ened condensed milk delivered by multiple schedules. m five seconds to 16 minutes. Positive contrast did not the rate af responding emitted during a variable interval Fe schedut:e was not greater when the other component was identicd variable interval schedule. The function that ng to component duration atso differed from that reported by past studies ing keys_ Finding such functional differences suggests that behavioral iated by dif#erent mechanisms for different responses. It also implies er &@erences in the rates of responding, nor differences in the parts of the body past functional differences in the observation of cmtrast









isn sies of b&auioraE contrast predict that contrast will occur in a similar manner (e.g. Herrnstein, 1970; Hinson and Staddon, 1978; McSweeney, 1987; but see a&so.the Additive Theories, e.g. Rachlin, 1973). Recent data have e theories by showing that contrast generally varies inversely with compohen pigeons peck keys (McSweeney, 1982) and directly with component ratisaa when


ce ts: F.K. 991644820,



McSweeney, c&4.


et al.,


or rats press levers

Department of Psychology, Washington

State University,

and Afkfvilfe, ‘39911.


questions the general proCe=J dismissed as artifacts of melhodologica ceiling effects, failures of discriminati response, but they cannot explain wh cxxltra5t



for different


resents differ in fundamental ways. The factors that produce the fun~i~~a~ mined

because of their potential

to be criticai:



results were found

di i

for lever pressin

pigeons. Factors related to response topography dlo a were found for key pecking and treadle pressing by The present experiment tests two possible first, contrast might change differently for r and treadle pressing, than for those that em apparatus,

such as key pecking.



emitted at low rates, such as lever or trea such as key pecking. To test these hypotheses, rats were trained to press a key with their sallou& If ei the preceding hypotheses is correct, then the size of positive ke~+press o~~tirast shoul decrease with increases in component duration as it does for key pecking. Key rate. like key pecking, employs the head and, as will be shown, ,occ~-s at a The current experiment uses a within-session method of measuring avioral contrast. Behavioral contrast is an inverse relation between the Crateof responding emitted constant component of a multiple schedule and tIhe conditions of rein by the other component. The across-sessions procedure, used in mos modifies reinforcement frequency in one component across successive line to contrast and then back to baseline (e.g. h&Sweeney et a!., 1 within-session, procedure measures contrast and baseline within sing! A within-session procedure was used in this study because of the rat. Each across-session measurement of contrast requires e in each of the three phases (baseline, contrast, baseline), sessions. As a result, the length of the experiment may e when five or more measurements of contrast are desired. addresses this problem by reducing the amount of time re cornast to approximately 30 sessions. The results of this st resub of several past studies of key pecking and lever or trea withinw~ion procedure. This comparison could not be dure were used here.


aterials an Subjects The subjects were five experimentally-naive rats bred from Sprague-Dawley stock in the Johnson Tower vivarium at Washington State University. The rats were approximately 120

rimentai endssure forrats, equip seated 3 cm from each side ~9 ear and the fight was opaque. A 2.5 cm A 3.5 cm fever, which extended 2 em right tight. A 6 cm diameter opening r. The opening was centered in the panel, 4 cm above the tus was ewcltxed in a sound-attenuating chamber. A SYM microcomputer, esented the experimental events and recorded the data. A from outside the apparatus.

d by successive approximations to press the key with their snouts. on a multiple schedule that delivered high rates of reinforcement in nents. Rates of reinforcement were gradually reduced until the subjects multipIe variable interval 15-s (Vi 15-s) VI 15-s schedule. ent began, each experimental session was divided into two halves that uccessively. During the first (baseline) half, a multiple V1 15-s VI 15-s ed to al! subjects. During the second (contrast) half, a multiple VI 15-s presented. The availability of the component listed first was ng the light located above the key. The component listed second was e Eight off. The components alternated uration varied across experimental conditions. The following durations following order: 60 s (201, 30 s (40), 5 s (240), 3 min (8) and 16 (2). The number of components presented in each of the baseline ahd contrast phases e experimental session appears in parentheses after each duration. This number varied with component ation to prevent some sessions from t jecoming extremely long. Each was presented for 30 sessions. component chat All reinforcers were scheduled according to a 25-interva ,I series constructed according to e procedure outlined in Fleshier and Hoffman (1962). Reinforcers consisted of 5 s of access to a dipper that contained approximately 0.2 ml of sweetened condensed milk mixed one to one with water. Sweetened condensed milk was used in order to provide a food that was difficult to manipulate with the paws. Subjects could manipulate solid rlanipulations transferred to the response key, reinforcers such as Noyes pellets. ti &I then subjects might press the key with their paws instead of with their snouts. Reinforcers that were scheduled but not collected before a component changed were held over for the next presentation of that component. Sessions were conducted daily, five

to six times per week. A light appearing above the tever served as a houselig illceminated throughoui al; sessions. ante this procedure had been conducted.


It was identical to the first except that

a second sequence of sc a multiple V3 2-min VI 2-mm sc

was presented appeared

in the first half of the session. A multiple VT 2-min extin re ccr in the second half. The following component durations

following order: 60, 30 and 960 s. Throughout this paper, the schedule presented during the first ha18of the VI 15-s VI ‘s5-s or ~~~~~~~

referred to as the baseline schedule (multiple

2-min); the schedule presented during the second VI 15-s extinction or multiple Vi 2-min extinction).

half, as the contrait s&e The component

that is

across phases will be referred to as the constant component (e.g. the The component that is changed will be referred to as the changing co 15-s and extinction component).

Table I presents the mean rates of responding (presses per min) emitted by eat and by the mean of all subjects during each component of each multiple VI 15-s and multiple VI 15-s extinction schedule. Rates were calculated by dividing the nu responses emitted during a component by the time for which that component was available. The time of reinforcement availability was excluded from all calculations. Response rates have been averaged over the last five sessions for which each component duration was available. Occasionally subjects failed to obtain the same number of reinforcers in the constant components of the baseline and contrast schedules. Contrast cannot be studied for those sessions because Lontrast represents a change in the rate of responding that occurs even when the rate of reinforcement provided by that response remains constant. To avoid t problem, the last five sessions were sought during which the rate of reiniorcement obtained from the constant component remained constant from the baseline to the contrast schedule. Data from those sessions are reported in Table I and are indicated by asterisks. Table I shows that baseline response rates changed across conditions. The mean baseline response rate during the constant component varied from 91.2 to 150.7 responses per min. Response rate during the changing component varied from 83.9 to 130.2 responses per min. These changes may represent an effect of component duration on the baseline rates of responding. Alternatively, they may represent a fluctuation in responding over time Response rates often double between the presentation of a schedule and its recovery when subjects respond on multiple schedules (e.g. !&Sweeney et al., 1986; Spealman and Gollub, 1974). Such large changes in response rates imply that the baseline rate of responding must be measured each time that contrast is measured, as it was here. Table 1 shows that the present procedure controlled behavior. Responding during the changing component decreased when extinction replaced the VI 15-s schedule for all subjects and al1 component durations. The t-tests for matched pairs showed that these decreases were statistically significant for all component durations (t(3) = 5.67, 5-s components; t(3) = 7.55, 30-s components; t(3) = 7.27, 60-s components; t(3) = 6.87, 3-min components; t(3)= 8.96, 16-min components). Throughout this paper, results will be considered to be significant when P < 0.05.



r mid

a 5-s etiindion


CQ!lStXkt changing

emitteei derringeach


of each mukipk

VI 15-s Vf


128.1 110.5





90.6 70.0

210.6 41.4



7 20.8

39.1 98.0 38.5

133.2 117.3



2-I 3

a 77.6

189.7 109.1


111.0 101.6












9-l .4

23 7

156.0 141.3

69.5 * 69.4 *

59.3 * 2.4 *

108.3 *

114.3 *

87.7 * 8.9 *

100.2 * 97.2 *

98.2 17.0 83.6 * 3.9 *

Subject 935 Duration



constant changing





Basefine 60.5 54.5

Mean Contrast 43.5 36.6

Saseline 91.2 83.9

constant changing

123.7 *

79.2 *


105.2 *

25.0 *


constant changing







constant changing

93.0 80.9

71.7 10.4


constant changing

83.4 * 69.9 *’

Contrast 77.2 61 .O 127.5 31.7 97.3 38.8


100.2 18.6

54.4 *



6.0 *



* These data are not from the last five sessions

Positive contrast did not occur for any component duration. The rate of responding emitted during the constant, VI 15-s, component increased from the baseline to the contrast schedule in only two of the 20 cases reported in Table 1 for individual subjects (subject 934, the 60 -s and 3-min components). Aithough constant-component response rates generally decreased, these decreases were not statistically significant for any component duration except I6-min (t(3) = 2.67, 5-s components; t(3) = 2.63, 30-s components; t(3) = 1.99, 60-s components, !([3 j = 1.39, 3-min components; t(3) = 4.70, 16-min components). This one significant decrease represents negative induction defined as a decrease in the rate of responding for a constant reinforcer that occurs when the conditions of reinforcement provided by the other component worsen.

118 TABLE 2 Rates of responding (presses per min) during each component and multiple VI 2-min extinction schedule

of each m1.11tipk VI 2-min


Duration 30-s







931 Contrast



Basesine 29.1









































.3 4.5

Subject 934 Duration




constant changing

14.5 10.2

9.3 4.5

13.7 10.1














935 Contrast





7.4 4.2

19.8 16.4

10.0 4.5
















1 .o



Table 2 presents the rates of responding (presses per min) emitted by each su during each component of each multiple V1 2-min VI 2-min or multiple VI 2-min extinction schedule. Response rates have been calculated as in Table 1. Again, baseline response rates varied substantially across conditions. Response rates during the constant component varied from 8.9 to 19.8 responses per min. Response rates during the changing component varied from 5.8 to 16.4 responses per min. The procedure also controlled behavior. The rate of responding during the changing component decreased when extinction replaced the VI 2-min schedule for all subjects responding at each component duration. The t-tests for matched pairs show that these decreases were statisticL!y significant for each component duration (t(4) = 2.93, 30-s components; t(4) = 3.27, 60-s components; t(4) = 6.48, 16-min components). Negative induction occurred instead of positive contrast. The rates of responding emitted during the constant, VI 2-min, component decreased rather than increased from the baseline to the contrast schedule for all subjects and all component durations except for subject 934, the 60-s components. The t-tests for matched pairs showed that these decreases were statistically significant for each component duration (t(4) = 3.29, 30-s components; t(4) = 3.36, 60-s components; t(4) = 3.74, 16-min components). Figure 1 presents the size of negative induction (ratios less than 1 .O)or positive contrast (ratios greater than 1 .O) plotted as a function of component duration in seconds for the median of all subjects. The size of induction or contrast was calculated by dividing the rate

Fig. I. Csntmt

ratios as a functioer of component

duration in seconds. Contrast

ratios were

ing the rate of responding emitted during the constant component of the contrast e by the rate of respon ing emitted during the constant component of the bzseiine schedule. e for the median of a19subjects responding when the VI 15-s (inverted triangles) and VI 2-min (squares) schedules were used as components. The triangles present the results of a similar study which examined lever pressing by rats (McSweeney and Melville, 1991). The circles present the results of a similar study which examined key pecking by pigeons (McSweeney and Melville, 1988, Experiment 3). Ratios greater than 1 .O represent positive contrast. Ratios less than 1 .O represent

negative induction.

of responding emitted during the constant component of the contrast schedule by the rate of responding emitted during the same component of the baseline schedule. The inverted triangles present the results for the multiple VI 15-s VI 15-s and multiple VI 15-s extinction schedules. The squares present the results for the multiple VI 2-min VI 2-min and multiple VI 2-min extinction schedules. The triangles present the results for a similar experiment for lever pressing (McSweeney and Melville, 1991). The circles present the results from a similar experiment for key pecking by pigeons (McSweeney and Melville, 1988, Experiment 3). These experiments will be discussed later. Medizris have been reported and nonparametric statistics will be used because these measures are ratios. If greater deviations from 1 .O are considered to be greater induction, then Fig. 1 shows that induction was larger for the multiple VI 2-min VI 2-min than for the multiple VI 15-s VI 15-s baseline schedule. This was also trrre when the data were examined only for the four subjects that responded on all schedules. When only those data were considered, the median size of negative induction for the VI 15-s components was 0.84, 0.92 and 0.80 for the 30-, 60- and 960-s component durations, respectively. The median size of negative induction for the VI 2-min components was 0.54, 0.77 and 0.54 for the same component durations. Friedman nonpsrametric analyses of variance showed that the sizes of induction for the VI 15-s and 2-rnir; baselines were significantly different for the 30- (Friedman Test Statistic = 4.00, df = 1) and 960- (Friednxn Test Statistic = 4.00, df= 1 ), but not for the 60-s (Friedman Test Statistic = 1 .OO, di’= I) components. Figure 1 also shows that the greatest negative induction occurred for the longest and shortest components with smaller induction for intermediate duration. A Friedman nonparametric analysis of variance applied to the ratios for individual subjects was significant

for the multiple $4 2-min Vl 2-rnhI


bblt not for the multiple Vl 15-s VI 15-s df = 4. Wilcoxon Signed Rank Tests a 2-min schedules showed that inductiun


components W = ~.04 and for the 968- th size of induction did not differ for th

Failure to find contrast Positive behavioral contrast did not occur when rats pressed keys condensed milk. Instead, contrast generally failed to OCCUT when the mdt I 5-s schedule served as baseline and significant negative induaion occur multiple VI 2-min VI 2-min schedule was used. The present failure to observe contrast must be interpretrz!

careier8ly. T~CW are atitilf~-

tuai explanations for failures to observe any phenomena. Many such ex@ present results can be dismissed, however. They account for sr?me, but no The failure to find contrast cannot be attributed to a ceiling effect. Contest observed if response rates were high during the constant components of the schedules. High response rates could not increase much when subjects move contrast schedules. This might account for some failures to observe contrast in Table the mean response rates emitted during the constant components of the baseline sch rarely exceeded 20 responses per min in Table 2. Yet contrast did not occur fo schedules. The failure to find contrast cannot be attributed to difficulty rates of responding (e.g. Davison and Ferguson, 1978). The respo frequently exceeded 100 responses per min, but contrast was n The failure to find contrast did not result from a general insensitivity reinforcement (e.g. Staddon, 1982). Instead, pressing changed sensitive1 reinforcement. It decreased with decreases in the baseline rate of reinforcement between Tables 1 and 2. it also decreased consistently when extinction @aced t in both Tables 1 and 2. The fai!ure to find contrast was not an artifact of poor dis nation tween components of the contrast schedules. To test this idea, a iminati ratio calculated by dividing the rate of responding emitted during the constant component of each contrast schedule by the sum of the rates of responding emitted during bot compone;xs. The median discrimination ratio for the multiple VI 15-s extinction schedule was-only 0.56 when the components were 5 s long. This is close to a ratio of 0.50 which indicates the absence of discrimination. However, the median discrimination ratio was 0.94 for the same schedule when components were 16 min long. This is close to the ratio of 1.0 that indicates perfect discriminatitin. Yet positive contrast did not occur for either component duration. The failure to find contrast was not an artifact of fatigue. The contrast schedule was presented after the baseline in the present experiment. The increase in response rate that is positive contrast would be difficult to detect if response rates decreased across the session due to fatigue. Fatigue may have played a role in the responding reported in Table 1 which often exceeded

100 responses per min; fatigue should not be as apparent

in Table 2


min, Netletihefes, omtrast

rger,, not smalkr,

in Table


across the session

tiarkm wdd reinforcers.


likely for Tabfe 2 kcowever, again, contrast

negative induction was larger for Table 2 than for contrast cannot be attributed to inherent but unknown flaws ure.

Figure f shows that sizeable psitive contrast has been

[ever pressing using the within-session

procedure. The results

ese responses were obtained using a within-session procedure resent one. A mukipte VE t 5-s VI 15-s schedule appeared in the first half of a multipIe L”i 15-s extinction schedule appeared in the second half. mated: the same component duration and number of components per for those responses except that a IO-min component replaced the key pecking. The stimuli that signalted the components were not ta! response operandum in any of these studies. ry, the present results show that positive key-press contrast does not occur for rent component durations and two baseline rates of reinforceare similar to those which do reveal positive contrast for key sing by pigeons and for lever pressing by rats when a within-session procedure similar to that used in the present study is also employed. The failure to serve contrast is not an artifact of obvious methodological problems, although such problems can never be entirely ruled out. The present results show that positive contrast may fail to occur for key pressing under conditions that produce contrast for other responses. They do not show that contrast never occurs for key pressing. Future studies should examine the generality of the present failure to find contrast. Key-press contrast has not been studied using conventional two-component schedules, although Williams (I 990) has obtained results that are both similar to and different from the present ones using three- and four-component multiple schedules. Differences in contrast across responses Figure I presents the functions relating contrast or induction to component duration for three different responses. Comparisons among thes_0 functions must be done carefully. All attempts to compare across responses are inherently flawed. Functional differences may always be attributed to procedural differences among the studies used to measure them. Even when as many procedural factors as possible have been held constant, procedures may still differ in important, but as yet unidentified ways. However, the functions reported in Fig. 1 provide as good a comparison across responses as is currently available. The orxedures used to measure these functions were similar. As mentioned, ail used a wrthin-sessions procedure, the same schedules of reinforcement, most of the same compcnent durations and number of components per session, and ail stimuli were located away from the instrumental response operandum. The differences among the functions are most easily attributed to differences in either the type

ptf&q5 and ~w~~~~n~~ con Jhe pmsj&$ljty that cfjffe cannot



little control


However, pa9 resUlt3 indicate

over the observation

grain for treadle pressing, McSweeney and Melville, 1991). if it is accepted that the results presented in Fig. 1 re~~wsent JI contrast for different responses, then the present results have i for contrast. As argued earlier, one possible explanation between

key pecking and lever or treadle

pressing is that contrast is

for responses that are emitted at high rates (e.g. key pecking) rates (e.g. lever or treadle pressing). The present results reject the key at high rates when V1 15-s components were used, yet the do not resemble those obtained for key pecking. The present results also reject the idea that contrast occurs ditiere6ntly 16s~res employ the limbs (e.g. treadle and ie\*sr pressing) than for those that emp therefore, the consummatory apparatus (e.g. key pecking). If this gene nosing by rats should have shown properties similar to those shown by pigeons. This did not occur. Key pecking by pigeons readily shsvb/s co;n&ast o of parameters used in the present study (e.g. McSweeney, 19 not show contrast for any of these parameters. The present subjects were not watched continually throughout tltilis ex theless, several considerations assure that they did press the key wilth their paws. First, occasional observation of the subjects sh snouts, usually by licking the key. Second, the subjects we snouts. Although pigeons that are shaped to press 2 txeaddc with a switch to pecking, the change in the response topography is acccionn increases in response rates. No such dkcontinuities i experiment. Third, the response rates reported in Ta represent pressing rather than nosing. Rats usually press responses per min. The rates of pressing reported in per min Finally, McSweeney and Melville (1991 b s procedure ,imilar to the present one. As indlicat lever-press contrast was observed for some of the present corn positive contrast should have been observed if tke rab ha key. Future studies should seek other explanations responses. Future studies should also examine using an across-sessions procedure. Past resslks procedure produce similar results. For exa reported using the across- (McSweeney, I Melville, 1988, Experiment 31 procedures and 1.44 for 30-s compsnenb and I .W and

for the similarities wh indicate that the within-

Fkquson, A.,, 2978. -Trw efk?cts WI schedur,~.

ifferent component

I. Exp. Ana

962. A progression n the 1~

response requirements


hav., 29: 283-295. for generating


schedules. 1.

of effect. j. Exp. Anal. Behav., t 3: 243-266. 978_ Behavioral competition: A mechanism for schedule


f 2ai2: 632-434. 1982. Positive and negative contrast as a function of component duration for key Exp. Anat. Behav., 37: 287 -293. n by reinforcement, a model for multiple-schedule behavioral cowUrast_ Behav. Process, 15: 191-209. c%veeney,

F.K.,, Dougan,

j.D-., I ii-g&, f. and Farmer, V.A., 1986. Behavioral contrast as a function of rate of reinforcement. Anim. Learn. Behav., 14: 173-l 83. y, F.K. and MeLviFte,. C.L., 1988. Positive contrast as a function of component duration

~&in-session procedure. Behav. Process, 16: 21-41. ~~~~~~~~~,, F.K. and Metvitte, C.L., 1991. Behavioral contrast as a function

of component


for ileveapressing using a within-session procedure. Anim. Learn. Behav., 19: 71-80. achiiin, H., 1973. Contrast and matching. PsychoI. Rev., 80: 217-234. &maw, RD. and GoKub, L.R., 1974. Behavioral interactions in multiple variable-interval sched-

_ Exp. Anaf. BebA, 22: 471-481. ~.E.R., 1982. Behavioral competition,




In: M.L.



stein and H. E&En (EditorsI, Quantitative Analyses of Behavior, Matching and Maximizing Accounts WoI. 21. Babtinger Publishing Company, Cambridge, ,Massachusetts, pp. 243-262. A_, 1983. Another took at contrast in multiple schedules. J. Exp. Anal. Behav., 39: 1990. Absence of anticipatory ;., 53. 395-407.

contrast in rats trained on multiple schedules. J. Exp.

Failure to find positive key-press contrast for milk reinforcers using a within-session procedure.

Rats pressed keys for sweetened condensed milk delivered by multiple schedules. Component duration varied from five seconds to 16 minutes. Positive co...
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