FOR BETTER OR WORSE: EFFECT OF UPCOMING REINFORCER TYPE ON RATS' LEVER PRESSING FOR LOW-CONCENTRATION SUCROSE REINFORCERS

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1 The Psychlgical Recrd, 21, 51, FOR BETTER OR WORSE: EFFECT OF UPCOMING REINFORCER TYPE ON RATS' LEVER PRESSING FOR LOW-CONCENTRATION SUCROSE REINFORCERS JEFFREY N. WEATHERLY University f Nrth Dakta JASON E. STOUT, CAROLYN S. DAVIS, and CAM L. MELVILLE McNeese State University Recent research has demnstrated that fd-pellet reinfrcement upcming in the secnd half f an experimental sessin can increase perant respnse rates fr lw-cncentratin liquid-sucrse reinfrcers in the first half. The present study investigated whether an upcming reinfrcer ther than fd pellets culd prduce a similar increase (Experiment 1) and whether a decrease in respnding wuld be bserved by wrsening the upcming cnditins f reinfrcement (Experiment 2). Experiment 1 demnstrated that rats' rates f lever pressing fr 1 r 5% liquid-sucrse reinfrcers delivered by a randm-interval 6-s schedule during the first half f a 5-min sessin were similarly increased by the delivery f fd r sucrse pellets in the secnd half. Experiment 2 demnstrated that rates f lever pressing fr 5% liquid-sucrse reinfrcers in the first half f the sessin were increased and decreased by upcming fd-pellet and 1 % liquid-sucrse reinfrcement, respectively. The results frm bth experiments represent inductin (i.e., a direct relatinship between rate f respnding and the upcming cnditins f reinfrcement). They suggest that the inductin may be bth general (i.e., nt limited t specific upcming reinfrcers) and symmetrical (i.e., manifesting in either an increase r a decrease in respnding), the latter f which prvides a ptential link t the study f behaviral cntrast. Behaviral cntrast (Reynlds, 1961) is said t ccur when a subject's rate f respnding in ne, unchanged, cmpnent f a multiple schedule f reinfrcement varies inversely with changes in the cnditins f reinfrcement in anther cmpnent (McSweeney & Nrman, 1979). Behaviral cntrast prcedures typically emply pigens t peck keys fr reinfrcers delivered at high rates n multiple schedules that have shrt cmpnents which deliver the same reinfrcer and that alternate many times We thank the Department f Bilgical and Envirnmental Sciences at MSU fr its willingness t share labratry space and Frances K. McSweeney fr her insightful cmments n an earlier versin f this manuscript. Crrespndence regarding this article shuld be directed t Jeffrey N. Weatherly, Ph.D., Department f Psychlgy, University f Nrth Dakta, Grand Frks, NO ( jeffrey-weatherly@und.ndak.edu).

2 63 WEATHERLY ET AL. during the experimental sessin (see McSweeney & Weatherly, 1998, fr a review). When these prcedures are emplyed, cntrast reliably results. Hwever, if ne deviates frm these prcedural details, then the ppsite f cntrast (Le., inductin) may ccur. Weatherly, Stut, McMurry, Rue, and Melville (1999, Experiment 2), fr instance, had rats lever press n a multiple schedule with lng cmpnents (Le., 3 min) that delivered different reinfrcers (5% liquid sucrse r a fd pellet). Specifically, subjects respnded in 6-min sessins in which 5% liquid-sucrse reinfrcers were delivered during the first 3 min f the sessin and 45-mg fd-pellet reinfrcers were delivered during the secnd 3 min. Acrss cnditins, rate f fd-pellet reinfrcement was varied frm a variable-interval (VI) 15-s t a VI 24-s schedule in the secnd half f the sessin while the rate f sucrse reinfrcement in the first half was held cnstant at a VI 6-s schedule. Results shwed that rates f respnding fr sucrse in the first half f the sessin increased (inductin 1 ), rather than decreased (cntrast), as the rate f fd-pellet reinfrcement increased. In ther wrds, the rats' rates f respnding fr the liquid sucrse varied directly with the upcming cnditins f reinfrcement. Weatherly, Stut, Rue, and Melville (2) replicated this inductin. They had rats respnd in 5-min sessins in which liquid-sucrse reinfrcers were available during the first 25 min f the sessin and, acrss cnditins, either the same sucrse reinfrcers r fd pellets were available during the secnd 25 min. Higher rates f respnding were bserved in the first half f the sessin when fd pellets, rather than sucrse reinfrcers, were delivered in the secnd half. The results als shwed that the sie f inductin varied inversely with the cncentratin f sucrse. Upcming fd-pellet reinfrcement had a large effect n respnding fr 1 % sucrse (which maintains very lw rates f respnding). It had little t n effect n respnding fr 25% sucrse (which maintains mderate t high rates f respnding). Weatherly, Davis, and Melville (2) attempted, acrss three experiments, t pinpint prcedural factrs cntributing t this inductin. They investigated whether the presence r absence f a discriminative stimulus fr the secnd half f the sessin (Experiment 1) r the predictability f the timing f the switch frm liquid-sucrse t fd-pellet reinfrcement (Experiment 2) wuld alter the inductin. Neither did. Hwever, their Experiment 3 demnstrated that the certainty f the 1This result can be cnsidered inductin fr tw reasns. First, increasing the upcming rate f fd-pellet reinfrcement represented an increase in the cnditins f reinfrcement. Secnd, fd-pellet reinfrcement, in itself, represented an increase in the cnditins f reinfrcement cmpared t lw-cncentratin (i.e., 1% r 5%) liquid-sucrse reinfrcement. This latter claim can be supprted by several pieces f evidence. Fd pellets maintain higher rates f perant respnding at the same rate f reinfrcement than d lwcncentratin sucrse reinfrcers (e.g., see Weatherly et ai., 1999). On a cncurrent schedule f reinfrcement in which bth fd-pellet and liquid-sucrse reinfrcers are available at an equal rate, rats prprtin mre respnses fr fd pellets than fr sucrse (unpublished research frm ur labratry). Finally, negative successive cntrast (see Flaherty, 1996) is bserved when subjects are switched frm fd-pellet t liquid-sucrse reinfrcement (Weatherly & Multn, 21).

3 UPCOMING REINFORCER 631 upcming fd-pellet reinfrcement was influential. Acrss cnditins, the prbability that fd-pellet reinfrcement wuld be delivered in the secnd half f the sessin was varied frm. t 1.. Inductin was bserved fr respnding fr sucrse in the first half f the sessin when the prbability f fd reinfrcement in the secnd half f the sessin was 1. (cmpared t when it was.). Altering the prbability f upcming fd-pellet reinfrcement (.25,.5, &.75) altered the inductin. Increases in the rate f respnding acrss the first half f the sessin, measured by the slpe, were directly related t the prbability f upcming fd-pellet reinfrcement. Thus, the results f Weatherly, Davis, et al. (2) suggested that the prbability, and nt discriminability r timing, f upcming differential reinfrcement cntributes t the inductin. The abve studies demnstrate that upcming fd-pellet reinfrcement can increase rate f respnding fr lw-cncentratin sucrse reinfrcers. Hwever, their results are limited. Each used upcming fd-pellet reinfrcement t prduce inductin. Mrever, each prduced psitive inductin (i.e., respnse rates were increased by imprving the upcming cnditins f reinfrcement). Thus, they can nt speak t whether an upcming reinfrcer ther than a 45-mg fd pellet can prduce inductin r whether inductin may als manifest in a decrease in respnding when the upcming cnditins f reinfrcement are wrse than the current cnditins (i.e., negative inductin). The present study was designed t make these determinatins. Making them was deemed imprtant fr several reasns. First, determining whether different upcming reinfrcers can prduce psitive r negative inductin will help determine the generality f the effect. If nly psitive inductin is bserved and is nly prduced in rats when fdpellet reinfrcement is upcming, then the effect is very limited. On balance, if bth psitive and negative inductin can be prduced by different upcming reinfrcers, then the scpe f the effect is ptentially very brad. Secnd, by determining whether different upcming reinfrcers can prduce psitive r negative inductin, a theretical link between the present effect and that f behaviral cntrast may be established. Mre specifically, it has been argued that behaviral cntrast is a symmetrical effect (i.e., alteratins f the same factr, such as rate f reinfrcement, can prduce either psitive r negative behaviral cntrast; see McSweeney & Weatherly, 1998). If the present effect is related t behaviral cntrast (e.g., the inverse f the cnditins that prduce behaviral cntrast), then ne wuld expect that the present inductin wuld als be symmetrical. Tw experiments were cnducted. Experiment 1 attempted t determine whether an upcming reinfrcer ther than a Nyes fd pellet culd prduce inductin. Experiment 2 attempted t determine whether psitive r negative inductin wuld be bserved when the upcming cnditins f reinfrcement were better r wrse, respectively, than the current cnditins.

4 632 WEATHERLY ET AL. Experiment 1 Previus studies (e.g., Weatherly, Davis, et ai., 2; Weatherly, Stut, et ai., 2) have demnstrated that rats respnding fr lwcncentratin liquid-sucrse reinfrcers (e.g., 1 % r 5% sucrse) will display increased respnse rates when fd-pellet (P. J. Nyes, Frmula All), rather than the same liquid-sucrse, reinfrcement will be available in the secnd half f the sessin. The present experiment investigated whether upcming sucrse-pellet (P. J. Nyes, Frmula F) reinfrcement wuld prduce a similar effect. A sucrse pellet was chsen fr several reasns. Fr ne, this type f pellet varies frm the previusly used fd pellet in a number f ways, including clr, taste, nutritinal value, and calric cntent. Secndly, sucrse pellets als maintain higher perant rates f respnding than lw-cncentratin liquid-sucrse reinfrcers (unpublished pilt data frm ur labratry). Thus, it was ptentially pssible t cmpare any inductin prduced by upcming sucrse pellets t that prduced by upcming fd pellets. Methd Subjects. The subjects were 8 experimentally naive male Sprague Dawley rats btained frm Charles River Labratries. Subjects were hused individually, had free access t water (nly) in the hme cage, and experienced a 14/ hr light/dark schedule (6-2). Subjects were maintained at apprximately 85% f their free-feeding bdy weight via pstsessin feedings when necessary r by daily feedings n days in which sessins were nt cnducted. Apparatus. The apparatus was an experimental chamber fr rats (MED Assciates, ENV-8) that measured 23.5 by 3.5 by 21 cm. A s cm respnse lever was lcated 1.5 cm frm the left edge f the frnt panel and 8 cm abve the grid flr. The lever extended 2 cm int the chamber. A frce f apprximately.25 N was required t depress the lever. An identical lever was symmetrically lcated n the right side f the frnt panel. It was nt used in the present experiment. A 2.5-cm diameter stimulus light was lcated 6.5 cm abve each lever. A 5- by 5-cm aperture, which culd allw access t reinfrcement, was centered n the frnt panel, 5 cm abve the flr. A liquid-drp dispenser (MED Assciates, ENV-21 A) and a pellet dispenser (MED Assciates, ENV- 23) were lcated behind the frnt panel. Bth culd deliver reinfrcers int a 2.5-cm diameter cup recessed within the aperture. A huselight was centered n the back wall f the chamber, 2 cm belw the ceiling. The experimental chamber was hused in a sund-attenuating chamber, with a ventilating fan masking nise frm the utside. Experimental events were prgrammed and data were recrded by an IBM-cmpatible cmputer that was running MED-State sftware and cnnected t a MED Assciates interface. The cmputer and experimental chamber were lcated in the same rm.

5 UPCOMING REINFORCER 633 Prcedure. Subjects were trained t press the left lever using shaping by successive apprximatins. Once each subject had pressed the lever mre than times, it was placed n the experimental prcedure. Subjects respnded in sessins that were 5 min in length. Pressing the left lever was reinfrced n a randm-interval (RI) 6-s schedule thrughut the sessin. Reinfrcers were prgrammed at a prbability f.167 every 1 s as lng as a reinfrcer was nt already scheduled fr delivery. That is, a scheduled reinfrcer needed t be cllected befre the next interreinfrcer interval began. Neither the sessin timer nr the interreinfrcer interval advanced during the delivery f a reinfrcer. The huselight and the light abve the left lever were illuminated thrughut the sessin. Three types f cnditins were cnducted. In ne (Suc-Suc),.2 ml f liquid sucrse served as the reinfrcer in bth halves f the sessin. In anther (Suc-FdPel), liquid-sucrse reinfrcers were delivered in the first 25 min f the sessin and a 45-mg Nyes fd pellet (Frmula All) served as the reinfrcer in the secnd 25 min. In the third cnditin (Suc SucPel), liquid-sucrse reinfrcers were delivered in the first half f the sessin and a 45-mg Nyes sucrse pellet (Frmula F) served as the reinfrcer in the secnd half. Each cnditin was cnducted fr a ttal f 2 sessins with sessins cnducted daily, 5 t 7 days per week. The subjects were divided int tw grups. The first grup respnded fr 1 % liquid sucrse (v/v mixed with tap water). The secnd grup respnded fr 5% liquid sucrse. One pair f subjects in each grup received the fllwing rder f cnditins: Suc-SucPel, Suc-FdPel, and Suc-Suc. The ther pair f subjects received the reverse rder f cnditins. Once each grup had cmpleted the three cnditins, the sucrse cncentratin fr which they respnded was reversed. Each pair f subjects then respnded in the same three cnditins, but in the reverse rder as they experienced them when respnding fr the previus sucrse cncentratin. Results and Discussin Figure 1 presents the results bserved during cnditins in which subjects respnded fr 1 % (tp graph) r 5% sucrse (bttm graph). It presents rate f respnding (in respnses/min) acrss successive 5-min intervals f the sessin during the Suc-Suc (clsed squares), Suc-SucPel (pen squares), and Suc-FdPel (clsed circles) cnditins. Each functin represents the mean fr all subjects respnding during the final five sessins f a particular cnditin. The errr bars represent the standard errr f the mean fr respnding during that particular 5-min interval. The data in Figure 1 leave several visual impressins. First, inductin was apparent in the first half f the sessin. Fr bth cncentratins f sucrse, respnse rates in the first half f the sessin were higher when either sucrse pellets r fd pellets served as the reinfrcer in the secnd half than when the same liquid sucrse served. Secnd, the

6 634 WEATHERLY ET AL. 1% SUCROSE 25 ~ ~ "- 2 (!) 15 C V) UJ UJ l- «S % SUCROSE 25 Z 2 :i "- ~ (!) C V) 15 UJ UJ l- «S Sue- Sue ~ Sue- SuePel -- Sue-FdPel S-MIN INTERVAL Figure 1. Rates f respnding (in respnses/min) acrss successive 5-min intervals f the sessin fr the mean f all subjects respnding fr 1 % (tp graph) r 5% sucrse (bttm graph) when liquid sucrse (clsed squares), sucrse pellets (pen squares), r fd pellets (clsed circles) served as the reinfrcer in the secnd half f the sessin. Errr bars represent the standard errr f the mean acrss subjects fr respnding in that particular 5-min interval.

7 UPCOMING REINFORCER 635 presence f inductin did nt vary nticeably between pellet types. Third, bth types f pellets maintained higher rates f respnding in the secnd half f the sessin than did either liquid-sucrse cncentratin. Statistical analyses cnfirmed these impressins. Respnding frm the first half f the sessin was analyed by cnducting a three-way (Sucrse cncentratin x Secnd-half reinfrcer x 5-min interval) repeated measures analysis f variance (ANOVA) n the rates f respnding f individual subjects. Results f this analysis shwed significant main effects f sucrse cncentratin, F(1, 7) = 12.68, P <.1, and secnd-half reinfrcer, F(2, 14) = 8.57, P <.1, indicating that subjects respnded significantly higher fr the 5% sucrse than fr the 1 % sucrse and that rates f respnding in the first half f the sessin varied with the type f reinfrcer delivered in the secnd half, respectively. The nly ther effect t reach significance was the interactin between secnd-half reinfrcer and 5-min interval, F(8, 56) = 3.24, P <.1, indicating that respnding changed acrss the first 25 min f the sessin differently when different reinfrcers were delivered in the secnd half f the sessin. Results frm this analysis, and all that fllw, were cnsidered significant at p <.5. Althugh the significant main effect f secnd-half reinfrcer suggests that inductin was bserved fr respnding in the first half f the sessin, it des nt necessarily mean that the upcming sucrse pellets prduced a similar inductin effect as did the upcming fd pellets. In ther wrds, the significant main effect culd have been prduced by the fd pellets alne. T make this determinatin, a fllw-up three-way (Sucrse cncentratin x Pellet type x 5-min interval) repeated measures ANOVA was cnducted using data frm the first half f the sessin f the Suc-SucPel and Suc Fd Pel cnditins. The main effect f sucrse cncentratin was again significant, F(1, 7) = 6.31, P <.5, indicating that 5% sucrse maintained higher respnse rates than did 1 % sucrse. Hwever, the main effect f pellet type was nt significant, indicating that respnding in the first half f the sessin did nt differ between these cnditins. The main effect f 5-min interval was significant, F(4, 28) = 3.95, p <.5, indicating that respnse rates changed acrss the first half f the sessin when pellets were delivered in the secnd half f the sessin. N ther effects reached Significance. These results thus supprt the cnclusin that inductin was bserved when subjects respnded fr either 1 % r 5% sucrse and that bth types f pellets prduced a similar inductin effect. Respnding in the secnd half f the sessin was analyed by cnducting a three-way (Sucrse cncentratin x Secnd-half reinfrcer x 5-min interval) repeated measures ANOVA n respnding frm all three cnditins. In this analysis, the main effects f secnd-half reinfrcer, F(2, 14) = 11.16, P <.1, and f 5-min interval, F(4, 28) = 5.22, p <.1, were significant, indicating that the different reinfrcers maintained different respnse rates and that thse rates changed acrss the secnd half f the sessin, respectively. The interactins between sucrse cncentratin and secnd-half reinfrcer, F(2, 14) = 7.54, p <.1, and between secnd-half

8 636 WEATHERLY ET AL. reinfrcer and 5-min interval, F(8, 56) = 4.67, P <.1, were als significant. These results suggest that the different reinfrcers maintained different abslute rates f respnding between the 1 % and 5% sucrse cnditins and that different within-sessin changes in respnding were bserved in the secnd half f the sessin fr the different reinfrcers, respectively. Again, the main effect f secnd-half reinfrcer des nt allw fr a cnclusin n whether the different pellet types maintained different respnse rates. Therefre, a fllw-up three-way (Sucrse cncentratin x Pellet type x 5-min interval) repeated measures ANOVA was cnducted n respnding frm the secnd half f the sessin f the Suc-SucPel and Suc-FdPel cnditins. Results indicated that the main effect f 5-min interval, F(4, 28) = 5.75, P <.1, was significant, indicating that rates f respnding changed acrss the secnd half f the sessin in these cnditins. Hwever, n ther effects reached significance, indicating that the different types f pellets did nt differ in the behavir they maintained in the secnd half f the sessin. The results f Experiment 1 suggest that inductin prduced by upcming reinfrcement is nt limited t the use f fd pellets. Similar inductin effects were bserved when a slid sucrse pellet served as the upcming reinfrcer. Besides increasing the ptential generality f the present inductin effect, these results may als prvide sme insight int the factrs that cntrl (r d nt cntrl) the effect. Intuitively, the present results wuld seem t run cntrary t the idea that a difference in taste between the liquid sucrse and the upcming reinfrcer plays a rle in the inductin. That is, a switch frm liquid sucrse t a slid sucrse pellet prduced inductin. Such a cnclusin cannt be unequivcally accepted, hwever, because subjects' behavir was similarly supprted by bth the sucrse and the fd pellets. Thus, it is nt pssible t cnclude that subjects discriminated between the tw. Hwever, because the tw types f pellets did differ in a number f ways (e.g., taste, clr, calric cntent, etc.), it seems mre likely that they maintained similar rates f behavir because they had a similar "value" than because subjects did nt discriminate between them. Such a cnclusin implies that the inductin effect may then be influenced by the "value" f the upcming reinfrcer. If such were the case, then ne might expect that an upcming reinfrcer with a lwer "value" than the present ne might prduce a negative inductin effect (Le., a decrease in respnse rate). Experiment 2 explred this idea. Experiment 2 Experiment 2 was designed in an attempt t prduce negative inductin. As nted abve, all previus studies n the present effect, including Experiment 1, have prduced a psitive inductin effect by bettering the upcming cnditins f reinfrcement. Negative inductin was attempted by wrsening f the upcming cnditins f reinfrcement by delivering a reinfrcer in the secnd half f the sessin that maintained a lwer rate f respnding than the ne delivered in the first.

9 UPCOMING REINFORCER 637 Methd Subjects and apparatus. The subjects were 4 experimentally naive male Sprague-Dawley rats. They were btained, hused, and maintained as were thse in Experiment 1. The apparatus was a chamber fr rats (Med Assciates, ENV-7) measuring 28 by 22 by 28 cm. It was a tw-lever chamber, nearly identical t that used in Experiment 1. One difference was that tw types f liquid reinfrcers culd be delivered t the recessed cup via tw 22-gauge pipettes, each cnnected t a separate syringe pump (Med Assciates, PHM-) lcated utside f the sund-attenuating chamber. A secnd difference was that a nse-pke apparatus, which was nt used, was centered n the back wall. The nse-pke pening was 2 cm in diameter, with the bttm f the pening being lcated 1.5 cm abve the flr. Prcedure. Subjects were trained t press the left lever using shaping by successive apprximatins. Once each subject had pressed the lever mre than times, it was placed n the experimental prcedure. The prcedure was mdeled after that used by Weatherly, Stut, et al. (2). Subjects respnded in sessins that were 5 min in length. Pressing the left lever was reinfrced n a RI 6-s schedule thrughut the sessin, with reinfrcers prgrammed identically as in Experiment 1. The huselight and the light abve the left lever were illuminated thrughut the sessin. Tw sets f cnditins, cnstant and randm, were cnducted. In all cnditins, the reinfrcer in the first half (25 min) f the sessin was.2 ml f a 5% liquid-sucrse slutin. In the cnstant cnditins, the type f reinfrcer delivered in the secnd half f the sessin (25 min) was held cnstant acrss 2 cnsecutive sessins. Three such cnditins were cnducted. Type f secnd-half reinfrcer was either.2 ml f 5% sucrse (5% - 5%), a 45-mg Nyes fd pellet (5% - FP), r.2 ml f 1% sucrse (5% - 1%). One-percent sucrse was chsen because it maintains significantly lwer respnse rates than des 5% sucrse (e.g., see results f Experiment 1). In randm cnditins, the type f reinfrcer delivered in the secnd half f the sessin was determined randmly prir t each sessin. This cnditin was cnducted until each f the three pssible reinfrcers had been delivered in the secnd half f the sessin a ttal f 2 times. Because the prcedure was truly randm, sme reinfrcers were presented in the secnd half f the sessin mre than 2 times befre anther reinfrcer had been presented 2 times. Sessins subsequent t the 2th sessin fr a particular reinfrcer were nt analyed. Tw subjects experienced the cnstant cnditins prir t the randm cnditins. The ther pair f subjects received the reverse rder. Within each pair f subjects, 1 subject experienced the cnstant cnditins in the fllwing rder: 5% - FP, 5% - 1 %, and 5% - 5%. The ther subject received the reverse rder. Sessins were cnducted daily, 5 t 7 days per week.

10 638 WEATHERLY ET AL. Results and Discussin Figure 2 presents the rates f respnding fr 5% sucrse acrss successive 5-min intervals in the first half f the sessin (Intervals 1 thrugh 5) fr each cnditin. It was cnstructed similarly t Figure 1. Each functin represents the mean fr all subjects when 1% sucrse (pen circles), 5% sucrse (clsed squares), r a fd pellet (pen triangles) served as the reinfrcer in the secnd half f the sessin. The tp graph presents the results frm the cnstant cnditins. The middle graph presents first-half respnding frm the randm cnditins in which the different reinfrcers were delivered in the secnd half f the sessin. The bttm graph presents the same data as the middle graph but recalculated in terms f the type f reinfrcer that was presented in the secnd half f the sessin in the previus sessin (see explanatin that fllws). The tp graph f Figure 2 demnstrates that respnding in the first half f the sessin during the cnstant cnditins varied as a functin f upcming reinfrcer. Rate f respnding fr 5% sucrse was lwest when 1 % sucrse served as the reinfrcer during the secnd half f the sessin. Respnse rates were highest and increased mst steeply during the first half f the sessin, when fd pellets were delivered in the secnd half f the sessin. Statistical analyses supprted these impressins. A tw-way (Secnd-half reinfrcer by 5-min interval) repeated measures ANOVA, cnducted n respnse rates fr individual subjects in the cnstant cnditins, prduced a significant main effect f secnd-half reinfrcer, F(2, 6) = 8.62, P <.5; main effect f 5-min interval, F(4, 12) = 19.46, P <.1 ; and interactin term, F(8, 24) = 3.73, P <.1. These results indicate that average rate f respnding in the first half f the sessin differed when different reinfrcers were delivered in the secnd half f the sessin, that rates f respnding changed acrss the first half f the sessin, and that the changes were different when different reinfrcers were delivered in the secnd half f the sessin, respectively. T better elucidate the effects in the tp graph f Figure 2, fllw-up tw-way (Secnd-half reinfrcer by 5-min interval) repeated measures ANOVAs were cnducted n each pssible pair f cnstant cnditins. Of particular interest were the main effects f secnd-half reinfrcer, which indicate whether the upcming reinfrcer prduced different rates f respnding in the first half f the sessin, and the interactin terms, which indicate whether the upcming reinfrcer prduced different changes in respnding during the first half. Cmparisn f the 5% - 1 % and 5% - 5% cnditins prduced a main effect f secnd-half reinfrcer that apprached significance, F(1, 3) =.1, P <.51, and a nnsignificant interactin. Cmparisn f the 5% - 5% and 5% - FP cnditins prduced a nnsignificant main effect f secnd-half reinfrcer but a significant interactin term, F(4, 12) = 3.68, P <.5. Finally, cmparisn f the 5% - 1 % and 5% - FP cnditins prduced a significant main effect f secndhalf reinfrcer, F(1, 3) = 37.13, P <.1, and interactin term, F(4, 12) = 5.35, P <.1. In general, these results suggest that the negative inductin

11 UPCOMING REINFORCER FIRST-HALF RESPONDING CONSTANT CONDITIONS " is U> w w!c Z i e. " is U> w W l e " is U> w W l e 15 O+-----r---~-----r----~--_, RANDOM CONDITIONS PRESENT SESSION'S REINFORCER RANDOM CONDITIONS PREVIOUS SESSION'S REINFORCER l%suc - 5OY.SUC _ RJOOPEL 4 5-MIN INTERVAL Figure 2. Rates f respnding (in respnses/min) acrss successive 5-min intervals in the first half f the sessin fr the mean f all subjects respnding fr 1 % (pen circles), 5% (clsed squares), r fd-pellet (pen triangles) reinfrcement in the secnd half f the sessin. The tp graph presents respnding during the cnstant cnditins. The middle graph presents respnding during the randm cnditins when a certain reinfrcer was delivered in the secnd half f the present sessin. The bttm graph presents respnding during the randm cnditins when a certain reinfrcer was delivered in the secnd half f the previus sessin. Errr bars represent the standard errr f the mean acrss subjects fr respnding in that particular 5-min interval.

12 64 WEATHERLY ET AL. effect is best described as a change in the verall rate f respnding. The psitive inductin effect, hwever, is best described as change in the pattern f respnding acrss the first half f the sessin.. The middle graph f Figure 2 shws that the differences in the tp graph were the result f cnsistent experience with thse reinfrcers ccurring in the secnd half f the sessin. When subjects received the same ttal amunt f experience with the same secnd-half reinfrcers, but n a randm basis, respnding during the first half f the sessin did nt vary with type f upcming reinfrcer. This impressin was tested by cnducting a tw-way (Secnd-half reinfrcer by 5-min interval) repeated measures ANOVA n respnse rates fr individual subjects in the first half f the sessin fr the randm cnditins in which a particular secnd-half reinfrcer was delivered during the present sessin. The analysis prduced a significant main effect f 5-min interval, F(4, 12) = 7.82, P <.1, indicating that respnse rates changed acrss the first half f the sessin. Hwever, neither the main effect f secnd-half reinfrcer nr the interactin term was significant, indicating that neither the average rate f respnding nr the pattern f respnding differed in the first half f the sessin when different reinfrcers were randmly delivered in the secnd half. Because the type f upcming reinfrcer in the randm cnditins was unpredictable, it wuld seem unreasnable t expect that it wuld alter respnding during the first half f the sessin. It wuld seem mre reasnable t suspect that the reinfrcer delivered in the secnd half f the previus sessin may have influenced respnding in the first half f the subsequent sessin. The bttm graph presents first-half respnding frm the randm cnditins based upn the previus sessin's secndhalf reinfrcer. An identical tw-way ANOVA was cnducted n these data, which prduced the identical results. The main effect f 5-min interval, F(4, 12) = 6.99, P <.1, was significant, but the main effect f secnd-half reinfrcer and interactin term were nt, indicating that the previus sessin's secnd-half reinfrcer did nt cntrl subjects' respnding in the first half f the next sessin. Figure 3 presents results frm the secnd half f the sessin. As expected, the different reinfrcers delivered in the secnd half f the sessin maintained different rates f respnding. It presents the rates f respnding that were bserved acrss successive 5-min intervals (Intervals 6 thrugh ) f the secnd half f the sessin in the cnstant (tp graph) and randm (bttm graph) cnditins when subjects respnded fr 1% sucrse (pen circles), 5% sucrse (clsed squares), r fd pellets (pen triangles). Figure 3 was cnstructed similarly t Figure 1. The data in Figure 3 were analyed by cnducting a three-way (Cnditin x Secnd-half reinfrcer x 5-min interval) repeated measures ANOVA n the respnse rates f individual subjects in the cnstant and randm cnditins. This analysis prduced a significant main effect f secnd-half reinfrcer, F(2, 6) = 5.67, P <.5, indicating that the different reinfrcers supprted different rates f respnding. The interactin between secnd-half reinfrcer and 5-min interval was als significant,

13 UPCOMING REINFORCER ~ 5 ~ CJ is Q. en W II.. W ~ c SECOND-HALF RESPONDING CONSTANT CONDITIONS / I RANDOM CONDITIONS 6 Z ~ ~ CJ is Q. en W II.. w ~ C r--! I t ~ f t! ~ a a %SUC 5%SUC 6 F<XX>PEL I I """II 5 MIN INTERVAL Figure 3. Rates f respnding (in respnses/min) acrss successive 5-min intervals in the secnd half f the sessin fr the mean f all subjects respnding fr 1% (pen circles), 5% (clsed squares), r fd-pellet (pen triangles) reinfrcement. The tp graph presents respnding during the cnstant cnditins. The bttm graph presents respnding during the randm cnditins. Errr bars represent the standard errr f the mean acrss subjects fr respnding in that particular 5-min interval. Nte that the rdinate scale varies frm that in Figure 2.

14 642 WEATHERLY ET AL. F(8, 24) = 2.56, P <.5, indicating that different changes in respnse rates were bserved in the secnd half f the sessin fr the different reinfrcers. N ther main effects r interactins reached significance. General Discussin A number f recent studies (e.g., Weatherly, Davis, et ai., 2; Weatherly, Stut, et ai., 2) have reprted that fd-pellet reinfrcement upcming in the secnd half f the sessin prduces an increase in liquid-sucrsereinfrced respnse rates in the first half (Le., inductin). The results f the present study replicated and extended this finding. Experiment 1 demnstrated that an upcming reinfrcer ther than a fd pellet can prduce an increase in sucrse-reinfrced respnding. Experiment 2 demnstrated that a decrease in sucrse-reinfrced respnding may be bserved when the reinfrcer delivered in the secnd half f the sessin maintains a lwer rate f respnding than that delivered in the first half. The present results thus increase the generality f the inductin by shwing that the effect is nt limited t just ne type f upcming reinfrcer (Le., fd pellets). Mrever, by shwing that the inductin may manifest in either an increase r decrease in respnse rate, the results suggest that the present inductin effect shares the quality f symmetry with its seemingly ppsite effect, behaviral cntrast. Althugh the present study appears t braden the generality f the inductin effect, it culd ptentially be argued that the present results d nt represent inductin at all. Rather, by delivering reinfrcers f different "values" in the secnd half f the sessin, the value f the entire sessin might have been altered. Thus, respnse rates in the first half f the sessin may have differed acrss cnditins because subjects were treating sessins as single units with differing values rather than displaying inductin. Althugh the present results cannt rule ut this explanatin, several pieces f evidence wuld seem t run cunter t it. Fr ne, althugh the present study used a mixed schedule f reinfrcement, the present effect has been bserved using multiple schedules f reinfrcement, which prvide vert discriminative stimuli t allw subjects t discriminate the first and secnd halves f the sessin (Weatherly et ai., 1999; Weatherly, Davis, et ai., 2). Secndly, the behavir f subjects in the present study suggests that they differentiated between the reinfrcers delivered in the first and secnd halves f the sessin. As can be seen in Figure 1, and by cmparing Figures 2 and 3, when a change in reinfrcer type ccurred at the midpint f the sessin, rates f respnding changed as well. Bth f these pieces f evidence suggest that subjects were nt treating the sessin as a single unit. That is nt t say, hwever, that the present effect was nt a prduct f a change in the "value" f the reinfrcer delivered in the first half f the sessin. Such a change in the value f the liquid-sucrse reinfrcer may have ccurred if Pavlvian cnditining played a rle in the current effect. One pssible rle is that the liquid-sucrse reinfrcers and/r the stimuli

15 UPCOMING REINFORCER 643 that were present during their delivery served as cnditined stimuli fr the upcming differenti~1 reinfrcement. Fr example, when these stimuli predicted upcming fd-pellet reinfrcement, they might have becme cnditined exciters and thus facilitated respnding by prducing arusal (e.g., Killeen, 1995). Likewise, when they predicted upcming 1 % sucrse reinfrcement, they might have becme cnditined inhibitrs and thus suppressed respnding, pssibly by causing withdrawal frm the perandum (e.g., Wasserman, Franklin, & Hearst, 1974). This explanatin appears cnsistent with the finding that the effect sie f the inductin varies inversely with sucrse cncentratin (Weatherly, Davis, et ai., 2; Weatherly, Stut, et ai., 2). One might expect high cncentratins f sucrse t serve as strng reinfrcers (i.e., uncnditined stimuli) and therefre be less likely t serve as cnditined stimuli (thus eliminating the inductin). On balance, lw cncentratins f sucrse may serve as weak reinfrcers and thus may have enhanced ptential t serve as cnditined stimuli. The idea that Pavlvian cnditining plays a rle in multiple-schedule interactins is nt new (e.g., Gamu & Schwart, 1973). It shuld thus be pssible t use existing prcedures designed t measure the influence f Pavlvian cnditining in such situatins (e.g., Williams, 1991) t determine its rle in the present effect. One explanatin fr the present inductin effect frwarded by Weatherly, Davis, et al. (2) and Weatherly, Stut, et al. (2) seems t be challenged by the results f Experiment 2. That explanatin was that the inductin prduced by upcming fd-pellet reinfrcement was the result f anticipatry respnses fr fd pellets being added t respnses fr the presently available liquid-sucrse reinfrcers. In ther wrds, subjects may have started t respnd fr the fd pellets prir t their availability. Thus, these respnses, added t thse fr the currently available sucrse, may have prduced a rate f respnding higher than that bserved when fdpellet reinfrcement was nt upcming. Althugh the same explanatin culd ptentially explain the results f Experiment 1 and thse frm Experiment 2 when fd-pellet reinfrcement was upcming, it seems t fail t explain the decrease in respnding bserved in Experiment 2 when 1 % sucrse reinfrcement was upcming. That is, it seems unlikely that upcming 1 % sucrse reinfrcement subtracted frm respnding fr presently available 5% sucrse reinfrcement. Althugh the results f Experiment 2 suggest that respnse rates in the first half f the sessin can be increased r decreased depending n the type f reinfrcer in the secnd half f the sessin, the ntin that these effects are symmetrical remains pen t questin. The increase in respnding in the first half f the sessin when fd-pellet, rather than the same 5% sucrse, reinfrcement ccurred in the secnd half can be best described as a difference in respnse pattern. Hwever, the effect f upcming 1 % sucrse reinfrcement was ne f respnse rate. When these results are cupled with thse f Weatherly et al. (1999, Experiment 4), wh fund that respnse rates fr fd pellets in the first half f the sessin were insensitive t changes in the rate f 5% liquid-sucrse reinfrcement in the

16 644 WEATHERLY ET AL. secnd half, it appears that further research is required befre ne can cnclude whether r nt the inductin is indeed symmetrical. Finally, it shuld be nted that the inductin bserved in the present study was bserved under cnditins that maintained lw baseline rates f respnding. This bservance may suggest that the inductin effect is peculiar t situatins that supprt lw respnse rates. In fact, finding that little r n inductin is bserved when rats respnd fr 25% liquid-sucrse reinfrcers (Weatherly, Davis, et ai., 2; Weatherly, Stut, et ai., 2), which maintain relatively high rates f respnding, is cnsistent with such a suggestin. This ptential limitatin in generality, hwever, des nt necessarily diminish the ptential imprtance f the inductin. That is, althugh the inductin may be limited t situatins in which there is little r weak reinfrcement, it culd be argued that these are exactly the situatins that mst interest psychlgists and behavir analysts. References FLAHERTY, C. F. (1996).lncentive relativity. New Yrk: Cambridge University Press. GAMZU, E., & SCHWARTZ, B. (1973). The maintenance f key pecking by stimulus-cntingent and respnse-independent fd presentatin. Jurnal f the Experimental Analysis f Behavir, 5, KILLEEN, P. R. (1995). Ecnmics, eclgics, and mechanics: The dynamics f respnding under cnditins f varying mtivatin. Jurnal f the Experimental Analysis f Behavir, 64, MCSWEENEY, F. K., & NORMAN, w. D. (1979). Defining behaviral cntrast fr multiple schedules. Jurnal f the Experimental Analysis f Behavir, 32, MCSWEENEY, F. K., & WEATHERLY, J. N. (1998). Habituatin t the reinfrcer may cntribute t multiple-schedule behaviral cntrast. Jurnal f the Experimental Analysis f Behavir, 69, REYNOLDS, G. S. (1961). Behaviral cntrast. Jurnal f the Experimental Analysis f Behavir, 4, WASSERMAN, E. A, FRANKLIN, S. R., & HEARST, E. (1974). Pavlvian appetitive cntingencies and apprach vs. withdrawal t cnditined stimuli in pigens. Jurnal f Cmparative and Physilgical Psychlgy, 86, WEATHERLY, J. N., DAVIS, C. S., & MELVILLE, C. L. (2). Inductin with upcming fd-pellet reinfrcement. Learning and Mtivatin, 31, WEATHERLY, J. N., & MOULTON, P. L. (21). The effect f fd-pellet reinfrcement n rats' rates f lever pressing fr 1 % sucrse reinfrcers acrss several "cntrast" prcedures. Learning and Mtivatin, 32, WEATHERLY, J. N., STOUT, J. E., MCMURRY, A S., RUE, H. C., & MELVILLE, C. L. (1999). Within-sessin respnding when different reinfrcers are delivered in each half f the sessin. Behaviural Prcesses, 46, WEATHERLY, J. N., STOUT, J. E., RUE, H. C., & MELVILLE, C. L., (2). The effect f secnd-half reinfrcer type n respnding fr sucrse in the first half f the sessin. Behaviural Prcesses, 49, WILLIAMS, B. A (1991). Behaviral cntrast and reinfrcer value. Animal Learning & Behavir, 19,