Postreinforcement pause duration varies within a session and with a variable response requirement but not as a function of prior revolutions.
Abstract: The current study examined the variables that influence postreinforcement pause (PRP) duration in rats when wheel running serves as the reinforcing consequence. The relationship between revolutions and PRP duration when revolutions were manipulated within a session and the effect of changing the response requirement from fixed to variable on PRP duration were evaluated. The operant was lever pressing and the reinforcer was the opportunity to run for a programmed number of revolutions. Results failed to show a positive correlation between revolutions run and PRP duration but did show that PRP duration was decreased by a variable response requirement. Analysis also revealed a within-session decrease in PRP duration. The implications of these findings for understanding the factors that influence PRP duration with wheel-running reinforcement are discussed.

Key words: wheel-running reinforcement, postreinforcement pause, revolutions, variable ratio, rat, lever press
Article Type: Report
Subject: Rats (Behavior)
Rattus (Behavior)
Reinforcement (Psychology) (Research)
Author: Belke, Terry W.
Pub Date: 03/22/2011
Publication: Name: The Psychological Record Publisher: The Psychological Record Audience: Academic Format: Magazine/Journal Subject: Psychology and mental health Copyright: COPYRIGHT 2011 The Psychological Record ISSN: 0033-2933
Issue: Date: Spring, 2011 Source Volume: 61 Source Issue: 2
Topic: Event Code: 310 Science & research
Geographic: Geographic Scope: Canada Geographic Code: 1CANA Canada
Accession Number: 255541971
Full Text: Learning more about the factors that influence postreinforcement pause (PRP) duration is important in the experimental analysis of behavior, particularly with respect to understanding differences between qualitatively different reinforcers. PRP duration can index changes in the value of a reinforcing consequence; however, it can also reflect inhibitory postreinforcement effects and/or excitatory stimulus effects (Bonem & Crossman, 1988; Perone & Courtney, 1992; Schlinger, Derenne, & Baron, 2008). The current study investigated the effect of a variable response requirement and number of revolutions on PRP duration in rats responding on levers for the opportunity to run in a wheel as a reinforcer.

It is commonly believed that PRPs tend to be minimal or nonexistent on variable-ratio (VR) schedules (Schlinger et al., 2008), although this appears to be a function of the size of the lowest ratio value in a schedule (Schlinger, Blakely, & Kaczor, 1990). The unpredictable nature of the requirement leads animals to respond more quickly following the termination of a reinforcer. In contrast, when the requirement is fixed, animals typically pause for a long period following the termination of a reinforcer--the duration of the pause varying with the response requirement (Schlinger et al., 2008). While minimal or very short pauses have been demonstrated on VR schedules using conventional reinforcers, they have yet to be shown with wheel-running reinforcement. With the exception of Premack, Schaeffer, and Hundt's (1964) study that used fixed-ratio (FR) schedules, most studies of wheel-running reinforcement have used fixed-interval (FI) schedules (e.g., Belke, 2000; Belke & Dunbar, 1998; Belke & Hancock, 2003; Collier & Hirsch, 1971) and response-initiated variable-interval (VI) schedules (e.g., Belke, 1997; Belke & Heyman, 1994). Response-initiated VI schedules have been used because the longer PRPs following the termination of wheel running would either time out or greatly reduce the programmed reinforcement interval, particularly in the case of short average-duration VI schedules (Belke & Dunbar, 1998). Requiring a response to initiate the reinforcement interval ensures that the schedule is experienced as programmed rather than as a continuous reinforcement schedule. Regardless of this point, PRPs have yet to be described with VR schedules using wheel-running reinforcement. Showing that PRPs are shortened when the ratio requirement is changed from fixed to variable would be a small contribution but consistent with Collier and Hirsch's (1971) conclusion that wheel running generates schedule effects similar to those with more conventional reinforcers.

Of greater importance is the need to understand the relationship between revolutions and PRP duration. If a revolution is considered the unit of value with wheel running, then this relationship would be equivalent to that between reinforcer magnitude and PRP duration with more conventional reinforcers. Because revolutions vary with duration of opportunity to run, the relationship between revolutions and PRP duration should be similar to that between wheel-running reinforcer duration and PRP duration. This latter relation has been investigated with wheel-running reinforcement.

Previous research has shown that as wheel-running reinforcer duration increases, PRP duration increases. Premack et al. (1964) showed that pause duration on a FR 10-lick schedule increased as duration of a contingent opportunity to run increased from 2 s to 30 s. Belke (1997) showed that median PRP duration on a tandem FR 1 VI 30-s schedule (i.e., a response-initiated VI schedule) increased as the duration of an opportunity to run increased across 30 s, 60 s, and 120 s. Belke and Dunbar (1998) found a similar systematic relationship for rats responding on FI schedules with wheel-running reinforcer durations of 15 s, 30 s, and 90 s.

Although a relationship between wheel running as a reinforcer and PRP durations is clearly evident, assessment of the relative role of excitatory effects and inhibitory aftereffects (e.g., satiation or fatigue) is not as clear. Excitatory stimulus effects refer to the effect of a stimulus signaling an upcoming reinforcer on the initiation of responding on a schedule of reinforcement. That is, PRP durations would be shorter in the presence of a stimulus signaling an upcoming reinforcer of greater value. Inhibitory aftereffects refer to factors following the termination of a reinforcer that would affect subsequent initiation of responding (Perone & Courtney, 1992). With respect to wheel running as a reinforcer, fatigue would be an example of an inhibitory aftereffect that could affect PRP duration.

Previous investigations of excitatory stimulus effects and inhibitory aftereffects with wheel-running reinforcement have produced inconsistent results. Using a multiple-schedule procedure in which different stimuli signaled whether the upcoming reinforcer was sucrose or wheel running, Belke and Hancock (2003) found that as the wheel-running duration increased from 15 s to 90 s, PRPs increased in duration in the presence of a stimulus signaling wheel running but not in the presence of a stimulus signaling sucrose. This finding was attributed to a decrease in the excitatory effect of wheel running rather than an increase in inhibitory aftereffects because it occurred in the presence of a stimulus signaling an upcoming wheel-running reinforcer, but not when the previous reinforcer was wheel running and the signaled upcoming reinforcer was sucrose. Note, however, that in this procedure, different wheel-running reinforcer durations were not differently signaled.

In 2007, Belke investigated the effect of different wheel-running reinforcer durations with the durations signaled by different stimuli. The signaled reinforcers were opportunities to run for 10 s and 50 s. In contrast to Belke and Hancock's (2003) results, PRP duration varied as a function of the duration of the previous reinforcer but did not differ as a function of stimuli signaling different durations. PRP duration in this study was a function of inhibitory aftereffects, and there was no evidence for a difference in excitatory stimulus effects. The absence of a difference in excitatory stimulus effects as a function of wheel-running reinforcer duration is consistent with indifference on concurrent VI schedules of wheel-running reinforcement leading to different durations of opportunities to run (Belke, 2006a).

The other procedure employed to investigate the relationship between wheel running as a reinforcer and PRP durations is a within-session procedure. Rather than being exposed to a single duration within a session, as is the case with a between-condition procedure, subjects are exposed to multiple reinforcer durations within a session, and the immediate effects on the subsequent PRPs are determined.

Belke (2000) investigated the relationship between revolutions run within a reinforcement interval and the duration of the immediately following PRP. Results showed a small positive correlation between revolutions run and the subsequent PRP duration. Closer examination showed that this positive correlation was the result of shorter PRPs for the lowest number of revolutions (i.e., zero to nine), with little difference in PRPs following revolutions greater than nine. In summary, both within-session and between-condition procedures show positive relationships between revolutions/reinforcer duration and PRP duration; however, the relationship is clearly stronger when assessed using a between-condition procedure.

One reason for this may be that there are variables that covary with the manipulated variable in a within-session procedure that do not in a between-condition procedure. With respect to the current study, one of these variables is more general while the other may be specific to wheel running. McSweeney and colleagues (McSweeney, Hinson, & Cannon, 1996; McSweeney & Murphy, 2009) have shown that response rates vary within sessions, showing patterns of increasing rates, decreasing rates, or rates that increase and then decrease. Habituation to the sensory aspects of reinforcement leads response rates to decrease, whereas sensitization does the opposite. According to this theory, these processes alter the effectiveness of reinforcers within a session, and if this is the case, PRPs may also change. Unfortunately, with the exception of McSweeney, Roll, and Weatherly (1994), little attention has been paid to within-session changes in PRPs. In their study, PRPs increased within a session on FI and differential-reinforcement-of-low-rate schedules while local response rates showed the opposite pattern. McSweeney et al. (1994) noted that previous studies of PRPs on FI schedules showed either no change (Palya, 1992) or increases (Collier, 1962) in pauses.

The other variable specific to wheel running is the aversion that precedes wheel running and the rewarding effect that follows it. Previous research shows that rats develop an aversion to a flavor (Lett & Grant, 1996; Lett, Grant, & Gaborko, 1998; Salvy, Heth, Pierce, & Russell, 2004), food (Salvy, Pierce, Heth, & Russell, 2003), or place (Masaki & Nakajima, 2008) that occurs prior to an opportunity to run and a preference for a flavor (Hughes & Boakes, 2008) or place (Lett, Grant, Byrne, & Koh, 2000; Lett, Grant, & Koh, 2001, 2002; Lett, Grant, Koh, & Smith, 2001) that immediately follows the termination of a period of running. Generalization of these effects to an experimental session might predict that the rats would show longer pauses at the beginning of a session and shorter pauses at the end.

The present study, like Belke (2000), investigated the relationship between revolutions run and the duration of the immediately following PRP. However, rather than defining a reinforcer in terms of an interval during which the rat could run, it was defined as a number of revolutions to be run. The importance of this difference is that it eliminates variance between subjects in terms of the number of revolutions run during an interval of the same duration. For example, during a 30-s wheel-running reinforcement interval, one rat might run 15 revolutions while another might run 23 revolutions. When PRP duration is subsequently analyzed by revolutions, the former rat would contribute a data point to a different grouping than the latter. The other advantage of defining a wheel-running reinforcer in terms of revolutions is that the range of revolutions over which the relationship is examined can be precisely controlled. In Belke (2000), the number of revolutions in the longest interval of 240 s varied between 92 and 147 revolutions across rats.

Method

Subjects

Five female Long Evans rats obtained from Charles River Breeding Laboratories in St. Constant, Quebec, Canada served as subjects in this experiment. At the beginning of the experiment the rats were approximately 5 months old. Each rat was housed in a separate polycarbonate cage (480 mm x 270 ram x 220 mm) covered with a steel wire lid equipped with a food cradle and water apparatus. The colony room was maintained at 20 [degrees]C with a 12-hour light/dark cycle, lights on at 7:30 a.m. The diet consisted of RMH 3000 lab chow and free access to water. Rats were fed once a day immediately after each running session and were maintained at a target weight of 260 +/- 10 g. The target weight was approximately 85% of an initial 300-g adult female body weight for this strain.

Apparatus

Experimental sessions occurred in four wire mesh activity wheels (one Wahmann and three LaFayette Instruments, model 86041) measuring 350 mm in diameter and 115 mm in width. Each wheel was housed in a sound-attenuated shell (610 mm x 530 mm x 485 mm) equipped with a fan to provide ventilation and to mask extraneous noise. A retractable lever (Med Associates ENV-112) was mounted directly at the 70-mm-by-90-mm opening of each wheel and not in a side cage. The lever extended 18 mm into the wheel through the opening and was located 80 mm above the running surface. The force required to close the lever micro-switches ranged from 16 to 24 N. Two 24V DC lights mounted at the front and back of the wheel at a height of 175 mm served to illuminate the wheel chamber. A microswitch attached to the wheel frame recorded wheel revolutions. A solenoid-operated brake was attached to the base of each wheel. When the brake was engaged, a rubber tip on the end of a metal shaft came in contact with the outer rim of the wheel causing it to stop. Control of experimental events and recording of data were handled by Digital DECpc computers interfaced to the wheels through their parallel ports.

Procedure

Rats had been trained to lever press in the running wheels prior to the present experiment. This training consisted of 20 days of free running for 30 min each day. After this period, a retractable lever in each wheel chamber was extended and the opportunity to run for 60 s was dependent upon a single lever press. With a single lever press, the lever was retracted, the brake was released, and the wheel was free to turn for 60 s. After 60 s, the brake was asserted and the lever was extended. A session ended after 30 reinforcers were obtained. Rats remained on this FR 1 schedule for 10 sessions. Following this, rats were exposed to a VR 3 schedule for 15 sessions and then a VR 5 schedule for 55 sessions. After 55 sessions, the rats were exposed to the first experimental condition. This condition consisted of a FR 30 schedule on which the reinforcer was the opportunity to run 60 revolutions (i.e., FR fixed revolution). Sessions terminated following the first response after the termination of the 10th reinforcer. The termination of the session was defined in this way in order to obtain the PRP following the last reinforcer. This PRP was required to analyze the effect of revolutions programmed in each reinforcer within a session on the duration of the subsequent PRP. This condition remained in effect for 40 sessions. In the second condition, the reinforcer was changed from a fixed number of revolutions to a variable number of revolutions (i.e., FR variable revolution) where the average number of revolutions was 60 (range: 3 to 198). This condition remained in effect for 40 sessions. Subsequently, in the third condition, the rats were returned to the FR fixed-revolution schedule used in the first condition for 40 sessions. In the final condition, the schedule of reinforcement was changed from FR 30 to VR 30 while the reinforcer remained the opportunity to run for 60 revolutions (i.e., VR fixed revolution). As with the others, this condition remained in effect for 40 sessions.

Dependent Measures

Lever presses, cumulative time spent lever pressing, cumulative PRP time, reinforcers, and session time were recorded for each session. Within a session, the initial latency to lever press following commencement of a session and the PRP following each reinforcer were recorded. The number of revolutions run in each wheel-running reinforcement period was recorded for the purpose of assessing the relationship between revolutions run and PRP.

Results

Examination of the data from the two exposures to the FR fixed-revolution condition (Conditions 1 and 3) revealed that PRPs were significantly higher in the second (M = 19.67 s) than in the first (M = 15.73 s), paired t(4) = 3.11, p = .03. To accommodate this, data from the FR variable-revolution condition (Condition 2) were compared to that from the FR fixed-revolution condition that preceded it (Condition 1), and data from the VR fixed-revolution condition (Condition 4) was compared to the FR fixed-revolution condition (Condition 3) that preceded it. Note that the initial latency to lever press following commencement of the session was not included in any analysis of PRPs because it is not a PRP.

Figure 1 shows the mean initial latency to lever press following session commencement and the mean PRPs following each of the 10 reinforcers within a session in Conditions 1 and 2. In general, across all animals and in both conditions, PRPs tended to decrease throughout the session. With respect to differences due to revolutions being fixed or variable, there appear to be no consistent differences. PRPs were higher in the variable-revolution condition for rats NC7 and NC8 but not for the remaining animals. No systematic difference would necessarily be expected since any effect of a specific number of revolutions on the duration of the subsequent PRP would average out for any given reinforcer over the 10 sessions that were used to generate the means. An analysis of the correlation between PRP duration and the number of the revolutions run for each reinforcer over those 10 sessions is required to see this relationship. A repeated-measures ANOVA with condition and reinforcer as within-subject variables revealed no main effect of condition, F(1, 4) = 0.42, ns, and no interaction, F(9, 36) = 1.54, ns, but did show a main effect of reinforcer order, F(9, 36) = 9.0, p < .001.

[FIGURE 1 OMITTED]

Table 1 shows the correlations between PRPs and the number of revolutions from the last 10 sessions in the FR variable-revolution condition (Condition 2). Inspection of the correlations shows no evidence of a consistent relationship between revolutions and PRPs. Correlations for rats NC7, NC8, and NC11 were not significant, whereas the correlations for NC13 and NC14 were significant, but in opposite directions. For NC13, greater numbers of revolutions were associated with shorter PRPs; for NC14, greater numbers of revolutions were associated with longer PRPs. For data from all rats combined, the correlation was not significant.

To assess the possibility that a relationship between revolutions and PRPs was obscured by the within-session effect on PRPs, partial correlation coefficients were calculated with the effect of reinforcer order partialled out. Inspection of these correlations shows that they did not differ substantively from the uncontrolled correlations.

Figure 2 shows the initial latency to respond and subsequent PRPs in the second FR 30 fixed-revolution 60 (Condition 3) and the VR 30 fixed-revolution 60 (Condition 4) schedules for each rat and the group. In general, as observed in Figure 1, PRPs tended to decrease across the session in both conditions with the exception of PRPs in the FR fixed-revolution condition for NC13. For this rat, PRPs did not appear to change systematically across the session. With respect to the effect of the ratio requirement being variable as opposed to fixed, as would be expected, PRPs were lower with the variable response requirement. A repeated-measures ANOVA with condition and reinforcer order as within-subject variables revealed significant main effects of condition, F(1, 4) = 68.17, p = .001, and reinforcer order, F(9, 36) = 12.51, p < .001, but not a significant interaction, F(9, 36) = 1.88, ns. Mean PRPs when the ratio requirement was fixed and variable were 19.67 s and 10.41 s, respectively.

[FIGURE 2 OMITTED]

Discussion

The objectives of this study were to describe the effect of a variable response requirement on PRP duration between conditions and the relationship between revolutions and PRP duration within a session. With respect to the first objective, as expected, changing the response requirement from fixed to variable reduced average PRP duration by 50%. Thus, as observed with more conventional reinforcers, PRPs were shorter when the response requirement was unpredictable and longer when it was constant. In and of itself, this finding is unremarkable; however, it is of interest to note that although the response requirement was variable, PRP duration was still over 10 s on average. This contrasts with the finding that PRPs on a comparable schedule with a more conventional reinforcer (i.e., a food pellet) are minimal or nonexistent. This suggests that across qualitatively different reinforcers, the relative contributions of excitatory aspects of the reinforcer and inhibitory aftereffects to PRP duration vary. With wheel-running reinforcement, the longer PRP duration could be a function of differences in either of these two factors. Certainly, running for 60 revolutions takes longer and requires more effort than eating a 45-mg food pellet, which would suggest the possibility that fatigue might play a role in the long PRPs observed with wheel-running reinforcement. Satiation would be another possibility, if an animal can become satiated for this reinforcer (Belke, 2006b). Differences in excitatory aspects of reinforcers could also play a role. Previous research with wheel-running reinforcement suggests that the value of an opportunity to run is approximately equivalent to that of a 2.5% sucrose solution (Belke & Hancock, 2003; Belke, Pierce, & Duncan, 2006). Weaker excitatory effects might lead an animal to take longer to initiate responding to obtain the next reinforcer. The stronger excitatory effects of a higher concentration of sucrose would likely lead to quicker initiation of responding. Further investigation of wheel-running reinforcement is required to determine the relative roles of these factors in the longer PRPs observed with wheel running.

With respect to the second objective, the current study failed to replicate the weak positive correlation between revolutions and PRP duration previously observed by Belke (2000). The observation of a substantive within-session decrease in PRPs in the current study, which was not assessed in Belke's (2000) study, suggested a possible reason for this failure, that is, the within-session effect obscured the predicted relation. However, partial correlation coefficients did not support this possibility.

How the reinforcer was defined might be another possibility. Although the author assumed that defining the reinforcer in terms of revolutions rather than time available to run would provide stronger evidence of a relationship between revolutions run and PRPs, the opposite may be the case. When the reinforcer is defined in terms of a number of revolutions to be run, there is no cost associated with pausing within the reinforcement interval. In contrast, when the reinforcer is defined as a time available to run, pausing within the reinforcement interval reduces the number of revolutions run. If pausing during the reinforcement interval is more likely when the reinforcer is defined in terms of revolutions, then this would diminish the contribution of inhibitory aftereffects such as fatigue to PRP duration. That is, on a reinforcer defined by a substantive number of revolutions, a rat might pause during the wheel-running interval and alleviate fatigue before resuming running. This could potentially eliminate the relationship between revolutions run and the subsequent PRP. Future research is required to determine if rats behave differently within the reinforcement interval when the reinforcer is defined in terms of revolutions rather than time.

The failure to find a relationship between revolutions and PRP duration in the current study despite clear evidence of a relationship using a between-conditions procedure is not unique to wheel running as a reinforcer. A similar inconsistency was noted by Bonem and Crossman (1988) with respect to a relationship between reinforcer magnitude and PRP duration with more conventional reinforcers. Specifically, Lowe, Davey, and Harzem (1974) showed that when sucrose concentration was varied within a session, PRPs on a FR schedule increased as sucrose concentration increased. In contrast, when magnitude was varied between conditions, PRP duration either decreased with reinforcer magnitude (Meunier & Starratt, 1979) or did not vary (Harzem, Lowe, & Davey, 1975). Similar inconsistencies appeared with respect to the effect of magnitude on response rates. This issue has yet to be resolved. Bonem and Crossman (1988) stated, "Whether within-session manipulations are sufficient to produce a magnitude effect has not been satisfactorily answered; that is, the within-session factor has not been isolated from other potentially controlling factors" (p. 356). With respect to wheel-running reinforcement, these "other potentially controlling factors" have also yet to be determined. Type of schedule, response contingency, within-session changes in wheel-running rate, number of reinforcers per session, and so forth may all play a role in determining the observed pattern of PRPs within a session. The effect of these variables and potential interactions between them are likely to be quite complex.

Although this study failed to find a positive relationship between revolutions and PRP duration, it did reveal a within-session decrease in PRPs on FR and VR schedules of wheel-running reinforcement with the reinforcement schedule held constant that has not been previously documented. Belke (1996) showed that PRPs decreased throughout a session of lever pressing for wheel-running reinforcement; however, in this case, the schedule of reinforcement (i.e., tandem FR 1 VI) varied within a session. Belke suggested that the changes in PRPs were not a function of changes in the schedule requirement because lever-pressing rates, but not PRPs, varied with changes in the reinforcement schedule. Instead, PRPs varied inversely with wheel-running rates. Wheel-running rates increased while PRPs decreased, suggesting the possibility that the changes in PRPs were related to changes in wheel-running rates.

In contrast to the present study, Belke (1997) found no systematic changes in PRPs within a session. In that study, rats responded on tandem FR 1 VI 30-s schedules of wheel-running reinforcement, with the opportunity to run varying across durations of 30 s, 60 s, and 120 s. Wheel-running and local lever-pressing rates, averaged over successive groupings of four reinforcers, systematically increased throughout a session; however, PRPs showed no systematic linear trends. As suggested previously, the other factors that play a role in determining PRP duration within a session need to be determined in order to better understand differences between qualitatively different reinforcers.

With respect to this goal, it was suggested earlier in this article that within-session changes in PRP duration might reflect habituation and/or sensitization to the sensory properties of reinforcement (McSweeney & Murphy, 2009) or an initial aversion to wheel running that gives way to a rewarding aftereffect (Lett, Grant, Koh, & Smith, 2001). While the current findings are silent with respect to the former explanation, they are not with respect to the latter. The problem with the explanation that longer PRPs at the beginning of a session may be due to an aversion to wheel running is that the initial latencies to lever press following commencement of the session were longer than the PRPs following the first reinforcer for some rats but shorter for others. If rats were experiencing an aversion to stimuli predictive of a session of wheel running, one would expect that this initial latency would be the longest, as the aversion would be strongest. Clearly this was not the case.

In summary, the current study showed that PRP duration decreased when the response requirement was changed from fixed to variable, but it did not change with variation in the number of revolutions between reinforcers within a session. Thus, while PRP duration appears to be related to wheel running when duration of opportunity to run is varied across conditions, this relationship is not evident when the number of revolutions during a reinforcement interval is varied within a condition. Although PRP duration did not vary with revolutions, it systematically decreased within the session. Further research is required to elucidate variables controlling PRP duration within a session for both conventional reinforcers as well as unconventional reinforcers such as wheel running.

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Terry W. Belke

Mount Allison University

This research was supported by Grant 0GP0170022 from the Natural Sciences and Engineering Research Council of Canada.

Correspondence concerning this article should be addressed to Terry W. Belke, Psychology Department, Mount Allison University, Sackville, New Brunswick, Canada, E4L 1C7. E-mail: tbelke@mta.ca
Table 1

Correlations Between PRPs and Revolutions in the Variable-Revolution
Condition (VR Revs) and Partial Correlation Coefficients Between PRPs
and Revolutions in the Variable-Revolution Condition (pVR Revs) With
the Effect of Reinforcer Order Partialled out for Each Animal and the
Group

Rat    VR Revs  pVR Revs

NC7      .15      .17
NC8     -.01     -.01
NC11     .17      .18
NC13    -.24 *   -.15
NC14     .26 *    .25 *
Group    .05      .06

* p < .05.
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