Behavioural Processes 43 (1998) 153 – 162
The effects of stopping and restarting a session on within-session patterns of responding Cari B. Cannon a,b,*, Frances K. McSweeney b a
Department of Psychology: Experimental, Box 90086, Duke Uni6ersity, Durham, NC 27708 -0086, USA b Washington State Uni6ersity, Pullman, WA, USA Received 8 October 1997; received in revised form 9 January 1998; accepted 13 January 1998
Abstract Rats pressed levers for sweetened condensed milk reinforcers delivered according to a multiple variable-interval 1-min, variable-interval 1-min schedule during 60-min baseline sessions. The obtained pattern of responding was an early-session increase in responding followed by a relatively constant rate of responding during the remainder of the session. In the experimental conditions of experiment 1, sessions were interrupted by a blackout or timeout lasting either 5, 10, or 30 min. Responding following 5- or 10-min interruptions resembled the constant rate of responding late in the baseline sessions. However, responding increased before 30-min interruptions and increased again after the interruption. This change in the pattern of responding was best described as a ‘restarting’ of the within-session pattern of responding. In experiment 2, a 30-min blackout interrupted sessions. However, reinforcers were presented intermittently during the blackout. Responding did not restart following the 30-min blackout-with-reinforcers condition. These results suggest that the mere presentation of reinforcers is sufficient to maintain within-session changes in responding. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Blackout; Lever press; Multiple schedule; Timeout; Variable-interval schedule; Within-session responding
1. Introduction The rate at which animals perform a simple operant response (e.g. lever press) changes systematically within experimental sessions: increasing, decreasing, or increasing and then decreasing * Corresponding author. Tel.: + 1 919 6605652; fax: +1 919 6605726; e-mail: [email protected]
within the session (McSweeney et al., 1990, 1994b, 1995b). Within-session changes in responding have been widely observed with many different responses and stimuli and across species ranging from nematodes to humans (Rankin and Broster, 1992; McSweeney and Roll, 1993; Roll et al., 1996). As argued elsewhere, within-session changes in responding deserve study because they have important implications for methodology and
0376-6357/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0376-6357(98)00009-6
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theory on conditioning (McSweeney and Roll, 1993; McSweeney et al., 1996a). To give just one example, within-session changes in responding challenge molar theories of behavior (Herrnstein, 1970) because they imply that the primary variable used in these theories, rate of responding averaged over the session, masks regularities in behavior at a more molecular level. In the past, within-session changes in responding have been acknowledged, but they have been treated as problems to be controlled, rather than as phenomena worthy of study in their own right. For example, animals are given time to adapt to the apparatus (Papini and Overmier, 1985) or they are given several ‘warm-up’ trials before the session is started (Hodos and Bonbright, 1972). Presumably, this is done because responding at the beginning of the session differs from responding later in the session. The present experiments were designed to identify the factors that are necessary to restore within-session patterns of responding to their state at the beginning of the session. Research on within-session changes often confounds the effects of factors related to responding and reinforcement with those of other factors related to the passage of time in the session (McSweeney, 1992). That is, as time passes in the session, the number of responses emitted and reinforcers delivered increases. The present experiments separated the effects of responding and reinforcement from other effects related to the passage of time in the session by allowing time to elapse independently of these other factors. In experiment 1, sessions were stopped by retracting the lever, withholding reinforcers, and turning off the discriminative stimulus alone (timeout) or the discriminative stimulus and the houselight (blackout). In experiment 2, sessions were interrupted by a blackout similar to the blackout in experiment 1 except that reinforcers were delivered intermittently during the blackout. In both experiments, post-interruption responding was examined to determine whether the early-session increasing pattern of responding had been ‘restarted’. If a factor related to responding, reinforcement or discriminative stimuli produces the within-session changes in responding then remov-
ing these factors should restore the conditions that were present at the beginning of the session and consequently restart the within-session response pattern. If another unknown variable related to the passage of time in the session controls the within-session pattern, then post-interruption responding should always resemble the pattern of responding at the same absolute time in the baseline session.
2. Experiment 1 Experiment 1 presented timeouts and blackouts of different durations in order to differentiate between two possible explanations for early-session increases in operant responding. First, earlysession increases in responding may be triggered by some variable (e.g. turning on the lights) that occurs at the beginning of the session. In that case, the duration of an interruption would not matter. All that would matter would be that that variable was also restored after the interruption. For example, during blackouts, the houselight was turned off and then turned on at the end of the blackout. If the houselight served as a cue for early session increases in responding, then restoring this variable after the blackout should restart the within-session pattern of responding. Second, early-session increases in responding might occur because some variable recovered over the time between sessions. In that case, the duration of the interruption might matter. If a variable is recovering gradually, then recovery might not have taken place by the end of short interruptions but would have at the end of long interruptions.
2.1. Materials and methods 2.1.1. Subjects The subjects were ten naive, male Sprague– Dawley derived rats bred in the Johnson Tower Vivarium at Washington State University. All subjects were approximately 90 days old at the start of the experiment and were maintained at 85% of their free-feeding body weight established at the start of the experiment. They were housed in individual cages with water freely available.
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2.1.2. Apparatus The apparatus was a standard two-lever experimental enclosure for rats, 24.5× 22.5 × 20.5 cm. Three sides and the ceiling of the enclosure were made of metal. The door was made of Plexiglas. The apparatus was enclosed in a sound-attenuating chamber. A 5× 6-cm opening, which allowed access to a 5-s presentation of a dipper containing 0.5 ml of sweetened condensed milk mixed 1:2 with water, was centered on the front panel 0.5 cm from the floor. Two 1.5 ×3.5-cm levers were located 5.5 cm from this opening, one on each side of it. The levers were located 5 cm above the floor and extended 1.5 cm into the enclosure. A 1.5-cm diameter light was located 8 cm above each lever. The houselight was another 1.5-cm diameter light, located in the center of the ceiling. A SYM microcomputer, located in another room, presented the experimental events and recorded the data. 2.1.3. Procedure Pressing the left lever was shaped by the method of successive approximations. The rate of reinforcement was gradually decreased until subjects responded on a VI 1-min schedule. The experimental procedure employed a multiple VI 1-min VI 1-min schedule of food reinforcement in which components alternated every 5 min. The light above the left lever was illuminated during the first component, but not during the second. Reinforcer availability under each VI schedule was arranged according to a 25-interval Fleshler and Hoffman (1962) series. Sessions ended when 12 components (60 min) had been presented. The session and component timers stopped, and the light above the left lever went out during reinforcement. Sessions were conducted five to six times per week. Five of the ten subjects began the experiment by responding in 30 baseline sessions. During the baseline, the 60-min sessions were uninterrupted by a blackout. Next, the subjects responded during sessions with a single blackout which occurred 15 min into the session. During the blackout, the houselight and the light above the left lever were turned off, the experimental lever was retracted and no reinforcers were presented for the follow-
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ing amounts of time, presented in the following order: 5, 30, 10 min. Each of these blackout durations was presented for 30 sessions. Finally, the rats responded in 15 return-to-baseline sessions. When all the above conditions were conducted, the other five subjects repeated the experiment except that timeouts were used instead of blackouts. The timeout conditions were identical to blackout conditions except that the houselight remained illuminated throughout the session. After all the timeout conditions were presented, the rats responded on 30 return-to-baseline sessions.
2.2. Results and discussion Figs. 1 and 2 present the proportion of totalsession responses during successive 5-min components in the session for individual subjects responding in the baseline, return-to-baseline, blackout (Fig. 1) and timeout (Fig. 2) conditions. Proportions were calculated by dividing the number of responses in a component by the total number of responses in the session. Proportions are presented so that differences in the absolute rates at which different rats responded do not obscure the similarities in the pattern of responding within the session (see McSweeney et al., 1996a for a discussion about using proportions). However, the proportions can be converted to absolute response rates using the data in Table 1. The table presents the mean number of responses in the session for individual rats responding in each condition of experiments 1 and 2. Proportions are larger for longer timeout or blackout conditions because they were calculated over fewer components. However, this does not complicate the interpretation of the data because no comparisons were made between absolute sizes of proportions across conditions. These results and all the results that follow are the means of the last five sessions for each condition. One-way repeated measures analyses of variance conducted on the proportion of responses showed that the within-session changes in responding were significant for all conditions: F(11,44)=8.35, blackout-baseline; F(10,40)= 5.82, blackout-5-min; F(9,36)= 7.27, blackout-10-
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min; F(5,20)= 6.11, blackout-30-min; F(11,44) = 2.32, blackout-baseline-return; F(11,44) = 7.07, timeout-baseline; F(10,40) = 5.14, timeout-5-min; F(9,36)=3.54, timeout-10-min; F(5,20) = 16.63, timeout-30-min; F(11,44) = 11.07, timeout-baseline-return. Here and throughout this manuscript results were considered significant if P B 0.05. Visual inspection of Figs. 1 and 2 suggests that changing the length of the blackout or timeout changed the pattern of responding within the session. Fig. 3 shows this result more clearly. It presents the proportion of total-session responses during successive 5-min components for the mean
Fig. 2. The proportion of total session responses during successive 5-min components for five individual rats during the baseline and timeout conditions of Experiment 1. Proportions were calculated and results are reported as in Fig. 1.
Fig. 1. The proportion of total session responses during successive 5-min components for five individual rats during the baseline and blackout conditions of Experiment 1. Proportions were calculated by dividing the number of responses in a component by the total number of responses in the session. Each graph presents the results for a different condition. Each function presents the results for a different subject averaged over the last five sessions of each condition.
of all subjects responding during baseline, blackout, and timeout conditions. Each function represents the results for a different blackout condition or a corrected baseline. Proportions were calculated only for those components in which the blackout was not presented. Baselines were corrected to contain only responding in the comparable components for the 5-, 10-, and 30-min blackouts and timeouts. Fig. 3 shows that responding was usually lowest in the first 5 min of baseline sessions, increased to a peak and then stabilized at a relatively constant rate by about 15 min into the session. The pattern of responding for the 5-min and 10-min blackout conditions was usually similar to the baseline pattern of responding. Rats 5C, 5E and 5005 deviated from this pattern somewhat. These rats
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Table 1 Mean number of responses (rounded to the nearest whole number) per session for individual rats responding in each condition of experiments 1 and 2 Condition
Experiment 1 – blackout conditions Baseline 5-min blackout 10-min blackout 30-min blackout Return to baseline
Experiment 1 – timeout conditions Baseline 5-min timeout 10-min timeout 30-min timeout Return to baseline
Experiment 2 – blackout-with-reinforcers conditions Baseline 30-min blackout Return to baseline
Subject 5A
5B
5C
5D
5E
1730 1021 807 474 830
1856 1434 1700 1352 2411
1633 724 895 453 1005
2217 1357 1611 882 1813
2788 855 1942 1009 1851
5001
5002
5003
5004
5005
943 1833 2351 1581 3140
716 1141 1137 634 1051
478 702 968 379 1208
582 1510 835 718 600
502 705 491 325 386
51
52
53
54
55
2460 342 320
4507 1325 3427
666 534 2301
3562 859 1988
1660 433 819
Means were calculated over the last five sessions for which each condition was available.
showed slight increases in responding after 10min blackouts or timeouts. A different pattern of responding occurred when sessions were stopped with a 30-min blackout or timeout. In these conditions, responding usually increased early in the session as it had during the baseline and then it increased again after the 30-min blackout or timeout. This pattern of responding was found for nine of the 10 subjects. For two subjects (5C and 5004) the increase in responding after the blackout or timeout was as large or larger than the increase in responding before the blackout or timeout. Two-way (condition×5-min component) repeated measures ANOVAs compared the proportion of responding in each blackout or timeout condition with its corrected baseline. The main effect of component was significant for the 5-min blackout F(10,40)=9.28, 10-min blackout F(9,36)= 10.79, 30-min blackout F(5,20) = 12.09, 5-min timeout F(10,40) = 11.10, 10-min timeout F(9,36)=8.78, and the 30-min timeout F(5,20) =
20.31, indicating that responding changed significantly across the session in all conditions. A significant interaction was found between the 30-min blackout condition and the baseline F(5,20)= 3.06, and between the 30-min timeout and the baseline F(5,20)= 3.08. The significant interaction indicates that the pattern of responding in the 30-min blackout and timeout conditions differed from the baseline pattern of responding. The interactions between the baseline and the 5-min blackout F(10,40)= 1.35, the 10min blackout F(9,36)= 2.08, the 5-min timeout F(10,40)= 0.76, and the 10-min timeout F(9,36)= 2.01 were not significant indicating that the pattern of responding in these conditions did not differ from the baseline pattern. Thus, a 30min and not a 5- or 10-min blackout or timeout apparently ‘restarted’ the within-session pattern of responding. The main effect of condition is not presented because the use of proportions ensured that the proportions totaled 1.00 in all conditions.
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Fig. 3 also shows that the results for timeout and blackout conditions were similar. Finding no differences between the results for timeouts and blackouts implies that a factor related to turning on the lights at the start of a session contributes little to within-session changes in responding. The results further suggest that a factor that is removed by timeouts and blackouts (e.g. component stimuli, responding, and/or reinforcers) is necessary to maintain within-session changes in responding and the effects of this factor dissipated during 30-min timeouts and blackouts.
Fig. 3. The proportion of total session responses during successive 5-min components for the mean of all rats during the baseline, blackout, and timeout conditions in Experiment 1. Each function presents the results for a different timeout or baseline condition. Proportions were calculated only for those components in which the blackout or timeout was not presented. Baselines were corrected to contain only responding in the comparable components for blackouts and timeouts.
3. Experiment 2 Data from experiment 1 indicated that the simple passage of time in the session was not sufficient to produce within-session changes in responding when 30-min interruptions were presented. That is, responding did not resume after a blackout or timeout as it would have if no interruption had occurred. This suggests that a factor that was removed by the interruption was necessary to maintain the within-session changes in responding. Discriminative stimuli, responding, and reinforcers were all removed during interruptions. Of these factors, the presence of reinforcers seems the most likely factor necessary for maintaining within-session changes in responding (McSweeney, 1992; McSweeney and Hinson, 1992). For instance, manipulating aspects of the reinforcer such as its size (Cannon and McSweeney, 1995; Roll et al., 1995) or the frequency of its presentation (McSweeney et al., 1994a) influences the form of the within-session changes in responding. However, manipulations that affect responding (McSweeney, 1992; McSweeney and Johnson, 1994; McSweeney et al., 1995a,c) usually do not affect within-session changes in responding and manipulations affecting discriminative stimuli (e.g. lever lights) do not produce clear effects (Weatherly and McSweeney, 1995). Therefore, the present experiment restored reinforcers during the interruption. If the presentation of reinforcers is sufficient to maintain within-session changes in responding, then providing reinforcers during an interruption should prevent responding from ‘restarting’ after the interruption. In experiment 2, a 30-min blackout was presented after 15 min had elapsed in the session when rats pressed a lever for sweetened condensed milk. During the blackout, reinforcers were presented on a variable time (VT) 1-min schedule. If the presence of reinforcers is sufficient to produce within-session changes in responding, then response rates should continue after the blackout as if no interruption occurred.
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3.1. Materials and methods 3.1.1. Subjects The subjects were five naive Sprague – Dawley rats similar to those used in experiment 1. 3.1.2. Apparatus The apparatus was the same as that used in experiment 1 except that experimental events were controlled and data were collected using MED Associates software on an IBM-compatible computer. 3.1.3. Procedure Procedural details were identical to those used in experiment 1 with the following modifications. A baseline condition was followed by a blackout condition and then by a return-to-baseline. Each of these conditions was presented for 30 sessions. During the baseline condition, no blackouts occurred during the session. During the blackout condition, a 30-min blackout occurred after 15 min had elapsed in the session. During the blackout, the houselight and discriminative stimulus were turned off and the lever was retracted. Reinforcers were presented during the blackout on a VT 1-min schedule delivered according to a 25-interval Fleshler and Hoffman (1962) series. 3.2. Results and discussion Fig. 4 presents the proportion of total-session responses in successive 5-min components in the session for individual subjects responding in the baseline, return-to-baseline and blackout-with-reinforcers conditions. The proportions are larger for the blackout-with-reinforcers condition because they were calculated over fewer components. However, this does not complicate the interpretation of the data because absolute sizes of proportions were not compared across conditions. Visual inspection of Fig. 4 suggests that for rats 52, 53, 54 and 55, baseline responding was usually lowest in the first 5 min of the session. Within the next 5 or 10 min, responding increased and then remained at a relatively constant rate for the remainder of the session. Rat 51 deviated from
Fig. 4. The proportion of total session responses during successive 5-min components for five individual rats during the baseline and blackout-with-reinforcers conditions of Experiment 2. Proportions were calculated and results are reported as in Fig. 1.
this pattern of responding. For this rat, responding remained constant throughout the session in baseline 1 and decreased throughout the session in baseline 2. One-way repeated measures ANOVAs conducted on the proportion of responses showed that the within-session changes in responding were significant for the baseline F(11,33)= 2.15, return-to-baseline F(11,33)= 4.37, and the blackout-with-reinforcers condition F(5,15)= 3.99. The data from rat 51 were excluded from the ANOVAS because this rat’s data differed from the other rats. Visual inspection of Fig. 4 suggests that the pattern of responding in the blackout-with-reinforcers condition was usually similar to the baseline pattern of responding. For rats 52, 53 and 55, responding increased from its lowest level at the start of the session and then by 15 min into the session, leveled off at a relatively constant rate. Rat 51’s within-session pattern of responding decreased and then increased before the blackout and then remained relatively constant after the blackout. Only rat 54 showed a rising pattern of responding after the blackout-with-reinforcers. In
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contrast, nine of the 10 rats showed this pattern after the 30-min blackout in experiment 1. Fig. 5 directly compares the baseline and the blackout-with-reinforcers condition. It presents the proportion of total-session responses during successive 5-min components for the mean of all subjects responding during the baseline and the blackout-with-reinforcers condition. The baseline was corrected to contain only responding in comparable components of the blackout-with-reinforcers condition. Fig. 5 shows that the pattern of responding in the blackout-with-reinforcers condition was similar to the pattern of responding in the baseline. Responding in both conditions increased before the blackout and remained relatively constant after the blackout. A two-way (condition× 5-min component) repeated measures ANOVA was conducted to compare the corrected baseline and the blackout-with-reinforcers condition. The main effect of component was significant, indicating that responding changed during the session F(5,12) =12.31. The interaction F(5,15)= 1.81 was not significant indicating that the pattern of responding did not differ across
Fig. 5. The proportion of total session responses during successive 5-min components for the mean of all rats responding during the baseline and the blackout-with-reinforcers conditions in Experiment 2. Proportions were calculated and results are reported as in Fig. 3.
these conditions. The main effect of condition is not presented because the use of proportions ensured that the proportions totaled 1.00 in every condition. The data in Figs. 4 and 5 suggest that delivering food intermittently during a 30-min blackout was sufficient to prevent the ‘restarting’ of the withinsession pattern of responding that occurred for 30-min blackouts and timeouts in Figs. 1 and 2. More specifically, in Figs. 1 and 2, responding usually increased before the 30-min blackout or timeout and then increased again after the blackout or timeout. This differed from the obtained baseline pattern of responding. However, in the present experiment, responding increased before the 30-min blackout-with-reinforcers and then remained at a relatively constant rate after the blackout. This pattern of responding resembled the baseline pattern of responding.
4. General discussion In experiment 1, responding increased before 30-min blackouts or timeouts and then increased again after the blackout or timeout. This change in the pattern of responding from baseline, was best described as a ‘restarting’ of the within-session pattern of responding. Restarting did not occur when interruptions were 5 min or 10 min. These data suggest that a variable was recovering gradually during interruptions because the earlysession rising pattern of responding was restored after 30 min, but not after the shorter durations. The lack of a difference between timeouts and blackouts further indicated that the illumination of the chamber at the start of a session contributes little to within-session changes in responding when rats press levers for sweetened condensed milk. Finding that responding reset after 30-min interruptions is similar to the results reported during ‘warmup’ for avoidance (Hineline, 1978). Warmup refers to the situation where animals receive more shocks early in an avoidance session than later in the session. In Hineline’s (Hineline, 1978) experiment, rats pressed levers to avoid shock and the duration of a timeout between two
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half-sessions of the avoidance session was varied. Hineline reported that warmup in the second halfsession did not occur if no time elapsed between the two half-sessions. The size of the warmup increased with increasing intervals from 0 to 30 min. Similarly, in the current experiments, ‘warmup’ for positive reinforcement did not occur for 5-or 10-min interruptions but it occurred when interruptions were 30 min. Experiment 2 suggested that the intermittent presentation of food during a 30-min interruption prevented the restarting observed for 30-min interruptions in experiment 1. These results indicate that the presentation of reinforcers is sufficient to maintain within-session changes in responding. This finding is consistent with the warmup for avoidance literature. During warmup for avoidance, if pre-session shocks are delivered, they reduce the within-session warmup (Hoffman et al., 1961). This finding is also consistent with other data showing that reinforcers are an important determinant of within-session changes in responding for positive reinforcement (McSweeney et al., 1996b). For instance, several studies have demonstrated that higher rates of reinforcement produce larger within-session changes in responding (McSweeney, 1992; McSweeney et al., 1994a). Larger reinforcers also produce larger within-session changes in responding when high rates of reinforcement are presented (Cannon and McSweeney, 1995; Roll et al., 1995). Also, changing the rate of reinforcement within the session changes the form of the within-session changes in responding (McSweeney et al., 1995b). It is clear that a factor related to reinforcement influences within-session changes in responding. However, it is not clear how to characterize this effect. For example, satiation (Killeen, 1995; Palya and Walter, 1997), habituation (McSweeney et al., 1996a) or a combination of both of these factors (Swithers and Hall, 1994) may contribute to within-session changes in responding. Theoretical work is needed to clarify the definitions of these terms and empirical research is needed to determine the relative contribution of each of these variables. The present results differ from those reported by McSweeney and Johnson (1994). In some of
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their conditions, pigeons responded in 2 successive sessions that were separated by blackouts spent inside of the experimental enclosure. The blackouts were 0-, 10-, or 30-min long in different conditions. Responding reset in the second session for every condition regardless of the duration of the blackout. However, several methodological differences between the two studies may account for these different results. For example, McSweeney and Johnson (1994) used pigeons whereas the current experiments used rats. Pigeons may begin to roost when the houselight is turned off at the start of the blackout. Future research is needed to compare the effects of blackouts and timeouts on rats and pigeons.
Acknowledgements C.B.C. was supported during the data collection phase of this work by a grant from the National Science Foundation awarded to F.K.M. She was supported during the writing phase of this work by grants from the National Science Foundation and the National Institute of Mental Health awarded to John Staddon. The authors thank John Roll, Jeff Weatherly, and Sam Swindell for their assistance with data collection.
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The effects of stopping and restarting a session on within-session patterns of responding Cari B. Cannon a,b,*, Frances K. McSweeney b a
Department of Psychology: Experimental, Box 90086, Duke Uni6ersity, Durham, NC 27708 -0086, USA b Washington State Uni6ersity, Pullman, WA, USA Received 8 October 1997; received in revised form 9 January 1998; accepted 13 January 1998
Abstract Rats pressed levers for sweetened condensed milk reinforcers delivered according to a multiple variable-interval 1-min, variable-interval 1-min schedule during 60-min baseline sessions. The obtained pattern of responding was an early-session increase in responding followed by a relatively constant rate of responding during the remainder of the session. In the experimental conditions of experiment 1, sessions were interrupted by a blackout or timeout lasting either 5, 10, or 30 min. Responding following 5- or 10-min interruptions resembled the constant rate of responding late in the baseline sessions. However, responding increased before 30-min interruptions and increased again after the interruption. This change in the pattern of responding was best described as a ‘restarting’ of the within-session pattern of responding. In experiment 2, a 30-min blackout interrupted sessions. However, reinforcers were presented intermittently during the blackout. Responding did not restart following the 30-min blackout-with-reinforcers condition. These results suggest that the mere presentation of reinforcers is sufficient to maintain within-session changes in responding. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Blackout; Lever press; Multiple schedule; Timeout; Variable-interval schedule; Within-session responding
1. Introduction The rate at which animals perform a simple operant response (e.g. lever press) changes systematically within experimental sessions: increasing, decreasing, or increasing and then decreasing * Corresponding author. Tel.: + 1 919 6605652; fax: +1 919 6605726; e-mail: [email protected]
within the session (McSweeney et al., 1990, 1994b, 1995b). Within-session changes in responding have been widely observed with many different responses and stimuli and across species ranging from nematodes to humans (Rankin and Broster, 1992; McSweeney and Roll, 1993; Roll et al., 1996). As argued elsewhere, within-session changes in responding deserve study because they have important implications for methodology and
0376-6357/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0376-6357(98)00009-6
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theory on conditioning (McSweeney and Roll, 1993; McSweeney et al., 1996a). To give just one example, within-session changes in responding challenge molar theories of behavior (Herrnstein, 1970) because they imply that the primary variable used in these theories, rate of responding averaged over the session, masks regularities in behavior at a more molecular level. In the past, within-session changes in responding have been acknowledged, but they have been treated as problems to be controlled, rather than as phenomena worthy of study in their own right. For example, animals are given time to adapt to the apparatus (Papini and Overmier, 1985) or they are given several ‘warm-up’ trials before the session is started (Hodos and Bonbright, 1972). Presumably, this is done because responding at the beginning of the session differs from responding later in the session. The present experiments were designed to identify the factors that are necessary to restore within-session patterns of responding to their state at the beginning of the session. Research on within-session changes often confounds the effects of factors related to responding and reinforcement with those of other factors related to the passage of time in the session (McSweeney, 1992). That is, as time passes in the session, the number of responses emitted and reinforcers delivered increases. The present experiments separated the effects of responding and reinforcement from other effects related to the passage of time in the session by allowing time to elapse independently of these other factors. In experiment 1, sessions were stopped by retracting the lever, withholding reinforcers, and turning off the discriminative stimulus alone (timeout) or the discriminative stimulus and the houselight (blackout). In experiment 2, sessions were interrupted by a blackout similar to the blackout in experiment 1 except that reinforcers were delivered intermittently during the blackout. In both experiments, post-interruption responding was examined to determine whether the early-session increasing pattern of responding had been ‘restarted’. If a factor related to responding, reinforcement or discriminative stimuli produces the within-session changes in responding then remov-
ing these factors should restore the conditions that were present at the beginning of the session and consequently restart the within-session response pattern. If another unknown variable related to the passage of time in the session controls the within-session pattern, then post-interruption responding should always resemble the pattern of responding at the same absolute time in the baseline session.
2. Experiment 1 Experiment 1 presented timeouts and blackouts of different durations in order to differentiate between two possible explanations for early-session increases in operant responding. First, earlysession increases in responding may be triggered by some variable (e.g. turning on the lights) that occurs at the beginning of the session. In that case, the duration of an interruption would not matter. All that would matter would be that that variable was also restored after the interruption. For example, during blackouts, the houselight was turned off and then turned on at the end of the blackout. If the houselight served as a cue for early session increases in responding, then restoring this variable after the blackout should restart the within-session pattern of responding. Second, early-session increases in responding might occur because some variable recovered over the time between sessions. In that case, the duration of the interruption might matter. If a variable is recovering gradually, then recovery might not have taken place by the end of short interruptions but would have at the end of long interruptions.
2.1. Materials and methods 2.1.1. Subjects The subjects were ten naive, male Sprague– Dawley derived rats bred in the Johnson Tower Vivarium at Washington State University. All subjects were approximately 90 days old at the start of the experiment and were maintained at 85% of their free-feeding body weight established at the start of the experiment. They were housed in individual cages with water freely available.
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2.1.2. Apparatus The apparatus was a standard two-lever experimental enclosure for rats, 24.5× 22.5 × 20.5 cm. Three sides and the ceiling of the enclosure were made of metal. The door was made of Plexiglas. The apparatus was enclosed in a sound-attenuating chamber. A 5× 6-cm opening, which allowed access to a 5-s presentation of a dipper containing 0.5 ml of sweetened condensed milk mixed 1:2 with water, was centered on the front panel 0.5 cm from the floor. Two 1.5 ×3.5-cm levers were located 5.5 cm from this opening, one on each side of it. The levers were located 5 cm above the floor and extended 1.5 cm into the enclosure. A 1.5-cm diameter light was located 8 cm above each lever. The houselight was another 1.5-cm diameter light, located in the center of the ceiling. A SYM microcomputer, located in another room, presented the experimental events and recorded the data. 2.1.3. Procedure Pressing the left lever was shaped by the method of successive approximations. The rate of reinforcement was gradually decreased until subjects responded on a VI 1-min schedule. The experimental procedure employed a multiple VI 1-min VI 1-min schedule of food reinforcement in which components alternated every 5 min. The light above the left lever was illuminated during the first component, but not during the second. Reinforcer availability under each VI schedule was arranged according to a 25-interval Fleshler and Hoffman (1962) series. Sessions ended when 12 components (60 min) had been presented. The session and component timers stopped, and the light above the left lever went out during reinforcement. Sessions were conducted five to six times per week. Five of the ten subjects began the experiment by responding in 30 baseline sessions. During the baseline, the 60-min sessions were uninterrupted by a blackout. Next, the subjects responded during sessions with a single blackout which occurred 15 min into the session. During the blackout, the houselight and the light above the left lever were turned off, the experimental lever was retracted and no reinforcers were presented for the follow-
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ing amounts of time, presented in the following order: 5, 30, 10 min. Each of these blackout durations was presented for 30 sessions. Finally, the rats responded in 15 return-to-baseline sessions. When all the above conditions were conducted, the other five subjects repeated the experiment except that timeouts were used instead of blackouts. The timeout conditions were identical to blackout conditions except that the houselight remained illuminated throughout the session. After all the timeout conditions were presented, the rats responded on 30 return-to-baseline sessions.
2.2. Results and discussion Figs. 1 and 2 present the proportion of totalsession responses during successive 5-min components in the session for individual subjects responding in the baseline, return-to-baseline, blackout (Fig. 1) and timeout (Fig. 2) conditions. Proportions were calculated by dividing the number of responses in a component by the total number of responses in the session. Proportions are presented so that differences in the absolute rates at which different rats responded do not obscure the similarities in the pattern of responding within the session (see McSweeney et al., 1996a for a discussion about using proportions). However, the proportions can be converted to absolute response rates using the data in Table 1. The table presents the mean number of responses in the session for individual rats responding in each condition of experiments 1 and 2. Proportions are larger for longer timeout or blackout conditions because they were calculated over fewer components. However, this does not complicate the interpretation of the data because no comparisons were made between absolute sizes of proportions across conditions. These results and all the results that follow are the means of the last five sessions for each condition. One-way repeated measures analyses of variance conducted on the proportion of responses showed that the within-session changes in responding were significant for all conditions: F(11,44)=8.35, blackout-baseline; F(10,40)= 5.82, blackout-5-min; F(9,36)= 7.27, blackout-10-
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min; F(5,20)= 6.11, blackout-30-min; F(11,44) = 2.32, blackout-baseline-return; F(11,44) = 7.07, timeout-baseline; F(10,40) = 5.14, timeout-5-min; F(9,36)=3.54, timeout-10-min; F(5,20) = 16.63, timeout-30-min; F(11,44) = 11.07, timeout-baseline-return. Here and throughout this manuscript results were considered significant if P B 0.05. Visual inspection of Figs. 1 and 2 suggests that changing the length of the blackout or timeout changed the pattern of responding within the session. Fig. 3 shows this result more clearly. It presents the proportion of total-session responses during successive 5-min components for the mean
Fig. 2. The proportion of total session responses during successive 5-min components for five individual rats during the baseline and timeout conditions of Experiment 1. Proportions were calculated and results are reported as in Fig. 1.
Fig. 1. The proportion of total session responses during successive 5-min components for five individual rats during the baseline and blackout conditions of Experiment 1. Proportions were calculated by dividing the number of responses in a component by the total number of responses in the session. Each graph presents the results for a different condition. Each function presents the results for a different subject averaged over the last five sessions of each condition.
of all subjects responding during baseline, blackout, and timeout conditions. Each function represents the results for a different blackout condition or a corrected baseline. Proportions were calculated only for those components in which the blackout was not presented. Baselines were corrected to contain only responding in the comparable components for the 5-, 10-, and 30-min blackouts and timeouts. Fig. 3 shows that responding was usually lowest in the first 5 min of baseline sessions, increased to a peak and then stabilized at a relatively constant rate by about 15 min into the session. The pattern of responding for the 5-min and 10-min blackout conditions was usually similar to the baseline pattern of responding. Rats 5C, 5E and 5005 deviated from this pattern somewhat. These rats
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Table 1 Mean number of responses (rounded to the nearest whole number) per session for individual rats responding in each condition of experiments 1 and 2 Condition
Experiment 1 – blackout conditions Baseline 5-min blackout 10-min blackout 30-min blackout Return to baseline
Experiment 1 – timeout conditions Baseline 5-min timeout 10-min timeout 30-min timeout Return to baseline
Experiment 2 – blackout-with-reinforcers conditions Baseline 30-min blackout Return to baseline
Subject 5A
5B
5C
5D
5E
1730 1021 807 474 830
1856 1434 1700 1352 2411
1633 724 895 453 1005
2217 1357 1611 882 1813
2788 855 1942 1009 1851
5001
5002
5003
5004
5005
943 1833 2351 1581 3140
716 1141 1137 634 1051
478 702 968 379 1208
582 1510 835 718 600
502 705 491 325 386
51
52
53
54
55
2460 342 320
4507 1325 3427
666 534 2301
3562 859 1988
1660 433 819
Means were calculated over the last five sessions for which each condition was available.
showed slight increases in responding after 10min blackouts or timeouts. A different pattern of responding occurred when sessions were stopped with a 30-min blackout or timeout. In these conditions, responding usually increased early in the session as it had during the baseline and then it increased again after the 30-min blackout or timeout. This pattern of responding was found for nine of the 10 subjects. For two subjects (5C and 5004) the increase in responding after the blackout or timeout was as large or larger than the increase in responding before the blackout or timeout. Two-way (condition×5-min component) repeated measures ANOVAs compared the proportion of responding in each blackout or timeout condition with its corrected baseline. The main effect of component was significant for the 5-min blackout F(10,40)=9.28, 10-min blackout F(9,36)= 10.79, 30-min blackout F(5,20) = 12.09, 5-min timeout F(10,40) = 11.10, 10-min timeout F(9,36)=8.78, and the 30-min timeout F(5,20) =
20.31, indicating that responding changed significantly across the session in all conditions. A significant interaction was found between the 30-min blackout condition and the baseline F(5,20)= 3.06, and between the 30-min timeout and the baseline F(5,20)= 3.08. The significant interaction indicates that the pattern of responding in the 30-min blackout and timeout conditions differed from the baseline pattern of responding. The interactions between the baseline and the 5-min blackout F(10,40)= 1.35, the 10min blackout F(9,36)= 2.08, the 5-min timeout F(10,40)= 0.76, and the 10-min timeout F(9,36)= 2.01 were not significant indicating that the pattern of responding in these conditions did not differ from the baseline pattern. Thus, a 30min and not a 5- or 10-min blackout or timeout apparently ‘restarted’ the within-session pattern of responding. The main effect of condition is not presented because the use of proportions ensured that the proportions totaled 1.00 in all conditions.
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Fig. 3 also shows that the results for timeout and blackout conditions were similar. Finding no differences between the results for timeouts and blackouts implies that a factor related to turning on the lights at the start of a session contributes little to within-session changes in responding. The results further suggest that a factor that is removed by timeouts and blackouts (e.g. component stimuli, responding, and/or reinforcers) is necessary to maintain within-session changes in responding and the effects of this factor dissipated during 30-min timeouts and blackouts.
Fig. 3. The proportion of total session responses during successive 5-min components for the mean of all rats during the baseline, blackout, and timeout conditions in Experiment 1. Each function presents the results for a different timeout or baseline condition. Proportions were calculated only for those components in which the blackout or timeout was not presented. Baselines were corrected to contain only responding in the comparable components for blackouts and timeouts.
3. Experiment 2 Data from experiment 1 indicated that the simple passage of time in the session was not sufficient to produce within-session changes in responding when 30-min interruptions were presented. That is, responding did not resume after a blackout or timeout as it would have if no interruption had occurred. This suggests that a factor that was removed by the interruption was necessary to maintain the within-session changes in responding. Discriminative stimuli, responding, and reinforcers were all removed during interruptions. Of these factors, the presence of reinforcers seems the most likely factor necessary for maintaining within-session changes in responding (McSweeney, 1992; McSweeney and Hinson, 1992). For instance, manipulating aspects of the reinforcer such as its size (Cannon and McSweeney, 1995; Roll et al., 1995) or the frequency of its presentation (McSweeney et al., 1994a) influences the form of the within-session changes in responding. However, manipulations that affect responding (McSweeney, 1992; McSweeney and Johnson, 1994; McSweeney et al., 1995a,c) usually do not affect within-session changes in responding and manipulations affecting discriminative stimuli (e.g. lever lights) do not produce clear effects (Weatherly and McSweeney, 1995). Therefore, the present experiment restored reinforcers during the interruption. If the presentation of reinforcers is sufficient to maintain within-session changes in responding, then providing reinforcers during an interruption should prevent responding from ‘restarting’ after the interruption. In experiment 2, a 30-min blackout was presented after 15 min had elapsed in the session when rats pressed a lever for sweetened condensed milk. During the blackout, reinforcers were presented on a variable time (VT) 1-min schedule. If the presence of reinforcers is sufficient to produce within-session changes in responding, then response rates should continue after the blackout as if no interruption occurred.
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3.1. Materials and methods 3.1.1. Subjects The subjects were five naive Sprague – Dawley rats similar to those used in experiment 1. 3.1.2. Apparatus The apparatus was the same as that used in experiment 1 except that experimental events were controlled and data were collected using MED Associates software on an IBM-compatible computer. 3.1.3. Procedure Procedural details were identical to those used in experiment 1 with the following modifications. A baseline condition was followed by a blackout condition and then by a return-to-baseline. Each of these conditions was presented for 30 sessions. During the baseline condition, no blackouts occurred during the session. During the blackout condition, a 30-min blackout occurred after 15 min had elapsed in the session. During the blackout, the houselight and discriminative stimulus were turned off and the lever was retracted. Reinforcers were presented during the blackout on a VT 1-min schedule delivered according to a 25-interval Fleshler and Hoffman (1962) series. 3.2. Results and discussion Fig. 4 presents the proportion of total-session responses in successive 5-min components in the session for individual subjects responding in the baseline, return-to-baseline and blackout-with-reinforcers conditions. The proportions are larger for the blackout-with-reinforcers condition because they were calculated over fewer components. However, this does not complicate the interpretation of the data because absolute sizes of proportions were not compared across conditions. Visual inspection of Fig. 4 suggests that for rats 52, 53, 54 and 55, baseline responding was usually lowest in the first 5 min of the session. Within the next 5 or 10 min, responding increased and then remained at a relatively constant rate for the remainder of the session. Rat 51 deviated from
Fig. 4. The proportion of total session responses during successive 5-min components for five individual rats during the baseline and blackout-with-reinforcers conditions of Experiment 2. Proportions were calculated and results are reported as in Fig. 1.
this pattern of responding. For this rat, responding remained constant throughout the session in baseline 1 and decreased throughout the session in baseline 2. One-way repeated measures ANOVAs conducted on the proportion of responses showed that the within-session changes in responding were significant for the baseline F(11,33)= 2.15, return-to-baseline F(11,33)= 4.37, and the blackout-with-reinforcers condition F(5,15)= 3.99. The data from rat 51 were excluded from the ANOVAS because this rat’s data differed from the other rats. Visual inspection of Fig. 4 suggests that the pattern of responding in the blackout-with-reinforcers condition was usually similar to the baseline pattern of responding. For rats 52, 53 and 55, responding increased from its lowest level at the start of the session and then by 15 min into the session, leveled off at a relatively constant rate. Rat 51’s within-session pattern of responding decreased and then increased before the blackout and then remained relatively constant after the blackout. Only rat 54 showed a rising pattern of responding after the blackout-with-reinforcers. In
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contrast, nine of the 10 rats showed this pattern after the 30-min blackout in experiment 1. Fig. 5 directly compares the baseline and the blackout-with-reinforcers condition. It presents the proportion of total-session responses during successive 5-min components for the mean of all subjects responding during the baseline and the blackout-with-reinforcers condition. The baseline was corrected to contain only responding in comparable components of the blackout-with-reinforcers condition. Fig. 5 shows that the pattern of responding in the blackout-with-reinforcers condition was similar to the pattern of responding in the baseline. Responding in both conditions increased before the blackout and remained relatively constant after the blackout. A two-way (condition× 5-min component) repeated measures ANOVA was conducted to compare the corrected baseline and the blackout-with-reinforcers condition. The main effect of component was significant, indicating that responding changed during the session F(5,12) =12.31. The interaction F(5,15)= 1.81 was not significant indicating that the pattern of responding did not differ across
Fig. 5. The proportion of total session responses during successive 5-min components for the mean of all rats responding during the baseline and the blackout-with-reinforcers conditions in Experiment 2. Proportions were calculated and results are reported as in Fig. 3.
these conditions. The main effect of condition is not presented because the use of proportions ensured that the proportions totaled 1.00 in every condition. The data in Figs. 4 and 5 suggest that delivering food intermittently during a 30-min blackout was sufficient to prevent the ‘restarting’ of the withinsession pattern of responding that occurred for 30-min blackouts and timeouts in Figs. 1 and 2. More specifically, in Figs. 1 and 2, responding usually increased before the 30-min blackout or timeout and then increased again after the blackout or timeout. This differed from the obtained baseline pattern of responding. However, in the present experiment, responding increased before the 30-min blackout-with-reinforcers and then remained at a relatively constant rate after the blackout. This pattern of responding resembled the baseline pattern of responding.
4. General discussion In experiment 1, responding increased before 30-min blackouts or timeouts and then increased again after the blackout or timeout. This change in the pattern of responding from baseline, was best described as a ‘restarting’ of the within-session pattern of responding. Restarting did not occur when interruptions were 5 min or 10 min. These data suggest that a variable was recovering gradually during interruptions because the earlysession rising pattern of responding was restored after 30 min, but not after the shorter durations. The lack of a difference between timeouts and blackouts further indicated that the illumination of the chamber at the start of a session contributes little to within-session changes in responding when rats press levers for sweetened condensed milk. Finding that responding reset after 30-min interruptions is similar to the results reported during ‘warmup’ for avoidance (Hineline, 1978). Warmup refers to the situation where animals receive more shocks early in an avoidance session than later in the session. In Hineline’s (Hineline, 1978) experiment, rats pressed levers to avoid shock and the duration of a timeout between two
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half-sessions of the avoidance session was varied. Hineline reported that warmup in the second halfsession did not occur if no time elapsed between the two half-sessions. The size of the warmup increased with increasing intervals from 0 to 30 min. Similarly, in the current experiments, ‘warmup’ for positive reinforcement did not occur for 5-or 10-min interruptions but it occurred when interruptions were 30 min. Experiment 2 suggested that the intermittent presentation of food during a 30-min interruption prevented the restarting observed for 30-min interruptions in experiment 1. These results indicate that the presentation of reinforcers is sufficient to maintain within-session changes in responding. This finding is consistent with the warmup for avoidance literature. During warmup for avoidance, if pre-session shocks are delivered, they reduce the within-session warmup (Hoffman et al., 1961). This finding is also consistent with other data showing that reinforcers are an important determinant of within-session changes in responding for positive reinforcement (McSweeney et al., 1996b). For instance, several studies have demonstrated that higher rates of reinforcement produce larger within-session changes in responding (McSweeney, 1992; McSweeney et al., 1994a). Larger reinforcers also produce larger within-session changes in responding when high rates of reinforcement are presented (Cannon and McSweeney, 1995; Roll et al., 1995). Also, changing the rate of reinforcement within the session changes the form of the within-session changes in responding (McSweeney et al., 1995b). It is clear that a factor related to reinforcement influences within-session changes in responding. However, it is not clear how to characterize this effect. For example, satiation (Killeen, 1995; Palya and Walter, 1997), habituation (McSweeney et al., 1996a) or a combination of both of these factors (Swithers and Hall, 1994) may contribute to within-session changes in responding. Theoretical work is needed to clarify the definitions of these terms and empirical research is needed to determine the relative contribution of each of these variables. The present results differ from those reported by McSweeney and Johnson (1994). In some of
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their conditions, pigeons responded in 2 successive sessions that were separated by blackouts spent inside of the experimental enclosure. The blackouts were 0-, 10-, or 30-min long in different conditions. Responding reset in the second session for every condition regardless of the duration of the blackout. However, several methodological differences between the two studies may account for these different results. For example, McSweeney and Johnson (1994) used pigeons whereas the current experiments used rats. Pigeons may begin to roost when the houselight is turned off at the start of the blackout. Future research is needed to compare the effects of blackouts and timeouts on rats and pigeons.
Acknowledgements C.B.C. was supported during the data collection phase of this work by a grant from the National Science Foundation awarded to F.K.M. She was supported during the writing phase of this work by grants from the National Science Foundation and the National Institute of Mental Health awarded to John Staddon. The authors thank John Roll, Jeff Weatherly, and Sam Swindell for their assistance with data collection.
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