase in the constant component, producing behavioral contrast (see some of the results for rats). Although the results of the present study and of several others (McSweeney et al., 2003b, in press-a, in press-b; Swindell, McSweeney, & Murphy, 2003) suggest that changes in habituation to the reinforcer contribute to the behavioral interactions observed during multiple schedules, the results do not establish the size or generality of this contribution. Many factors undoubtedly contribute to behavioral interactions (e.g., Williams, 1983). Habituation to the reinforcer is only one of them. A great deal of additional research will be needed to determine the importance of the contribution of habituation.
Acknowledgments The conduct of these experiments and the preparation of the manuscript was partially supported by Grant RO1 MH6170 from the National Institute of Mental Health to F.K.M. Portions of these data were part of a thesis submitted by Duane M. Isava to the Department of Psychology at Washington State University in partial fulWllment of the requirements for the Masters Degree. Portions of these data were also presented at the 26th annual meeting of the Association for Behavior Analysis, May, 2000, in Washington, DC.
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