Western Michigan University ScholarWorks at WMU Dissertations Graduate College 4-1988 Self-Control: Effects of Ratio Size, Intra-Delay Reinforcers, and Response Requirement Elbert Q. Blakely Jr. Western Michigan University Follow this and additional works at: http://scholarworks.wmich.edu/dissertations Part of the Experimental Analysis of Behavior Commons Recommended Citation Blakely, Elbert Q. Jr., "Self-Control: Effects of Ratio Size, Intra-Delay Reinforcers, and Response Requirement" (1988). Dissertations. 2190. http://scholarworks.wmich.edu/dissertations/2190 This Dissertation-Open Access is brought to you for free and open access by the Graduate College at ScholarWorks at WMU. It has been accepted for inclusion in Dissertations by an authorized administrator of ScholarWorks at WMU. For more information, please contact maira.bundza@wmich.edu.
SELF-CONTROL: EFFECTS OF RATIO SIZE, INTRA-DELAY REINFORCERS, AND RESPONSE REQUIREMENT by Elbert Q. Blakely, Jr. A D issertation Submitted to the Faculty of The Graduate College in p a rtia l fulfillm ent of the requirements for the Degree of Doctor of Philosophy Department of Psychology Western Michigan University Kalamazoo, Michigan April 1988
SELF-CONTROL: EFFECTS OF RATIO SIZE, INTRA-DELAY REINFORCERS, AND RESPONSE REQUIREMENT E lbert Q. Blakely, J r., Ph.D. Western Michigan U niversity, 1988 This study te ste d the following hypotheses: (a) The p re fe re n c e r e v e r s a l phenomenon w ill be found when the d elay s to re in fo rc e m e n t a re d e fin e d by f i x e d - r a t i o schedules, (b) The preference rev ersal phenomenon w ill be observed when in tra -d e la y re in fo rce rs are programmed, and (c) Imposing response requirements during the delay to reinforcem ent w ill a ffe c t preference fo r a larg er delayed re in fo rce r over a sm aller more immediate re in fo rc e r. In Experiment 1, pigeons chose between two schedules, each a s e q u e n c e o f two f i x e d - r a t i o sc h e d u le s. The second schedule of one sequence offered a small re in fo rce r and the second schedule of the a lte rn a tiv e offered a la rg e r re in fo rc e r. The l a t t e r sequence always re q u ire d more re sp o n se s than the fo rm er. The i n i t i a l f i x e d - r a t i o schedules, which were always equal, programmed access to g r a i n o r a h o p p e r f l a s h. The r e s u l t s showed th a t preference sh ifte d from th e sequence w ith the sm a lle r r e in f o r c e r to the sequence w ith the la rg e r re in fo rce r ( i. e., preference rev ersal) as the size of the i n i t i a l f i x e d - r a t i o increased. Whether food or a hopper flash
followed the in itia l fixed-ratio did not greatly affect this relation. When the duration of the hopper flash was in c re a se d in o th e r c o n d itio n s, responding for the sequence with the larger re in fo rce r increased. These re s u lts showed the preference reversal phenomenon when the delays to reinforcement were defined by fixed-ratio s c h e d u le s and when i n t r a - d e la y r e in f o r c e r s were programmed. Thus, the phenomenon is not re s tric te d to procedures in which single time-based schedules define the delays. In Experiment 2, pigeons chose between two fixed-ratio schedules or two fixed-time schedules. schedule of each pair programmed 2-s access to grain; the One other, 8-s access to grain. Preference for the schedule with the 8-s reinforcer decreased in some conditions when subjects moved from a choice between the fixed-ratios to a choice between the fixed-time schedules. These results suggest th a t p re fe re n c e fo r a la rg e r more delayed reinforcer is dependent on the type of schedule used to define the delays to reinforcement.
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O rder N um ber 8811386 Self-control: Efifects o f ratio size, intra-delay reinforcers, and response requirem ent Blakely, Elbert Q., Jr., Ph.D. Western Michigan University, 1988 UMI 300 N. Zeeb Rd. Ann Arbor, MI 48106
ACKNOWLEDGEMENTS Thanks go to my parents, Elbert and Mary Blakely, for their undying support and encouragement throughout my educational experience, and to Dr. Jack Michael, Dr. R. Wayne Fuqua, and Dr. Alan Poling for th e ir sagacious guidance during my work at Western Michigan University. I also thank Dr. Poling for his efforts in teaching me to write and conduct research, and Dr. David Lyon for the generous financial assistance throughout my education at Western Michigan University. Elbert Q. Blakely, Jr. XX
TABLE OF CONTENTS ACKNOWLEDGEMENTS... ix LIST OF TABLES... iv LIST OF FIGURES... v CHAPTER I. INTRODUCTION... 1 Background of the Study... 1 Intra-D elay Choice Procedures... 4 Pre-Delay Choice Procedures... 6 Focus of the Study... 15 I I. EXPERIMENT 1... 18 Method... 18 S u b jects... 18 Apparatus... 18 Procedure... 19 R esults and D iscussion... 24 I I I. EXPERIMENT 2... 31 Method... 31 S u b jects... 31 Apparatus... 31 Procedure... 31 Results and D iscussion... 33 IV. GENERAL DISCUSSION... 40 BIBLIOGRAPHY... 45 i i i
LIST OF TABLES 1. The Number of Sessions and Key Color Associated with the Sequence Offering the Larger Reinforcer at Each Value of the In itia l Fixed-Ratio Schedule for Each Bird... 23 2. The Number of Sessions and Key Color Associated with the Varied Schedule in Each Condition for Each Bird... 34 XV
LIST OF FIGURES 1. The Percentage of Choices to the Sequence with the Larger Reinforcer as a Function of the Size of the In itia l FR Schedule. Data Points Represent the Mean of the Last Five Sessions of Egch Condition; Vertical Lines, the Range Across the Five Sessions... 25 2. The Percentage of Choices to the Sequence with the Larger Reinforcer as a Function of Hopper Flash Duration for Individual Pigeons. Data are Presented for Each of the Last Five Sessions of Each Condition... 27 3. The Percentage of Choices Directed to the Varied Schedule as a Function of the Size of the Varied FR Schedule. Data Points Represent the Mean of the Last Five Sessions of Each Condition; Vertical Lines, the Range Across the Five Sessions. Asterisks (*) Indicate the FR Schedules That Were Re-presented After Exposure to the Chain FR 1 FT Schedule... 35
CHAPTER I INTRODUCTION Background of the Study In tra d itio n a l form ulations of s e lf-c o n tro l, the locus of control resides within the person. According to Freud s in te rp re ta tio n, the ego re s tra in s im pulses, thereby postponing gratification (Karoly, 1982). Klein (1976) also p o sits th a t the ego develops ad ap tiv e controls which determine and control behavior. Mikulas (1986) suggests fu rth e r th at "Associated with s e l f - c o n tro l...is a sense of a personal, individualized self which is the s e a t of aw areness and the agent of volitional change" (p. 302). But the "self" and its many aliases are at best, hypothetical causal v ariab les of l i t t l e or no p r a c t i c a l v alu e in p r e d ic tin g and controlling behavior (Skinner, 1953). In behavioral interpretations, however, self-control is a special class of behavior th a t is a fu n c tio n of e n v iro n m e n ta l v ariab les. Because of the focus on contingencies of reinforcement (Brigham, 1982; Rachlin, 1974; Skinner, 1953), such an approach a ffo rd s o p p o rtu n ities for conditioning s e lf-c o n tro l behaviors. I t is in th is s p ir it th at behavioral research has in v estig ated the 1
en v iro n m en tal v a r ia b le s of w hich s e l f - c o n t r o l i s a function. In one b e h a v io ra l model of s e lf-c o n tro l, behavior t h a t i s d e te r m in e d by im m e d ia te c o n se q u e n c e s and r e la tiv e ly in s e n s itiv e to more delayed outcomes is termed "im pulsive" ( A in s lie, 1974). O v ereatin g, e x c e s s iv e smoking and drinking, and aggression are examples of such behavior. The problem fo r the in d iv id u al and fo r others is th a t th e d elay ed consequences of impulsive behavior a r e o f t e n h a rm fu l ( e. g., i n j u r y, i l l n e s s ), and u n fo rtu n a te ly, do not s u ffic ie n tly weaken the behavior. Behavior th a t is a function of delayed, and o fte n more im p o rta n t consequences, i s s a id to exem plify " s e l f - co n tro l" (R a ch lin, 1974). Foregoing a d e lic io u s but fatten in g d e sse rt, avoiding a drinking establishm ent, and counting to "10" to prevent a temper tantrum are examples of s e l f - c o n t r o l. U n fo rtu n a te ly, s e l f - c o n t r o l can be i n f r e q u e n t b e c a u s e t h e r e a r e im m e d ia te a v e r s i v e consequences of such b e h av io r (Rachlin, 1974), or the immediate consequences of im p u lsiv e b eh av io r p r e v a i l. The b e n e f i c i a l lo n g - te r m consequences of im p u lsiv e behavior make i t im p o rta n t to fin d ways to encourage s e lf-c o n tro l. Two b e h av io ral approaches to studying s e lf-c o n tro l are noteworthy. In one, s u b je c ts are tr a in e d to use p ro ced u res, such as s e lf reinforcem ent, m anipulation of
3 discrim inative s tim u li, and s e lf-p u n ish m e n t, t h a t are d e r iv e d from t h e i r l a b o r a t o r y c o u n te r p a r ts. Such p ro ced u res presum ably in c re a s e th e e f f e c t i v e n e s s of delayed consequences, although th e ir mechanisms of action are unclear. Some in te rp re t th e ir actions as id e n tic a l to those of the laboratory counterparts (Bandura, 1976); o th e rs p o s it more complex p ro c e sse s (C a ta n ia, 1975; Goldiamond, 1976). In th e second approach, which is employed in th e p re s e n t stu d y, s u b je c ts are given a ch o ice between a l e s s p re fe rre d rein fo rcer and a more p r e f e r r e d, but d elay ed, r e in f o r c e r ( e. g., R a c h lin & G reen, 1972; M ischel & Ebbeson, 1970). Choosing the former is said to be "impulsive"; choosing th e l a t t e r, " s e lf - c o n tr o l" (R a ch lin, 1974). The conditions under which subjects show s e l f - c o n t r o l, o r, in o th e r words, s e l e c t the more p referred delayed re in fo rc e r, are then examined. Subjects are not train ed to use a p a rtic u la r s e l f - c o n t r o l s k i l l, such as s e lf - r e in f o r c e m e n t, lik e those used in the f i r s t approach ju s t discussed. Early research in v estig ated subject c h a ra c te ris tic s th a t were co rrelated with preference for la rg e r delayed consequences. For example, Mischel and Metzner (1962) gave children of d iffe re n t ages a choice between a small candy b ar d e liv e red immediately, or a la rg e r candy bar delivered l a t e r ( i. e., from 1 day to 4 weeks a f t e r ). R e s u lts showed th a t p re fe re n c e for the la rg e r delayed
4 candy increased with age and with scores on in te llig e n c e t e s t s. The r e s u l t s of o th e r re s e a rc h suggested th a t p re fe re n c e fo r l a r g e r d e la y e d c o n se q u e n c e s can be p re d ic te d by c u ltu ra l group membership (Mischel, 1958). Although c o rre la tio n a l, these studies suggest th a t s e lf - c o n tr o l may be s tro n g ly determ ined by h is to r y. What sp e c ific v ariab les are im portant were not in v e s tig a te d and are, as y et, unknown. More recent research has manipulated c h a ra c te ris tic s o f th e c h o ic e s i t u a t i o n, among them th e te m p o ra l v ariab les of and behavioral requirements during the delay to reinforcem ent. In v estig ato rs have used two g e n e ra l procedures. Intra-D elay Choice Procedures In in tra-d elay procedures, subjects e ith e r wait for a preferred re in fo rce r th a t is delayed, or respond during the delay for a le ss preferred immediate re in fo rce r. The general research question i s the fo llo w in g : Given an opportunity to wait for a preferred delayed re in fo rc e r, under what conditions w ill subjects instead choose a le ss p referred immediate re in fo rce r during the delay? Some research m anipulated the tim e to th e la r g e r re in fo rc e r (Logue & Pena-Correal, 1984). Pigeons chose between 6-s access to food delayed by ^ s and 2-s access to food delayed by 0.11 s. Pecks during the delay to the
5 la rg e r r e in f o r c e r im m ediately produced 2 -s access to food. When ^ was low ( i. e.,.11 s ), most i n i t i a l choices were for the la rg e r delayed re in fo rc e r. As ^ increased ( i. e., to 6 s ), pecking during th e d elay in c re a s e d in almost one-half of the b ird s. Thus, as the differen ces in the delays increased ( i. e., as the delay to the la rg e r food d e liv e ry i n c r e a s e d ), resp o n d in g fo r th e sm aller re in fo rce r during the delay also increased. S tu d ies have shown th a t events or behavior during the delay a ffe c t s e lf-c o n tro l. Children could wait for a p r e f e r r e d re in f o r c e r ( i. e., a tr e a t id e n tifie d in prete stin g ), or signal fo r a le ss p referred re in fo rce r th a t was d e liv e re d im m ediately a f t e r the signal (Mischel & Ebbeson, 1970). S u b je c ts w aited lo n g e r fo r th e more p referred re in fo rc e r when the re in fo rc e rs were concealed than when they were v is ib le. Using a sim ilar procedure, children waited longer for the more preferred rein fo rcer when they were in stru c te d to think about "fun" a c tiv itie s (e.g., singing, swinging) than when they were in stru cted to th in k about th e r e i n f o r c e r s, or i f they were not in stru c te d a t a l l (M ischel, Ebbeson, & Zeiss, 1972). In ad d itio n, more w aiting was observed when the c h ild re n were provided with toys during the delay. These re s u lts su g g est th a t w a itin g f o r a more p r e f e r r e d d e la y e d re in fo rce r may be increased by "d istra ctio n s" ( i. e., the e m is s io n of i n c o m p a tib le b e h a v i o r s u n r e l a t e d to
6 re in fo rc e r consumption). Sim ilar re s u lts were reported with pigeons (Grosch & Neuringer, 1981). A more p referred and l e s s p r e f e r r e d type of g ra in were i d e n t i f i e d d u rin g pre-experim ental sessions. During experim ental s e s s io n s, th e p r e f e r r e d g ra in was a v a il a b le a f t e r a d e la y ; the le ss-p re fe rre d grain was av ailab le during the delay, but was contingent on key-pecks. Waiting for the preferred grain occurred more often when the grain was concealed than when i t was v i s i b l e. More w a itin g was a l s o o b s e rv e d when a concurrent response and reinforcem ent schedule ( i. e., FR 20) was a v ailab le. In another phase, some bird s earned access to the p referred grain by not responding for 3 s. When l a t e r t e s t e d, th e se b ir d s w aited lo n g er fo r the p referred g rain, which was contingent on w aiting fo r 20 s, than b ird s w ith a h i s t o r y of pecking for the le ss p r e f e r r e d im m e d ia te g r a i n. T h u s, a h i s t o r y o f re in fo rcem en t fo r behavior incom patible with responding for the le ss-p re fe rre d grain increased w a itin g fo r the preferred delayed grain. Pre-Delay Choice Procedures P re -d e la y ch o ice p ro ced u res assay responding for larg e and small re in fo rce rs without perm itting responding f o r th e s m a lle r r e in f o r c e r d u rin g the d e la y s. The general research question with th is class of procedures
is th is : Given a choice between a small re in fo rce r and one t h a t i s l a r g e r b u t more d e la y e d, u n d er w hat conditions w ill subjects choose the l a t t e r when a choice once-m ade can n o t be changed? In d i s c r e t e - t r i a l s procedures, th e choice is u s u a lly made by e m ittin g a s in g le resp o n se on one of two response operands a f te r which the selected delay is tim ed and th e a p p ro p ria te r e in f o r c e r i s d eliv e red. An i n t e r t r i a l in te rv a l (ITI) fo llo w s each t r i a l. " P r e f e r e n c e " f o r one o f th e a l t e r n a t i v e s is defined by the proportion of t r i a l s on which th a t a lte rn a tiv e was selected. A second assay is the concurrent chains schedule. The i n i t i a l schedules of each c h ain, which are a v a ila b le s im u lta n e o u sly, a re t y p i c a l l y v a r i a b l e - i n t e r v a l (VI) schedules. When the response requirement fo r e ith e r of the VI sch ed u les is completed, entry in to the term inal lin k schedule of the chain th a t was selected is accompanied by a s tim u lu s - c h a n g e. At t h i s tim e, the o th e r ch ain schedule is in o p e r a tiv e. R einforcem ent i s d e l i v e r e d a f t e r th e completion of the term inal lin k schedule, followed by a return to a choice between the i n i t i a l lin k sc h e d u le s. "P referen ce" f o r the term inal schedules is assessed by examining the d is trib u tio n of responding to the i n i t i a l lin k schedules (Reynolds, 1975). U n d er b o t h d i s c r e t e - t r i a l s p r o c e d u r e s and c o n c u rre n t-c h a in s schedules, the rein fo rcer delays are
8 u sually defined by the v alu es of tim e-b ased schedules ( i. e., th e te rm in a l schedules in the concurrent-chains sc h e d u le ). For example, fix e d -tim e (FT) o r f i x e d - i n t e r v a l (FI) schedules th a t o ffer d iffe re n t magnitudes of rein fo rcem en t are programmed. The l a r g e r delayed re in fo rce r is programmed by linking the schedule with the longer time requirement and the la rg e r food delivery; the sm aller more immediate rein fo rcer is arranged by linking the schedule with the shorter time requirem ent and the sm aller food delivery. Some re s e a rc h w ith pigeons in v estig ated d iffe re n t methods of introducing delays to re in fo rcem en t (Logue, R o d rig u e z, P e n a - C o r r e a l, & Mauro, 1984). Under a d i s c r e t e - t r i a l s p ro ced u re, pigeons chose between two schedules, one of which was an FT 6 s th a t delivered 6-s access to food, and the other, an FT t^ s th at delivered 2-s access to food. If ^ was slowly decreased from 6 to.1 s, the birds chose the larg er delayed rein fo rce, more o f t e n th a n when ^ was r a p i d l y d e c r e a s e d. When d i f f e r e n c e s in r e in f o r c e m e n t d e la y were g ra d u a lly in tro d u c e d in o th e r re s e a rc h, p re fe re n c e fo r l a r g e r delayed rein fo rce rs increased (Mazur & Logue, 1978; Logue & Mazur, 1981). O t h e r s t u d i e s m a n i p u l a t e d t h e t e m p o r a l c h a ra c te ris tic s of the delays to re in fo rcem en t. Green and Snyderman (1980) exposed pigeons to a Concurrent-
chains schedule with VI schedules in the i n i t i a l lin k s. The term inal schedules (the delays) were an FI ^ s th a t delivered 6-s access to food and an FI s th a t delivered 2-s access to food. The delays were varied while holding constant the proportional d ifferen ces between them. For example, in the 6:1 group, delays were 12 s/2 s ( i. e., FI 12 s & FI 2 s), 24 s/4 s, 60 s/10 s, and 120 s/20 s. Two o th e r groups were exposed to sch ed u les in which the p r o p o r tio n s were 3:1 and 3 :2. For th e 6:1 and 3:1 groups, responding fo r the large delayed food in creased w ith th e absolute duration of the delays; fo r the 3:2, group, th e re as an in v e rs e r e l a t i o n between th e two v a r i a b l e s. S u b s e q u e n t r e s e a r c h t h a t e l i m i n a t e d procedural confounds in the above study reported for a l l th re e groups increased responding for the larg e delayed food as absolute delays increased (Snyderman, 1983). Navarick and Fantino (1976) varied the delays while holding constant the absolute d ifferen ces between them. Pigeons were exposed Co a concurrent chain schedule with FT or FI sch ed u les in th e te rm in a l l i n k s. The tim e requirem ents for the two schedules were t_ s and t-10 s and the food d e liv e rie s were 6 s and 2 s, resp ectiv ely. R e s u l t s showed t h a t responding fo r the l a r g e r more delayed re in fo rce r increased with In some conditions, p r e f e r e n c e s h i f t e d from th e s m a ll more im m ed iate r e in f o r c e r to th e l a r g e r delayed r e i n f o r c e r. Adult
10 humans were exposed to a s im ila r p ro c e d u re. S u b je c ts chose between escape from white noise fo r short periods of time, or escape fo r a longer period of time th a t was presented a f te r a delay (Solnick, Kannenberg, Eckerman, & W aller, 1982). When the s h o rt escape was im m ediately a v a i l a b l e, s u b j e c t s c h o se t h i s a l t e r n a t i v e alm ost e x c lu s iv e ly. When a 1 5 -s d e la y was added to b o th a l t e r n a t i v e s, s u b je c ts p r e f e r r e d th e la r g e r d elayed esc a p e. Both s tu d ie s show t h a t when th e a b s o l u t e difference in the delays is held constant, increasing the d u ra tio n of th e d e lay s can s h i f t p r e f e r e n c e from a sm aller re in fo rce r to a la rg e r re in fo rc e r. Rachlin and Green (1972) re p o rte d s im ila r r e s u l t s when th e a b s o lu te d if f e r e n c e s in the delays were held constant. sch ed u le. Pigeons were exposed to a c o n c u rre n t-c h a in s I f the 25th response occurred on the rig h t key, a ^ s blackout was followed by a choice between 2-s access to food and 4-s access to food delayed by 4 s. If the ra tio was completed on the l e f t key, the ^ s blackout was followed by 4-s access to food delayed by 4 s. When ^ was low, subjects generally preferred the i n i t i a l lin k th a t produced a choice in the term inal lin k, and in th at lin k they always chose the sm aller immediate food. As ^ increased to 16 s, preference switched to the non-choice term inal lin k which offered only the large delayed food. According to the authors, the birds used a "commitment"
11 stra te g y. When ^ was la rg e, the b ird s chose the lin k in which a choice was unavailable, and th erefo re, they were committed to the la rg e r delayed re in fo r c e r because the sm aller more immediate re in fo rce r was unavailable. This commitment phenomenon was demonstrated in other research with humans (Solnick et a l., 1982). The e ffe c ts of varying delays while holding constant th e a b so lu te d if f e r e n c e s were a ls o s tu d ie d under an autoshaping procedure (Poling, Thomas, H a ll-j o h n so n, & Picker, 1985), Pigeons received pairin g s of 6-s blue key illu m in a tio n s follow ed by 3 -s a c c e ss to g r a i n, and p a ir in g s of 6-s red key illu m in a tio n s followed by 9-s access to grain. Access to grain was delivered a f te r, on th e av erag e, o n e -h a lf of the t r i a l s. The duration of both key illu m in atio n s was then in c re a se d by adding a constant to each. For two of three b ird s, responding to th e stim u lu s c o r r e la te d w ith la r g e r food d e l i v e r i e s in c re a s e d w ith th e d u ra tio n of the s tim u lu s. These re s u lts and those of previous in v estig atio n s (Navarick & F a n tin o, 1976; R achlin & Green, 1972; Solnick e t a l., 1982) show th a t s u b je c ts w ill o fte n s e l e c t a s m a ll r e i n f o r c e r t h a t i s immediately av ailab le over a la rg e r delayed re in fo rc e r. As both delays are incremented by a c o n s ta n t, resp o n d in g in creasin g ly s h ifts to the la rg e r more delayed re in fo rc e r. This re la tio n o b ta in e d under b o t h r e s p o n s e - d e p e n d e n t and r e s p o n s e - in d e p e n d e n t
12 procedures. Sim ilar re s u lts were reported when s p a tia l distance was manipulated (Boehme, Blakely, & Poling, 1986). In a tw o -lan e runway ex p erim en t, r a t s were given a choice between one p e lle t delivered immediately upon entry into the goal box, and four p e lle ts delivered 8 s a f te r goal box entry. When runway length was 40 cm, subjects e ith e r p re fe rre d the smaller- immediate re in fo rce r or they were in d iffe re n t; when runway length was increased to 240 cm, a l l s u b j e c t s s t r o n g l y p re fe rre d the l a r g e r delayed re in fo rce r. Because s p a tia l distance, e f f o r t, and time to re in fo rce m e n t co v aried in th is study, the singular e ffe c ts of each were unclear. The e ffe c ts of e f f o r t, independent of those of time to re in fo rc e m e n t, were e lu c id a te d in a re c e n t study (Grossbard & Mazur, 1986). Pigeons were given a choice between a constant FR schedule th a t delivered 2-s access to grain and an ad ju stin g FR th a t offered 6-s access to grain. The constant FR v a rie d a cro ss c o n d itio n s ; the a d j u s t i n g FR was v a r i e d w ith in each s e s sio n u n t i l subjects chose the two schedules equally o ften. Values of the a d ju s tin g FR at which subjects were in d iffe re n t were id e n tifie d fo r each value of the constant FR. In o th e r c o n d itio n s, FT sch ed u les were yoked to the FR schedules such th a t the value of the FT schedule equaled the average run time under the FR schedules. Summarizing
13 re s u lts from many conditions, Grossbard and Mazur (1986) concluded th a t "... t h e tim e to th e la rg e r e in f o r c e r tended to be s lig h tly longer when the a lte rn a tiv e s were two FR schedules than when they were two FT schedules" (p. 312). This im plies th a t the re la tiv e value of the la rg e r re in fo rce r is g reater under two FR schedules than under two FT schedules, and th a t e ffo rt is a f a c to r in determining responding fo r la rg e r delayed re in fo rce rs. Many of the above studies showed th a t s e lf-c o n tro l i s a f u n c t i o n of c h a r a c t e r i s t i c s of the d elay s to reinforcem ent. Other research reported d iffe re n t re su lts (L ogue, P e n a - C o r r e a l, R o d rig u e z, & K abela, 1986). Subjects earned points th a t could be exchanged for money a f t e r each session. The delays to money delivery were manipulated in various ways. The r e s u l t s showed th a t subjects usually chose the la rg e r re in fo rce r irresp e ctiv e o f th e d e l a y s. In f a c t, when q u e r ie d a f t e r th e experiment, subjects reported th a t they attempted to earn the maximum number of points in each session (a ls o see B la k e ly, S t a r i n, & P o lin g, in p r e s s ). The a u th o rs suggested th a t the n a tu re of th e r e in f o r c e r in th e ir experiment may have strongly affected the re s u lts : "All of th e p rev io u s experim ents w ith pigeons used food- d ep riv ed pigeons and food as the re in fo rce r. In such s itu a tio n s, depending on the degree of food deprivation, there might be some advantage to obtaining food quickly"
14 (p. 172). I t s h o u ld be n o te d t h a t many re s e a rc h e r s have generated mathematical models in an e f f o r t to d e sc rib e an d p r e d i c t b e h a v i o r u n d e r v a r i o u s s e l f - c o n t r o l procedures. For example, one was suggested by Baum and Rachlin (1969) and is presented below; R1 VI A1 X D2 R2 V2 A2 X D1 This e q u a tio n g e n e r a lly h o lds th a t when an organism chooses between two schedules, the r a tio of the responses to two a lte rn a tiv e s (R1/R2), or the r a tio of the values of the a lte rn a tiv e s (V1/V2), is d ire c tly re la te d to ra tio of the magnitudes of reinforcem ent (A1/A2), and inversely r e la te d to th e r a t i o s of th e d e lay s of reinforcem ent (D2/D1) of the two a lte r n a tiv e s. The e x te n t to which t h i s and o th e r equations p red ict observed behavior has been addressed in many s tu d ie s ( e. g., N avarick, 1932; N avarick & F a n tin o, 1976; Snyderraan, 1983; Snyderman & G reen, 1980). The r e s u l t s of some r e s e a r c h ( e. g., Navarick & Fantino, 1976; Snyderraan, 1983) have suggested th a t the e q u atio n has lim ite d p r e d ic tiv e u t i l i t y and o th e r more complex e q u atio n s have been proposed (see Snyderman, 1983). The present study w ill not examine the g e n e ra lity of any p a rtic u la r equation, but instead w ill f o c u s on g e n e r a l f u n c t i o n a l r e l a t i o n s w i t h o u t
15 q u a n tita tiv e analyses. Focus of the Study The in v e s tig a tio n s above show t h a t s e l f - c o n t r o l, when defined as choosing a p referred delayed consequence over a le ss p referred more immediate consequence, is a f u n c tio n of e n v iro n m en tal v a r i a b l e s, among them the te m p o ra l c h a r a c t e r i s t i c s of th e d e l a y s, re s p o n s e r e q u ir e m e n ts d u r in g th e d e l a y, and n a t u r e o f th e re in fo rce r. More s p e c ific a lly, a major finding is th a t s e lf-c o n tro l is a function of where, in tim e, the subject is required to make the choice. When the choice point is close in time to contact with the immediate rein fo rce r, th a t re in fo rce r is selected. But when the choice point is tem porally separated from the re in fo rc e rs, by adding a constant to both delays, responding s h if ts to the larg er re in fo rce r (see R achlin, 1974). This preference rev ersal has been shown in many studies (e.g., Navarick & Fantino, 1976; Rachlin & Green, 1972; Solnick e t a l., 1982). Experim ent 1 o f the p re s e n t study w i l l exam ine fu r th e r the preference rev ersal phenomenon. In studies th a t demonstrated the phenomenon, the delays were defined by single time-based schedules (Green, F isher, Perlow, & Sherman, 1981; Navarick & Fantino, 1976; Rachlin & Green, 1972). I t i s, t h e r e f o r e, unclear the ex ten t to which preference re v e rsa l is procedure-bound. In an e ffo rt to
16 extend th is research. Experiment 1 of the present study- d e fin e d th e delays with two FR schedules. A d is c re te - t r i a l s procedure (see Hall-Johnson & P o lin g, 1984) was employed in which pigeons chose between two schedules, each com prising a sequence of two FR sch e d u le s. The seco n d sch ed u le of one sequence was an FR 5, which offered 2-s access to g rain, and th a t of the a lte rn a tiv e was an FR 45 which o ff e re d 8 -s access to g rain. The c h o ic e p o i n t was t e m p o r a l l y s e p a r a t e d fro m th e r e in f o r c e r s by in c r e a s in g th e s iz e o f the i n i t i a l FR s c h e d u le of each s e q u e n c e. The e f f e c t s o f t h i s manipulation were studied when completion of the i n i t i a l FR was followed by (a) a hopper fla sh, and (b) access to food. This procedure assayed the e ffe c ts on s e lf-c o n tro l of adding equal ra tio s to two FR schedules of d iffe re n t s iz e s, and o f f e r i n g d i f f e r e n t m a g n itu d e s o f reinforcem ent, and the extent to which such e ffe c ts were attenuated by re in fo rce rs presented a f te r the i n i t i a l FR. In Experiment 1, m anipulating FR s iz e v a rie d both tim e to reinforcem ent and e f f o r t. The e ffe c ts of each v a r i a b l e w ere, t h e r e f o r e, u n c l e a r. E x p e rim e n t 2 attem pted to separate th e ir e ffe c ts. In i t, pigeons were given a choice between an FR 5 schedule th at offered 2-s access to g rain, and a varied FR schedule th a t offered 8- s access to g rain. The value of the varied FR schedule was m anipulated a c ro ss c o n d itio n s. A fter exposure to
17 each varied FR value, yoked FT schedules were programmed to assay the effects on self-control of deleting response requirements during the delay to reinforcem ent. The re s u lts of previous research suggests that self-control should decrease when subjects move from a choice between two FR schedules to a choice between two yoked FT schedules (Grossbard & Mazur, 1986).
CHAPTER II EXPERIMENT 1 Method Subjects F o u r W h ite C a r n e a u x p i g e o n s, m a in ta in e d a t approximately 80% of th e ir free-feed in g weights, serv ed as s u b je c ts. Two birds were experim entally naive (PI, P 3 ), and th e o t h e r two had re s p o n d e d u n d e r r a t i o schedules in a previous experiment (P2, P4). Each bird was in d iv id u ally housed with unlim ited access to w ater and g r i t in a constantly illum inated room. Apparatus Four Lehigh Valley E lectronics operant conditioning chambers, measuring 32 cm long, 36 cm h ig h, and 35 cm wide, were used. Three response keys, 2.5 cm in diameter and 5.5 cm ap art, were mounted on the front wall 23 cm from the flo o r. Each could be transillum inated red, or b lu e /g re e n, and o p e ra te d by a f o r c e of 0.2 N. An aperture centered on the fro n t wall 7.5 cm from the flo o r perm itted feeding from a grain hopper. When raised, the hopper was illum inated by a 7-W bulb and provided access 18
19 to m ixed-grain. A 7-W bulb c e n t r a l l y lo c a te d on th e c eilin g provided ambient illu m in atio n, and a w hite-noise generator provided masking noise. A PD P -8A m i n i c o m p u t e r ( D i g i t a l E q u ip m e n t C o rp o ra tio n, M aynard, MA) e q u ip p e d w ith SUPERSKED s o f t w a r e ( S t a t e S y s t e m s, K a la m a z o o, MI) and e l e c t r o m e c h a n i c a l i n t e r f a c i n g c o l l e c t e d d a ta and scheduled experim ental events. Procedure Subjects were exposed to a v ariab le time (VT) 45-s schedule of grain d e liv e rie s u n til each re lia b ly ate from the hopper. B irds then re c e iv e d p a ir in g s of 6-s key illu m in atio n s, and 3-s access to grain presented under a VT 4 5 - s s c h e d u l e. R e s p o n s e s had no program m ed consequence. Key c o lo r (red or b lu e /g re e n ) and key p o s i t i o n ( r i g h t, c e n t e r, o r l e f t ) w ere random ly determined on each t r i a l with the r e s tr ic tio n th a t each c o l o r / p o s i t i o n com bination occurred an equal number of times each session. l i g h t / f o o d p a ir in g s. Sessions term inated a f t e r 36 key- When s u b je c ts key-pecked on at le a s t 80% of the t r i a l s, they were exposed to fix e d -ra tio (FR) schedules of grain delivery programmed on one of the three keys. The schedule, i n i t i a l l y FR 1, was gradually increased to FR 65 over a number of sessions. The key color and p o sitio n were randomly determined although each
20 c o lo r/p o sitio n combination occurred equally often within sessions. R einforcers were followed by a 15-s i n t e r t r i a l i n t e r v a l ( I T I ), and s e s s i o n s te r m i n a t e d a f t e r 36 re in fo rc e rs. When a l l 36 ra tio s were completed w ithin 45 minutes in th ree c o n se c u tiv e s e s s io n s, th e experim ent proper began. In th e experiment proper, subjects responded under two sequences of FR schedules, each sequence comprising two FR schedules. In one sequence, the second schedule was always an FR 5 followed by 2-s access to grain; in th e o th e r, i t was an FR 45 follow ed by 8-s access to g r a in. The i n i t i a l sch ed u les of each sequence were always eq u al and were v a rie d as described below. The sequence with the 8-s re in fo rce r always required 40 more re sp o n se s than th e sequence w ith the 2 -s r e in f o r c e r. E ig h teen fo rc e d -e x p o su re t r i a l s were fo llo w ed by 18 ch o ice t r i a l s in each session. During forced-exposure t r i a l s, one of th e t h r e e keys was l i g h t e d red or blue/green, and one of the two sequences was programmed on th a t key. Each sequence was programmed on 9 forced- exposure t r i a l s each session. During choice t r i a l s, two of the three keys were lig h te d, were presented sim ultaneously. and th e two sequences A fter a response to one of the illum inated keys, the other key was darkened and th a t sequence was in o p erativ e. Both forced-exposure and choice t r i a l s were followed by an ITI, the d u ra tio n of
21 which was a d ju ste d su ch t h a t th e o v e r a l l r a t e s of reinforcem ent offered by two sequences were equal. Key lig h ts were darkened and resp o n ses had no programmed consequence d u rin g th e IT I. Each key c o lo r /p o s itio n com bination, in fo rce d exposure and c h o ic e t r i a l s, occurred an equal number of times per session. Sessions term inated a fte r 36 t r i a l s or 60 minutes, whichever came f i r s t. In Phase 1, the i n i t i a l FR schedule was FR 1, FR 5, FR 20, FR 35, or FR 50. Each bird received a l l values, but in a d i f f e r e n t i r r e g u l a r o rd er across conditions. Two conditions were programmed at each i n i t i a l FR value. In th e food c o n d itio n, 3 -s access to g ra in follow ed completion of the i n i t i a l FR of each sequence. In the hopper f la s h co n d itio n, a 0.25-s hopper fla sh followed completion of the i n i t i a l FR. The order of the food and hopper f la s h c o n d itio n s a t each i n i t i a l FR value was randomly determined. As the data were co llected in Phase 1, i t was clea r th a t when the i n i t i a l schedule of each sequence was an FR 1, responding was biased more toward the sequence with the la rg e r re in fo rce r ( i. e., 8-s access to grain) under the food condition than under the hopper flash condition. Increased responding fo r th e sequence w ith th e la r g e r re in fo rce r under the food condition might be a function of th e food d e l i v e r i e s p e r s e. But im p o sin g food
22 d e liv e rie s also increased the delay to the 2 -s and 8-s re in fo rc e rs because the duration of the food d e liv e rie s was 2.75-s more than t h a t of th e hopper f la s h e s. To in v e stig ate the e ffe c t of adding a 2.75-s delay to the 2- s and 8-s re in fo rc e rs. Phase 2 manipulated the duration of the hopper fla sh. The sequences were arranged in the same way as in Phase 1, and th e i n i t i a l schedule was always FR 1. In the f i r s t condition, completion of the FR 1 was followed by a 3 s hopper fla sh. In the second condition, the hopper flash was 0.25 s; in the th ird, 3 s. Following th is sequence of c o n d itio n s, P3 was r e exposed to the 0.25 s, and then th e 3 s c o n d itio n, because there were large differences between th e f i r s t and second exposure to the 3 s condition. In both p h ases, th e dependent v a ria b le was th e p e rc en ta g e of choice t r i a l s on which the sequence with the la r g e r r e in f o r c e r was s e le c te d. C o n d itio n s were changed a f t e r a minimum of 10 sessions, and when there was no v i s i b l e tr e n d in th e d a ta o v e r th e l a s t 5 s e s s io n s. The key color associated with each sequence varied across birds and was occasionally changed a f t e r exposure to both c o n d itio n s a t a p a rtic u la r FR value. The number of sessions and key color co rrelated with the sequence with the la rg e r rein fo rcer in each condition are presented in Table 1.
23 Table I The Number of Sessions and Key Color Associated with Sequence O ffering the Larger R einforcer a t Each Value of the I n i t i a l Fixed-Ratio Schedule fo r Each Bird Subiect PI P2 P3 P4 I n i t i a l FR Phase 1 FR 1 10 (25) 24 (10) 47 (10) 24 (35) Blue/Green Blue/Green Red Red FR 5 24 (14) 17 (22) 10 (27) 32 (13) Blue/Green Blue/Green Blue/Green Red FR 20 10 (12) 32 (48) 32 (27) 23 (10) Red Blue/Green Blue/Green Red FR 35 10 (16) 19 (19) 44 (20) 14 (19) Blue/Green Red Red Red FR 50 10 (10) 12 (22) 22 (38) 30 (44) Red Blue/Green Red Red Phase 2 3 s H.F. 38 24 27 (14) 33 Red Blue/Green Red Red 0.25 s H. F. 40 10 10 (10) 10 Red Blue/Green Red Red 3 s H.F. 20 16 29 34 Red Blue/Green Red Red Note. For Phase 1, the f i r s t number of the p air and the number in parentheses rep resen ts the number of sessions in the food and hopper f la s h c o n d itio n, r e s p e c tiv e ly. For Phase 2, th e number in parentheses represents the number of sessions in re-exposures to th a t condition.
24 R esults and Discussion Figure 1 shows fo r each subject the p e rc e n ta g e of choices to the sequence with the la rg e r re in fo rc e r in the hopper fla s h and food conditions as a function of i n i t i a l FR siz e. Each data p o in t represents the mean of the la s t 5 sessions in each condition, and v e r tic a l lin e s through each point rep resen t the range. In both the hopper flash and food conditions, the percentage of c h o ic e s fo r the sequence with the la rg e r re in fo rce r ( i. e., se lf-c o n tro l) increased with the size of the i n i t i a l FR for a l l b ird s. A ll b i r d s showed p r e f e r e n c e r e v e r s a l, alth o u g h the i n i t i a l FR v alu e a t which th e s h i f t o ccu rred v a rie d s u b s t a n t i a l l y a c ro ss b ir d s. For P I, th e in crease in responding fo r the sequence with the l a r g e r r e in f o r c e r was p ro p o rtio n ally sm aller than th a t for the other b ird s, because p re fe re n c e f o r the sequence w ith th e sm aller r e i n f o r c e r a t low i n i t i a l FR v a lu e s a p p ro a c h e d in d ifferen c e. Casual observations su g g ested th a t t h is b ird showed a preference fo r the rig h t key, p a rtic u la rly under the food condition, a t lower values of the i n i t i a l FR. N evertheless, the re la tio n between i n i t i a l FR size and s e lf-c o n tro l s t i l l obtained. I t should be noted th a t when the i n i t i a l schedule was FR 1, there was more s e lf - c o n tr o l fo r a l l b ird s under the food than under the hopper fla sh condition.
100 P3 eo -o 8 ii. «2 Ï.S P 85 " (3 K 100 ft 60- P2 # - - # HOPPER FLAtH 0-0 FOOD P4 40-90 20 INITIAL FR SIZE 35 60 20 INITIAL FR SIZE 36 60 Figure 1. The Percentage of Choices to the Sequence with the Larger Reinforcer as a Function of the Size of the In itia l FR Schedule. Data Points Represent the Mean of the Last Five Sessions of Each Condition; V ertical Lines, the Range Across the Five Sessions, Ln
26 Figure 2 shows th e p ercen tag e of ch o ices to th e sequence w ith th e l a r g e r r e in f o r c e r as a function of hopper flash duration. Data are presented from each of the l a s t five sessions fo r individual b ird s. For PI, P2, and P4, th e sequence w ith th e la r g e r r e in f o r c e r was chosen more often when the hopper fla sh was 3 s than when i t was 0.25 s. The d ifferen ces were on the order of 30-40%. For P3, th ere was only a small d ifferen ce when the hopper fla sh was f i r s t decreased from 3 s to 0.25 s, but a much larg er d ifferen ce upon subsequent exposure to the two conditions. In th is study, reinforcem ent delays were defined by FR schedules. Equal response requirements were added to the FR 5 and FR 45 schedules by increasing the i n i t i a l FR s iz e. The r e s u l t s showed t h a t as i n i t i a l FR s i z e increased, preference sh ifte d from the sequence with the sm aller r e in f o r c e r to th e sequence w ith th e re in fo rc e r th at also required more responses. l a r g e r Previous research has reported s im ila r r e s u l t s when th e delays were d e f in e d u s in g o t h e r s c h e d u le s. For example, Navarick and Fantino (1976) programmed two FT (FT t s & FT t-10 s) or two FI (FI t s & FI t-10 s) schedules as term inal lin k s of a c o n c u rre n t-c h a in s sch ed u le. The l a r g e r r e i n f o r c e r was programmed under the form er schedule of each p a ir. As ^ increased, responding to the schedule offering the la rg e r rein fo rcer increased. In
3 3 HOP FI. 0 25 S HOP FL 3 S HOP FL. 3 S HOP FL 0.29 8 3 8 HOP.FL. HOP.Ft. P3 80 (/> UJ i I BO 4 0 «20-80 P2 P4 i 40-20- Figure 2. L A S T 9 S E S S IO N S The Percentage of Choices to the Sequence with the Larger Reinforcer as a Function of Hopper Flash Duration for Individual Pigeons. Data are Presented for Each of the Last Five Sessions of Each Condition. fo -vl
28 many cases, preference re v e rsa l was o b ta in e d. R ach lin and Green (1972) programmed a b la ck o u t of t_ seconds before contact with the term inal lin k s of a concurrent- chains schedule. As ^ increased, preference sh ifte d from th e s m a l le r to th e l a r g e r more delayed r e i n f o r c e r. F in a lly, Boehme, Blakely, and Poling (1986) m anipulated runway le n g th. R ats chose between 1 p e lle t delivered immediately a fte r goal box entry, and 4 p e lle ts delivered 8 s a f t e r g o al box en try. When distance to both goal boxes was in c re a se d from 40 cm to 240 cm, p re fe re n c e sh ifte d from the sm aller to the la rg e r re in fo rce r. Taken t o g e t h e r, th e r e s u l t s of t h e s e s t u d i e s show t h a t i r r e s p e c t i v e of how th e d e lay s are d e fin e d, adding a c o n s ta n t to both d elay s s h i f t s p r e f e r e n c e from th e s m a l l e r r e i n f o r c e r to th e l a r g e r, more d e la y e d, re in fo rc e r. When the delays were defined in non-temporal dimensions (e.g., r a tio size, d ista n c e ), however, time to reinforcem ent and e ffo rt covary. Therefore, the singular e ffe c ts of each are unclear in these in v e stig atio n s. When the i n i t i a l sch ed u le was an FR 1, in th e present study, there was more responding for the sequence with the la rg e r re in fo rce r under the food condition than u n d e r t h e h o p p e r f l a s h c o n d i t i o n. Two p o s s i b l e in te rp re ta tio n s may be offered for th is re s u lt. F ir s t, because th e hopper fla s h e s were only 0.25 s and food d e liv e rie s were 3 s, the 8-s and 2 -s r e in f o r c e r s were
29 delayed an ex tra 2.75 s in the l a t t e r c o n d itio n. With the 8-s re in fo rc e r, th is change represented a re la tiv e ly small p ro p o rtio n al increase in delay to re in fo rc e m e n t. With th e 2 -s r e in f o r c e r, the p ro p o rtio n al increase was la rg e r and probably more d iscrim inable. Thus, control by the 2 -s r e in f o r c e r might be expected to be atten u ated. D ata from P h ase 2 s u p p o r t t h i s i n t e r p r e t a t i o n. Increasing the duration of the hopper fla sh from 0.25 s to 3 s gen erally decreased preference fo r th e sequence with the 2-s re in fo rc e r. Second, access to grain a f te r the i n i t i a l FR might somehow obscure the d if f e r e n tia l e f f e c t s of th e second FRs. The present experiment did not evaluate the e ffe c ts of a cc e ss to g ra in p er se on c o n tr o l by th e second schedules. To answer th is question, the e ffe c ts of 3-s access to g rain should be compared to th o se of a 3-s hopper fla s h. Previous research with sequences of ra tio schedules showed, however, th a t the second ra tio schedule in a sequence d id a f f e c t ch o ice when access to grain follow ed com pletion of th e i n i t i a l s c h e d u le o f th e sequence (Poling, Blakely, P e lle tie r e, & Picker, 1987). That study and the present research demonstrate th a t food d e l i v e r i e s do n o t c o m p le te ly o b s c u r e c o n t r o l by subsequent schedules. In p re v io u s s e l f - c o n t r o l r e s e a r c h, delays were defined by s in g le s c h e d u le s. reinforcem ent The p re s e n t
30 study extends th is research by showing s im ila r r e s u l t s d elay s were d e fin e d w ith a sequence of two schedules. Moreover, re in fo rc e rs were in terp o sed d u rin g th e delay ( i. e., a f te r the i n i t i a l FR), which consisted of the time re q u ire d to com plete an FR s c h e d u le. T h at c h o ic e resp o n ses were s e n s i t i v e to d if f e r e n tia l contingencies embedded in such complex d e lay s i s n o tew o rth y. The p r e f e r e n c e r e v e r s a l phenomenon i s not dependent on defining the delays with single time-based schedules.
CHAPTER III EXPERIMENT 2 Method Subjects Four W hite C arneaux p ig e o n s, m aintained a t approximately 80% of their free-feeding weights, served as subjects. Two (S3, S4) were experimentally naive; the other two (S2, S2) responded under ra tio schedules in previous research. Each bird was individually housed with free access to water and g r it in a c o n sta n tly illuminated room. Apparatus The apparatus used in Experiment 1 was also employed in Experiment 2. Procedure The same procedures as those in Experiment 1 were used to train the birds to eat from the hopper, peck the key, and complete FR 65 schedules. In the experiment proper, forced exposure tria ls, choice t r i a l s, and the ITI were arranged in a manner identical to that described 31
32 in Experiment 1. In the p re se n t ex p erim en t, s u b je c ts chose between a constant and a varied schedule. In the f i r s t condition, the constant and varied schedules were FR schedules. The constant FR schedule in th is and a l l other conditions was an FR 5 th a t provided 2-s access to g ra in. The varied schedule was e ith e r FR 5, FR 20, FR 35, FR 50, or FR 65 and each programmed 8-s access to g r a in. The four s u b je c ts received a d iffe re n t varied schedule in th is f i r s t condition. In the next condition, th e c o n sta n t and v a rie d FR sch ed u les were changed to y o k e d c h a i n FR 1 FT s c h e d u l e s t h a t p r o v i d e d, r e s p e c tiv e ly, 2 -s and 8 -s access to grain. Under the yoked schedules, completion of the FR 1 schedule darkened key lig h ts and in itia te d the FT schedule, the completion of which produced access to grain and entry in to the ITI. The time requirements of the constant and varied FT were equal to the average run time of the constant and varied FR schedule, resp ectiv ely, in the la s t 5 sessions of the previous condition. The run time on a given t r i a l was the time between the f i r s t response and la s t response of th e FR. Average run tim es f o r each s c h e d u le were computed by dividing the to ta l run time by the number of forced t r i a l s on which the subject responded under th a t schedule. In th e rem ainder of the c o n d itio n s, b ird s were exposed to a l l other values of the varied FR schedule in