BRIEF REPORTS. Associative competition in operant conditioning: Blocking the response-reinforcer association

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1 Psychnmic Bulletin & Review /999,6 (4),6/8-623 BRIEF REPORTS Assciative cmpetitin in perant cnditining: Blcking the respnse-reinfrcer assciatin BEN A. WILLIAMS University f Califrnia, San Dieg, La Jlla, Califrnia Naiverats were trained t leverpress with a 3D-sec delay-f-reinfrcement cntingency frm the start f training. In Experiment 1, the delay interval fr different grups f subjects included a signal in the first 5 sec, a signal in the last 5 sec, r n signal at any time. Rats with the signal at the start f the delay interval learned mst rapidly. Rats with the signal at the end f the delay failed t learn. In Experiment 2, a chice prcedure was used, in which each f tw levers was assciated with its wn 3D-sec delay f reinfrcement. The delay fr ne lever included a 5-secsignal at the end fthe delay.the delay fr the secnd lever had n signal in its 3D-sec delay. Preference was in favr f the lever withut the signal in the delay interval. The results demnstrate that the acquisitin f new respnse can be blcked in a manner analgus t the blcking f Pavlvian cnditining. N empirical finding in the study fanimal learninghas been fgreater theretical imprtance than the phenmenn fblcking. When a stimulus (A) is pairedwiththe uncnditined stimulus (US), and then A is cmpunded with a new stimulus (B) and this cmpund (AB) is als paired with the same US, subsequent tests invlving B alne shw a substantially decreased level fperfrmance, relative t cntrl cnditins in which the AB cmpund is paired with the US withut the prir training with A alne. The mst influential interpretatin f blcking, dating back t its initial reprt by Kamin (1968), has been based n the premise that tempral cntiguity between a cnditined stimulus (CS) and US is nt sufficient t prduce an assciatin between them. Instead, the CS must als prvide infrmatin abut the US's ccurrence. Accrdingly, the US is already predicted by the presence f stimulus A, s that the additin fstimulus B prvides n further infrmatin and thus is nt assciated with the US. (But nte that "nnassciative" interpretatins fblcking have been prpsed: e.g., Miller & Matzel, 1988.) The idea that simple pairings fthe CS and US are insufficient t prduce learning has had majr theretical ramificatins. It serves as the fundatin principle fthe well-knwn thery f learning f Rescrla and Wagner (1972) and has been extendedt such human learning situatinsas categrylearning (e.g., Gluck & Bwer, 1988) and cntingency judgments (e.g., Vallee- Turangeau, Baker, & Mercier, 1994). Mre generally, it pses imprtant questins at the neurphysilgical level, where the This research was supprted by NSF and NIMH grants t the University f Califrnia, San Dieg. Reprint requests shuld be addressed t B. A. Williams, Department fpsychigy, University fcaiifrnia, San Dieg, La Jlla, CA ( bawilliams@ucsd.edu). theretical analysis f learning has been predicated n the assumptin that tempral cntiguity is sufficient fr learning t ccur. The great majrity fresearch supprting the idea that tempral cntiguity is nt sufficient fr learning t ccur has been cnducted with Pavlvian cnditining prcedures, but the idea has als been supprted in studies f instrumental/perant learning. In the first such experiment reprted, Williams (1975) presented pigens a 5-sec keylight with the cntingency that a single peck during the keylight presentatin caused the delivery fthe fd reinfrcer 10 sec later. Different cnditins were then defined by the events during the delay-f-reinfrcement interval. Substantial respnding ccurred t the keylight when the delay interval was blank (i.e., had the same stimulus cnditins as thse fr the intertrial interval), whereas the presentatinf a secnd stimulus during the last few secnds f the interval greatly reduced the level fkeylight respnding. Subsequent investigatins fthe same general prcedure (Williams, 1978, 1982; Williams, Prestn, & de Kervr, 1990) have prduced the same pattern f results. Apparently, therefre, cntrary t traditinal interpretatins (Skinner, 1948), the tempral relatin between the respnse and reinfrcer is nt sufficient t predict when a respnse-reinfrcer assciatin will be acquired. The nvelty f previus putative demnstratins f blcking fthe respnse-reinfrcer assciatin has been questined by the cncern that the blcking effects seen are due t Pavlvian cnditining, nt perant cnditining, because fthe similarity fthe prcedure t autshaping. Arguments against this interpretatin were presented by Williams (1978), based primarily n the finding that when the respnse cntingency was remved, with Cpyright 1999 Psychnmic Sciety, Inc. 618

2 BLOCKING RESPONSE-REINFORCER ASSOCIATIONS 619 fd then delivered autmatically at the end f the 10 sec delay regardless f the bird's behavir, the evidence fr blcking was eliminated. That is, the level fkeypecking was reduced fr bth cnditins but the level frespnding was slightly higher when the signal appeared during the delay interval. Hwever, even that demnstratin has been questined as a sufficient basis fr the claim that blcking f the respnse-reinfrcer assciatin has ccurred, because perant cntingencies may themselves affect the Pavlvian cntingencies that are learned (Mre, 1973). Thus, iffd is delivered nly after a respnse has ccurred, the animal must be attending t the keylight n thse trials. Withut a respnse cntingency, in cmparisn, the subject may be attending t any number f stimulus features in the envirnment ther than the CS. The present study was undertaken in the attempt t avid this alternative interpretatin, in rder that a mre cnvincing demnstratin f blcking f the respnsereinfrcer assciatin might be prvided. This new apprach was inspired by the reprt f Lattal and Gleesn (1990) that experimentally naive subjects will acquire a new respnse even when the respnse-cntingent reinfrcer is delayed up t 30 sec. Given that acquisitin is pssible with such delayed reinfrcement cntingencies, it becmes feasible t determine hw signals at the end f the delay interval affect the rate at which such acquisitin ccurs. In Experiment 1, three grups frats were expsed t a 30-sec delay-f-reinfrcement cntingency with respect t leverpressing. Fr the cntrl cnditin, n stimulus change ccurred at any time during the delay interval. Fr the blcking cnditin, a 5-sec huselight was presented during the last 5 sec f the 30-sec delay. Technically speaking, this is an vershadwing prcedure rather than a blcking prcedure, because the signal was nt pretrained prir t cmmencing the delayf-reinfrcement cntingency. Hwever, this distinctin is purely semantic: Because fthe differences in their tempral separatins frm fd, the signal-fd assciatin was learned mre rapidly than the respnse-reinfrcer assciatin. Thus, nly a few learning trials were necessary t prduce cnditining t the signal equivalent t that which wuld have ccurred had prir training with the signal been prvided. A third cnditin, marking, was als included (cf. Lieberman, McIntsh, & Thmas, 1979), in which the 5-sec huse light ccurred during the first 5 sec f the delay interval. This cnditin was included t determine the extent t which the lcatin f the huselight in the delay interval was a critical determinant f its effects. EXPERIMENT 1 Methd Subjects. Twenty-fur experimentally naive male albin Sprague Dawley rats. ranging in age frm 6 weeks t 4 mnths, were hused in individual wire-mesh cages in a clny rm with a 14:10-h light:dark cycle. Experimental sessins ccurred during the light prtin fthe cycle, usually frm 8 a.m. t 2 p.m. Water was freely available in the hme cages, while fd deprivatin was maintained by allwing free access t lab chw fr 2 h beginning apprximately 5 min after the end fthe experimental sessins. Apparatus. Tw identical experimental chambers were cnstructed, each frm a Culburn mdular shell that included a grid flr, tw Plexiglas sidewalls, and ceiling, frnt, and rear walls f aluminum. The height f the interir chamber was apprximately 29 cm; its width was 24 cm, and its depth was 31 cm. The interir chamber was enclsed within a larger exterir enclsure fr sund insulatin. Masking nise was prvided by a ventilating fan. Lcated in the center fthe frnt panel, 3 cm abve the flr, was the recessed fd well, int which 45-mg Nyes pellets were presented frm a Gerbrands pellet dispenser. On the left side f the frnt panel 15 cm abve the flr was munted a stainless steel retractable lever (Culburn Mdel E21-03), 3.7 cm wide, 0.3 cm thick, and extending 3.0 cm int the chamber when nt in the retracted psitin. A minimum frce f 0.25 N was required fr peratin. Munted in the center fthe frnt panel just belw the ceiling was a huselight mdule (Culburn Mdel EII-OI) equipped with a 28-V miniature bulb (n. 1828). Prcedure. Thrughut training each experimental sessin was 30 min lng. During the first tw sessins, the leverwas retracted frm the chamber and free fd was presented n a respnse-independent variable-time (VT) 60-sec schedule. On the third sessin, the lever was extended, the VT schedule was terminated, and fd culd be earned by leverpresses that were fllwed by a 30-sec delay-freinfrcement interval, terminating with the delivery f the fd pellet. Leverpresses while the delay timer was perating had n scheduled effect, s that the maximum number f reinfrcers per sessin was 60. Training cntinued fr 20 experimental sessins. Different grups f subjects were defined by the stimuli during the delay interval. Fr the n-signal cnditin, n exterceptive stimuli were presented during the delay. Fr the blcking cnditin, the huselight was presented during the last 5 sec f the delay, whereas fr the marking cnditin, the huse light appeared during the first 5 sec fthe delay. Results Figure 1 shws the mean number fbtained reinfrcers fr each fthe three experimental grups. The number f reinfrcers is used as the measure f the degree f cnditining, rather than respnse rate, because respnse rate was mre variable acrss subjects. Hwever, the effects t be reprted als ccurred fr the respnse rate measure. It is clear that the marking cnditin with the huselight signal at the beginning f the delay interval prduced the mst rapid acquisitin, whereas the blcking cnditin, with the signal at the end fthe interval, prduced the slwest acquisitin. Indeed, n evidence f acquisitin ccurredfr the blcking cnditin. The cntrl cnditin, in which n signal ccurred at any time during the delay, was intermediate. Thus, the marking cnditin facilitated learning relative t the cntrl, whereas the blcking cnditin retarded it. These results were analyzed statistically with a twway analysis fvariance (ANOVA). The effect fexperimental grups was significant [F(2,21) = 15.6,p <.01], as was the sessins variable [F(9,189) = 13.8,p <.01]. The interactin term was als significant [F(l8, 189) =

3 620 WILLIAMS :::::l :r:... Q) Q.... en 30 Q) ~ 0-C 'Q) 20 Cl: -0- NSig Marlling Blcking Blcks f Tw Sessins Figure 1. Mean number f reinfrcers btained per tw-sessin blck frm the 30-sec delay-f-reinfrcement cntingency fr each grup in Experiment ,p <.01]. Analysis fsimple effects shwed that the sessins variable was significant fr the marking and cntrl cnditins, indicating the acquisitinfleverpressing with cntinued training. The sessins variable was nt significant (F < 1) fr the blcking cnditin, indicating that n acquisitin ccurred. Tw-way cmparisns with a Newman-Keuls test revealed that all three experimental grups were significantly different frm each ther. A secnd apprach t assessing the significance f signals in the delay interval is the number fsubjects in each grup that attained the learning criterin f a cumulative ttal f 100 reinfrcers ver the curse f the 20 sessins f training. All 8 subjects in the marking cnditin reached that criterin, with a mean f 11.1 sessins. Only 4 f 8 subjects in the cntrl cnditin attained the criterin, with thse that did requiring a mean f 12.8 sessins. Nne f the subjects in the blcking cnditin attained the criterin. Discussin The present results prvide a strng demnstratin f cmplete blcking frespnse acquisitin by signals appearing at the end f a delay-f-reinfrcement interval when the reinfrcer is cntingent n the respnse. This result is similar t thse f previus studies f blcking f the respnse-reinfrcer assciatin (cf. Williams, 1975, 1978, 1982; Williams et ai., 1990), despite the fact that a very different experimental preparatin was invlved. Unlike the earlier studies, which used pigens in discriminated perant prcedures, here the bject f the blcking effect was the acquisitin f a nvel instrumental respnse in a free-perant setting. The results thus prvide prima facie evidence that the blcking f the respnse-reinfrcer assciatin is an empirical phenmenn f cnsiderable generalityand thus must be addressed fr any general thery f blcking t be cmplete. The present results als shw that signals at the nset fthe delay-f-reinfrcement interval facilitate rather than retard the learning fnew respnses. This effect is reminiscent f the marking effect first reprted by Lieberman et al. (1979), despite majr differences in prcedure. The results are als similar t thse f Schaal and Branch (1988), wh demnstrated that a brief stimulus change at the nset f an therwise unsignaled delay interval maintained behavir at a level near that f behavir that was immediately reinfrced. The fact that marking ccurs in the present respnse acquisitin prcedure suggests an alternative explanatin fthe results fthe "blcking" cnditin in terms f respnse cmpetitin. The cncept f marking implies that respnses ccurring just prir t the huse light signal shuld be remembered better at the time the reinfrcer is delivered. Behavirs ther than leverpresses may

4 BLOCKING RESPONSE-REINFORCER ASSOCIATIONS 621 have been adventitiusly marked by the huselight, and thus were mre greatly strengthened than such nnleverpressing behavir in the cntrl cnditin in which the signal was nt presented. Accrdingly, the effect f the signal at the end fthe delay interval may have been simply t increase the level f respnse cmpetitin. Such an explanatin seems unlikely because any cmpeting behavir shuld have extinguished when it ccurred in the absence fthe leverpress respnse, because fd became available nly when leverpressing had ccurred. Nevertheless, Experiment 2 addressed this alternative explanatin. EXPERIMENT 2 One methd fassessing the ptential rle frespnse cmpetitin caused by the marking fcmpeting behavirs is t emply a chice prcedure. T the extent that blcking effects ccur in chice prcedures in which ne respnse has the signal in its delay freinfrcement while the alternative respnse has n signal, such blcking cannt be ascribed t respnse cmpetitin, because strengthening cmpeting respnses shuld reduce bth chice alternatives equally. If blcking is fund in a chice prcedure, it als rules ut the pssibility that the signal preceding the reinfrcer has altered the prperties f the backgrund cues (e.g., by reducing their ability t prduce arusal). Any change in the status fthe backgrund cues presumably shuld affect bth respnse alternatives equally, given that they share the same backgrund. Experiment 2 thus extended the prcedure used in Experiment I t include tw separate respnse levers, each f which prduced a fd pellet after a 30-sec delay interval. The tw respnses differed nly in terms f whether the signal ccurred during the last 5 sec fthe delay interval. Methd Subjects. Twenty-tw experimentally naive female Sprague Dawley rats served as subjects. The subjects were maintained as described fr Experiment I. Apparatus. A standard tw-lever rat chamber, different frm that used in Experiment I, with glass sidewalls, sheet metal ceiling, frnt walls, and rear walls, and a grid flr, was hused inside a sund-attenuating larger chamber equipped with a ventilating fan. The interir f the chamber was 30.5 cm wide X 20.3 cm high X 22.9 cm lng. Tw retractable levers (BRS/LVE Mdel RRL-015), which prtruded 1.5 cm int the chamber when extended and which required a minimum frce f 0.3 N fr depressin, were munted n the frnt wall fthe chamber, spaced 9 cm apart measured frm side t side. The nly feedback fr a leverpress was the sund fthe micrswitch inside the lever husing. Directly between, and 6.5 cm belw, the levers was a fd receptacle in which culd be drpped 45-mg Nyes pellets (imprved Frmula A) that served as the reinfrcer. Between, and 3.3 cm abve, the levers was a 28- V pilt light, cvered by a glass translucent cver. Prcedure. On the first sessin ftraining, subjects were placed in the experimental chamber withut prir training, but with three t five fd pellets placed in the pellet receptacle and bth respnse levers extended int the chamber. The experimental cntingencies used fr the entire experiment were in effect frm the nset ftraining. Respnses t either the left r the right lever initiated a 30-sec delay timer that terminated with the delivery f fd. During this delay interval, additinal respnses t either lever had n scheduled effect. Fr the delay interval prduced by ne lever, a 5-sec presentatin fthe huselight ccurred during the last 5 sec fthe interval; fr the delay interval prduced by the alternative lever, n signal was presented during the delay. The subjects were randmly assigned t tw grups, ne with the signal fllwing respnses t the right lever, and a secnd with the signal fllwing respnses t the left lever. Experimental sessins cntinueduntil 50 reinfrcers were btained, r until 45 min had elapsed, whichever came first. Training cntinued fr 25 sessins. Results Figure 2 shws the mean percentage f the ttal reinfrcers that were btained by respnses t the right lever. At the start ftraining, bth experimental cnditins had a significant bias tward the right lever, which resulted in apprximately 70% fthe chicest the right lever regardless f which delay interval cntained the signal. When the signal appeared nly during the delay cntingent n respnses t the left lever, chices f the right lever then increased ver the curse fthe 20 sessins ftraining; when the signal appeared nly during the delay cntingent n respnse t the right lever, chices fthe right lever decreased ver training. These results were analyzed with a tw-way ANOVA. The effect f the lcatin f the signal was significant [F(I,20) = 7.54,p<.05]. The effect fsessinsftraining was nt significant [F(4,80) = 1.67, p >.05]. The interactin between the tw variables was als significant [F(4,80) = 7.06,p <.01]. Thus, a significant preference develped ver training fr the lever withut the signal in its delay interval. Discussin The results f Experiment 2 shw that preference fr a chice alternative is strngly reduced by a signal ccurring during the last prtin fa delay-f-reinfrcement interval prir t the reinfrcer cntingent n that respnse. T the extent that the results fexperiments I and 2 depend n the same mechanisms, the demnstratin f blcking in the chice prcedure used in Experiment 2 excludes any interpretatin f the effect in terms frespnse cmpetitin, r in terms f changes in the mtivatinal prperties f the backgrund stimuli, because bth such effects shuld have been equated fr the tw chice alternatives. It is imprtant t recgnize that the results btained in bth Experiments 1 and 2 are in the ppsite directin frm that anticipated n the basis ftraditinal cnceptins f delay f reinfrcement. The signal presumably pssessed cnditined reinfrcement prperties because fits tempral cntiguity with fd and thus shuld have functinally shrtened the delay interval in which it appeared. Cnsequently, a slight preference shuld have ccurred fr the alternative with the signal during the last 5 sec fthe 30-sec delay, because its delay freinfrcement was nly 25 sec. The strng effect in the ppsite directin implies that the signal in the delay interval sme-

5 622 WILLIAMS.9 -.c 0) a: - r.n CO 0.c - c t a. ọ Signal n Right -0- Signal n Left ~--~--..._ r , 5 Blcks f 5 Sessins Figure 2. The percentage f the ttal number f reinfrcers that was btained by chices f the right lever in Experiment 2. One grup f subjects had a 5-sec signal in the delay interval fllwing a chice f the right lever; the secnd grup had the signal in the delay interval fllwing a chice fthe left lever. hw interfered with the ability fthe delayed reinfrcer t strengthen chice respnding, the mst plausible interpretatin fthis interference being that the signal cmpeted with the respnse fr assciatin with the reinfrcer. The prcedure used here t demnstrate blcking f the respnse-reinfrcer assciatin differs frm the usual Pavlvian study f blcking in tw ways. The first is the use f serial stimulus cmpunds rather than simultaneus cmpunds. Here the stimulus with prir cnditining that prevented the acquisitin f instrumental respnding ccurred after the target event (the respnse) had already ccurred. By cntrast, in Pavlvian prcedures, the pretrained stimulus is typically presented simultaneusly with the the target (a secnd stimulus). Hwever, blcking with serial stimulus cmpunds has been demnstrated with several different Pavlvianprcedures (Gibbs, Kehe, & Grmezan, 1991, Experiment 3; Khler & Ayres, 1982; Pearce, Nichlas, & Dickinsn, 1981, Experiment 4). That is, when a trace cnditining prcedure is used, inserting a secnd stimulus between the target CS and the US reduces acquisitin t the target CS. It shuld be nted that this finding is the ppsite t that which wuld be expected n the basis f secndrder cnditining, just as the present reprt is cntrary t the expected effects f cnditined reinfrcement. In bth cases, whether the prcedure prduces blcking r secnd-rder cnditining (r cnditined reinfrcement) depends critically n the tempral relatinships amng the elements fthe training situatin (fr a discussin f this issue, see Gibbs et ai., 1991, and Williams, 1982). In general, the ptimal prcedure fr prducing blcking rather than cnditined reinfrcement is that used in the present study, in which the interspersed stimulus is temprally distant frm the target event (here a respnse) but cntiguus with the reinfrcer. In any event, the present findings are cnsistent with previus studies with Pavlvian cnditining in shwing that blcking can be btained with sequentially presented assciative elements. The secnd difference between the present prcedure and Pavlvian studies f blcking was the use f a respnse cntingency determining the delivery f the reinfrcer. In Pavlvian prcedures, the delivery fthe US (the reinfrcer) ccurs regardless fthe animal's behavir, s that the same number f US presentatins is presented regardless f whether blcking des r des nt ccur. In the present study, by cntrast, the fd reinfrcers were presented nly when the perant respnse had been emitted; ifblcking ccurred, fd was nt delivered. The presencefthe respnse-reinfrcer cntin-

6 BLOCKING RESPONSE-REINFORCER ASSOCIATIONS 623 gency prvides a strng incentive fr respnding t ccur, assuming that the respnse-reinfrcerassciatin has been acquired. Thus, accunts f blcking in terms f perfrmance rules, rather than in terms fan actual failure f acquisitin (e.g., Miller & Matzel, 1988), can be applied nly with cnsiderable difficulty. In that sense, the ccurrence fblcking frespnse-reinfrcerassciatinsprvides significant supprt fr the inadequacyfmere tempral relatinships t predict when learning will ccur. REFERENCES GIBBS, C. M., KEHOE, E. J., & GORMEZANO, I. (1991). Cnditining f the rabbit's nictitating membrane respnse t a CSA-CSB-US serial cmpund. Manipulatins fcsb's assciative character. Jurnal fexperimental Psychlgy: Animal Behavir Prcesses, 17, GLUCK, M. A., & BOWER, G. H. (1988). Frm cnditining t categry learning: An adaptive netwrk mdel. Jurnal fexperimental Psychlgy: General, 117, KAMIN, L. J. (1968). "Attentin-like" prcesses in classical cnditining. In M. R. Jnes (Ed.), Miami sympsium n the predictin fbehavir: Aversive stimulatin (pp. 9-33). Cral Gables, FL: University f Miami Press. KOHLER, E. A., & AYRES, J. J. B. (1982). Blcking with serial and simultaneus cmpunds in a trace cnditining prcedure. Animal Learning & Behavir, 10, LATTAL, K. A., & GI.EESON, S. (1990). Respnse acquisitin with delayed reinfrcement. Jurnal fexperimental Psychlgy: Animal Behavir Prcesses, 16, LIEBERMAN, D. A., McINTOSH, D. c., & THOMAS, G. V.(1979). Learning when reward is delayed: A marking hypthesis. Jurnal fexperimental Psychlgy: Animal Behavir Prcesses, 11, MILLER, R. R, & MATZEL,L. D. (1988). The cmparatr hypthesis: A respnse rule frthe expressin fassciatins. In G. H. Bwer (Ed.), The psychlgy flearning and mtivatin (Vl. 22, pp ). San Dieg: Academic Press. MOORE, B. R. (1973). The rle f directed Pavlvian reactins in simple instrumental learning in the pigen. In R. Hinde & J. S. Hinde (Eds.), Cnstraints n learning (pp ). New Yrk: Academic Press. PEARCE, J. M., NICHOLAS, D. J., & DICKINSON, A. (1981). The ptentiatin effect during serial cnditining. Quarterly Jurnal fexperimental Psychlgy, 33B, RESCORLA, K. A., & WAGNER, A. R. (1972). A thery f Pavlvian learning: Variatins in the effectiveness freinfrcement and nnreinfrcement. In A. H. Black & W. F.Prkasy (Eds.), Classical cnditining ll: Current research and thery (pp ). New Yrk: Appletn-Century-Crfts, SCHAAL, D. W, & BRANCH, M. N. (1988). Respnding fpigens under variable-interval schedules funsignaled, briefly signaled, and cmpletely signaled delays t reinfrcement. Jurnal fthe Experimental Analysis fbehavir, SO, SKINNER, B. E (1948). "Superstitin" in the pigen. Jurnal fexperimental Psychlgy, 38, VALLEE-TOlJRANGEAU, E, BAKER, A. G., & MERCIER, P. (1994). Discunting in causality and cvariatin judgements. Quarterly Jurnal fexperimental Psychlgy, 478, WILLIAMS, B. A. (1975). The blcking freinfrcement cntrl. Jurnal fthe Experimental Analysis fbehavir, 24, WILLIAMS, B. A. (1978). Infrmatin effects n the respnse-reinfrcer assciatin. Animal Learning & Behavir, 6, WILLIAMS, B. A. (1982). Blcking the respnse-reinfrcer assciatin. In M. Cmmns, R. Herrnstein, & A. Wagner(Eds.), Quantitative analyses fbeha vir: V!' 3. Acquisitin (pp ). Cambridge, MA: Ballinger. WILLIAMS, B. A., PRESTON, R. A., & DEKERVOR, D. E. (1990). Blcking fthe respnse-reinfrcer assciatin: Additinal evidence. Learning & Mtivatin, 21, (Manuscript received April 29, 1998; revisin accepted fr publicatin December 18, 1998.)