Journal of Experimental Psychology: Animal Behavior Processes 197S, Vol. 104, No. 2, 134-148

Some New Perspectives on Conditioned Hunger Susan Mineka University of Pennsylvania One theory holds that appetitive drives such as hunger and thirst are not conditionable because of their slow onset. However, recent evidence has shown only transitory conditioning of appetitive drives even with rapid onset. Such experiments may have failed because: (a) Exteroceptive conditioned stimuli (CSs) used in past experiments may be less easily accociated with the internal hunger drive than are interoceptive taste cues. Experiments 1-3 provided some support for this hypothesis, (b) The dependent measures used in past experiments may not be valid. Experiments 4 and 5 suggested that changes in the rate of bar pressing on an operant extinction curve following probe CSs for hunger may be a more sensitive and valid index of conditioned appetitive drive. However, the elusive and transitory nature of these results demands a reexamination of the basic difference between appetitive and aversive drives, which lies in the mode of their onset and control and which, given adaptive considerations, can account for their widely different conditionability.

For 30 years or more, learning theorists have questioned whether or not two of the primary appetitive drives—hunger and thirst —are conditionable to external cues in the same way as fear. A parallel between the conditionability of appetitive and aversive drives has been considered important for theories of acquired motivation and personality development that are based on the concept of conditioned drive (e.g., Dollard & Miller, 1950). Yet Cravens and Renner (1970), in a recent review of the conditioned appetitive drive literature, conclude that only 5 out of 20 published experiments in the last 20 years show positive results for the conditioning of hunger or thirst. Because unsuccessful attempts are less likely This research was supported by U. S. Public Health Service Grant MH-04202 to R. L. Solomon. The author was supported by a National Science Foundation Predoctoral Fellowship during the course of this research. The author would like to thank Francis Irwin, Martin E. P. Seligman, R. L. Solomon, and Mark Starr for their helpful comments on an earlier version of this manuscript. Special thanks go to R. L. Solomon whose advice, support, and encouragement immeasurably improved this research, although he does not agree with all the ideas presented here. Requests for reprints should be sent to S. Mineka, Department of Psychology, Charter and Johnson Streets, University of Wisconsin, Madison, Wisconsin S3706.

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to be published than are successful ones, Cravens and Renner then question the validity of the positive findings. Their critique of the methodological issues in this area is reasonable: (a) They doubt the sensitivity or validity of the most commonly used dependent variable—eating or drinking rate— particularly when time limits are placed on the animal, (b) They note that only rarely has the reliability of the dependent measure in such experiments been assessed, (c) They point out that we do not know whether the animal did discriminate or could have discriminated the external stimuli and/or their internal drive stimuli. These facts have typically been assumed rather than empirically demonstrated. As reasonable as this criticism of the conditioned appetitive drive literature may appear, most people, psychologists and laymen alike, would probably still argue that there are many stimuli in our everyday life that seem to act as positive conditioned stimuli (CS + s) for hunger and thirst, even though in humans of normal weight they probably do not substantially affect total caloric intake (Schachter, 1971). It may be that such CS + s should be considered as conditioned incentive stimuli that contribute to appetite rather than to drive per se, because they have probably gained their motivational ca-

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pacity through direct association with food ral deprivation as the UCS. Weight loss and drink. In any case, it should be noted was not used as an index of drive because that most of these CS+s are in the modal- of methodological problems in equalizing ities of taste and smell; in contrast, visual weight loss for testing. In addition, weight and auditory stimuli, such as the sight of loss could not be used in the within-subjects, the dinner table or the refrigerator or the discriminative conditioning design, which sound of the dinner bell, do not seem to was chosen for all experiments because it serve as CS+s for hunger or appetite. Yet would provide the most powerful demonall of the experiments cited by Cravens and stration of two discriminable cues arousing Renner that have attempted to condition two different levels of drive. appetitive drive states per se (i.e., experiEXPERIMENT 1 ments in which the CS+s are associated only with the drive state and not with food Experiment 1 was an attempt to condior drink) have used external visual or audi- tion hunger, using external color cues as tory CSs for hunger and thirst. CSs for different levels of hunger drive. Recent work on conditioned aversive The paradigm is similar to that used in sevstates, such as nausea and illness induced eral of the experiments summarized by by x-irradiation or various poisons, suggests Cravens and Renner (1970). It should be that the nature of the conditioned stimulus noted that any differences in eating in the (CS) used in an experiment must not be presence of the two CSs during testing must arbitrary if the optimal conditionability of be attributed to conditioned drive rather the drive is to be demonstrated. Garcia and than to conditioned incentive effects because Koelling (1966) have shown, for example, the CSs are never present during the anithat whereas flashing lights and tones can mal's eating periods throughout the course easily be associated with shock, tastes are of conditioning. For conditioned incentive very difficult to associate with shock as an effects to occur, the CSs would have to be unconditioned stimulus (UCS). Con- associated with eating during conditioning. versely, tastes are easily associated with nausea and illness, whereas lights and tones Method are much less easily associated with nausea Subjects and apparatus. Twenty male albino and illness. Numerous other examples of rats of the Sprague-Dawley strain obtained from such selective associations are now available Holtzman Co., 90-110 days old at the start of the (see Rozin & Kalat, 1971; Seligman & experiment, were housed individually with ad-lib Hager, 1972; and Shettleworth, 1973, for water. The conditioning apparatus consisted of two reviews). It is highly plausible that similar kinds of boxes: (a) 10 black wooden boxes (12 X selectivity of associations exist in the condi- 12 X 12 in. [30 X 30 X 30 cm]) with a metal floor tioning of appetitive drive states. Whereas and a screen top and (b) 10 transparent plastic fear may be easily conditioned to external cages (Keyco Co., Inc., 7 X 11 X 54 in. [18 X 28 X 13 cm]) with perforated metal lids to allow for cues because it is an externally elicited drive ventilation. Each plastic cage was enclosed in in conditioning experiments, hunger and a larger white cardboard box (12X IS X 10 in. thirst, being internally aroused, may be more [30X38X25 cm]). Procedure. After being gentled, weighed, and easily conditioned to more interoceptive the 20 subjects were divided into two cues such as tastes or smells (cf. Woods, marked, groups of 10 each, which were matched for weight. Makous, & Hutton, 1969, for the first dem- All subjects were deprived of food 19 hr before onstration of this with conditioned hypo- the first day of conditioning. On each of the 20 days of conditioning, one group, the white-black glycemia). was placed in the white box for 30 min The following series of experiments ex- group, with no food available after 19-hr food deprivaplores the question of whether the modality tion. This deprivation condition will be referred of the CS and the index used for conditioned to as the strong drive condition and the associated as the strong drive conditioned stimulus. drive are, in fact, important in demonstrat- box Thirty minutes after being returned to the homeing the conditioning of hunger, using natu- cage subjects were given ad-lib access to food

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half of each group were tested first in their weak drive box and second in their strong drive box. Day 23 was a reconditioning day, following the normal conditioning procedure. Days 24 and 25 were test days identical to those described above: Latency and intake were recorded at 5-min intervals for 20 min after 3-hr deprivatidn. Day 26 was a reconditioning day, followed by days 27 and 28, which were test days at 1-hr instead of 3-hr deprivation.

Results WEAK DRIVE CS

STRONG DRIVE CS

FIGURE 1. Mean amount eaten for black-white (filled circle) and white-black (open circle) groups in Experiment 1 in the presence of strong drive and weak drive CSs. (Purina Lab Chow) for 1 hr. They were then given 1 hr of food deprivation before being placed for 30 min in the black box. This deprivation condition will be referred to as the weak drive condition and the associated box as the weak drive conditioned stimulus. Again, 30 min after being returned to the home cage subjects were given ad-lib access to food for 1 hr. This was followed by 19-hr deprivation, to complete the 24-hr cycle. The other group, the black-white group, received the same treatment except that they spent their first 30-min conditioning session of each day under strong drive in the black box and the second session under weak drive in the white box. During the last 3 days of conditioning, subjects were fed wet mash (50% dry food powder, 50% water) instead of the regular lab chow during their two 1-hr eating sessions. This was to habituate them to wet mash before the test days. Testing. On the last day of conditioning, subjects were deprived as usual after their second feeding. On the first day of testing the animals were fed at about their usual time for 1 hr (no conditioning session preceded the feeding, however) and then deprived for 3 hr. Half the animals in each group were then placed in their strong drive box (white for the white-black group and black for the black-white group), with about 90 g of wet mash. Latency to the first bite was measured, and the amount of food eaten was measured at 5-min intervals for 20 min. This required removing the food for a few seconds after 5, 10, and 15 min of the 20-min eating period. The rats were then returned to their home cages and deprived until the next day, when they were again fed at their usual time, deprived for 3 hr, and tested in their weak drive box (black for the white-black- group, white for the black-white group) in the same manner. Animals in the other

No evidence for conditioned hunger was found in either of the groups. Subjects did not eat more in the strong drive box than they did in the weak drive box (23.3 vs. 23.7 g). In addition, the latency to the first bite of food was not significantly shorter in the strong drive than in the weak drive box (45.6 vs. 40.7 sec). On the first 3-hr test, only 12 out of 20 subjects ate more in the strong drive than in the weak drive box when 20-min totals are compared, and only 7 out of 20 when the first 5 min are compared. Regardless of which drive it had been associated with, a tendency to eat more in the smaller, more confining box was noted, especially during the first 5 min (10.6 vs. 8.6 g, p < .05, sign test, two-tailed).1 Figure 1 shows the number of grams eaten for both groups (whiteblack and black-white) in the strong drive and weak drive conditions. The second and third tests also showed no evidence for conditioning, but did show a tendency for subjects to eat more in the smaller white box. Latencies to the first bite were never significantly different between the two groups. Discussion Another apparently well-controlled attempt to condition hunger, using natural deprivation as the UCS, can now be added to the list of failures compiled by Cravens and Renner (1970). The design of this experiment is very similar to that used in a successful experiment by Wright (1965), differing only in the length of the conditioning sessions and in their number and order 1 These and all subsequent statistical tests are two-tailed.

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(Wright used 24 1-hr sessions in each box hair brush at the start of each session and with more swabs again halfway through the 30spread randomly over 36 days, and tested two min session. Both sessions were spent in the eating over a 1-hr period instead of for 20 white boxes. In Experiment 2a, one group of min). That these apparently minor varia- eight subjects (peppermint-lemon) received peptions should make such a difference is per- permint as its strong drive CS and lemon as its haps surprising, but given the past history weak drive CS, and a second group of eight subjects (lemon-peppermint) received the opposite. of the problem, not unusual. Wike, Cour, In Experiment 2b one group of eight subjects and Mellgren (1967) reported one success (maple-orange) received maple as its strong drive and two failures in their attempts to replicate CS and orange as its weak drive CS, and a secWright, also with only minor variations in ond group of eight subjects (orange-maple) rethe opposite. procedure. It also seems unlikely that the ceived Testing. After the 18th day of conditioning, one confounding variable in this experiment subjects were deprived as usual until the 19th day, —box size—could account for the negative when they were fed at about the same time as results because only 7 out of 10 subjects ate usual for 1 hr. Then after 3-hr deprivation they more in their strong drive box even when were tested in the conditioning chamber: In Experiment 2a, half of the subjects from each group it was the smaller white box. had four swabs of peppermint solution painted Perhaps, as was suggested in the introduc- on their tongues, were placed in the box for 5 min tion, the failure to condition hunger in Ex- with no food (allowing time for the CS to arouse periment 1 and in many previous experi- the appropriate hunger drive), and were then given to about 90 g of wet mash for 20 min. The ments was due to the nature of the CS access other half of the subjects from each group had employed. Since tastes have been shown four swabs of lemon solution painted on their to be much more effective than visual and tongues for the first test session. Latency to eat auditory cues as CSs for nausea and illness and the amount eaten for each S-min interval were (Garcia & Koelling, 1966), it is possible measured as in Experiment 1. After overnight deprivation, 1-hr access to food, that tastes may also be more effective than and 3-hr deprivation, the other half of the test visual and auditory cues as CSs for hunger. was completed: Subjects tested with peppermint Experiment 2 examines the possibility of on the first day were tested with lemon on the conditioning hunger with the same basic second day and vice versa. Day 21 was a reconditioning day, followed by tests at 3-hr deprivation paradigm as that used in Experiment 1, but on Days 22 and 23. Day 24 was also a recondiuses distinctive flavors rather than external tioning day, followed by tests under satiation (imcolor cues as CSs for the two different hun- mediately after the 1-hr feeding period) on Days 25 and 26. The testing procedure in Experiment ger levels. EXPERIMENTS 2A AND 2s Method Subjects and apparatus. Thirty-two male albino rats like those described in Experiment 1 were the subjects of this experiment. They were housed individually and maintained on ad-lib water. The apparatus consisted of 16 clear plastic test cages enclosed in the larger white cardboard boxes described in Experiment 1. The solutions used as CSs in Experiment 2a were \% McCormick's peppermint extract in water, and 1% McCormick's lemon extract in water. In Experiment 2b the CSs were 1% McCormjck's maple extract in water and 1% McCormick's orange extract in water. Procedure. The procedure was nearly identical to that described in Experiment 1, except that instead of being placed in two different boxes for the two conditioning sessions each day, subjects received one of two different tastes painted on their tongues twice with two swabs of a camel's

2b was identical to Experiment 2a except that the second test (Days 22 and 23) was under satiation conditions and the third test (Days 25 and 26) under 3-hr deprivation. As in Experiment 2a, half of each group received its strong drive CS on the first day of each test, and the other half received weak drive CS first; on the second day of each test, all subjects received the opposite taste.

Results Experiment 2a. On the first test subjects ate reliably more after their strong drive taste was administered than after their weak drive taste (24 vs. 17.5 g, df = 1, 28, p < .022, sign test). Analysis of variance revealed a significant drive effect, F (1, 28) = 8.3, p < .01, and a nonsignificant taste effect, F = 1.32. A trend toward an interaction of the two main factors did exist, reflecting perhaps a general appetizing effect of

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FIGURE 2. Mean amount eaten for peppermintlemon (filled circle) and lemon-peppermint (open circle) groups in Experiment 2a after strong drive and weak drive CS presentations.

vs. 21 g). Analysis of variance revealed a marginally significant taste main effect, (F 1, 28) = 2.9, p < .10, and a nonsignificant drive main effect (F — 1.8). Orange seemed to. have a general appetizing effect as did lemon in Experiment 2a, so that it did not serve well as a weak drive CS. These results are presented in Figure 3. As in Experiment 2a, 5-min tests yielded results in the same direction as did the total scores, except that in the first 5 min maple strong drive was greater than orange weak drive (10.5 vs. 8.4). Tests 2 and 3 again revealed no significant differences for either group. As in Experiment 2a, latencies to eat did not differ between strong and weak drive conditions (33.3 vs. 52.4 sec for the first test).

lemon, F = 3.5, p < .10. Figure 2 presents Discussion these results. It can be seen from betweenThe results of the first test of Experiment group comparisons that peppermint was a 2a gave strong evidence for the hypothesis good CS for both strong and weak drive that taste cues serve as better CSs for dif(group averages: peppermint strong drive, ferent levels of the hunger drive than do ex24.8 vs. peppermint weak drive, 14.1, U — ternal color cues (Experiment 1). This 8, p < .01, Mann-Whitney U test), but might be expected from the results of Garcia lemon was not nearly as good a CS (group and Koelling (1966) and others who have averages: lemon strong drive 23.1 vs. lemon demonstrated that tastes are more easily conweak drive, 20.9, U = 27, ns). ditioned to internal states of nausea and The results from the second and third tests malaise than are exteroceptive cues. reveal that the conditioning had extinguished The results of Experiment 2b are not as by the second and third tests, although dif- straightforward as those of Experiment 2a, ferences in the right direction remained. The differences for each 5-min period of 30 the first test were all in the same direction, oORANGE,--° MAPLE although not significant. Latencies did not differ significantly in the strong drive and UJ weak drive conditions in any of the three •MAPLEcf tests (23.7 vs. 26.0 sec, for the first test). ORANGE 15 Experiment 2b. The results of the first 10 test reveal that the discriminative conditioning procedure was effective in only the cc C5 orange-maple group: Subjects having orange i as their strong drive CS ate more after strong drive CS presentation than they did WEAK STRONG after weak drive CS (maple) presentation DRIVE DRIVE (26.1 vs. 16 g, /> < .055, Walsh test). SubCS CS jects in the maple-orange group did not, on FIGURE 3. Mean amount eaten for maple-orange the other hand, eat more after strong drive (filled circle) and orange-maple (open circle) CS (maple) presentation than they did after groups in Experiment 2b after strong drive and weak drive CS (orange) presentation (19.7 weak drive CS presentations.

CONDITIONED HUNGER perhaps because the tastes used are not as salient CSs as are the tastes used in Experiment 2a (Kalat & Rozin, 1970). They do, nevertheless, tend to support the conclusion that tastes are potentially effective CSs in the conditioning of hunger. Thus the use of more prepared or relevant CSs than have been used in past experiments may increase the possibility of conditioning hunger. But as Cravens and Renner (1970) have pointed out, this area of research has been rife with initial successes followed by failures of replication (Calvin, Bickell, & Sperling, 1953; Siegel & Macdonnell, 1954; Wike, Cour, & Mellgren, 1967; Wright, 1965). Experiments 3a and 3b replicate Experiment 2a, with the addition of a control group to assess whether a strong drive CS increases the amount eaten above an appropriate baseline and whether a weak drive CS decreases the amount eaten below an appropriate baseline (conditioned satiety). The appropriate baseline was provided by a control group that had experience with the conditioning situation under 3-hr deprivation only—the deprivation level under which all subjects were tested. EXPERIMENTS 3A AND 3s

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TABLE 1 MEAN GRAMS EATEN AFTER CONDITIONED STIMULUS (CS) FOR GROUPS IN EXPERIMENT 3A Group

Peppermint CS

Lemon CS

Peppermint-lemon Lemon-peppermint Control

22.4 (SD) 20.7 (WD) 16.0

19.1 (WD) 16.2 (SD) 17.9

Note. SD = strong drive; WD = weak drive.

The procedure for Experiment 3b was slightly modified from that of Experiment 3a in two respects. Conditioning lasted 28 days instead of only 18, and subjects had more exposure to the CS. Subjects received three swabs of the tastes painted on their tongue (instead of two) at three 10-min intervals in the 30-min conditioning session. A control group identical to that in Experiment 3a was also included. Subjects in this control group had 14 exposures to each taste (again at 3-hr deprivation) in a random order over the 28 c'ays of conditioning for the experimental groups. Testing. The testing procedure for Experiment 3a was identical to that described for Experiment 2 except that only one test was run (Days 19 and 20). During testing, subjects in Experiment 3b also received a second exposure to the CS 10 min after the onset of their 20-min eating period. The amount eaten was measured only twice (at the 5- and 20-min interval) in these experiments. Latencies to eat were not measured in these experiments.

Results

Experiment 3a. No evidence for the discriminative conditioning of hunger was found Subjects and apparatus. Forty-eight naive male in this replication of Experiment 2a. Subalbino rats like those described in Experiment 1 jects did not eat more after their strong were the subjects in this experiment. The ap- drive tastes than after their weak drive paratus was identical to that described for Extastes(19.3 vs. 19.9 g), and differences were periment 2. Procedure. The procedure for Experiment 3a in the predicted direction only for the pepwas identical to that of Experiment 2a, except that permint-lemon group (22.4 vs. 19.1 g). a third control group of eight subjects was in- There was a trend, however, for the expericluded. This group spent only one 30-min session in the conditioning chamber each day, always un- mental subjects (peppermint-lemon' and der 3-hr food deprivation. Subjects were deprived lemon-peppermint) to eat more than the overnight (17-hr deprivation), fed for 1 hr in the control subjects when averages across two home cage, deprived for 3 hr, and then placed in test days were compared (19.6 for experithe conditioning chambers for 30 min after receiving either peppermint or lemon as a CS. They mental subjects vs. 17 for control subjects, were then fed again for 1 hr, thus completing the U = 40, ns, Mann-Whitney U test). This 24-hr cycle at the same time as the two experi- difference was significant when comparing mental groups. The two taste CSs were admin- experimentals and controls following pepistered in a random order over the 18 days (all permint alone (U - 29, p < .05). Table 1 subjects in this group received the same taste on any given day), giving a total of nine 30-min ex- presents these results. Experiment 3b. Again no evidence for posures to each CS. CSs were again \% peppermint and 1% lemon solutions. the discriminative conditioning of hunger Method

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140 TABLE 2

MEAN GRAMS EATEN AFTER CONDITIONED STIMULUS (CS) FOR GROUPS IN EXPERIMENT 3B Group

Peppermint CS

Lemon CS

Peppermint-lemon Lemon-peppermint Control

21.1 (SD) 23.5 (WD) 14.1

23.1 (WD) 24.7 (SD) 18.7

Note. SD = strong drive; WD = weak drive.

was found. In the two experimental groups no systematic differences between the strong drive CS and weak drive CS conditions were found (22.9 vs. 23.3 g). As in Experiment 3a, however, the experimental subjects (peppermint-lemon and lemon-peppermint) ate more than their control counterparts (averaged across their 2 test days) : 23.1 g for experimentals vs. 16.4 g for controls, U = 27, p < .05, Mann-Whitney U test. Again, experimentals ate more after peppermint than did controls (U = 30, p < .05, Mann-Whitney U test). The same trend for lemon was not significant (U = 42). Table 2 presents these results. Discussion Experiments 3a and 3b failed to replicate the initial success of Experiment 2a in demonstrating the discriminative conditioning of hunger. No reason for this failure suggests itself, because Experiment 3a was an exact replication, and Experiment 3b attempted to maximize the possibility of demonstrating conditioned hunger by increasing both CS exposure and the number of conditioning days. There was, however, some evidence in these experiments for the conditioning of hunger to the whole conditioning situation. The control groups, which had only been in the conditioning situation under a low degree of hunger (3-hr deprivation) ate less than did the experimental groups, which had been in the conditioning situation under both high and low degrees of hunger (19- and 1-hr deprivation). The distinctive elements of the strong and weak drive CSs (the two different flavors) may have been obscured by the many common elements (e.g., the same test chamber, a gustatory cue, and the

same somewhat aversive CS administration procedure). As a consequence, the relatively high level of hunger may have been conditioned only to the situational cues. The reader, at this point, might wonder why no evidence for conditioned hunger was found in Experiment 1 when conditioning to the general situation seems to have occurred in Experiments 3a and 3b. It is of course possible that, compared to a control group similar to those of Experiments 3a and 3b, the experimental groups in Experiment 1 might have shown similar generalized conditioned hunger. Independent evidence exists, however, that the black and white boxes used as CSs in that experiment were highly discriminable in a conditioned emotional response (CER) paradigm (Mineka, Note 1). This minimizes the possibility that a blurred discrimination was the cause of the results of that experiment. In Experiments 3a and 3b it seems more likely that the many elements common to the strong and weak drive CS sessions could have obscured the sole difference—flavor. However, until similar groups are run, each receiving a different element of the compound CS, the real plausibility of this hypothesis (that the gustatory element of the CS was critical) will not be known. The possibility also exists, as has been previously suggested by Cravens and Renner (1970), that the dependent measure used in these experiments is not a very sensitive or valid one, especially since there is a time limit on eating (Davis & Keehn, 1959; Moll, 1959). Konorski's (1967) formulations on the hunger drive also suggest that eating may be a poor index of drive. According to Konorski, the food UCS or CS (e.g., food in the mouth) elicits the hunger antidrive which in turn inhibits the hunger drive (unconditioned response). "During the consummatory food response the hunger drive itself is temporarily inhibited, to be restored with rebound after the response is over, unless food has been presented ad libitum and the subject becomes completely satiated" (1967, p. 46). In addition, the hunger conditioned response, indexed according to Konorski by "restlessness, increased sensi-

CONDITIONED HUNGER tivity to external taste stimuli, and hunger contractions of the stomach" (p. 277), is inhibited by both a food UCS and a food CS. To the extent that Konorski is correct in postulating that hunger and conditioned hunger are suppressed during eating (even though they may rebound between bites), eating seems to become a less theoretically sound measure of the hunger drive. What, then, might be a more sensitive and reliable index of conditioned hunger? Konorski's formulations suggest that such a measure should not be confounded by inhibition of the drive during eating, or at least that such inhibition should be minimal and allow for maximal rebound of the hunger drive between bites of food. Two such measures might be operant rate and resistance to extinction on a partial reinforcement schedule in a simple bar-press situation. Clark (1958) showed that variable interval (VI) 1-, 2-, and 3-min rates increased with increasing levels of deprivation (1, 3, 5, 7, 10, 20, and 23 hr). And Perin (1942), Yamaguchi (1951), and others have shown that resistance to extinction is also increased with increasing levels of deprivation. These two dependent measures of unconditioned drive, which are not confounded by the inhibition of drive during long bouts of eating, perhaps ought to be good measures of conditioned drive. Experiment 4 explores the use of these two measures as indices of conditioned drive, again using tastes as CSs for strong and weak levels of drive. EXPERIMENT 4 In line with the reasoning of Rescorla and Solomon (1967), CSs for appetitive or aversive drives should produce changes in the rate of ongoing operant behavior. That CS+s predicting shock produce marked suppression in the rate of bar pressing for food is well documented; such suppression, in fact, is commonly used as a reliable and valid index of the fear drive. The effect of CSs for the hunger drive on the rate of bar pressing for food seems, however, never to have been tested. Experiment 4 tests the effects of such CSs on the rate of bar pressing for food, both in maintenance and in extinction.

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Method Subjects and apparatus. Sixteen male albino rats like those described in the previous experiments were used as subjects in this experiment. The conditioning apparatus was identical to that used in Experiments 2 and 3. There were also four identical Skinner boxes with inside dimensions of H i X l O i X l g in. ( 2 9 X 2 7 X 4 6 cm). The sides were aluminum; the door and the back were Plexiglas. The floors consisted of 18 aluminum rods, 4 in. (.3 cm) in diameter, running parallel to the sides. The boxes were contained in lightproof, sound-attenuating ice chests and were ventilated by fans. The food cup was in the center of a side wall, 4 in. (1.3 cm) above the floor. A retractable bar was located li in. (4 cm) above the top of the food cup. The bar was 2 in. (5 cm) wide and protruded I in. (2 cm) from the side wall into the chamber. A press of the bar delivered a 45-mg Noyes pellet to the food cup on the appropriate schedule. An opening 4 in. (10 cm) to the right of the food cup was covered with green frosted glass, behind which a light was located. The light was always on when the boxes were- being used. White noise was delivered through a loudspeaker 2 in. (5 cm) to the left of the food cup during all sessions. Procedure. Rats were deprived overnight and placed in the Skinner boxes with several Noyes pellets available on the bar and in the food cup. Subjects were left in the box until they started bar pressing, initially on continuous reinforcement schedule, followed by VI 1-min training. Several subjects who did not pick up the response were deprived again overnight and left in the boxes until they learned to bar press. When all subjects had learned the response under these fairly high levels of deprivation, they were trained for five daily 55-tnin sessions under 3-hr deprivation. After being deprived overnight, they were fed for 1 hr in their home, cages, deprived for 3 hr, and then placed for their SS-min VI 1-min session in the Skinner boxes. The feeding and deprivation times were staggered to accommodate all 16 subjects in the four Skinner boxes, After the last day of VI training, subjects were divided into two groups matched according to their rates on the last 2 days. After being deprived overnight (19-21 hr), the first day of conditioning began. The conditioning procedure was identical to that described in Experiment 2a. For 18 days, subjects received one daily session in the test cage with their strong drive CS and one daily session with their weak drive CS. CSs were again 1% peppermint and \% lemon solutions and were administered as two swabs on the tongue at three 10-min intervals in each 30-min session. One group received peppermint as its strong drive CS and lemon as its weak drive CS. The second group received the opposite. After the last day of conditioning, subjects were deprived overnight, fed for 1 hr, deprived for 3

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hr, and then given one SS-min reacquisition session in the Skinner boxes. They were again deprived overnight, fed for 1 hr the following day, and then deprived for 3 hr before their first test session. Testing. Half of the subjects in each group of eight were tested at this 3-hr deprivation under maintenance conditions (two maintenance groups of four subjects each) and half of each group of eight were tested under extinction conditions (two extinction groups of four subjects each). In the maintenance groups half the subjects (two per group) were given their strong drive CS and half were given their weak drive CS (four swabs of the taste painted on the tongue) before being placed in the Skinner boxes for a 20-min session. Subjects were briefly removed from the boxes and CSs were administered again after 10 min of the session had elapsed. On the second day of testing subjects received the opposite CS both before and 10 min into a similar session. In the extinction groups testing was the same as for the animals tested under maintenance except that no food pellets were delivered. After the 20-min extinction session with the two CS presentations, a 35-min reacquisition session took place, with no CSs. On the second test day subjects received the CS that they had not received on the first. After the second test day, all subjects received a reconditioning day in the test chambers. Two more test days followed this reconditioning day, but all subjects were tested in extinction in the manner described above.

Results and Discussion Maintenance test. Those subjects receiving their first test sessions under maintenance conditions did not press more after their strong drive CS than after their weak drive CS. A pilot study had indicated that there were unconditioned drive effects on the rate of bar pressing at 1- and 19-hr deprivation under maintenance conditions. Thus, failure to find evidence for conditioned drive under these conditions could indicate either that different levels of drive had not been conditioned to the different taste CSs or that the bar-pressing index was not sensitive enough to test for such conditioning. Extinction test.2 Although there was an overall trend for subjects to press more after their strong drive CS than after their weak 2

Results are combined here for the two tests in extinction for half the subjects and the one extinction test for the other subjects who had first been tested under maintenance conditions.

drive CS (76.7 vs. 70.3 bar presses in 20 min), these results were confounded by the tendency to press less on the second than on the first day of each test (60.9 vs. 86.4 bar presses in 20 min). Thus, for example, subjects who received their strong drive CS first (strong drive-weak drive group) were likely to bar press more after their strong drive CS than after their weak drive CS, but this difference was confounded with the tendency for all subjects to press more on their first test day. To overcome this difficulty in analyzing the results, ratios were computed for comparison purposes of both strong drive CS bar presses/weak drive CS bar presses and first-day bar presses/secondday bar presses. Note that these two ratios are identical for those subjects receiving their strong drive CS first and their weak drive CS second (strong drive-weak drive group). A tendency for the group receiving their weak drive CS first (weak drivestrong drive group) to show a smaller ratio of first-/second-day bar presses than the strong drive-weak drive group indicates that the CS drive effects were interacting with the first-/second-day effects. These ratios were compared for 20-min totals and for each S-min subtotal. For the 20-min totals, there was a nonsignificant trend for the strong drive-weak drive group to have a higher first-/secondday ratio than the weak drive-strong drive group (1.51 vs. 1.19, f / > 4 2 , Mann-Whitney U test). When the 20-min total was broken down into four 5-min subtotals, it became evident that the differences between strong drive-weak drive and weak drivestrong drive groups only began to emerge by the third 5-min subtotal. Thus, first-/ second-day ratios did not differ for either of the first and second 5-min subtotals (1.36 vs. 1.45; 1.42 vs. 1.56). But by the third 5-min subtotal, subjects had started to press more after their strong drive CS than after their weak drive CS and first-/second-day ratios began to reveal this (strong driveweak drive 1.37 vs. weak drive-strong drive 1.09, U = 47, ns). By the fourth 5-min subtotal, subjects were pressing reliably more after their strong drive CS than after their

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weak drive CS (15.2 vs. 10 bar presses in 5 mm, p < .022, sign test). The first-/ second-day ratios also revealed a significant drive effect in this subtotal (strong driveweak drive 2.35 vs. weak drive-strong drive 1.04, 17 = 24, P < .02, Mann-Whitney U test). Table 3 clarifies these results for the fourth 5-min subtotal. Thus, some indication was found that the rate of bar pressing in extinction may be a sensitive index of conditioned drive effects, as a pilot study had shown it was for unconditioned drive effects. In spite of the design problems in this kind of experiment (which seem to be due to the different extinction curves generated on successive days of extinction despite 35-min reacquisition sessions after each 20-min extinction session), a replication of the present results was deemed necessary before further refinements on the procedure were attempted. Experiment 5 replicates Experiment 4, except that all subjects are tested only under extinction conditions. EXPERIMENT 5 Method Subjects and apparatus. The subjects in this experiment were 20 naive male albino rats (80 days old) like those described in the previous experiments. The apparatus was identical to that described in Experiment 4. Procedure. The procedure was nearly identical to that described for Experiment 4. After acquisition of the bar-press response, all subjects were given five SS-min sessions on a VI 1-min schedule while under 3-hr deprivation. Subjects were then divided into two groups matched for their bar-pressing rates on the last 2 days of VI training. An 18-day conditioning period followed, in which all subjects were on the same 24-hr feeding schedule as has been used in Experiments 2-4. Two subjects in each group of 10 were placed in the Skinner boxes (with the bar retracted) rather than in the plastic test cages for their two conditioning sessions each day. CSs were again 1% peppermint and \% lemon solutions. Following the last (18th) day of conditioning, subjects were deprived as usual, but on the following day were fed for 1 hr and deprived for 3 hr before a 55-min reacquisition session in the Skinner boxes. They were again deprived overnight, fed for 1 hr the following day, and deprived for 3 hr before their first test session. Testing. The first test session consisted of a 20-min extinction period in the Skinner boxes fol-

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TABLE 3 MEAN NUMBER OF BAR PRESSES IN LAST S MIN FOR GROUPS IN EXPERIMENT 4s Group

SDCS

WD CS

SD-WD WD-SD

18.8 11.5

8.0 12.0

Note. SD = strong drive; WD = weak drive; CS = conditioned stimulus.

lowed by a 35-min reacquisition session. Taste CSs were administered as four swabs on the tongue at the start of the extinction session and again after 10 min. Half the subjects in each group received their strong drive CS on the first test day and half their weak drive CS. On the second test day, the session was run in an identical manner, except that each subject received the taste CS it had not received on the previous day. The following day was a reconditioning day in the conditioning chambers (Skinner boxes for two subjects in each group). The 3rd and 4th test days were run in the same fashion, with each subject receiving the same taste on the 3rd day that it had on the 2nd, and the same taste on the 4th day that it had on the 1st.

Results and Discussion As in Experiment 4, subjects tended to press more after their strong drive taste than after their weak drive taste on the first test (101.9 vs. 86 barpresses in 20 min, p < .092, sign test).3 Table 4 presents these results. One striking difference between the results of this experiment and those of Experiment 4 was that there was no 'great tendency for the animals to bar press more on the first than on the second extinction test. In addition, the largest differences appeared in the last two 5-min subtotals only for the weak drive-strong drive group. In fact, the strong drive-weak drive group even showed a slight tendency to press more in the weak drive than in the strong drive condition in the last two 5-min subtotals. The results of the second test (Test Days 3 and 4) after reconditioning revealed no evidence of a conditioned drive effect (61 vs. 81.9 bar presses in 20 min). 3

Five subjects (two from the peppermint-lemon and three from the lemon-peppermint group) were excluded from the data analysis because of various equipment problems during their tests.

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TABLE 4 MEAN NUMBER OF BAR PRESSES IN 20 MIN FOR GROUPS IN EXPERIMENT 5 Group

SD CS

WD CS

SD-WD WD-SD

98.4 105.4

78.7 93.4

Note. SD = strong drive; WD = weak drive; CS = conditioned stimulus.

The results of this experiment support the conclusion of Experiment 4 that the barpressing rate in extinction may be a more sensitive index of conditioned drive than the amount eaten in the same period of time. However, the different pattern of the results in this experiment, for which there was no apparent reason, indicates that the reliability of this measure is uncertain. These results resemble those of Weissman (1972), who reported that a discriminative stimulus (SD) for the availability of water reinforcement could later produce increments in responding in a free operant situation when the animals were satiated on water. Thus an SD for water reinforcement could produce an apparent motivational effect in water-satiated animals. It seems likely, however, that this increase in responding was due to an incentive effect because the SD had been selectively paired with water reinforcement rather than with the thirst drive per se. So, although Weissman's results are similar to those of the present experiment, they appear to stem from a conditioned incentive rather than from a conditioned drive effect. GENERAL DISCUSSION The results of the experiments reported here suggest that further research on conditioned appetitive drives must take into account such variables as CS modality when attempting to determine the limits of the conditionability of such drives. Exteroceptive color cues may be easily associated with an externally elicited fear drive but not with an internally induced appetitive drive. And, on the other hand, the results of these experiments have suggested that interoceptive taste cues may be more easily associated

with the internal hunger drive than with the externally elicited fear drive (Mineka, Note 1). In fact, interoceptive CSs such as those used by Russian investigators (Bykov, 1959; Razran, 1961) might indeed be the most easily associated with internal hunger cues. Such conditioning, once established, tends to be fairly stable and resistant to extinction. In addition, such CSs are everyday events in the life of the organism, and no attention to external cues is necessary for such conditioning to occur. Experiments 4 and 5 also point to the importance of using a new dependent measure when attempting to measure a conditioned appetitive drive. Analogous measures for fear conditioning have been used for some time now (Rescorla & LoLordo, 1965; Rescorla & Solomon, 1967). In these experiments CS+s for shock and CS—s for no shock are typically imposed on Sidman avoidance baselines or on appetitive operant baselines. Modulations in Sidman rate or in rates of bar pressing for food when the CSs are presented are presumed to reflect increases and decreases in "fear." In our experiments, different CSs for high and low hunger drive are presented during extinction of an appetitive operant. Increases and decreases in the bar-pressing rate in extinction when CSs are presented are presumed to reflect increases and decreases in conditioned hunger drive. Such a dependent measure may be more valid and sensitive than the amount of eating. But perhaps the most striking point to emerge from this series of experiments is the elusiveness of the "conditioned hunger" phenomenon, even with tastes as CSs. In Experiment 3, two attempts to replicate Experiment 2a failed to demonstrate discriminative conditioning of hunger, although some evidence for a generalized conditioned hunger to the whole testing situation was observed. In Experiment 5 the attempt to replicate the second half of Experiment 4 revealed the same general effect as had Experiment 4: CSs for high hunger drive as compared to CSs for low hunger drive tended to increase the bar-pressing rate in extinction. But the specific pattern of the

CONDITIONED HUNGER results was quite different in the two experiments. In addition, in all of these experiments any conditioning that existed on the first test trial typically disappeared by the second test, even if a reconditioning day was given between the two tests. So even this new evidence for the conditioning of hunger, using modality-relevant CSs and a theoretically sounder dependent measure, indicates that such conditioning is at best an ephemeral effect. This is also consistent with Booth's (1973) recent finding of only transitory conditioned satiety in rats to taste cues associated with a food of high caloric density. The critical reader may now wonder whether the elusiveness of the conditioned hunger phenomenon in these experiments is a mere excuse for inadequately controlled experiments. Similar patterns of results in past experiments—initial successes followed by problems in replication—may also be attributed to failure to bring the phenomenon under adequate control. The author feels, however, that this conclusion is not justified because of the large number of such occurrences in the past. But even more importantly, it would merely postpone to a future date an examination of the reasons why the phenomenon is indeed so difficult to produce reliably. % What is the current status of conditioned appetitive drives? Cravens and Renner (1970) adopt the commonly cited explanation for the difficulty of conditioning these drives as compared to the ease of conditioning aversive drives such as fear: Because of their slow onset, typically over a period of many hours, it may be difficult to associate any specific external cues with the onset, or even the continuation of, the drive states. If one assumes that the difficulty of conditioning appetitive drives stems directly from their slow onset, one must predict that with rapid induction of the drives, now possible with injections, conditioning of the drives will be possible (cf. D'Amato, 1970, p. 316). If, on the other hand, the conditioning of such drives is just as difficult and unreliable

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using rapid onset, some other reason must be invoked to explain the difficulty in conditioning. A number of attempts have now been made to condition hunger and thirst using rapid induction of these drives through various techniques, and the results do not support the slowness-of-onset explanation. Andersson and Larsson (1956) failed to condition thirst or drinking in goats using rapid onset of thirst via hypothalamic brain stimulation as the UCS. More recently Balagura (1968) claimed to have conditioned insulin-induced hunger. Siegel and Nettleton (1970) have shown, however, that what was conditioned was actually a response to decrease insulin-induced stress: Their results do not support the idea that insulin-induced hunger can be classically conditioned (although the actual hypoglycemia does seem to be conditionable; c.f. Woods, Makous, & Hutton, 1969). Huston and Brozek (1972) have also reported their failure to condition eating elicited by lateral hypothalamic brain stimulation, even after as many as 800 CS-UCS pairings. These experiments constitute evidence against the idea that appetitive drives are not conditionable solely because of their slow onset. A recent series of experiments by Seligman and Mineka constitutes even stronger evidence against this hypothesis (Mineka & Seligman, 1975; Mineka, Seligman, Hetrick, & Zuelzer, 1972; Seligman, Ives, Ames, & Mineka, 1970; Seligman, Mineka, & Fillit, 1971). These authors have shown that thirst (or drinking) induced rapidly by unnatural means (hypertonic-saline or isotonic-procaine) is highly conditionable to an exteroceptive compound CS. However, when the thirst is induced rapidly by angiotensin, a natural precursor of extracellular thirst (Epstein, Fitzsimons, & Rolls, 1970), the result is little and ephemeral conditioning. Along with the fact that these authors have never been able to condition thirst using natural deprivation as the UCS, these results suggest that even if drinking is induced rapidly, but in a natural or normal fashion, it is not likely to become strongly conditioned to external cues. The successful conditioning that does occur with unnatural UCSs (including insulin-induced

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hunger) may result from the aversive properties of those UCSs and thus not represent conditioning of real thirst or hunger. Although hunger and thirst do indeed differ from fear in gradualness of onset, the experiments cited above seem to rule out this difference as the major cause of the large diferences in conditionability between these two different kinds of drives. It seems that in their eagerness to find parallels between conditioned aversive and conditioned appetitive drives, learning theorists have overlooked the basic differences between these two kinds of drives which can account for their widely different conditionability. This basic difference lies in the mode of onset and control of these different drives. Hunger and thirst are internally elicited, internally discriminated, internally experienced, and largely controlled by internal homeostatic mechanisms. One can make a distinction between the following two aspects of the states of hunger and thirst: (a) an animal's feeling of hunger or thirst and (b) an animal's food- or water-seeking behavior which may be caused by those feelings of hunger or thirst. The former state, feeling hungry or thirsty per se, does not provoke attention to or scanning of external cues; animals can discriminate their own internal drive state (see Holies, 1967; Webb, 1955) and do not use external cues to tell them when or how hungry or thirsty they are. The second aspect, which involves food- or water-seeking behavior in the hungry or thirsty animal, involves scanning of external cues by its very nature. However, such scanning of external cues to find food or water will not result in the conditioning of hunger or thirst per se to those cues; i.e., those cues will not elicit hunger or thirst in the nonhungry or nonthirsty animal. In addition, from an evolutionary standpoint, easy conditioning of these drives to external cues could upset homeostatic regulation, and would probably only very rarely be of adaptive value in the life history of most organisms. On the other hand, fear, being largely externally elicited, is likely to provoke scanning of external cues. And of course, from an evolutionary standpoint the conditioning of fear to cues which have been

associated with danger or pain may quite literally be a matter of life and death. If different learning processes such as sensitization, classical conditioning, and instrumental conditioning have evolved in a fashion similar to other behaviors and to morphology as Razran has recently suggested (1971), it seems likely that they may also have evolved to modify behavior only in areas where modification of that behavior is of adaptive significance to the organism. Thus the classical conditioning of aversive drives such as fear and illness is obviously of great value in the defensive ("preparatory-protective") activities of the organism but its value is much more dubious in the appetitive ("preparatory-alimentary") activities of the organism (Konorski, 1967). Strong homeostatic control mechanisms seem instead to fulfill this role for hunger and thirst. The interesting possibility exists that some of these homeostatic controls are themselves conditioned. Weisinger and Woods (1971) have demonstrated, for example, that aldosterone-elicited sodium appetite is probably the result of conditioning. In conclusion, learning theorists interested in conditioned drive will not find close parallels between aversive and appetitive drives. The critical difference between these two kinds of drives lies not in the speed of their onset, but rather in the way in which they are typically discriminated and controlled. Awareness of these basic differences leads to a better understanding of the elusiveness of the conditioned hunger or thirst phenomena, even when relevancy or preparedness is maximized and when a more sensitive dependent measure is used in attempts to condition these drives. With respect to hunger and thirst, classical conditioning does undoubtedly play a role in determining both our appetities and some of our short-term regulatory processes (cf., Woods, Decke, & Vasselli, 1974). Future research should probably be directed toward the roles that classical conditioning can play in some of the short-term regulatory processes of the organism and in the development of various appetities rather than toward the classical conditioning of the drive states per se (see D'Amato, 1974, for a similar argument).

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Mineka, S., & Seligman, M. E. P. Conditioned drinking as avoidance learning. Journal of Comparative and Physiological Psychology, 1975, 88, 69-80. Mineka, S., Seligman, M. E. P., Hetrick, M., & Zuelzer, K. Poisoning and conditioned drinking. Journal of Comparative and Physiological Psychology, 1972, 79, 377-384. Moll, R. P. The effect of drive level on acquisition of the consummatory response. Journal of Comparative and Physiological Psychology, 1959, 52, 116-119. Perin, C. T. Behavior potentiality as a joint function of the amount of training and degree of hunger at the time of extinction. Journal of Experimental Psychology, 1942, 30, 93-113. Razran, G. The observable unconscious and the inferrable conscious in current Soviet psychophysiology: Interoceptive conditioning, semantic condiitoning, and the orienting reflex. Psychological Review, 1961, 68, 81-147. Razran, G. Mind in evolution. Boston: Houghton Mifflin, 1971. Rescorla, R. A., & LoLordo, V. M. Inhibition of avoidance behavior. Journal of Comparative and Physiological Psychology, 1965, 59, 406-412. Rescorla, R. A., & Solomon, R. L. Two-process learning theory: Relationships between Pavlovian conditioning and instrumental learning. Psychological Review, 1967, 74, 151-182. Rozin, P., & Kalat, J. Specific hungers and poison avoidance as adaptive specializations of learning. Psychological Review, 1971, 78, 459486. Schachter, S., Some extraordinary facts about obese humans and rats. American Psychologist, 1971, 26, 129-144. Seligman, M. E. P., & Hager, J. Biological boundaries of learning. New York: Appleton-Century-Crofts, 1972. Seligman, M. E. P., Ives, C, Ames, H., & Mineka, S. Conditioned drinking and its failure to extinguish : Avoidance, preparedness, or functional autonomy? Journal of Comparative and Physiological Psychology, 1970, 71, 411-419. Seligman, M. E. P., Mineka, S., & Fillit, H. Conditioned drinking induced by hypertonic saline, isotonic procaine, and angiotensin. Journal of Comparative and Physiological Psychology, 1971, 77, 110-121. Shettleworth, S. J. Constraints on learning. In D. S. Lehrman, R. A. Hinde, & E. Shaw, (Eds.), Advances in the study of behavior (Vol. 4). New York: Academic Press, 1972. Siegel, P, S., & MacDonnell, M. F. A repetition of the Calvin-Bicknell-Sperling study of conditioned drive. Journal of Comparative and Physiological Psychology, 1954, 47, 250-252. Siegel, S., & Nettleton, N. Conditioning of insulin-induced hyperphagia. Journal of Comparative and Physiological Psychology, 1970, 72, 390393.

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Webb, W. B., Drive stimuli as cues. Psychological Reports, 1955, 1, 287-298. Weisinger, R. S., & Woods, S. C. Aldosteroneelicited sodium appetite. Endocrinology, 1971, 89, 538-544. Weissman, A. Elicitation by a discriminative stimulus of water-reinforced behavior and drinking in water satiated rats. Psychonomic Science, 1972, 28, 155-156. Wike, E. L., Cour, C., & Mellgren, R. Establishment of a learned drive with hunger. Psychological Reports, 1967, 20, 143-145. Woods, S. C., Decke, E., & Vasselli, J. R. Meta-

bolic hormones and regulation of body weight. Psychological Review, 1974, 81, 26-43. Woods, S. C., Makous, W., & Hutton, R. A. Temporal parameters of conditioned hypoglycemia. Journal of Comparative and Physiological Psychology, 1969, 69, 301-307. Wright, J. H. Test for a learned drive based on the hunger drive. Journal of Experimental Psychology, 1965, 70, 580-584. Yamaguchi, H. G. Drive (D) as a function of hours of hunger (h).. Journal of Experimental Psychology, 1951, 42, 108-117. (Received July 16, 1974)