Food anticipation and lever-directed activities in rats

LEARNING

AND

MOTIVATION

15,

12-36 (1984)

Food Anticipation and Lever-Directed Activities in Rats GRAHAM

C. L. DAVEY AND GARY G. CLELAND The City University, London

Three experiments are described which elaborate some of the conditions under which rats will contact and manipulate a periodically presented retractable lever. Experiment 1 demonstrated that (i) initial manipulative oral and manual contact with the lever was facilitated if the rat had previous experience of food delivery in the experimental chamber; (ii) persistence in contacting the lever on successive presentations was a function of whether food continued to be presented in the experimental environment; and (iii) food satiation significantly reduced the tendency of the rat to contact the lever even though an expectancy of food had previously been established under conditions of food deprivation. Experiment 2 suggested’ that the tendency to approach and contact the lever was in part a function of the local moment-to-moment conditional probability of food delivery. Experiment 3 found that the probability of contacting the lever was higher during presentation of an auditory CS signaling a high rate of food delivery than during stimuli signaling no food at all. These results are interpreted as suggesting that the foodsignaling aspects of an appetitive CS and that CS’s ability to generate signaldirected behaviors are experimentally separable properties.

It has now been known for some years that animals will frequently approach and contact conditioned stimuli (CSs) which signal impending response-independent appetitive reinforcers (UCSs). This phenomenon has been called autoshaping (Brown & Jenkins, I%@, sign-tracking (Hearst & Jenkins, 1974), and more simply, signal-directed or signal-centered behavior (e.g. Jenkins, Ban-era, Ireland, & Woodside, 1978). It has been observed in a wide variety of species (cf. Schwartz & Gamzu, 1977), and appears to obey Pavlovian rather than instrumental associative rules (cf. Hearst, 1979). However, there is still much debate about the true nature of the performance rules which underlie signal-centered behavior. Some recent performance models have alluded to the importance of the nature of the CS in determining sign-tracking (e.g. Cleland & Davey, 1982, 1983; TimRequests for reprints should be addressed to Graham Davey, Department of Social Science and Humanities, The City University, Northampton Square, London, ECIV OHB. UK. 12 0023-9690184 $3.00 Copyright All rights

0 1984 by Academic Press. Inc. of reproduction in any form reserved.

FOOD

ANTICIPATION

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AUTOSHAPING

13

berlake, 1983; Timberlake & Grant, 1975). That is, the CS may serve two purposes: (i) as a signal for food and (ii) as a natural releaser for phylogenetically preorganized foraging behaviors. In relation to the second purpose, an important component of these foraging behaviors is approach to, and contact with, the releasing CS. For instance, for hungry rats, a retractable or illuminable lever CS may release investigative foraging behaviors which result in manual and oral manipulation of the CS (Atnip, 1977; Boakes, 1977; Davey & Cleland, 1982; Davey, Oakley, & Cleland, 1981; Davey, Phillips, & Cleland, 1981; Holland, 1980; Peterson, 1975) and, indeed, manipulation of small, often inedible, objects is a characteristic of free-living feeding rats (Barnett, 1956; Ewer, 1971). However, using a restrained conspecific as the CS generates CS-directed activities characteristic of social feeding (Timberlake, 1983; Timberlake & Grant, 1975); and small moving objects-such as a rolling ball bearing CS-appear to release chase and pounce responses characteristic of predatory feeding (Boakes, Poli, Lockwood, & Goodall, 1978; Timberlake, Wahl, & King, 1982). Using localizable CSs which do not appear to be natural releasers for preorganized appetitive approach and contact behaviors does not seem to generate any CS approach (Cleland & Davey, 1983; Powell, Kelly, & Santisteban, 1975; Timberlake, 1983). Whereas much of the available evidence is consistent with a view of signal-centered behavior which stresses interaction between the animal’s need state and the ecological relevance of the CS to that species, there is still much about this approach that needs to be clarified. One of the most important tasks is to elaborate the conditions under which the CS will act as a releaser for appetitive activities and what factors determine the strength of this tendency. If one takes the view that signal-centered behavior reflects the release of a functionally preorganized feeding behavior system (e.g., Timberlake, 1983) then much importance will be attached to the way in which stimuli, responses and internal motivational states are related (Baerends, 1976). For instance, while a state of hunger is an important contributor to the strength of signal-centered behavior (Cleland & Davey, 1982) it is not sufficient to generate substantial approach and contact with a CS that is not paired with food (cf. Atnip, 1977; Davey et al., 1981; Locurto et al., 1976; Peterson, Ackil, Frommer, & Hearst, 1972; Stiers & Silberberg, 1974). Therefore, according to this behavior systems account there must be some attribute of the CS-UCS contingency which establishes and maintains the status of the CS as a releaser for CS approach and contact. There are two immediately obvious possibilities: (i) the CS-UCS pairings ensure that hunger becomes a conditioned state which is elicited only during CS presentation (cf. Davey, Cleland, & Oakley, 1982; Konorski, 1967), and (ii) the animal learns about the distribution of food during CS and no-CS periods and the ability of the CS to act as a releaser is a direct function of this learning about the moment-

14

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to-moment probability of food delivery (i.e., it is a function of the animal’s anticipation of food). In both cases the CS serves two, relatively independent, functions: as a signal for food and as a releaser for appetitive activities. We have addressed the first possibility elsewhere (Cleland & Davey, 1982) and this present paper represents an attempt to investigate the second possibility-that the tendency to approach and contact a signal for food is dependent on the moment-to-moment probability of food in the environment and it is the anticipation of food based on knowledge of these probabilities which acts conjointly with the animal’s deprivation state to prime the CS as a releaser for appetitive behaviors. EXPERIMENT

1

The first experiment is designed to investigate the effects on retractable lever contact in rats of (i) feeding the rat in the experimental chamber prior to presenting the lever, (ii) presenting the lever while the rat is either food satiated or food deprived, (iii) presenting the lever in a contiguous relationship with food, (iv) in the same context as food but explicitly without any contiguous relationship to it, or (v) alone without any food presentations. Of particular interest in this experiment are any differences in the rate of lever contact on early trials between subjects which have previously been fed in the experimental chamber and those which have not. If a generalized “expectancy” of food is important in priming the lever to act as a releaser for approach and manipulative activities, then those subjects which have had experience of food delivery in that environment should exhibit a stronger tendency to contact the lever than subjects which have had no experience of food delivery. Method Subjects

The subjects were 30 male hooded Lister rats, experimentally naive and approximately 90 days old at the outset of the experiment. Throughout the experiment (with the exception of the SATIATION groupsee below) all rats were maintained at 80% of their ad-lib body weights. Apparatus

The experimental chambers were purpose-built Skinner boxes marketed by Campden Instruments Ltd., the internal dimensions of which are reported in Davey et al. (1981). Situated in one wall of the chamber was a central reinforcer-tray recess which was 5.0 cm high and 4.0 cm wide. A Perspex flap covered this recess which, when pushed, could be used to record tray entries via a microswitch connected to the top of the flap. Reinforcement was provided in the form of a single 45-mg food pellet

FOOD ANTICIPATION

AND AUTOSHAPING

15

delivered into the food tray and accompanied by a brief flash of the tray light. Situated 3.0 cm to the left of the tray was a retractable lever. When extended, the lever projected 2.2 cm into the chamber, was 3.8 cm wide, and was located 13.5 cm from the ceiling and 4.0 cm from the grid floor. When retracted the lever was flush with the wall of the chamber. The lever took approximately 0.5 set to extend fully into the chamber, and trials were timed from the onset of insertion to the onset of retraction. A small house light situated on the ceiling of the chamber provided general illumination throughout each session. The chambers were housed in sound-attenuating boxes with the front door open to permit observation of the subjects through closed-circuit TV. A closed-circuit TV camera was positioned in front of each box throughout the whole experiment and this relayed TV pictures of the subjects to observers in an adjoining room. The experiment was controlled and data were collected by solidstate logic programming equipment situated in the adjoining room. Procedure The 30 subjects were divided into five groups of 6 subjects and these groups were labelled (i) PAIRED, (ii) UNPAIRED, (iii) FOOD EXPECTANCY, (iv) NO FOOD EXPECTANCY, and (v) SATIATION. The experiment consisted of two parts: (i) magazine training, and (ii) autoshaping testing, and groups differed according to the conditions they experienced in these two phases. (i) PAIRED: Magazine training. Subjects in this group were given four sessions of magazine training during which food pellets were delivered on a variable-time 60-set schedule (this schedule had a maximum interval of 109 set and a minimum interval of 27 set). Each session lasted for approximately 20 min and the lever remained retracted during this phase. Testing: Following magazine training, subjects received five sessions of 20 trials each in which the lever was inserted into the chamber for 10 set immediately prior to each food delivery. Food was delivered on the same schedule as during training. (ii) UNPAIRED: Magazine training. This was identical to that received by the PAIRED group. Testing: These subjects then received five sessions in which lo-set lever insertions and food deliveries were each scheduled on independent VT 60-set schedules. A session lasted for 20 lever insertions and at no time was food delivered during lever insertion or for at least 5 set after lever retraction. (iii) FOOD EXPECTANCY: Magazine training. This was identical to that received by the PAIRED group. Testing: These subjects were given five sessions in which lo-set lever insertions were scheduled according to the VT 60-set schedule. No food deliveries occurred during these five sessions and each session lasted for 20 lever insertions.

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(iv) NO FOOD EXPECTANCY: Magazine training. These subjects were given four sessions during which the food tray light was flashed according to a VT 60-set schedule. No food was delivered during these sessions and a session lasted for approximately 20 min. Testing: This phase was identical to that received by the FOOD EXPECTANCY group. Five sessions consisted of 20 lever presentations and no food deliveries. (v) SATIATION: Magazine training. Like all other subjects during this first phase all subjects were food deprived to 80% of their ad-lib body weight; they were then given magazine training identical to that received by the PAIRED group. Testing: For three days following magazine training and throughout this testing phase, subjects were given free food in the home cage. Testing sessions were identical to those received by the FOOD EXPECTANCY GROUP with the exception that immediately prior to each of the five sessions, all SATIATION subjects were given 30-min access to 45mg food pellets. Immediately following each session they were given a further 30-min access to 45mg food pellets and the amount they consumed was recorded. All subjects in the UNPAIRED group were also given 30-min access to 45mg food pellets after each test session as a food-deprived comparison.

Observational

Procedures

During the five sessions of autoshaping testing for each group, the lever-directed responses of subjects during lever insertions were observed and analyzed according to preselected topographic categories. These categories were defined as follows: orienting: a rapid movement of the head toward the lever without necessarily contacting the lever: sn$jng: moving the nose around the lever with movement of the vibrissae characteristic of sniffing an object; mouthing: touching the lever with the mouth and making small nibbling movements; licking: contacting the lever with the tongue; biting: grasping the lever between the teeth; pawing: placing a paw on the top of the lever or grasping the lever between the paws. Each category was scored on the basis of the percentage trials on which at least one instance of the behavior occurred. The observational data described in the results section represent the observations of one observer, but when two observers independently scored the responses for two sessions there was 89% agreement between them, suggesting that the selected categories were reliable and objectively definable (see also Cleland & Davey, 1982; Davey & Cleland, 1982; Davey et al., 19811. In all statistical analyses a rejection criterion of p c.05 was used. Results Table 1 shows the quantitative data (contacts and tray entries per trial) for all five groups on each daily session of autoshaping testing. For the

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last two sessions the PAIRED group displayed levels of lever contact and food tray entry during lever presentation which were higher than those for the other four groups [F(4, 25) = 20.9591. Furthermore, the SATIATED group consumed significantly fewer food pellets than the UNPAIRED group on the post-session consumption tests [t(8) = 15.01 suggesting that the satiation procedure was effective in reducing appetite for food. For the purpose of analyzing the topographical data, trials were blocked into two groups of 5 for the first 10 trials (because of the desire to scrutinize more closely the effect of magazine training on early lever trials) and from thence onward in lo-trial blocks. Figure 1 shows the mean probability per trial block of the four main lever contact topographies observed (only one instance of licking was observed in all five groups and this behavior was eliminated from further analysis). The bottom right-hand panel of Fig. 1 shows that although Groups PAIRED, UNPAIRED, and NO FOOD EXPECTANCY exhibited lever biting, this was maintained only in the PAIRED group. An examination of the change in maintenance over trial blocks of mouthing, pawing, and sniffing suggested that the probability of one of these responses varied jointly with both group and trial blocks [F(72,675) = 2.1451. Groups PAIRED, UNPAIRED, and FOOD EXPECTANCY all produced similar levels of mouthing, pawing, and sniffing during early trials (Blocks 1A and 1B) which were

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FIG. 1. Probability of lever mouthing, pawing, sniffing, and biting for the testing phase of Experiment 1. These data are presented in successive blocks of 10 trials except for the first 10 trials which are divided into two blocks of 5 (IA = trials l-5, 1B = trials 6-10). (Crosses with broken line = PAIRED; open circles with solid line = UNPAIRED: filled circles with solid line = FOOD EXPECTANCY; crosses with solid line = NO FOOD EXPECTANCY: filled circles with broken line = satiation.)

FOOD ANTICIPATION

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AND AUTOSHAPING

all significantly higher than either the SATIATION or NO FOOD EXPECTANCY groups during this period [Tukey’s Q(5, 75) % 3.97; cf. Hinkle, Wiersma, & Jurs, 1979, p. 2721. Between trials lo-80 (blocks 1B to 8) the likelihood of a response in each category for the FOOD EXPECTANCY group steadily decreased to the NO FOOD EXPECTANCY/SATIATION level, so that only the PAIRED and UNPAIRED groups exhibited significantly higher response levels for mouthing, pawing, and sniffing [Q
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FIG. 2. Probability of lever orienting and a lever-directed response in the testing phase of Experiment 1. These data are presented in successive blocks of 10 trials except for the first 10 trials which are divided into two blocks of 5 (IA = trials I-5, 1B = trials 6-10). (Triangles with broken line = PAIRED; open circles with broken line = UNPAIRED; filled circles with solid line = FOOD EXPECTANCY; triangles with solid line = NO FOOD EXPECTANCY; filled circles with broken line = SATIATION.)

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DAVEYANDCLELAND

lower tendency to orient to the lever than the other four groups [Q(S, 500) > 3.861. However, by Block IB both the SATIATION and UNPAIRED groups had dropped to a level comparable to the NO FOOD EXPECTANCY group [Q(5, 500) + 3.861, whereas the PAIRED and FOOD EXPECTANCY groups were still orienting with a higher probability than the other three groups [Q(5, 500) > 3.861. In the case of general lever-directed responses, for the first five trials (Block 1A) the NO FOOD EXPECTANCY group exhibited a lower probability than the other four groups [Q(5, 500) > 4.6). However, a consistently high level of leverdirected responding was maintained in the PAIRED group, whereas the remaining three groups showed a decreasing trend which approaches the response level of the NO FOOD EXPECTANCY group [Page’s L test for trend, L(8, 6) > 1037; Page, 19631. Comparing all five groups over trials 91-100 (Block 10) showed that the PAIRED group directed significantly more responses to the lever than any of the other four groups. lQ(5, 500) > 4.61, and the unpaired group produced more lever-directed responses in these last 10 trials then the FOOD EXPECTANCY, NO FOOD EXPECTANCY, or SATIATED groups [(2(5, 500) > 4.61. Discussion

These results suggest that (i) initial manipulative or oral and manual contact with a retractable lever by a hungry rat is facilitated if the rat has had previous experience of food delivery in that environment, (ii) continuing to present food in the same environmental context as the lever, although never pairing the lever with food, extends the period of time that the rat will continue to contact the lever, and (iii) food satiation significantly reduces the tendency of the rat to manipulate the lever even though an expectancy of food has previously been established under conditions of food deprivation. Hence, if a rat has previously experienced food in the test environment this is likely to facilitate mouthing, pawing, and sniffing of a retractable lever on the first few presentations of this object, regardless of whether the lever has been paired with food (the autoshaping procedure) or whether or not food continues to be presented after the first lever presentation. Furthermore, when the PAIRED, UNPAIRED, and FOOD EXPECTANCY groups are compared across all 100 test trials, the level of levercontact behaviors in the FOOD EXPECTANCY group (who received no food during testing), falls to the level of the SATIATED and NO FOOD EXPECTANCY groups after only 20 trials. However, the UNPAIRED group took up to 80 trials to fall below the level of the PAIRED group, while after 100 trials the PAIRED group were still exhibiting significantly more lever-directed responses than all other groups (Fig. 2). This suggests that, even though the rat is in a state of food deprivation, lever-directed responses will rapidly decline if food ceases to be delivered.

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Continuing to deliver food-even though these deliveries are not correlated with lever presentation-substantially retards this decline. However, actually pairing lever presentations with food delivery produces the most durable lever-directed activity. EXPERIMENT

2

Experiment 1 demonstrated that, for rats, establishing an expectancy of food in the experimental environment facilitated manipulative contact with a subsequently presented retractable lever. Experiment 2 was designed to investigate the effect of the frequency of prior food delivery on the strength of this manipulative tendency. For instance, the rat’s tendency to contact and manipulate the lever may be a function of the average rate of food delivery computed over some preceding time period. If a direct relationship between rate of preceding food delivery and subsequent tendency to contact the lever exists, then this function would presumably reflect characteristics of the animal’s learned anticipation of food. Four groups of rats were given magazine training with differing rates of food delivery and subsequently given 30 presentations of the retractable lever in extinction. Method Subjects

The subjects were 24 male hooded Lister rats, experimentally naive and approximately 90 days old at the outset of the experiment. Throughout the experiment all rats were maintained at 80% of their ad-lib body weights. Apparatus

The experimental chambers, control apparatus, and observational niques were identical to those used in Experiment 1.

tech-

Procedure

The 24 subjects were divided into four groups of 6 subjects and the groups were labeled (i) VT30, (ii) VT4, (iii) VT1 and (iv) VT0 according to the schedule of food presentation they received during magazine training. Training. Each group received six daily sessions of magazine training, with a session lasting for 1 hr. Group VT30 received food on a variabletime 30-min schedule (minimum interreinforcement time = 5 min, maximum = 45 min); subjects in this group received two single-food-pellet deliveries per hourly session. Group VT4 received food on a variable-time 4-min schedule (minimum interreinforcement time = 60 set, maximum = 6 min 19 set). Group VT1 received food on a variable-time 1-min schedule (minimum interreinforcement time = 27 set, maximum = 1 min 49 set).

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DAVEYANDCLELAND

Group VT0 received no food at all, but were simply placed in the experimental chamber for 1 hr on each of the 6 session days. No retractable lever presentations occurred during magazine training. Testing. On the day following the six sessions of magazine training each subject was given 30 presentations of the retractable lever. Each presentation lasted for 10 set with a mean intertrial interval of 1 min (minimum IT1 = 40 set, maximum IT1 = 80 set). Subjects were observed during this session and their lever-directed behaviors were analyzed according to the same topographical categories as those used in Experiment 1, except that during the testing phase lever contacts were now monitored on a trial-to-trial basis via the drinkometer circuit connected to the lever, and entries into the food tray were recorded by the microswitch connected to a Perspex door covering the entrance to the food tray. The new levercontact response replaced the lever-directed response category used in Experiment 1. No food deliveries were made during this test session. Results Figures 3 and 4 show the probability of a lever contact and tray entry during lever presentation (Fig. 3), and the probability of the four levercontact topographies across trial blocks (Fig. 4). Examination of the change in maintenance of all of these behaviors over trial blocks suggested that the probability of a response covaried with trials blocks and VT group [F(19, 700) = 1.66571. Figure 3 shows that both VT1 and VT4 groups contacted the lever significantly more often during the initial 5 trials than either VT0 or VT30 [Q(4, 700) > 4.41. Group VT30 also contacted the lever significantly more LEVER 1.0

CONTACT

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1

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10 15

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TRAY LEVER

ENTRY DURING PRESENTATION

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FIG. 3. Probability of a lever presentation trial containing a lever contact (left panel) or a tray entry (right panel) in Experiment 2. Data are presented in successive blocks consisting of 5 trials per block. (Triangles with broken line = VT30; triangles with solid line = VT4; open circles with broken line = VTI; filled circles with solid line = VTO.)

FOOD ANTICIPATION MOUTHING 1.0

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FIG. 4. Probability of lever mouthing, pawing, sniffing, and biting in the testing phase of Experiment 2. Data are presented in successive blocks consisting of 5 trials per block. (Triangles with broken lines = VT30; triangles with solid lines = VT4; open circles with broken lines = VTI; filled circles with solid lines = VTO.)

often than the VT0 group during this period [Q(4, 700) > 3.631. After the initial 5 trials both the VT1 and VT4 groups showed a significant decreasing trend in lever-contact probability over succeeding trial blocks [Page’s L(6, 6) > 4861, whereas Group VT30 did not exhibit a decreasing trend across trial blocks [Page’s L(6, 6) < 4741. Following trial 21, contacts were significantly more probable in Group VT30 than in any other group [Q(4, 7009) > 4.41.

Across all trial blocks the probability of a tray entry during lever presentation in the VT0 group was close to zero and the other three groups exhibited significantly more tray entries than the VT0 group [Q(4, 700) > 4.41. Groups VT1 and VT4 showed a clear decreasing trend in tray entry over all trial blocks [Page’s L(6, 6) > 4741, whereas Group VT30 did not exhibit this significant decreasing trend [Page’s L(6, 6) < 4.741, but exhibited a constant intermediate level of tray entry from trial 6 onwards. Figure 4 shows the mean probability across trial blocks of the four lever-contact topographies. Mouthing in the VT1 and VT4 groups during early trials (l-5) was more frequent than in either VT0 or VT30 [Q(4, 700) > 4.41. Throughout succeeding trial blocks mouthing steadily decreased in VTO, VTl, and VT4 groups [Page’s L(6,6) > 4741, but was maintained at an asymptotic level in the VT30 group when by trial block 21-25 the probability of mouthing in the VT30 group was significantly higher than in the three other groups [Q(4, 700) > 3.631. For pawing, VT1 and VT4 groups showed a higher probability than Group VT0 over trials l-5 [Q(4, 700) > 3.631. For Group VT30 pawing remained at an asymptotic level over all trial blocks and over the last three blocks (trials 16-30) was significantly higher than for the other VT groups [Q(4, 700) > 4.41.

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Sniffing was the most frequent lever-directed response and took longer to extinguish than either mouthing or pawing. Groups VT1 and VT4 showed a higher probability of sniffing than VT0 or VT30 over trials l5 [Q(4,700) > 4.41. All groups except VT30 showed a decrease in sniffing across trial presentations [Page’s L(5, 6) > 2991. Figure 5 shows the mean lever contact duration per trial for each group as a function of successive trials. What this figure illustrates is the differential persistence of lever contacting over trials for each group. Group VT1 showed a peak of lever-contact activity around trial 4 and then rapidly extinguished to a low baseline level of contact by around trial 8. Group VT4 exhibited most lever-contact activity around trial 7 and then decreased to a fluctuating lower baseline level by around trial 13. Moreover, Group VT30 did not exhibit an obvious peak in lever-contact activity during early trials but showed a moderate, but fluctuating, tendency to contact the lever across all 30 trials. Group VT0 showed some levercontact activity up to trial 14, but this then approximated zero for the final trials. Discussion The results of this experiment suggest that after experience with relatively high food delivery rates (Groups VT1 and VT4), initial tendency to approach, contact, and manipulate a subsequently presented retractable lever is higher than when either relatively low food delivery rates precede lever presentation (Group VT30) or when no food experience is given (Group VTO). However, over 30 lever presentations, lever-contact activity decreased. rapidly in the high-density food-experienced groups (Groups VT1 and VT4), but persisted at a relatively stable intermediate level in the low-density food-experienced group (Group VT30). These data appear to suggest that the rat’s tendency to contact and manipulate the lever is not simply a function of the average rate of food delivery computed over some preceding time period. While this might determine the vigor of lever contact on initial lever-presentation trials it does not determine the persistence with which the subject will continue to contact the lever. However, a view which could account for both the initial vigor effects and the persistence of lever-directed behaviors is to assume that the tendency to contact the lever is determined by the moment-to-moment conditional probability of food delivery (see Fig. 5). For each of the magazine training schedules the maximum inter-food interval defined by the schedule indicates the amount of time the animal must wait without a food delivery until the probability of food delivery becomes 1.0. Similarly, the minimum inter-food interval determines the time the animal must spend without a food delivery until the probability of food becomes greater than zero. For Groups VT1 and VT4, the lever-

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FIG. 5. Mean lever contact duration for all four groups in Experiment 2 as a function of ordinal trial number (solid line). Broken line represents the conditional probability of food delivery at approximately that point of time into the testing session. (The conditional probability is calculated as p(t) = 1,/I,,,,, where p(t) is the probability of food after f set without food delivery, I, is the number of differing interreinforcement intervals defined by the VT schedule which are 6 r set, and I,,, is the total number of differing interreinforcement intervals defined by the VT schedule.)

contact duration functions in Fig. 5 appear to correspond to generalization gradients with a peak close to the time into the test session when according to the rat’s previous experience the conditional probability of a food delivery should be close to 1.0. For Group VT1 this is 1 min 49 set (i.e., during magazine training, a subject in this group has never spent longer than 1 min 49 set without a food delivery), and for Group VT4 the conditional probability of food delivery is 1.0 after 6 min 19 set without food. The maximum interfood interval for the VT30 group was 45 min and hence, by the end of the test session, the conditional probability of food had still not reached its maximum of 1.0 and therefore one might expect the tendency to contact the lever still to be relatively strong. This

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argument is also consistent with the data when applied to the minimum interfood interval for each group. Groups VT1 and VT4 experienced a minimum interfood interval of 27 set and 60 set, respectively, whereas, for Group VT30 this minimum interval was 5 min. Thus, for the latter group, the conditional probability of food was zero for no-food periods less than 5 min, and indeed, Group VT30 showed a significantly lower tendency to contact the lever on the first five trials (representing approximately the first 5 min of the test session) than Groups VT1 and VT4. In summary, Experiment 2 suggests that the tendency to approach and contact the lever is a function of the local moment-to-moment conditional probability of food delivery. Once this conditional probability has reached a maximum of 1.0, the tendency to contact the lever rapidly declines. This relationship is not a novel one in studies of appetitive conditioning. It has been observed during instrumental conditioning studies with complex variable interval schedules, where local rate of responding is found to be a function of local probability of reinforcement (cf. Catania & Reynolds, 1968, pp. 339-354), and also on fixed-interval schedules where the manipulandum is periodically presented throughout the interval (Dews, 1962; Wall, 1965). EXPERIMENT 3 It was suggested in the introduction that the CS in autoshaping studies may serve two experimentally separable functions: (i) as a signal for the imminent delivery of food, and (ii) as a releaser for species-specific foraging behaviors which involve approach to, and contact with, the CS. One obvious extrapolation from the results of Experiments 1 and 2 is consistent with this supposition: that since the ability of retractable lever presentations to elicit approach and contact behaviors is a function of the probability of food presentation, then stimuli which come to signal increases or decreases in the probability of food should also modulate the tendency to lever contact. Experiment 3 was designed to investigate this possibility by training rats to expect either high or low rates of food delivery during two discriminably different auditory CSs, and subsequently observing the tendency of subjects to contact a retractable lever presented during these stimuli. Method Subjects The subjects were six male hooded Lister rats, experimentally naive and approximately 90 days old at the outset of the experiment. Throughout the experiment all rats were maintained at 80% of their ad-fib body weights.

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Apparatus

The experimental chambers and control apparatus were identical to those used in Experiments 1 and 2, with the additional use of a small 5.5cm diameter speaker which was mounted on the ceiling of the conditioning chamber and used to present either a 70-db IOOO-Hz tone or 70-db IO-Hz clicks which were to be used as auditory stimuli. Procedure

Training

Each subject received 12 daily sessions in each of which they received two presentations of a tone CS and two presentations of a clicker CS. Each CS presentation lasted for 12 min and was separated by an IT1 of 10 min during which white noise of 70 db intensity was presented. For three of the subjects the tone signaled a high density of food (Hi) in which food pellets were delivered on a fixed-time (FT) I-min schedule, and the clicker signaled a low density of food (Lo) during which only two food pellets were delivered during CS presentation, and the time of food presentation during the Lo CS varied from trial to trial with a minimum interval between food deliveries of 1 min (maximum 10 min). For the remaining three subjects the clicker was paired with Hi-density food delivery and the tone with Lo-density food delivery. The order of presentation of tones and clickers during a session was randomized from day to day. Testing. On day 13 each subject received a normal training session with the addition that retractable lever presentations were programmed on a VT 60-set schedule (minimum IT1 = 27 set, maximum IT1 = 1 min 49 set) throughout the session. Each lever presentation lasted for 10 set and no food delivery was permitted during lever presentation or within 5 set of lever retraction. The order of Hi, Lo, and IT1 periods during this test session was randomized across animals with the proviso that two subjects received the Hi CS first, two subjects the Lo CS first, and two subjects the IT1 stimulus first. During the session the probability of contacts with the lever during Hi, Lo, and IT1 periods was recorded. Results Table 2 shows the percentage of lever presentations with a contact during Hi, Lo, and IT1 periods and the mean contacts per presentation during these periods. Although the individual subject variability in tendency to contact the lever was quite high, a significant conditions effect was observed (Freidman’s X,(2) = 6.684), and this revealed a significantly greater number of lever presentations with a contact in the Hi condition than during IT1 (T(6) = 0). Although five out of the six subjects also contacted the lever more frequently during the Hi than the Lo stimulus this was not significant at the 5% level. There was no significant difference between percentage lever contacts during the Lo stimulus and IT1 periods.

28

DAVEYANDCLELAND

TABLE 2 Percentage of Lever Presentations with a Contact and Rate of Lever Contact per Lever Presentation in Experiment 3 % Trials with a contact

Mean contacts/trial

Subject

Hi

Lo

ITI

Hi

Lo

ITI

R326 R327 R328 R329 R330 R331

39 8 11 20 11 20

16 0 31 8 9 3

13 4 7 4 5 8

0.88 0.08 0.11 0.28 0.26 0.23

0.54 0.0 0.63 0.14 0.09 0.03

0.52 0.08 0.28 0.06 0.05 0.13

Mean SE

18.1 24.64

11.1 k4.55

6.8 k 1.40

0.30 -c.ll

0.23 t.11

0.18 2.07

Finally, although the mean number of lever contacts per lever presentation was higher in the Hi and Lo conditions than the IT1 condition, these differences were not statistically significant (X,(2) = 1.826). Discussion The results of Experiment 3 suggest that although the level of lever contacting is relatively low compared with situations where the lever itself is scheduled as a predictor of food delivery, there is still a tendency to contact the lever more often during stimuli signaling relatively high rates of food than during stimuli signaling no food at all. This finding is consistent with the results of Experiment 1. Even though the lever is never paired with food delivery, conditions which still signal food delivery (e.g., UNPAIRED group) will facilitate lever-directed behavior. The finding that animals will tend to approach and contact small localizable stimuli which are presented during signaled periods of high reinforcement density is consistent with a number of other findings. First, Allaway (cited in Williams, 1981) found that even though presentations of a pecking key were redundant with regard to predicting food presentation, moderate to low levels of key pecking developed when the key was illuminated during an auditory signal for food (although no comparisons were made between levels of key pecking during a diffuse auditory CS+ and a CS, we have observed in our laboratory that in a design similar to that of Allaway, key-pecking rates are usually higher during a CSf than a CS). Secondly, when pigeons are given pairings of a diffuse auditory stimulus and food, and this training is followed by subsequent presentations of a compound CS, consisting of simultaneous presentation of the auditory CS and an illuminated pecking key, key pecking frequently develops (Schwartz, 1973; Jenkins, Barnes, & Barrera, 1981). Associative accounts of this procedure would predict that acquisition of key pecking CRs

FOOD

ANTICIPATION

AND

AUTOSHAPING

29

should be “blocked” because food is already adequately predicted by the auditory stimulus prior to compound training (Kamin, 1969). One possible implication of these violations of the blocking phenomenon in autoshaping is that key pecking occurs in such situations for reasons other than the fact that the key light becomes associated with food delivery. Consistent with these results and those from Experiment 3 in the present paper, is that pecking key illumination merely acts as a target (or releaser) for pecking responses whose threshold for occurrence is lowered by the presentation of the auditory stimulus which signals a high probability of food delivery. This possibility could be experimentally tested by subsequently using either the illuminated pecking key or the auditory stimulus to reinforce second-order conditioned responding to a new CS. If the pecking key has acquired associative strength despite the blocking procedure, it should generate conditioned responding to the new CS. If, however, it is simply acting as a releaser for responses which are primed by presentation of the auditory CS it should be incapable of reinforcing CRs to the new CS. Nevertheless, although independent presentation of signals for food would seem to increase the probability of contact with pecking keys and retractable levers, the rates of these directed behaviors are well below the rates observed when the pecking key and retractable levers are themselves the signal for food. Clearly there is some aspect of the leverfood or pecking key-food contingency which fosters high rates of directed action, one possibility being the facilitation of signal-directed action by the conditioned orienting response to the releaser when it also signals food (e.g., Buzsaki, 1982). GENERAL

DISCUSSION

This series of experiments has attempted to discover some of the conditions which will facilitate contact with discrete and periodic presentations of a retractable lever to rat subjects. The main findings are (i) that approach to and manipulation of a discretely presented retractable lever is facilitated if rats have previously experienced food in the experimental environment, and (ii) this tendency to approach and contact the lever appears to be a function of the probability of food delivery as determined by the animal’s previous experience with the temporal distributions of food delivery (Experiment 2) or by the presentation of exteroceptive stimuli which have acquired food-predicting significance through their pairing with food (Experiment 3). These findings have several implications for performance models of signal-centered behavior, but first it is worth discussing them in relation to more traditional behavioral processes.

30

DAVEY

AND

CLELAND

Sensitization Presenting the UCS (food) prior to presentations of the target stimulus (retractable lever) might simply represent an instance of “state” sensitization (Groves & Thompson, 1970). That is, the prior presentation of food in Experiments 1 and 2 may have aroused a nonspecific motivational state which either lowered the threshold for responding to ambient or novel stimuli or increased exploration of the environment. The possibility that prior feeding experience simply had its effect by activating a nonspecific arousal state is unlikely. The main reason for believing this is that density of prior food presentation did not bear a simple relationship to subsequent lever-contact probability (Experiment 2). In fact, persistence of levercontact behavior was an inverse function of the number of preceding food presentations; this is the opposite of the relationship generally found in sensitization studies (e.g., Fantino & Logan, 1979, p. 56; Groves, Lee, & Thompson, 1969; Thompson, Groves, Teyler, & Roemer, 1973). Interim or Adjunctive Behavior It is possible that the lever contact behaviors observed in this study might represent instances of that class of responses known as adjunctive behavior, a phenomenon which is frequently generated by schedules of food delivery (e.g., Falk, 1971; Staddon & Simmelhag, 1971). The findings from Experiments 2 and 3 make this interpretation unlikely: In both experiments the highest levels of lever-contact behavior were recorded at times when food probability was highest. This is in strict contrast with the temporal location of adjunctive behaviors, which are usually observed at times of lowest food probability. Frustration Amsel (1958, 1962) has suggested that when an expected reinforcer is unexpectedly omitted, a state of “frustration” is engendered which energizes any ongoing behavior. In the present experiments it may have been the case that lever-directed behavior was the result of “frustration” at the nondelivery of food at times when food was expected. This might have been so in Experiment 2 and in the FOOD EXPECTANCY group in Experiment 1. However, frustration cannot explain the high levercontact levels in groups PAIRED and UNPAIRED in Experiment 1 nor to the high-density food signal in Experiment 3. In these cases food was still being delivered at times when it was expected and so frustration should not be elicited in these situations. Since the lever-directed response topographies were markedly similar across all experiments in this study, and bore a striking resemblance to the investigative foraging behaviors observed in free-living rats (Barnett, 1956; Ewer, 1971) it would seem

FOOD ANTICIPATION

to be more parsimonious explain these results.

31

AND AUTOSHAPING

to search for a unitary appetitive

process to

Classical Conditioning of General Activity Levels

Some theorists have claimed that a motivational state becomes classically conditioned to reinforcement-paired stimuli and that this conditioned motivational state or “excitement” then energizes motor activity (Bindra & Palfai, 1967; Sheffield, 1966; Zamble, 1967). In the present study, then, those stimuli (either temporal or exteroceptive) which enabled the animal to anticipate food delivery may also have elicited a conditioned arousal state which increased the probability of contact with the retractable lever. However, this kind of theory has a particular inherent difficulty. For instance, data suggesting that appetitive Pavlovian CSs elicit motivational states which energize activity in a nonspecific way (e.g., by increasing the count of an activity meter) may simply reflect the operation of a specific, but unobserved, behavioral process. For example, it is well known that presentation of an appetitive Pavlovian CS will generally increase the vigor of ongoing instrumental responding (cf. Konorski, 1967; Rescorla & Solomon, 1967). but closer examination of this apparent “motivational” effect reveals an interaction between the location of the instrumental manipulandum and the location and localizability of the Pavlovian CS. If the CS location is identical to the location of the instrumental manipulandum, response facilitation will occur (e.g., LoLordo, McMillan, & Riley, 1974); if the CS is located some distance from the manipulandum, then response suppression occurs (e.g., Karpicke, Christoph, Peterson, & Hearst, 1977; Smith, 1974). Results such as these suggest that what was originally conceived of as a “motivational” phenomenon can be explained, at least in part, in terms of the interaction between two specific processes: instrumental responding and signal-directed behavior (cf. Davey, 1981, pp. 229-230; Karpicke et al., 1977). This raises a chicken and egg problem with regard to the data from the present study. While the results are consistent with an explanation in terms of a conditioned motivational state which increases general activity, such increase in general activity levels during stimuli predicting food may simply reflect the effect of these stimuli on species-specific search patterns and the rate of approach to and contact with discrete aspects of the environment while foraging for food. Topographically such behavior has the prima facie appearance of being nonspecific, but collectively it may represent an investigative foraging process with specific behavioral and functional attributes. Behavior Systems, Foraging Behavior, and Pavlovian Contingencies

In the introduction to this paper it was noted that ethologically oriented views of signal-directed responding emphasize two separably identifiable

32

DAVEYANDCLELAND

functions of the Pavlovian CS: (i) as a signal for a relatively high rate of food delivery, and (ii) as a natural releaser for species-specific foraging behaviors (Cleland & Davey, 1982; Davey & Cleland, 1983; Timberlake, 1983). Animals such as rats and pigeons will approach and investigate small objects when searching for food (Barnett, 1956; Ewer, 1971; Murton, 1971; Murton, Isaacson, & Westwood, 1963) and the localizable CSs used in autoshaping studies are tailor-made to elicit and support these behaviors. The results of the present study can be integrated with this account by suggesting that, in the rat at least, the strength of the appetitive activities released by a small manipulable object, such as a retractable lever, is a function of (i) the animal’s internal state (level of food deprivation), and (ii) the animal’s anticipation of food at that point in time. as embodied by learning of the temporal distribution of food or the significance of exteroceptive stimuli which predict food delivery. Hence in a simple autoshaping procedure this model implies that animals approach and contact the CS because they learn to anticipate food during CS presentation and this increases the likelihood of natural releasers, such as the CS itself, activating components of the animal’s feeding behavior system. Those modules of the phylogenetically pre-organized behavior system which are activated by stimuli such as retractable levers and illuminated pecking keys include approach and contact components (Timberlake, 1983) which characterize signal-directed behavior in rats and pigeons. In free-living animals such behaviors serve the function of bringing the animal into contact with food and identifying objects as edible or inedible. The fact that the food-signaling and behavior releasing properties of an appetitive CS are separable has a number of implications for views of signal-directed behavior. It implies that accounts of signal-directed behavior which appeal to the evolutionary benefits of approaching stimuli which signal food (e.g., Buzsaki, 1981; Hollis, 1982; Wasserman, 1981) are incomplete in that animals do not appear to possess an innate appetitive tracking reaction to food signals per se, but specific stimuli appear to have particular relevance to particular species. For instance, (a) rats will not approach a localizable auditory CS for food (Cleland & Davey, 1983; Harrison, 1979); (b) nonsocial feeders such as hamsters will not approach a conspecific CS for food (Timberlake, 1983); and (c) members of the crow family, Corvus bruchyrhynchos, will not contact an illuminated pecking key used as a CS for food (Powell et al., 1975). A behavior systems approach to these data would suggest that this is because in these cases the CS does not act as a releaser for appetitive approach behaviors: Since, (i) even when preying, free-living rats rarely detect food on the basis of auditory cues (cf. Ewer, 1971), (ii) hamsters are by nature solitary feeders, suggesting that they lack the social conspecific

FOOD ANTICIPATION

AND AUTOSHAPING

33

approach components to their feeding repertoire, and (iii) the crow is an opportunistic, omnivorous feeder which spends most of its time searching for large food items rather than pecking at small punctate visual stimuli as would the pigeon. Hence it is not the fact that a stimulus is an appetitive Pavlovian CS that engenders CS approach, but that the CS is also a releaser for species-specific appetitive activities which include approach and manipulate components. REFERENCES Amsel, A. The role of frustrative nonreward in continuous reward situations. Psychological Bufletin, 1958, 55, 102-119. Amsel, A. Frustrative nonreward in partial reinforcement and discrimination learning. Psychological Review, 1962, 69, 301-328. Atnip. G. W. Stimulus- and response-reinforcer contingencies in autoshaping, operant, classical and omission training procedures in rats. Journal of the Experimental Analysis of Behavior, 1977, 28, 59-69. Baerends, G. P. On drive, conflict, and instinct, and the functional organization of behavior. In M. A. Corner & D. F. Swaab (Eds.), Perspectives in Brain Research, 1976, 45, 427-447. Bamett, S. A. Behaviour components in the feeding of wild and laboratory rats. Befiavfartr, 1956, 9, 24-43. Bindra, A., & Palfai, T. Nature of positive and negative incentive motivational effects on general activity. Journal of Comparative and Physiological Psychology, 1967.63, 288297. Boakes, R. A. Performance of learning to associate a stimulus with positive reinforcement. In H. Davies & H. M. B. Hurwitz (Eds.), Operant-Pavlovian interactions. New York: Erlbaum, 1977. Boakes, R. A., Poli, M., Lockwood, M. J., & Goodall, G. A study of misbehavior: Token reinforcement in the rat. Journal of the Experimental Analysis of Behavior, 1978, 29, 115-134. Brown, P. L., & Jenkins, H. M. Auto-shaping of the pigeon’s key-peck. Journal of the Experimental

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analysis of fixed-interval discrimination. Journal of Comparatitv 1965, 60, 70-75. Wasserman, E. A. Response evocation in autoshaping: Contributions of cognitive and comparative-evolutionary analyses to an understanding of directed action. In C. M. Locurto, H. S. Terrace, & J. Gibbon (Eds.), Auroshaping and conditioning theory. New York: Academic Press, 1981. and Physiological

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Received June 8, 1983 Revised September 16, 1983

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