Journal of Comparative and Physiological 1975, Vol. 88, No. 2, 663-664
Psychology
WITHIN-SUBJECTS POSITIVE AND NEGATIVE CONTRAST EFFECTS IN RATS1 CHARLES F. FLAHERTY2 AND JOHN LARGEN Rutgers—The State University Rats were given alternating 1-min. access periods to 2 tubes containing either 32% or 4% sucrose solutions for daily 6-min. test sessions. Lick rate for 32% was higher under comparison (32 vs. 4) than noncomparison (32 vs. 32) conditions; and lick rate for 4% was lower under comparison conditions (4 vs. 32) than under noncomparison conditions (4 vs. 4). All sucrose conditions were varied within subjects and both positive and negative contrast were obtained with a small n. In addition to lick rate, intake and latency measures also revealed contrast effects. Deprivation conditions altered latency but not lick rate measures of contrast. Reducing the test session to 3 min. (alternating 30-sec. access periods) did not greatly affect contrast. Additional experiments provided evidence for distinct withindays and between-days contrast effects, as well as a between-groups contrast effect.
Contrast effects are often found when a this interpretation by showing that PCEs subject receives more than one level of re- may be obtained if procedures that obviate ward for performing a given task. These the influence of ceiling effects are used. For contrast effects are typically measured and example, Shanab and Biller (1972) and Melldefined by comparing the performance of gren (1972) have obtained PCE when the subjects receiving 2 levels of reward with delivery of reinforcement was delayed, a control subjects that receive only a single procedure that usually produces a general level of reward for performing the same task. slowing of response speed. In the present article we describe a proIf the performance of the multiple reward subjects for the larger reward is "better" cedure by which both PCEs and NCEs are (faster running in a runway, higher bar-press reliably obtained within the same subjects rate, etc.) than that of the control subjects and without the application of additional for the same level of large reward, then a variables such as delay of reinforcement. positive contrast effect (PCE) has occurred. EXPERIMENT 1 A negative contrast effect (NCE) is denned correspondingly. Extensive research with discrete-trials Method Subjects. Twelve male Sprague-Dawley rats contrast paradigms over the last 3 decades has indicated that NCEs are more readily weighing an average of 432 gm. and housed indiunder conditions of constant illumination obtained than PCEs (Black, 1968; Dunham, vidually were used as subjects. 1968). The failure to obtain PCEs has often Apparatus. Testing was conducted in a Plexibeen attributed to a measurement problem glas chamber measuring 30 X 25 X 25 cm. On one associated with ceiling effects (e.g., Bower, side of the chamber there were 2 centrally located 1961), and recent research has supported 1.5-cm.-diam. holes spaced 21.7 cm. apart and 4
cm. above the wire mesh floor. Two graduated This research was supported by a biological drinking cylinders, located outside the chamber, sciences support grant, by funds from the Rutgers were programmed so that either cylinder could Research Council, and by Grant No. MH-24612 automatically be moved into a drinking position in from the National Institute of Mental Health to which the orifice of the drinking spout was centhe first author. Appreciation is due to Kent tered in the 1.5-cm.-diam. hole, flush with the Bossange for his assistance in data collection and outside wall of the chamber. Pilot lights, mounted on either side of the chamber close to the drinking analysis in Experiments 3a and 3b. 2 Requests for reprints should be sent to Charles access holes, were illuminated whenever the F. Flaherty, Psychology Department, 88 College cylinder in closest proximity to that light was in Avenue, Rutgers University, New Brunswick, the drinking position. A contact relay circuit was used to measure the licking response. New Jersey 08903. 653 1
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CHARLES F. FLAHERTY AND JOHN LARGEN
Procedure. The rats were randomly assigned to 1 of 2 groups of 6 rats each. One group was maintained on ad-lib food and water for the duration of the experiment. The second group was deprived to 80% of its free feeding weight and maintained at that level by limited daily rations. Water was always available in the home cage. On each test day the rat was placed in the apparatus with only the left tube in the drinking position; it remained available for a 1-min. period starting from the time of the first lick. At the termination of this period the left tube retracted and the right tube moved in for a 1-min. period starting from the first lick. The right tube then retracted and the left tube again became available. This procedure continued for a total of 3 presentations of each tube. The sucrose solutions contained in the drinking bottles were varied systematically over a 4-day cycle. On 2 days both bottles contained the same solution, 32% on 1 day and 4% on the other. On the remaining 2 days one bottle contained the 32% solution and the other contained the 4% solution. On one of these days the 32% solution was in the left bottle and on the other day it was in the right bottle. Four cycles were presented, with a 3-day break between each cycle. Within each 4-day cycle the actual sequence of sucrose conditions was randomized. The solutions were prepared by weight (solute/solute + solvent) from commercial grade cane sugar and tap water. New solutions were prepared every 4 days and were presented at room temperature. Data recorded included (a) the number of licks made during each 1-min. period, and (b) the latency of the first lick when the bottles were changed. Only the last 4 min. of each session will be reported. The conditions of interest are comparisons of lick rates when (a) both bottles contained the 32% solution (32-32), (6) both bottles contained the 4% solution (4-4), and (c) one bottle contained the 32% solution and one bottle contained the 4% solution. The data for the latter condition were obtained by averaging across positions (and, therefore, days) to obtain lick rate for 32% when the alternative bottle contained 4% (32-4), and lick rate for 4% when the alternative bottle contained 32% (4-32).
Results One rat in the ad-lib group developed respiratory problems and was dropped from the experiment. Mean lick rates for the several sucrose presentation conditions are presented as a function of deprivation condition and test cycle in the upper panels of Figure 1. One result readily apparent in Figure 1 is that both PCE and NCE were obtained under both ad-lib and deprived conditions. That
is, the rats licked more for the 32 % sucrose when the alternative was 4 % than they did when the alternative was 32% (F = 16.26, df = 1/9, p < .005), and they licked less for 4 % when the alternative was 32 % than they did when the alternative was also 4% (F = 65.42, df = 1/9, p < .001). Furthermore, the contrast effects appear to be relatively permanent. The NCE obtained in the ad-lib group, in fact, showed some tendency to get larger across the 4 cycles of the experiment. In Cycle 1 the lick rate for 4% under contrast conditions was 63% of the lick rate under noncontrast conditions; the ratios for Cycles 2, 3, and 4 for this group were, respectively, 57%, 45%, and 31%. However, neither deprivation nor cycles nor any interactions approached significance in the analysis of NCE. The PCEs did show signs of becoming smaller across the 4 cycles. There was a significant Concentration X Cycles interaction (F = 34.27, df = 3/27 p < .001), reflecting the general increase in lick rate in the 32-32 condition but not in the 32-4 condition. It should be noted that there was probably a ceiling effect operating to hold down any tendency that may have existed for lick rate to increase in the 32-4 condition. The 3-way interaction between deprivation conditions, concentration, and cycles was also significant (F = 6.31, df = 3/27, p < .005). This interaction was likely due to a greater or more rapid increase in lick rate across cycles in the 32-32 condition by the deprived group than by the ad-lib group. Separate analyses of variance indicated that there was a reliable Concentration X Cycles interaction among the deprived rats (F = 5.85, df = 3/15, p < .01) but not among the ad-lib rats (F = 1.02). Within the deprived condition the PCE was significant in Cycles 1 and 3 but not in 2 and 4 (Fisher's least significant difference test, p = .05). In the ad-lib condition there was an overall PCE (F = 19.75, df = 1/4, p < .025) that did not vary as a function of the testing cycle (interaction, F = 1.02). It is also apparent from Figure 1 that the rats licked more on 32-32 days than they did on 4-4 days (F = 244.5, df = 1.9, p < .001). The deprived rats also licked slightly
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FIGURE 1. Lick rate and reciprocal latency as a function of deprivation state, sucrose concentration condition, and test cycle (Experiment 1). (In the lick rate measure, the curve labeled 32-4 refers to lick rate for 32% when the alternative bottle contains 4%, etc. In the reciprocal latency measure, 32-4 refers to the speed of switching to a bottle containing 32% sucrose and away from a bottle containing 4%, etc.)
more than the ad-lib rats on these control days, but the difference was not statistically reliable (F = 3.32, df = 1/9, p > .10). Examination of the data from the second and third minutes separately indicated that the effects of the independent variables were substantially the same in both minutes, and no conclusions were altered by taking the means of these last 2 min. Latency. The raw latency scores were transformed by dividing them into 10. The resultant "speed" scores were treated similarly to the lick data: the last 4 min. of each test condition were averaged to obtain 4 scores for each cycle, representing the average speed of switching to (making the first lick on) 32% when the alternative bottle also contained 32% (32-32), the mean speed
of switching to 32% and away from 4% (32-4), etc. These scores are presented in the bottom 2 panels of Figure 1 It is clear from Figure 1 that the speed data were consistent with the lick data in that both PCE (F = 11.05, df = 1/9, p< .01) and NCE (F = 35.08, df = 1/9, p < .005) were obtained. However, there were also several differences between the lick and latency measures, (a) The contrast effects were apparent within the first cycle when lick rate was the dependent variable, but did not develop until the second or third cycle with the latency measure. The slower development of contrast with the latency measure is reasonable since the rats had to learn what was essentially a position discrimination within each session. (b) The
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CHARLES F. FLAHERTY AND JOHN LARGEN
deprivation conditions had a greater effect on the latency measure than on the lick measure. The deprived rats showed generally faster overall speeds of switching bottles (NCE conditions, F = 23.67, df = 1/9, p < .005; PCE conditions, F = 5.17, df = 1/9, p < .05); and, in addition, the NCE was considerably larger in the deprived rats than in the ad-lib rats (F = 7.20, df = 1/9, p < .05). The positive contrast was not greater hi the deprived than in the ad-lib rats, although it is possible that a trend in this direction was developing at the termination of the experiment. With the lick measure there was relatively little effect of the deprivation conditions either on overall lick rate or on the contrast effects, (c) The speed of switching bottles was just as fast on control days when both bottles contained 4 % as on control days when both bottles contained 32 %; it was only on contrast days (4-32 or 32-4) that concentration was effective in altering the speed measure. With the lick measure, in comparison, lick rate was greater on 32-32 days than on 4-4 days, and the contrast effects were superimposed on these differential baselines. Discussion Reliable bidirectional contrast effects were obtained with 2 response measures: lick rate and latency to switch to alternative sucrose solutions when they became available. Both the PCEs and NCEs were long lasting, being maintained over some 16 days and repeated shifts in sucrose solutions, but the NCE appeared to be more stable than the PCE. Depriving the rats to 80% of their ad-lib weight did not greatly affect the consummatory response measure (licks) either in terms of absolute lick rates or in terms of the degree of contrast. The deprivation conditions did, however, influence the latency measure: deprived rats responded faster overall, and showed a considerably larger NCE than nondeprived rats. The effect of the deprivation variable on negative contrast is consistent with findings from a number of runway studies (e.g., Ehrenfreund, 1971; Flaherty & Kelly, 1973) and indicates that the latency measure in the present paradigm may be functionally similar to running speed
in more conventional instrumental paradigms. The differential effect of deprivation state on consummatory and instrumental response measures is also consistent with earlier data (e.g., Collier, 1962) The reliable occurrence of PCEs in the present study is at variance with the infrequent demonstration of positive contrast with other paradigms (Dunham, 1968). Because of this discrepancy we were particularly concerned with isolating possible artifactual sources of the PCE. The most likely source of artifactual positive contrast would be satiation under the 32-32 control conditions. That is, the difference between the 32-4 and 32-32 conditions may have been due to depressed performance on 32-32 test days, because of satiation, rather than to enhanced performance on 32-4 days. We think that the satiation interpretation of positive contrast is unlikely for 2 reasons. First, we have found in a number of studies in our laboratory that rats given the opportunity to lick a 32% sucrose solution for 5 min. per day from a single bottle show little indication of within-days satiation (e.g., Flaherty, Capobianco, & Hamilton, 1973). Second, if the rats were showing differential satiation (32-32 vs. 32-4), then the PCE should be larger in the fifth and sixth minutes than in the third and fourth minutes. This was not the case. However, in order to further preclude satiation as an interpretation of the PCE, we ran a second study in which the daily test session was only half as long—3 min. rather than 6 min. EXPERIMENT 2 Method Subjects. Five female albino rats of the SpragueDawley strain (purchased from Carworth Laboratories) were used as subjects. The rats were maintained at 80% of their ad-lib weight (X = 265 gm.). Apparatus. The apparatus was the same as in Experiment 1. Procedure. The procedure was the same as in Experiment 1 except for the time period that each bottle was available and the total session length. Each bottle was presented 3 times for a 30-sec. lick period on each occasion, and therefore the total session length, in terms of available lick time, was 3 min.
WITHIN-SUBJECTS CONTRAST EFFECTS
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slower when going from 32% to 4% than when switching back and fcrth between 4 % tubes (F = 14.53, df = 1/20, p < .005). The apparent interaction between lick condition and test cycle did not quite attain statistical reliability (F = 3.59, df = 1/20, p > .05). .As in Experiment 1, there were no reliable differences in switching speed between the 4-4 and 32-32 conditions. Discussion
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FIGURE 2. Mean lick rate and reciprocal latency measures as a function of sucrose concentration condition and test cycle (Experiment 2).
Results Analyses of the data were conducted in the same manner as in Experiment 1. As is evident in Figure 2, both positive and negative contrast were again obtained in both lick rate (top panel) and latency (bottom panel) measures. Statistical analyses of the lick data indicated a reliable PCE (F = 18.27, df = 1/20, p < .005) and a reliable NCE (F = 8.21, df = 1/20, p < .025). As in Experiment 1, there was no indication of an interaction between sucrose conditions and test cycle. There was also no overlap in lick rate between the 32-32 conditions and the 4-4 conditions. Statistical analyses of the speed data indicated a reliable PCE (F = 4.66, df = 1/20, p < .05), but the apparent interaction of this PCE with test cycles was not statistically reliable (F = l.G2,df = 1/20, p > .05). Examination of individual animal data showed that 3 of 5 animals showed a PCE in the first cycle, 5 of 5 in the second cycle, and 4 of 5 in the third cycle. Analysis of the negative contrast data indicated that the rats switched reliably
The results of Experiment 2 replicate those of Experiment 1 but with a session length only half as long. The brevity of the session length and consequent access to the 32% solution render unlikely the possibility that the PCE was due primarily to satiation in the 32-32 condition rather than enhancement in the 32-4 condition. However, the regular occurrence of the PCE is still puzzling given its infrequent occurrence under other test conditions. Experiment 3 was designed to test the hypothesis that there was something unique about the within-subjects test procedure that led to the occurrence of the PCE. Specifically, it is possible that the lick rate under the 32-32 control condition was depressed, for reasons other than satiation, and that the positive contrast was due largely to this reduced lick rate under the control condition. EXPERIMENT 3A The purpose of Experiment 3a was to measure the degree of positive and negative contrast against between-subjects control conditions. The design of the experiment was similar to the standard discrete-trials instrumental experiment for the investigation of simultaneous contrast effects (e.g., Bower, 1961). That is, there was one group that experienced only the high concentration (32%), one group that experienced only the low concentration (4%), and a third, differential, group that experienced both the 32 % and 4 % solutions. The occurrence of PCEs in Experiment 3a would indicate that the positive contrast obtained in Experiments 1 and 2 was not due to some peculiarity of the completely within-subjects test procedure.
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Method Subjects. Fifteen naive male Sprague-Dawley rats purchased from Carworth Laboratories were used as subjects. The rats were individually housed, maintained on a constant light cycle, and reduced to and maintained at 80% of their 427-gm. (mean) ad-lib weight. Apparatus. The apparatus was the same as that used in Experiments 1 and 2 except for the drinking tubes. In order to obtain an estimate of quantity of fluid intake, the drinking tubes were fashioned from small-diameter graduated burettes. The orifice of the drinking spout in these tubes was 5 mm. in diameter (in comparison to the 2.5-mm.-diam. orifices used in Experiments 1 and
apparent difference between the 32% and 4 % control groups was only marginally reliable (F = 3.78, df = 1/8, .05 < p < .10). Latency. Analysis of the reciprocal latency scores shown in Figure 5 indicated a reliable PCE (F = 6.77, df = 1/8, p < .05) but no NCE (F < 1.0), and no reliable difference between the 32% group and the 4% group (F = 4.55, df = 1/8, .05 < p < .10). Note 300
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Procedure. The rats were randomly assigned to 3 groups (n = 5). One group received a 32% sucrose solution in both bottles at all times (Group 32); a second group received a 4% sucrose solution in both bottles at all times (Group 4); and a third group received a 32% solution in one bottle and a 4% solution in the second bottle (Group 32-4). In this latter group the location of the 32% and 4% solution on the left or right was varied randomly across days. Testing was continued for 4 days. Intake measures for the total session only were estimated to the nearest milliliter. Other aspects of the procedure were the same as in Experiment 1.
Results
Mean lick rate as a function of test day, sucrose concentration condition, and period of the daily test session is presented in Figure 3. It is apparent that both PCEs and NCEs were again obtained. That is, the rats receiving both 32 % and 4 % sucrose licked at a higher rate for the 32 % solution than the rats that received only the 32 % solution, and at a lower rate for the 4 % solution than the rats that received only the 4 % solution. Both the PCE (F = 15.13, df = 1/8, p < .005) and the NCE (F = 15.33, df = 1/8, p < .005) were statistically reliable. There was a tendency for the PCE to be somewhat greater in the latter two thirds of the test session than in the first third (interaction, F = 7.21, df = 2/16, p < .01). This effect was due to both a higher lick rate in the contrast group and a lower lick rate in the control group. No other effects were statistically reliable. Intake. The intake data (presented in Figure 4) were consistent with the lick data. Both positive contrast (F = 13.87, df = 1/8, p < .01) and negative contrast (F = 18.14, df = 1/8, p < .005) were obtained. The
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between-subjects control conditions than the rate that had been obtained under the within-subjects control conditions in Experiments 1 and 2. Experiment 3b was designed to obtain a more direct comparison of the degree of contrast when measured against between-subjects and within-subjects control conditions. EXPERIMENT SB
Method Subjects and Apparatus. The subjects, apparatus, and maintenance conditions were the same as in Experiment 3a. Procedure. Procedural details were the same as DAY in Experiment 3a except for one design change: FIGURB 4. Mean sucrose intake per minute the group that had received both 32% sucrose and as a function of sucrose test condition and days 4% sucrose each day in Experiment 3a were now exposed to the 4-condition, 4-day cycle that was (Experiment 3a). used in Experiments 1 and 2. Experiment 3b was continued for 2 4-day cycles with the sequence that a comparison of Figure 5 with Figures of sucrose presentation randomized within each 1 and 2 indicates that the switching speeds cycle. The groups that received only 32% or only of the 4% group in Experiment 3a were 4% sucrose each day were continued as in Experi3a. considerably slower than the switching ment This design produced a total of 6 sucrose licking speeds obtained under the 4-4 within-sub- conditions: 32-32 between subjects (the single jects conditions of Experiments 1 and 2, value control group), 32-32 within, 32-4 within, and that this difference probably accounts 4-4 between (single value 4% group), 4-4 within, for the failure to obtain negative contrast and 4-32 within. The data obtained from the within-subjects testing conditions were reduced in the latency measure in Experiment 3a. and analyzed the same way as in Experiments 1 and 2.
Discussion The principal result of Experiment 3a was Results Lick rate. The lick rates obtained with the the occurrence of positive and negative contrast when measured against between-sub- various sucrose conditions are presented in jects control conditions. These results indi200 • o—o 32- 4 cate that the reliable occurrences of contrast o—o 32-32 ISO in Experiments 1 and 2 were not due to an •— • 4-32 ,p artifact of the within-subjects design em- o 160 • •— • 4 - 4 ployed. / 140 Experiment 3a also indicated that both 0 O / 120 positive and negative contrast are obtained ,'°~v /' with a measure of fluid intake as well as the — 0 100 \s / lick rate measure, and that the contrast B ,' effects are obtained throughout the 6-min. 8> 80 ' t test session, but with some tendency for < 60 the contrast to be smaller in the first third Ul 40 of the session than later in the session. The 20 results obtained with the latency measure differed somewhat from those of Experiments o'—j i 1 =-*•,— 1 and 2 in that a positive contrast but no negative contrast was obtained. The failure DAYS to find a negative contrast appears to be due FIGURE 5. Mean reciprocal latency to switch to an overall slower rate of switching in the to alternative solutions (Experiment 3a).
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CHARLES F. FLAHERTY AND JOHN LARGEN
second type of NCE. That is, the rats licking under the 4-4 within condition licked at a lower rate than the rats that experienced only the 4% solution (F = 14.10, df = 1/8, p < .01). 300 32-32B One final comparison involving the between-subjects control groups was made. This comparison indicated that the 4-4 be250 tween group licked at a reliably lower rate than the 32-32 between group (F = 45.27, df = 1/8, p < .001). There was no overlap 200 in lick rate under the within-subjects conV) X. trol conditions (32-32 within vs. 4-4 within). u Intake. The intake data directly paralleled the lick data in all cases but one. The excep150 < tion was in the positive contrast comparison Ul of intake under the 32-4 within condition 4-4 W as compared to intake under the 32-32 100 within condition. This comparison did not yield a statistically reliable main effect, but the interaction with cycles was reliable 50 (F = 95.27, df = 1/4, p < .005). Subsequent 4-32W analysis of this interaction with Fisher's least significant difference test (p = .01) indicated that intake under the 32-4 within condition was greater than intake under the 32-32 within condition in the second cycle CYCLE FIGURE 6. Mean lick rate as a function of only. The direction of this effect is similar to sucrose concentration condition and test cycle that evident with the lick measure (cf. Figure (Experiment 3b). (The curves labeled 32-32B and 5). 4-4B refer to groups that received only the indiLatency. The general trend of the reciprocated sucrose solutions. The curves labeled with a cal latency data was similar to that obtained W refer to animals that received all 4 combinations in the earlier experiments; however, not all of sucrose conditions.) contrast comparisons were statistically reliFigure 6. Examining the PCE first, it is able and those that were, were marginally so. apparent that the rats licking for the 32% These data are presented in Figure 7. The solution under the 32-4 within conditions PCE was marginally reliable when the 32-4 licked at a higher rate than they did under switching time was compared with Group the 32-32 within condition (F = 77.63, 32-32 between (F = 5.19, df = 1/8, p < .06) df = 1/4, p < .005) and at a higher rate but not when the within contrast conditions than rats exposed to only the 32 % solution (32-4 vs. 32-32) were compared (F < 1.00). (F = 21.43, df = 1/8, p < .005). There was The 32-32 within condition itself tended to no difference between the 32-32 within and produce faster switching than the 32-32 between condition (F = 4.71, df = 1/8, p < 32-32 between conditions (all Fs < 1.00). The NCEs were similar. The rats licking •07). The within-subjects NCE was marginally for the 4% solution under the 4-32 within reliable when the 4-32 within and 4-4 within condition licked at a lower rate than they did under the 4—4 within condition (F = conditions were compared (F = 7.36, df = 51.12, df = 1/4, p < .01) and at a lower 1/4, p < .07), but not when the 4-32 within rate than the rats exposed to only the 4% condition was compared with the 4—4 besucrose (F = 70.50, df = 1/8, p < .001). tween condition (F < 1.00). Switching under There was also what could be termed a the 4-4 within condition tended to be faster 350
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FIGURE 7. Mean reciprocal latency to switch to alternative solutions. (The curves labeled 32-32B and 4-4B refer to groups that received only the indicated sucrose solutions. The curves labeled with a W refer to animals that received all 4 combinations of sucrose conditions.)
than switching under the 4-4 between condition (F = 5.26, df = 1/8, p < .06). GENERAL DISCUSSION Both positive and negative contrast were again obtained in Experiment 3b. The results of Experiments 3a and 3b indicate quite strongly that the reliable positive and negative contrast obtained in Experiments 1 and 2 was not due in any artifactual sense to the within-subjects design. In fact, Experiment 3b indicated that there may be a second type of contrast effect operating in the withinsubjects design; specifically, the lick rate of the group that experienced all 4 sucrose combinations was depressed in the 4-4 conditions (4-4 within) relative to the group that experienced only the 4% solution (4-4 between). This between-groups contrast effect would tend to minimize the size of the within-subjects negative contrast since the 4-4 within condition is used as the control to measure the 4-32 within negative contrast. Despite this constraint, a reliable within-subjects negative contrast was found in all 3 relevant experiments (Experiments 1, 2, and 3b). The between-groups contrast effect must represent a contrast operating across days as a result of the rats' receiving
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the 4 % solution in a context (same apparatus, etc.) in which they sometimes receive 32% sucrose. The more powerful effect of the within-subjects negative contrast must result from the within-days comparison of the 2jsucrose solutions. A reasonable prediction from these data is that the between-groups contrast effect should be sensitive to manipulations of the intersession interval. That is, as the number of days interpolated between each test condition, particularly the 4-4 within and other within-subjects conditions, increases, then the magnitude of the between-groups contrast should decrease, i.e., the 4-4 within lick rate should approach that obtained under the 4-4 between conditions. Contrast effects have previously been shown to be sensitive to retention interval manipulations (e.g., Flaherty, Capobianco, & Hamilton, 1973; Gleitman & Steinman, 1964). The within-subjects contrast (i.e., 32-4 vs. 4-4 within), on the other hand, should be relatively insensitive to between-days manipulations (except for effects due to changes in the control groups, which should only enhance the size of the measured contrast), but these within-subjects contrast effects might be influenced by variations in the time between consecutive exposures to the sucrose solutions within each test session. That is, variations in the interbottle interval in the present paradigm should alter the size of the within-subjects negative contrast. The greater the interbottle interval, the smaller the contrast should be. It should be noted that this second type of contrast (between days) was stronger in the negative than in the positive contrast measures. In terms of comparison with other contrast paradigms, the present within-subjects paradigm is more similar to the simultaneous than to the successive contrast design (cf. Spear & Spitzner, 1966). This similarity is true on procedural grounds (multiple opportunities for comparison of the 2 rewards, as in the simultaneous paradigm, rather than only a single postshift comparison, as in the successive design) and also in terms of the results obtained with the different paradigms. Typically, the NCE found with the simultaneous design tends to be long lasting
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CHARLES F. FLAHERTY AND JOHN LARGEN
(cf. Bower, 1961; Flaherty, Riley, & Spear, 1973), whereas the contrast obtained with the successive shift design tends to last for only a few days subsequent-to the shift and tends not to occur after the first shift (e.g., Capaldi & Lynch, 1967; Vogel, Mukulka, & Spear, 1968). The contrast effects obtained with the present design appear to be long lasting and occur with repeated shifts. In this regard, it should be noted that the contrast effects develop much more gradually when estimated by the latency-to-switch measure than when estimated by a consummatory response measure (licks or intake). The failure to find indications of contrast with the latency measure in Experiments 3a and 3b that are as reliable and robust as those found in Experiments 1 and 2 is probably related to this sensitivity to training history. The rats in Experiments 3a and 3b were not exposed to a stable set of testing conditions as long as were the rats in either Experiment 1 or 2. The results of these experiments have some theoretical relevance. Subsequent to Crespi's (1942) demonstration of PCEs most of the research through the mid-1960s indicated that positive contrast was, at best, an elusive phenomenon. Because of these findings, theoretical efforts focused almost exclusively on NCEs (cf. Black, 1968; Dunham, 1968). One variety of explanation emphasized the contribution of generalization decrement (e.g., Capaldi & Lynch, 1967; Spear & Spitzner, 1966). That is, negative contrast was said to occur because a change in reward conditions also constituted a change in the stimulus conditions under which the subjects had learned the task, and this change in stimulus conditions led to a decrement in performance. The present results are relevant to the generalization decrement account of contrast in 2 ways. First, generalization decrement can only serve as an account of negative contrast, and the reliable occurrence of positive contrast in the present experiments emphasizes the limited applicability of the concept to contrast phenomena as a whole. Second, the effects of deprivation on the latency measure of negative contrast are not consistent with a generalization decrement interpretation. In Experiment 1 in-
creased deprivation was found to enhance the contrast effects (particularly negative contrast) as measured by the speed of switching from one bottle to its alternative. This enhancing effect of deprivation on negative contrast has also been found in a number of runway studies (Cleland, Williams, & DiLollo, 1969; Ehrenfreund, 1971; Flaherty & Kelly, 1973) and would seemingly be an effect opposite in direction from what would be expected if negative contrast were due to generalization decrement. This is so because degree of generalization has been shown to vary directly with degree of deprivation, at least in other contexts (cf. Kalish & Haber, 1965), and therefore more extreme deprivation states should retard the occurrence of negative contrast if generalization decrement were the major factor involved. A second theoretical approach to contrast effects has emphasized the contribution of presumed emotional factors. Crespi (1944) wrote that shifts in reward magnitude produced a heightened emotional drive: elation or joy, which enhanced ongoing behavior in the case of the comparison of large to smaller rewards, and anger or frustration, which subtracted from behavior in the case of the comparison of small to larger rewards. The mechanism by which these emotional responses came to affect behavior were not clearly specified by Crespi, but he did describe some of the behavior that was associated with the slower running observed under depression or negative contrast conditions. These behaviors included hesitant running, "attempts to jump out of the goal box," failure to consume the smaller reward, attempts at retracing in the runway, and "peering into the foodbox" (Crespi, 1942, 1944). No such obvious behavior patterns were apparently associated with the elation or PCE. As the data reported subsequent to those of Crespi (1942, 1944) generally failed to support the reliability of the PCE, explanatory attempts centered around the apparent disruptive emotional responses associated with NCEs. These explanations have often been framed within the structure of Amsel's (1958) frustration theory (e.g., Bower, 1961; Cleland et al., 1969; Ludvigson & Gay,
WITHIN-SUBJECTS CONTRAST EFFECTS
1967). For example, Bower reasoned that the lowered performance to the negative stimulus by the differential group (relative to a small reward control group) in a simultaneous contrast design might be the result of conflict between the anticipation of reward (rg in the Amsel model) and the anticipation of frustration (rf in the Amsel model). The frustration was said to occur because of generalization of expectancy (rg) from the positive stimulus to the negative stimulus. Similarly, Spence (1956) argued that the successive NCE was probably due to disruptive emotional effects occurring subsequent to a decrease in reward magnitude. The results of the present series of experiments as well as the results of other recent studies of positive contrast (e.g., Mellgren, 1972; Shanab & Biller, 1972) indicate that more attention must be given to presumed emotional correlates of positive contrast if theories based on emotional constructs are to have applicability for contrast phenomena in general. A recent study by Goldman, Coover, and Levine (1973) indicates that there may indeed be bidirectional physiological responses related to contrast. Specifically, Goldman et al. found that shifts to a less dense schedule of reinforcement (a successive negative contrast paradigm) produced elevated levels of plasma corticosterone, whereas shifts to a more dense schedule of reinforcement (a successive positive contrast paradigm) led to a decrease in plasma corticosterone below baseline levels. Whatever the theoretical mechanisms marshaled to explain contrast, it is clear from the present data that these mechanisms are going to have to account for relatively enduring positive as well as negative contrast, in particular a PCE that may be obtained without the application of special experimental techniques such as delay of reinforcement. The theoretical mechanisms must account for contrast effects that develop very rapidly and are relatively uninfluenced by deprivation state when measured by lick rate, but that develop more slowly, are more variable, and are more influenced by deprivation state when measured by the latency to switch between alternative solutions.
663
REFERENCES Amsel, A. The role of frustrative nonreward in nonoontinuous reward situations. Psychological Bulletin, 1958, 55,102-119. Black, R. W. Shifts in magnitude of reward and contrast effects in instrumental and selective learning: A reinterpretation. Psychological Review, 1968, 75, 114-126. Bower, G. H. A contrast effect in differential conditioning. Journal of Experimental Psychology, 1961, 62,196-199. Capaldi, E. J., & Lynch, D. Repeated shifts in reward magnitude: Evidence in favor of an associational and absolute (noncontextual) interpretation. Journal of Experimental Psychology, 1967, 75, 226-235. Cleland, E. A., Williams, M. Y, & DiLolIo, V. Magnitude of negative contrast effect in relation to drive level. Psychonomic Science, 1969, 15,. 121-122. Collier, G. Consummatory and instrumental responding as functions of deprivation. Journal of Experimental Psychology, 1962, 64, 410-414. Crespi, L. P. Quantitative variation in incentive and performance in the white rat. American Journal of Psychology, 1942, 55, 467-517. Crespi, L. P. Amount of reinforcement and level of performance. Psychological Review, 1944, 51, 341-357. Dunham, P. J. Contrasted conditions of reinforcement: A selective critique. Psychological Bulletin, 1968, 69, 295-315. Ehrenfreund, D. Effect of drive on successive magnitude shift in rats. Journal of Comparative and Physiological Psychology, 1971, 76, 418-423. Flaherty, C. F., Capobianco, S., & Hamilton, L. W. Effect of septal lesions on retention of negative contrast. Physiology and Behavior, 1973, 11, 625631. Flaherty, C. F., & Kelly, J. Effect of deprivation state on successive negative contrast. Bulletin of the Psychonomic Society, 1973,1, 365-367. Flaherty, C. F., Riley, E. P., & Spear, N. E. Effects of sucrose concentration and goal units on runway behavior in the rat. Learning and Motivation, 1973,4, 163-175. Gleitman, H., & Steinman, F. Depression effect as a function of retention interval before and after shift in reward magnitude. Journal of Comparative and Physiological Psychology, 1964, 57, 158160. Goldman, L., Coover, G. D., & Levine, S. Bidirectional effects of reinforcement shifts on pituitary adrenal activity. Physiology and Behavior, 1973, 10, 209-214. Kalish, H. I., & Haber, A. Prediction of discrimination from generalization following variations in deprivation level. Journal of Comparative and Physiological Psychology, 1965, 60, 125-128. Ludvigson, H. W., & Gay, R. A. An investigation of conditions determining contrast effects in dif-
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ferential reward conditioning. Journal oj Experimental Psychology, 1967, 75, 37-42. Mellgren, R. L. Positive and negative contrast effects using delayed reinforcement. Learning and Motivation, 1972, 3, 185-193. Shanab, M. E., & Biller, J. D. Positive contrast in the runway obtained following a shift in both delay and magnitude of reward. Learning and Motivation, 1972, 3,179-184. Spear, N. E., & Spitzner, J. H. Simultaneous and successive contrast effects of reward magnitude
in selective learning. Psychological Monographs, 1966, 80(10, Whole No. 618). Spehce, K. W. Behavior theory and conditioning. New Haven: Yale University Press, 1956. Vogel, J. R., Mukulka, P. J., & Spear, N. E. Effects of shift in sucrose and saccharine concentrations on licking behavior in the rat. Journal of Comparative and Physiological Psychology, 1968, 66, 661-666. (Received December 12,1973)
Psychology
WITHIN-SUBJECTS POSITIVE AND NEGATIVE CONTRAST EFFECTS IN RATS1 CHARLES F. FLAHERTY2 AND JOHN LARGEN Rutgers—The State University Rats were given alternating 1-min. access periods to 2 tubes containing either 32% or 4% sucrose solutions for daily 6-min. test sessions. Lick rate for 32% was higher under comparison (32 vs. 4) than noncomparison (32 vs. 32) conditions; and lick rate for 4% was lower under comparison conditions (4 vs. 32) than under noncomparison conditions (4 vs. 4). All sucrose conditions were varied within subjects and both positive and negative contrast were obtained with a small n. In addition to lick rate, intake and latency measures also revealed contrast effects. Deprivation conditions altered latency but not lick rate measures of contrast. Reducing the test session to 3 min. (alternating 30-sec. access periods) did not greatly affect contrast. Additional experiments provided evidence for distinct withindays and between-days contrast effects, as well as a between-groups contrast effect.
Contrast effects are often found when a this interpretation by showing that PCEs subject receives more than one level of re- may be obtained if procedures that obviate ward for performing a given task. These the influence of ceiling effects are used. For contrast effects are typically measured and example, Shanab and Biller (1972) and Melldefined by comparing the performance of gren (1972) have obtained PCE when the subjects receiving 2 levels of reward with delivery of reinforcement was delayed, a control subjects that receive only a single procedure that usually produces a general level of reward for performing the same task. slowing of response speed. In the present article we describe a proIf the performance of the multiple reward subjects for the larger reward is "better" cedure by which both PCEs and NCEs are (faster running in a runway, higher bar-press reliably obtained within the same subjects rate, etc.) than that of the control subjects and without the application of additional for the same level of large reward, then a variables such as delay of reinforcement. positive contrast effect (PCE) has occurred. EXPERIMENT 1 A negative contrast effect (NCE) is denned correspondingly. Extensive research with discrete-trials Method Subjects. Twelve male Sprague-Dawley rats contrast paradigms over the last 3 decades has indicated that NCEs are more readily weighing an average of 432 gm. and housed indiunder conditions of constant illumination obtained than PCEs (Black, 1968; Dunham, vidually were used as subjects. 1968). The failure to obtain PCEs has often Apparatus. Testing was conducted in a Plexibeen attributed to a measurement problem glas chamber measuring 30 X 25 X 25 cm. On one associated with ceiling effects (e.g., Bower, side of the chamber there were 2 centrally located 1961), and recent research has supported 1.5-cm.-diam. holes spaced 21.7 cm. apart and 4
cm. above the wire mesh floor. Two graduated This research was supported by a biological drinking cylinders, located outside the chamber, sciences support grant, by funds from the Rutgers were programmed so that either cylinder could Research Council, and by Grant No. MH-24612 automatically be moved into a drinking position in from the National Institute of Mental Health to which the orifice of the drinking spout was centhe first author. Appreciation is due to Kent tered in the 1.5-cm.-diam. hole, flush with the Bossange for his assistance in data collection and outside wall of the chamber. Pilot lights, mounted on either side of the chamber close to the drinking analysis in Experiments 3a and 3b. 2 Requests for reprints should be sent to Charles access holes, were illuminated whenever the F. Flaherty, Psychology Department, 88 College cylinder in closest proximity to that light was in Avenue, Rutgers University, New Brunswick, the drinking position. A contact relay circuit was used to measure the licking response. New Jersey 08903. 653 1
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CHARLES F. FLAHERTY AND JOHN LARGEN
Procedure. The rats were randomly assigned to 1 of 2 groups of 6 rats each. One group was maintained on ad-lib food and water for the duration of the experiment. The second group was deprived to 80% of its free feeding weight and maintained at that level by limited daily rations. Water was always available in the home cage. On each test day the rat was placed in the apparatus with only the left tube in the drinking position; it remained available for a 1-min. period starting from the time of the first lick. At the termination of this period the left tube retracted and the right tube moved in for a 1-min. period starting from the first lick. The right tube then retracted and the left tube again became available. This procedure continued for a total of 3 presentations of each tube. The sucrose solutions contained in the drinking bottles were varied systematically over a 4-day cycle. On 2 days both bottles contained the same solution, 32% on 1 day and 4% on the other. On the remaining 2 days one bottle contained the 32% solution and the other contained the 4% solution. On one of these days the 32% solution was in the left bottle and on the other day it was in the right bottle. Four cycles were presented, with a 3-day break between each cycle. Within each 4-day cycle the actual sequence of sucrose conditions was randomized. The solutions were prepared by weight (solute/solute + solvent) from commercial grade cane sugar and tap water. New solutions were prepared every 4 days and were presented at room temperature. Data recorded included (a) the number of licks made during each 1-min. period, and (b) the latency of the first lick when the bottles were changed. Only the last 4 min. of each session will be reported. The conditions of interest are comparisons of lick rates when (a) both bottles contained the 32% solution (32-32), (6) both bottles contained the 4% solution (4-4), and (c) one bottle contained the 32% solution and one bottle contained the 4% solution. The data for the latter condition were obtained by averaging across positions (and, therefore, days) to obtain lick rate for 32% when the alternative bottle contained 4% (32-4), and lick rate for 4% when the alternative bottle contained 32% (4-32).
Results One rat in the ad-lib group developed respiratory problems and was dropped from the experiment. Mean lick rates for the several sucrose presentation conditions are presented as a function of deprivation condition and test cycle in the upper panels of Figure 1. One result readily apparent in Figure 1 is that both PCE and NCE were obtained under both ad-lib and deprived conditions. That
is, the rats licked more for the 32 % sucrose when the alternative was 4 % than they did when the alternative was 32% (F = 16.26, df = 1/9, p < .005), and they licked less for 4 % when the alternative was 32 % than they did when the alternative was also 4% (F = 65.42, df = 1/9, p < .001). Furthermore, the contrast effects appear to be relatively permanent. The NCE obtained in the ad-lib group, in fact, showed some tendency to get larger across the 4 cycles of the experiment. In Cycle 1 the lick rate for 4% under contrast conditions was 63% of the lick rate under noncontrast conditions; the ratios for Cycles 2, 3, and 4 for this group were, respectively, 57%, 45%, and 31%. However, neither deprivation nor cycles nor any interactions approached significance in the analysis of NCE. The PCEs did show signs of becoming smaller across the 4 cycles. There was a significant Concentration X Cycles interaction (F = 34.27, df = 3/27 p < .001), reflecting the general increase in lick rate in the 32-32 condition but not in the 32-4 condition. It should be noted that there was probably a ceiling effect operating to hold down any tendency that may have existed for lick rate to increase in the 32-4 condition. The 3-way interaction between deprivation conditions, concentration, and cycles was also significant (F = 6.31, df = 3/27, p < .005). This interaction was likely due to a greater or more rapid increase in lick rate across cycles in the 32-32 condition by the deprived group than by the ad-lib group. Separate analyses of variance indicated that there was a reliable Concentration X Cycles interaction among the deprived rats (F = 5.85, df = 3/15, p < .01) but not among the ad-lib rats (F = 1.02). Within the deprived condition the PCE was significant in Cycles 1 and 3 but not in 2 and 4 (Fisher's least significant difference test, p = .05). In the ad-lib condition there was an overall PCE (F = 19.75, df = 1/4, p < .025) that did not vary as a function of the testing cycle (interaction, F = 1.02). It is also apparent from Figure 1 that the rats licked more on 32-32 days than they did on 4-4 days (F = 244.5, df = 1.9, p < .001). The deprived rats also licked slightly
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more than the ad-lib rats on these control days, but the difference was not statistically reliable (F = 3.32, df = 1/9, p > .10). Examination of the data from the second and third minutes separately indicated that the effects of the independent variables were substantially the same in both minutes, and no conclusions were altered by taking the means of these last 2 min. Latency. The raw latency scores were transformed by dividing them into 10. The resultant "speed" scores were treated similarly to the lick data: the last 4 min. of each test condition were averaged to obtain 4 scores for each cycle, representing the average speed of switching to (making the first lick on) 32% when the alternative bottle also contained 32% (32-32), the mean speed
of switching to 32% and away from 4% (32-4), etc. These scores are presented in the bottom 2 panels of Figure 1 It is clear from Figure 1 that the speed data were consistent with the lick data in that both PCE (F = 11.05, df = 1/9, p< .01) and NCE (F = 35.08, df = 1/9, p < .005) were obtained. However, there were also several differences between the lick and latency measures, (a) The contrast effects were apparent within the first cycle when lick rate was the dependent variable, but did not develop until the second or third cycle with the latency measure. The slower development of contrast with the latency measure is reasonable since the rats had to learn what was essentially a position discrimination within each session. (b) The
656
CHARLES F. FLAHERTY AND JOHN LARGEN
deprivation conditions had a greater effect on the latency measure than on the lick measure. The deprived rats showed generally faster overall speeds of switching bottles (NCE conditions, F = 23.67, df = 1/9, p < .005; PCE conditions, F = 5.17, df = 1/9, p < .05); and, in addition, the NCE was considerably larger in the deprived rats than in the ad-lib rats (F = 7.20, df = 1/9, p < .05). The positive contrast was not greater hi the deprived than in the ad-lib rats, although it is possible that a trend in this direction was developing at the termination of the experiment. With the lick measure there was relatively little effect of the deprivation conditions either on overall lick rate or on the contrast effects, (c) The speed of switching bottles was just as fast on control days when both bottles contained 4 % as on control days when both bottles contained 32 %; it was only on contrast days (4-32 or 32-4) that concentration was effective in altering the speed measure. With the lick measure, in comparison, lick rate was greater on 32-32 days than on 4-4 days, and the contrast effects were superimposed on these differential baselines. Discussion Reliable bidirectional contrast effects were obtained with 2 response measures: lick rate and latency to switch to alternative sucrose solutions when they became available. Both the PCEs and NCEs were long lasting, being maintained over some 16 days and repeated shifts in sucrose solutions, but the NCE appeared to be more stable than the PCE. Depriving the rats to 80% of their ad-lib weight did not greatly affect the consummatory response measure (licks) either in terms of absolute lick rates or in terms of the degree of contrast. The deprivation conditions did, however, influence the latency measure: deprived rats responded faster overall, and showed a considerably larger NCE than nondeprived rats. The effect of the deprivation variable on negative contrast is consistent with findings from a number of runway studies (e.g., Ehrenfreund, 1971; Flaherty & Kelly, 1973) and indicates that the latency measure in the present paradigm may be functionally similar to running speed
in more conventional instrumental paradigms. The differential effect of deprivation state on consummatory and instrumental response measures is also consistent with earlier data (e.g., Collier, 1962) The reliable occurrence of PCEs in the present study is at variance with the infrequent demonstration of positive contrast with other paradigms (Dunham, 1968). Because of this discrepancy we were particularly concerned with isolating possible artifactual sources of the PCE. The most likely source of artifactual positive contrast would be satiation under the 32-32 control conditions. That is, the difference between the 32-4 and 32-32 conditions may have been due to depressed performance on 32-32 test days, because of satiation, rather than to enhanced performance on 32-4 days. We think that the satiation interpretation of positive contrast is unlikely for 2 reasons. First, we have found in a number of studies in our laboratory that rats given the opportunity to lick a 32% sucrose solution for 5 min. per day from a single bottle show little indication of within-days satiation (e.g., Flaherty, Capobianco, & Hamilton, 1973). Second, if the rats were showing differential satiation (32-32 vs. 32-4), then the PCE should be larger in the fifth and sixth minutes than in the third and fourth minutes. This was not the case. However, in order to further preclude satiation as an interpretation of the PCE, we ran a second study in which the daily test session was only half as long—3 min. rather than 6 min. EXPERIMENT 2 Method Subjects. Five female albino rats of the SpragueDawley strain (purchased from Carworth Laboratories) were used as subjects. The rats were maintained at 80% of their ad-lib weight (X = 265 gm.). Apparatus. The apparatus was the same as in Experiment 1. Procedure. The procedure was the same as in Experiment 1 except for the time period that each bottle was available and the total session length. Each bottle was presented 3 times for a 30-sec. lick period on each occasion, and therefore the total session length, in terms of available lick time, was 3 min.
WITHIN-SUBJECTS CONTRAST EFFECTS
657
slower when going from 32% to 4% than when switching back and fcrth between 4 % tubes (F = 14.53, df = 1/20, p < .005). The apparent interaction between lick condition and test cycle did not quite attain statistical reliability (F = 3.59, df = 1/20, p > .05). .As in Experiment 1, there were no reliable differences in switching speed between the 4-4 and 32-32 conditions. Discussion
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FIGURE 2. Mean lick rate and reciprocal latency measures as a function of sucrose concentration condition and test cycle (Experiment 2).
Results Analyses of the data were conducted in the same manner as in Experiment 1. As is evident in Figure 2, both positive and negative contrast were again obtained in both lick rate (top panel) and latency (bottom panel) measures. Statistical analyses of the lick data indicated a reliable PCE (F = 18.27, df = 1/20, p < .005) and a reliable NCE (F = 8.21, df = 1/20, p < .025). As in Experiment 1, there was no indication of an interaction between sucrose conditions and test cycle. There was also no overlap in lick rate between the 32-32 conditions and the 4-4 conditions. Statistical analyses of the speed data indicated a reliable PCE (F = 4.66, df = 1/20, p < .05), but the apparent interaction of this PCE with test cycles was not statistically reliable (F = l.G2,df = 1/20, p > .05). Examination of individual animal data showed that 3 of 5 animals showed a PCE in the first cycle, 5 of 5 in the second cycle, and 4 of 5 in the third cycle. Analysis of the negative contrast data indicated that the rats switched reliably
The results of Experiment 2 replicate those of Experiment 1 but with a session length only half as long. The brevity of the session length and consequent access to the 32% solution render unlikely the possibility that the PCE was due primarily to satiation in the 32-32 condition rather than enhancement in the 32-4 condition. However, the regular occurrence of the PCE is still puzzling given its infrequent occurrence under other test conditions. Experiment 3 was designed to test the hypothesis that there was something unique about the within-subjects test procedure that led to the occurrence of the PCE. Specifically, it is possible that the lick rate under the 32-32 control condition was depressed, for reasons other than satiation, and that the positive contrast was due largely to this reduced lick rate under the control condition. EXPERIMENT 3A The purpose of Experiment 3a was to measure the degree of positive and negative contrast against between-subjects control conditions. The design of the experiment was similar to the standard discrete-trials instrumental experiment for the investigation of simultaneous contrast effects (e.g., Bower, 1961). That is, there was one group that experienced only the high concentration (32%), one group that experienced only the low concentration (4%), and a third, differential, group that experienced both the 32 % and 4 % solutions. The occurrence of PCEs in Experiment 3a would indicate that the positive contrast obtained in Experiments 1 and 2 was not due to some peculiarity of the completely within-subjects test procedure.
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CHARLES F. FLAHERTY AND JOHN LARGEN
Method Subjects. Fifteen naive male Sprague-Dawley rats purchased from Carworth Laboratories were used as subjects. The rats were individually housed, maintained on a constant light cycle, and reduced to and maintained at 80% of their 427-gm. (mean) ad-lib weight. Apparatus. The apparatus was the same as that used in Experiments 1 and 2 except for the drinking tubes. In order to obtain an estimate of quantity of fluid intake, the drinking tubes were fashioned from small-diameter graduated burettes. The orifice of the drinking spout in these tubes was 5 mm. in diameter (in comparison to the 2.5-mm.-diam. orifices used in Experiments 1 and
apparent difference between the 32% and 4 % control groups was only marginally reliable (F = 3.78, df = 1/8, .05 < p < .10). Latency. Analysis of the reciprocal latency scores shown in Figure 5 indicated a reliable PCE (F = 6.77, df = 1/8, p < .05) but no NCE (F < 1.0), and no reliable difference between the 32% group and the 4% group (F = 4.55, df = 1/8, .05 < p < .10). Note 300
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Procedure. The rats were randomly assigned to 3 groups (n = 5). One group received a 32% sucrose solution in both bottles at all times (Group 32); a second group received a 4% sucrose solution in both bottles at all times (Group 4); and a third group received a 32% solution in one bottle and a 4% solution in the second bottle (Group 32-4). In this latter group the location of the 32% and 4% solution on the left or right was varied randomly across days. Testing was continued for 4 days. Intake measures for the total session only were estimated to the nearest milliliter. Other aspects of the procedure were the same as in Experiment 1.
Results
Mean lick rate as a function of test day, sucrose concentration condition, and period of the daily test session is presented in Figure 3. It is apparent that both PCEs and NCEs were again obtained. That is, the rats receiving both 32 % and 4 % sucrose licked at a higher rate for the 32 % solution than the rats that received only the 32 % solution, and at a lower rate for the 4 % solution than the rats that received only the 4 % solution. Both the PCE (F = 15.13, df = 1/8, p < .005) and the NCE (F = 15.33, df = 1/8, p < .005) were statistically reliable. There was a tendency for the PCE to be somewhat greater in the latter two thirds of the test session than in the first third (interaction, F = 7.21, df = 2/16, p < .01). This effect was due to both a higher lick rate in the contrast group and a lower lick rate in the control group. No other effects were statistically reliable. Intake. The intake data (presented in Figure 4) were consistent with the lick data. Both positive contrast (F = 13.87, df = 1/8, p < .01) and negative contrast (F = 18.14, df = 1/8, p < .005) were obtained. The
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between-subjects control conditions than the rate that had been obtained under the within-subjects control conditions in Experiments 1 and 2. Experiment 3b was designed to obtain a more direct comparison of the degree of contrast when measured against between-subjects and within-subjects control conditions. EXPERIMENT SB
Method Subjects and Apparatus. The subjects, apparatus, and maintenance conditions were the same as in Experiment 3a. Procedure. Procedural details were the same as DAY in Experiment 3a except for one design change: FIGURB 4. Mean sucrose intake per minute the group that had received both 32% sucrose and as a function of sucrose test condition and days 4% sucrose each day in Experiment 3a were now exposed to the 4-condition, 4-day cycle that was (Experiment 3a). used in Experiments 1 and 2. Experiment 3b was continued for 2 4-day cycles with the sequence that a comparison of Figure 5 with Figures of sucrose presentation randomized within each 1 and 2 indicates that the switching speeds cycle. The groups that received only 32% or only of the 4% group in Experiment 3a were 4% sucrose each day were continued as in Experi3a. considerably slower than the switching ment This design produced a total of 6 sucrose licking speeds obtained under the 4-4 within-sub- conditions: 32-32 between subjects (the single jects conditions of Experiments 1 and 2, value control group), 32-32 within, 32-4 within, and that this difference probably accounts 4-4 between (single value 4% group), 4-4 within, for the failure to obtain negative contrast and 4-32 within. The data obtained from the within-subjects testing conditions were reduced in the latency measure in Experiment 3a. and analyzed the same way as in Experiments 1 and 2.
Discussion The principal result of Experiment 3a was Results Lick rate. The lick rates obtained with the the occurrence of positive and negative contrast when measured against between-sub- various sucrose conditions are presented in jects control conditions. These results indi200 • o—o 32- 4 cate that the reliable occurrences of contrast o—o 32-32 ISO in Experiments 1 and 2 were not due to an •— • 4-32 ,p artifact of the within-subjects design em- o 160 • •— • 4 - 4 ployed. / 140 Experiment 3a also indicated that both 0 O / 120 positive and negative contrast are obtained ,'°~v /' with a measure of fluid intake as well as the — 0 100 \s / lick rate measure, and that the contrast B ,' effects are obtained throughout the 6-min. 8> 80 ' t test session, but with some tendency for < 60 the contrast to be smaller in the first third Ul 40 of the session than later in the session. The 20 results obtained with the latency measure differed somewhat from those of Experiments o'—j i 1 =-*•,— 1 and 2 in that a positive contrast but no negative contrast was obtained. The failure DAYS to find a negative contrast appears to be due FIGURE 5. Mean reciprocal latency to switch to an overall slower rate of switching in the to alternative solutions (Experiment 3a).
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CHARLES F. FLAHERTY AND JOHN LARGEN
second type of NCE. That is, the rats licking under the 4-4 within condition licked at a lower rate than the rats that experienced only the 4% solution (F = 14.10, df = 1/8, p < .01). 300 32-32B One final comparison involving the between-subjects control groups was made. This comparison indicated that the 4-4 be250 tween group licked at a reliably lower rate than the 32-32 between group (F = 45.27, df = 1/8, p < .001). There was no overlap 200 in lick rate under the within-subjects conV) X. trol conditions (32-32 within vs. 4-4 within). u Intake. The intake data directly paralleled the lick data in all cases but one. The excep150 < tion was in the positive contrast comparison Ul of intake under the 32-4 within condition 4-4 W as compared to intake under the 32-32 100 within condition. This comparison did not yield a statistically reliable main effect, but the interaction with cycles was reliable 50 (F = 95.27, df = 1/4, p < .005). Subsequent 4-32W analysis of this interaction with Fisher's least significant difference test (p = .01) indicated that intake under the 32-4 within condition was greater than intake under the 32-32 within condition in the second cycle CYCLE FIGURE 6. Mean lick rate as a function of only. The direction of this effect is similar to sucrose concentration condition and test cycle that evident with the lick measure (cf. Figure (Experiment 3b). (The curves labeled 32-32B and 5). 4-4B refer to groups that received only the indiLatency. The general trend of the reciprocated sucrose solutions. The curves labeled with a cal latency data was similar to that obtained W refer to animals that received all 4 combinations in the earlier experiments; however, not all of sucrose conditions.) contrast comparisons were statistically reliFigure 6. Examining the PCE first, it is able and those that were, were marginally so. apparent that the rats licking for the 32% These data are presented in Figure 7. The solution under the 32-4 within conditions PCE was marginally reliable when the 32-4 licked at a higher rate than they did under switching time was compared with Group the 32-32 within condition (F = 77.63, 32-32 between (F = 5.19, df = 1/8, p < .06) df = 1/4, p < .005) and at a higher rate but not when the within contrast conditions than rats exposed to only the 32 % solution (32-4 vs. 32-32) were compared (F < 1.00). (F = 21.43, df = 1/8, p < .005). There was The 32-32 within condition itself tended to no difference between the 32-32 within and produce faster switching than the 32-32 between condition (F = 4.71, df = 1/8, p < 32-32 between conditions (all Fs < 1.00). The NCEs were similar. The rats licking •07). The within-subjects NCE was marginally for the 4% solution under the 4-32 within reliable when the 4-32 within and 4-4 within condition licked at a lower rate than they did under the 4—4 within condition (F = conditions were compared (F = 7.36, df = 51.12, df = 1/4, p < .01) and at a lower 1/4, p < .07), but not when the 4-32 within rate than the rats exposed to only the 4% condition was compared with the 4—4 besucrose (F = 70.50, df = 1/8, p < .001). tween condition (F < 1.00). Switching under There was also what could be termed a the 4-4 within condition tended to be faster 350
32-4 W
WITHIN-STJBJECTS CONTRAST EFFECTS 135 POSITIVE CONTRAST
NEGATIVE CONTRAST
120 O--_32-4W
875 Q UJ UJ 0.
32-32W
4-4W
60 45
"30
32-32B
4-4B
& 4-32W
CYCLES
FIGURE 7. Mean reciprocal latency to switch to alternative solutions. (The curves labeled 32-32B and 4-4B refer to groups that received only the indicated sucrose solutions. The curves labeled with a W refer to animals that received all 4 combinations of sucrose conditions.)
than switching under the 4-4 between condition (F = 5.26, df = 1/8, p < .06). GENERAL DISCUSSION Both positive and negative contrast were again obtained in Experiment 3b. The results of Experiments 3a and 3b indicate quite strongly that the reliable positive and negative contrast obtained in Experiments 1 and 2 was not due in any artifactual sense to the within-subjects design. In fact, Experiment 3b indicated that there may be a second type of contrast effect operating in the withinsubjects design; specifically, the lick rate of the group that experienced all 4 sucrose combinations was depressed in the 4-4 conditions (4-4 within) relative to the group that experienced only the 4% solution (4-4 between). This between-groups contrast effect would tend to minimize the size of the within-subjects negative contrast since the 4-4 within condition is used as the control to measure the 4-32 within negative contrast. Despite this constraint, a reliable within-subjects negative contrast was found in all 3 relevant experiments (Experiments 1, 2, and 3b). The between-groups contrast effect must represent a contrast operating across days as a result of the rats' receiving
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the 4 % solution in a context (same apparatus, etc.) in which they sometimes receive 32% sucrose. The more powerful effect of the within-subjects negative contrast must result from the within-days comparison of the 2jsucrose solutions. A reasonable prediction from these data is that the between-groups contrast effect should be sensitive to manipulations of the intersession interval. That is, as the number of days interpolated between each test condition, particularly the 4-4 within and other within-subjects conditions, increases, then the magnitude of the between-groups contrast should decrease, i.e., the 4-4 within lick rate should approach that obtained under the 4-4 between conditions. Contrast effects have previously been shown to be sensitive to retention interval manipulations (e.g., Flaherty, Capobianco, & Hamilton, 1973; Gleitman & Steinman, 1964). The within-subjects contrast (i.e., 32-4 vs. 4-4 within), on the other hand, should be relatively insensitive to between-days manipulations (except for effects due to changes in the control groups, which should only enhance the size of the measured contrast), but these within-subjects contrast effects might be influenced by variations in the time between consecutive exposures to the sucrose solutions within each test session. That is, variations in the interbottle interval in the present paradigm should alter the size of the within-subjects negative contrast. The greater the interbottle interval, the smaller the contrast should be. It should be noted that this second type of contrast (between days) was stronger in the negative than in the positive contrast measures. In terms of comparison with other contrast paradigms, the present within-subjects paradigm is more similar to the simultaneous than to the successive contrast design (cf. Spear & Spitzner, 1966). This similarity is true on procedural grounds (multiple opportunities for comparison of the 2 rewards, as in the simultaneous paradigm, rather than only a single postshift comparison, as in the successive design) and also in terms of the results obtained with the different paradigms. Typically, the NCE found with the simultaneous design tends to be long lasting
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CHARLES F. FLAHERTY AND JOHN LARGEN
(cf. Bower, 1961; Flaherty, Riley, & Spear, 1973), whereas the contrast obtained with the successive shift design tends to last for only a few days subsequent-to the shift and tends not to occur after the first shift (e.g., Capaldi & Lynch, 1967; Vogel, Mukulka, & Spear, 1968). The contrast effects obtained with the present design appear to be long lasting and occur with repeated shifts. In this regard, it should be noted that the contrast effects develop much more gradually when estimated by the latency-to-switch measure than when estimated by a consummatory response measure (licks or intake). The failure to find indications of contrast with the latency measure in Experiments 3a and 3b that are as reliable and robust as those found in Experiments 1 and 2 is probably related to this sensitivity to training history. The rats in Experiments 3a and 3b were not exposed to a stable set of testing conditions as long as were the rats in either Experiment 1 or 2. The results of these experiments have some theoretical relevance. Subsequent to Crespi's (1942) demonstration of PCEs most of the research through the mid-1960s indicated that positive contrast was, at best, an elusive phenomenon. Because of these findings, theoretical efforts focused almost exclusively on NCEs (cf. Black, 1968; Dunham, 1968). One variety of explanation emphasized the contribution of generalization decrement (e.g., Capaldi & Lynch, 1967; Spear & Spitzner, 1966). That is, negative contrast was said to occur because a change in reward conditions also constituted a change in the stimulus conditions under which the subjects had learned the task, and this change in stimulus conditions led to a decrement in performance. The present results are relevant to the generalization decrement account of contrast in 2 ways. First, generalization decrement can only serve as an account of negative contrast, and the reliable occurrence of positive contrast in the present experiments emphasizes the limited applicability of the concept to contrast phenomena as a whole. Second, the effects of deprivation on the latency measure of negative contrast are not consistent with a generalization decrement interpretation. In Experiment 1 in-
creased deprivation was found to enhance the contrast effects (particularly negative contrast) as measured by the speed of switching from one bottle to its alternative. This enhancing effect of deprivation on negative contrast has also been found in a number of runway studies (Cleland, Williams, & DiLollo, 1969; Ehrenfreund, 1971; Flaherty & Kelly, 1973) and would seemingly be an effect opposite in direction from what would be expected if negative contrast were due to generalization decrement. This is so because degree of generalization has been shown to vary directly with degree of deprivation, at least in other contexts (cf. Kalish & Haber, 1965), and therefore more extreme deprivation states should retard the occurrence of negative contrast if generalization decrement were the major factor involved. A second theoretical approach to contrast effects has emphasized the contribution of presumed emotional factors. Crespi (1944) wrote that shifts in reward magnitude produced a heightened emotional drive: elation or joy, which enhanced ongoing behavior in the case of the comparison of large to smaller rewards, and anger or frustration, which subtracted from behavior in the case of the comparison of small to larger rewards. The mechanism by which these emotional responses came to affect behavior were not clearly specified by Crespi, but he did describe some of the behavior that was associated with the slower running observed under depression or negative contrast conditions. These behaviors included hesitant running, "attempts to jump out of the goal box," failure to consume the smaller reward, attempts at retracing in the runway, and "peering into the foodbox" (Crespi, 1942, 1944). No such obvious behavior patterns were apparently associated with the elation or PCE. As the data reported subsequent to those of Crespi (1942, 1944) generally failed to support the reliability of the PCE, explanatory attempts centered around the apparent disruptive emotional responses associated with NCEs. These explanations have often been framed within the structure of Amsel's (1958) frustration theory (e.g., Bower, 1961; Cleland et al., 1969; Ludvigson & Gay,
WITHIN-SUBJECTS CONTRAST EFFECTS
1967). For example, Bower reasoned that the lowered performance to the negative stimulus by the differential group (relative to a small reward control group) in a simultaneous contrast design might be the result of conflict between the anticipation of reward (rg in the Amsel model) and the anticipation of frustration (rf in the Amsel model). The frustration was said to occur because of generalization of expectancy (rg) from the positive stimulus to the negative stimulus. Similarly, Spence (1956) argued that the successive NCE was probably due to disruptive emotional effects occurring subsequent to a decrease in reward magnitude. The results of the present series of experiments as well as the results of other recent studies of positive contrast (e.g., Mellgren, 1972; Shanab & Biller, 1972) indicate that more attention must be given to presumed emotional correlates of positive contrast if theories based on emotional constructs are to have applicability for contrast phenomena in general. A recent study by Goldman, Coover, and Levine (1973) indicates that there may indeed be bidirectional physiological responses related to contrast. Specifically, Goldman et al. found that shifts to a less dense schedule of reinforcement (a successive negative contrast paradigm) produced elevated levels of plasma corticosterone, whereas shifts to a more dense schedule of reinforcement (a successive positive contrast paradigm) led to a decrease in plasma corticosterone below baseline levels. Whatever the theoretical mechanisms marshaled to explain contrast, it is clear from the present data that these mechanisms are going to have to account for relatively enduring positive as well as negative contrast, in particular a PCE that may be obtained without the application of special experimental techniques such as delay of reinforcement. The theoretical mechanisms must account for contrast effects that develop very rapidly and are relatively uninfluenced by deprivation state when measured by lick rate, but that develop more slowly, are more variable, and are more influenced by deprivation state when measured by the latency to switch between alternative solutions.
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REFERENCES Amsel, A. The role of frustrative nonreward in nonoontinuous reward situations. Psychological Bulletin, 1958, 55,102-119. Black, R. W. Shifts in magnitude of reward and contrast effects in instrumental and selective learning: A reinterpretation. Psychological Review, 1968, 75, 114-126. Bower, G. H. A contrast effect in differential conditioning. Journal of Experimental Psychology, 1961, 62,196-199. Capaldi, E. J., & Lynch, D. Repeated shifts in reward magnitude: Evidence in favor of an associational and absolute (noncontextual) interpretation. Journal of Experimental Psychology, 1967, 75, 226-235. Cleland, E. A., Williams, M. Y, & DiLolIo, V. Magnitude of negative contrast effect in relation to drive level. Psychonomic Science, 1969, 15,. 121-122. Collier, G. Consummatory and instrumental responding as functions of deprivation. Journal of Experimental Psychology, 1962, 64, 410-414. Crespi, L. P. Quantitative variation in incentive and performance in the white rat. American Journal of Psychology, 1942, 55, 467-517. Crespi, L. P. Amount of reinforcement and level of performance. Psychological Review, 1944, 51, 341-357. Dunham, P. J. Contrasted conditions of reinforcement: A selective critique. Psychological Bulletin, 1968, 69, 295-315. Ehrenfreund, D. Effect of drive on successive magnitude shift in rats. Journal of Comparative and Physiological Psychology, 1971, 76, 418-423. Flaherty, C. F., Capobianco, S., & Hamilton, L. W. Effect of septal lesions on retention of negative contrast. Physiology and Behavior, 1973, 11, 625631. Flaherty, C. F., & Kelly, J. Effect of deprivation state on successive negative contrast. Bulletin of the Psychonomic Society, 1973,1, 365-367. Flaherty, C. F., Riley, E. P., & Spear, N. E. Effects of sucrose concentration and goal units on runway behavior in the rat. Learning and Motivation, 1973,4, 163-175. Gleitman, H., & Steinman, F. Depression effect as a function of retention interval before and after shift in reward magnitude. Journal of Comparative and Physiological Psychology, 1964, 57, 158160. Goldman, L., Coover, G. D., & Levine, S. Bidirectional effects of reinforcement shifts on pituitary adrenal activity. Physiology and Behavior, 1973, 10, 209-214. Kalish, H. I., & Haber, A. Prediction of discrimination from generalization following variations in deprivation level. Journal of Comparative and Physiological Psychology, 1965, 60, 125-128. Ludvigson, H. W., & Gay, R. A. An investigation of conditions determining contrast effects in dif-
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ferential reward conditioning. Journal oj Experimental Psychology, 1967, 75, 37-42. Mellgren, R. L. Positive and negative contrast effects using delayed reinforcement. Learning and Motivation, 1972, 3, 185-193. Shanab, M. E., & Biller, J. D. Positive contrast in the runway obtained following a shift in both delay and magnitude of reward. Learning and Motivation, 1972, 3,179-184. Spear, N. E., & Spitzner, J. H. Simultaneous and successive contrast effects of reward magnitude
in selective learning. Psychological Monographs, 1966, 80(10, Whole No. 618). Spehce, K. W. Behavior theory and conditioning. New Haven: Yale University Press, 1956. Vogel, J. R., Mukulka, P. J., & Spear, N. E. Effects of shift in sucrose and saccharine concentrations on licking behavior in the rat. Journal of Comparative and Physiological Psychology, 1968, 66, 661-666. (Received December 12,1973)
