the effect of early feeding experience on signal-directed response topography in the rat

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Physhdogy & Behavior, Vol. 32, pp. 11-15. Copyright © PergamonPress Ltd., 1984.Printedin the U.S.A. 0031-9384/84$3.00 + .00 The Effect of Early Feeding Experience on Signal-Directed Response Topography in the Rat GRAHAM C. L. DAVEY, 1 GARY G. CLELAND, 2 DAVID A. OAKLEY 3 AND JANET L. JACOBS 3 The City University, London UK Received 24 December 1982 DAVEY, G. C. L., G. G. CLELAND, D. A. OAKLEY AND J. L. JACOBS. The effect of early feeding experience on signal-directed response topography in the rat. PHYSIOL BEHAV 32(1) 11-15, 1984.--Ten Hooded Lister rats were divided from weaning into two groups of five. One group was fed exclusively on a liquid diet in the home cage (Complan-fed subjects), the other group was given normal laboratory chow (Chow-fed subjects). At approximately 60 days of age both groups were reduced to 80% of their ad lib body-weight and given pairings of a retractable lever (conditioned stimulus, CS) and response-independent food (unconditioned stimulus, UCS). In the first experimental condition a liquid UCS was used (condensed milk solution) and in later conditions this was substituted with a solid UCS (45 mg food pellet). Analysis of CS-directed behavior in the two groups suggested that (1) only very early in autoshaping training did feeding experience in the home cage influence the topography of signal-directed behavior, and subsequently (2) both Complan-fed and chow-fed subjects bit rather than licked the CS paired with a solid UCS and licked rather than bit the CS when it was paired with a liquid UCS. These results suggest that, in the long-term, the topography of signal-centered behavior in rats is more likely to be influenced by the nature of the reinforcer signalled by the CS than by the subject's feeding experiences during early development. Autoshaping Feeding experience Rats WHEN a localizable conditioned stimulus (CS) is paired with an appetitive reinforcer (unconditioned stimulus, UCS), animals will often approach and contact the CS. This phe- nomenon has variously been called autoshaping [3], sign- tracking [ 11], and signal-centered or signal-directed behavior (e.g., [13]). In particular, animals will contact the CS with a variety of appetitive responses relevant to the reinforcing UCS [6, 9, 14, 16]. Recent explanatory accounts of signal- directed responding have alluded to the pairing of CS and UCS acting to "conditionally release" phylogenetically pre- organized appetitive response patterns. In some accounts the response patterns released depend on the nature of the CS itself and whether that CS resembles a natural releaser for the response pattern (e.g., [16]). In other accounts it is claimed that the responses directed at the CS are released by the reinforcer itself (e.g. [20]). or more specifically by the memory of the UCS evoked by CS presentation (e.g., [5, 6, 19]). There is evidence to suggest that both of these accounts are to some extent true: the nature of signal-directed re- sponding does depend on the characteristics of the CS [5, 16, 17, 18], and also on the characteristics of the UCS [6,9]. Nevertheless, both versions of the "conditioned release" view stress the biologically preorganized nature of appetitive behavior patterns that are released by signals for imminent reinforcer delivery. One question raised by this emphasis on phylogenetic preorganization is how rigidily "pre-wired" the animal's reaction is to releasers in the autoshaping paradigm, and how readily these reactions can be modified by the animal's previous experience with food. For instance, very early in training on an autoshaping procedure, rats will con- tact a retractable lever CS in ways which are characteristic of general investigative foraging (cf., [1,10]), i.e., by pawing, sniffing and mouthing the CS. The nature of CS contact dur- ing these early sessions appears to be largely independent of the nature of the UCS in the autoshaping procedure and seems to reflect an expectancy of food in general rather than associative factors relating the CS to a specific food type [7]. The present study is designed to investigate how a rat's previous experience with food types affects the nature of subsequently acquired signal-centered behavior in an au- toshaping procedure. In particular, does a rat's exclusive experience with a particular food type (either liquid or solid) in the home cage influence the nature of food-related signal-directed responding early in autoshaping training? Secondly, given that a rat's experience at consuming food in the home cage can be largely restricted to a single basic topography (e.g., licking and swallowing) does that subject subsequently exhibit signal-directed behaviors which would ~Requests for reprints should be addressed to Graham Davey, Ph.D., Department of Social Science and Humanities, The City University, Northampton Square, London EC1V OHB. 2Supported by a bursary from the Wellcome Trust. 3Presently at University College, London. 11

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Physhdogy & Behavior, Vol. 32, pp. 11-15. Copyright © Pergamon Press Ltd., 1984. Printed in the U.S.A. 0031-9384/84 $3.00 + .00

The Effect of Early Feeding Experience on Signal-Directed Response Topography

in the Rat

G R A H A M C. L. D A V E Y , 1 G A R Y G. C L E L A N D , 2 D A V I D A. O A K L E Y 3 A N D J A N E T L. J A C O B S 3

The City Univers i ty , L o n d o n U K

Rece ived 24 D e c e m b e r 1982

DAVEY, G. C. L., G. G. CLELAND, D. A. OAKLEY AND J. L. JACOBS. The effect of early feeding experience on signal-directed response topography in the rat. PHYSIOL BEHAV 32(1) 11-15, 1984.--Ten Hooded Lister rats were divided from weaning into two groups of five. One group was fed exclusively on a liquid diet in the home cage (Complan-fed subjects), the other group was given normal laboratory chow (Chow-fed subjects). At approximately 60 days of age both groups were reduced to 80% of their ad lib body-weight and given pairings of a retractable lever (conditioned stimulus, CS) and response-independent food (unconditioned stimulus, UCS). In the first experimental condition a liquid UCS was used (condensed milk solution) and in later conditions this was substituted with a solid UCS (45 mg food pellet). Analysis of C S-directed behavior in the two groups suggested that (1) only very early in autoshaping training did feeding experience in the home cage influence the topography of signal-directed behavior, and subsequently (2) both Complan-fed and chow-fed subjects bit rather than licked the CS paired with a solid UCS and licked rather than bit the CS when it was paired with a liquid UCS. These results suggest that, in the long-term, the topography of signal-centered behavior in rats is more likely to be influenced by the nature of the reinforcer signalled by the CS than by the subject's feeding experiences during early development.

Autoshaping Feeding experience Rats

WHEN a localizable conditioned stimulus (CS) is paired with an appetitive reinforcer (unconditioned stimulus, UCS), animals will often approach and contact the CS. This phe- nomenon has variously been called autoshaping [3], sign- tracking [ 11], and signal-centered or signal-directed behavior (e.g., [13]). In particular, animals will contact the CS with a variety of appetitive responses relevant to the reinforcing UCS [6, 9, 14, 16]. Recent explanatory accounts of signal- directed responding have alluded to the pairing of CS and UCS acting to "conditionally release" phylogenetically pre- organized appetitive response patterns. In some accounts the response patterns released depend on the nature of the CS itself and whether that CS resembles a natural releaser for the response pattern (e.g., [16]). In other accounts it is claimed that the responses directed at the CS are released by the reinforcer itself (e.g. [20]). or more specifically by the memory of the UCS evoked by CS presentation (e.g., [5, 6, 19]). There is evidence to suggest that both of these accounts are to some extent true: the nature of signal-directed re- sponding does depend on the characteristics of the CS [5, 16, 17, 18], and also on the characteristics of the UCS [6,9]. Nevertheless, both versions of the "conditioned release" view stress the biologically preorganized nature of appetitive behavior patterns that are released by signals for imminent

reinforcer delivery. One question raised by this emphasis on phylogenetic preorganization is how rigidily "pre-wired" the animal's reaction is to releasers in the autoshaping paradigm, and how readily these reactions can be modified by the animal's previous experience with food. For instance, very early in training on an autoshaping procedure, rats will con- tact a retractable lever CS in ways which are characteristic of general investigative foraging (cf., [1,10]), i.e., by pawing, sniffing and mouthing the CS. The nature of CS contact dur- ing these early sessions appears to be largely independent of the nature of the UCS in the autoshaping procedure and seems to reflect an expectancy of food in general rather than associative factors relating the CS to a specific food type [7].

The present study is designed to investigate how a rat 's previous experience with food types affects the nature of subsequently acquired signal-centered behavior in an au- toshaping procedure. In particular, does a rat 's exclusive experience with a particular food type (either liquid or solid) in the home cage influence the nature of food-related signal-directed responding early in autoshaping training? Secondly, given that a rat's experience at consuming food in the home cage can be largely restricted to a single basic topography (e.g., licking and swallowing) does that subject subsequently exhibit signal-directed behaviors which would

~Requests for reprints should be addressed to Graham Davey, Ph.D., Department of Social Science and Humanities, The City University, Northampton Square, London EC1V OHB.

2Supported by a bursary from the Wellcome Trust. 3Presently at University College, London.

11

12 I)AVI'/Y t',1 ,41

normally be characteristic of consummatory behavior under unrestricted feeding circumstances (e.g., biting and gnaw- ing)? Thirdly, does this exclusive experience with either liq- uid or solid food in the home cage influence the nature of CS contact when either solid or liquid reinforcers are used dur- ing autoshaping training'?

METHOD

Subjects

The subjects were 10 experimentally naive male Hooded Lister rats which were taken from six litters of laboratory bred rats. At approximately 10 days post-partum all females and any excess males were culled from the litters leaving no more than 6 male pups per litter. From day 15 post-partum, when pups were starting to become active, all mothers were put on a liquid diet (described below) to ensure that pups did not begin to consume solid food before weaning. All pups were weaned at around 21 days. Using a split-litter design with no more than 2 pups from each litter, ten subjects were selected from the weaned pups and divided into two groups of five. One group of subjects received a liquid-only Complan-based diet in their home cages (Complan-fed sub- jects) and the remaining group were given standard labora- tory chow (Chow-fed subjects) plus a water bottle in their home cages. All subjects were housed individually in cages with bottom grids and a tray beneath. For the Complan-fed subjects this ensured that all spillage of Complan solution fell through the grid floor to the tray beneath rather than solidify- ing within the animals' reach. The liquid diet consisted of 275 g of strawberry or butterscotch flavored Complan (a pow- dered, complete food substitute marketed by Farley Health Products, UK and containing 4625 Kj of energy per 250 g dissolved in 400 ml of water). To this solution was added one egg, 10 ml of Vi-Daylin vitamin concentrate and additional water to produce a total stock volume of 600 ml. Prior to the introduction of the food deprivation regime each Complan- fed subject was given 120 ml of Complan solution daily. At approximately 60 days of age access to the Complan solution and chow pellets was limited in order to maintain the body- weights of all subjects at 80% of free-feeding weight. The experiment proper commenced when the weight of all ten subjects had reached this 80% level.

Apparatus

The experimental chambers were purpose-built rat con- ditioning chambers marketed by Campden Instruments Ltd. and whose internal dimensions are listed elsewhere [8].

Situated in one wall of the chamber was a central food- tray recess which was 5.0 cm high and 4.0 cm wide. A perspex flap covered this recess which, when pushed, re- corded tray entries via a microswitch hinged at the top of the flap. Reinforcement could be provided either in the form of a single 45 mg food pellet delivered into the food tray or 0.8 ml of 25% solution of Nestlr 's condensed milk delivered by a dipper mechanism to the floor of the tray. In its normal rest- ing position the dipper was flush with the floor of the tray. All reinforcer deliveries were accompanied by a brief flash of the food-tray light. Situated 3.0 cm to either side of the food tray were two retractable levers. When extended, the levers projected 2.2 cm into the chamber, were 3.8 cm wide and were located 13.5 cm from the ceiling and 4.0 cm from the

grid floor. When retracted, they were flush with the wall of the chamber, and, when extended into the chamber, lever contacts could be recorded by means of drinkometer circuits connected to each lever.

A small houselight 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 subjecls. Ob- servations were made using closed-circuit TV which was relayed to observers in an adjoining room. In the experimental room white noise was used to mask extraneous sounds. The experiment was controlled and data collected by solid-state logic programming equipment situated in an adjoining room.

Prot.t~dure

The experiment consisted of six phases. I. Adaptation. On the first day each subject was placed

in the experimental chamber for 20 minutes. The food tray door was propped open and the dipper was filled with condensed-milk solution.

2. Magazine training. For the next 5 days (sessions 1-5) each subject received magazine training in which the liquid reinforcer was delivered on a variable-time (VT) 100 sec schedule. At the end of this phase subjects were seen to be taking the reinforcer as soon as it was delivered.

3. Autoshaping: liquid UCS I. For the next l0 sessions (session 6-15) all subjects received deliveries of the liquid UCS immediately following a 10-see insertion of the left lever into chamber. Lever insertions were programmed on a VT 100 sec schedule and a session consisted of 20 lever-food pairings.

4. Autoshaping: solid UCS 1. For the following l(J ses- sions (session 16-25) all subjects received further presenta- tions of the left lever, but during this phase they were im- mediately followed by delivery of a solid 45 mg food pellet. Lever insertions were programmed in the same way as for phase 3.

5. Autoshaping: liquid UCS 2. For the following 10 ses- sions (sessions 26-35) all subjects were returned to pairings of the left lever with the liquid reinforcer, as in phase 3.

6. DifJ~,rential reversal: solid UCS 2. For the final 8 ses- sions (sessions 36-43) all subjects received solid pellet UCSs. These were each preceded by a 10-see insertion into the chamber of the right lever (CS +) on a VT 100-see schedule. In addition, the previously food-paired left lever was inserted on a similar, but independent VT 100-see schedule and was never followed by food (CS°). At no time were both levers present in the chamber simultaneously.

Before switching from one phase to another care was al- ways taken to ensure that the food tray was thoroughly cleaned and bore no traces of the UCS used in the previous phase.

Observation procedures. Observation of lever-directed behaviors were taken during the whole of sessions 3, 8 and 10 of phase 3 (liquid UCS 1), sessions 3, 8 and 10 of phase 4 (solid UCS 1), sessions 4, 8 and 10 of phase 5 (liquid UCS 2) and session 8 of phase 6 (Differential reversal, solid UCS 2). During these observation sessions lever-directed behaviors were analysed according to a number of pre-selected topo- graphic categories. These categories were: licking: contact- ing the lever with the tongue; biting: grasping the lever be- tween the teeth; mouthing: touching the lever with the mouth and making small nibbling movements; pawing: con- tacting the lever with one or more paws; stuffing: moving the

SIGNAL-DIRECTED RESPONSE TOPOGRAPHY 13

TABLE 1

GROUP DATA FOR COMPLAN-FED AND CHOW-FED RATS (BLOCKED OVER THE LAST 3 SESSIONS OF EACH CONDITION)

Solid UCS 2 Liquid UCS 1 Solid UCS 1 Liquid UCS '2

Complan Chow Response Complan Chow Complan Chow Complan Chow CS + CS ° CS + CS ° Measured

Mean 4.0 5.5 5.4 6.3 5.3 6.5 8.3 3.3 11.9 3.8 Contacts _+SEM _+0.5 _+ 1.5 _+ 1.5 _+0.2 _+0.6 _+0.3 ± 1.6 _+0.6 ± 1.5 ÷ 1.0 per trial

Mean 2.9 3.2 2.0 1.5 2.4 2.2 1.1 0.8 1.3 1.3 Tray Entries ±SEM _+0.3 _+0.5 _+0.4 _+0.4 _+0.3 _+0.4 _+0.3 _+0.1 _+0.2 _+0.2 per trial

Mean 0.5 0.4 0.9 0.8 0.7 0.7 0.8 1.0 0.6 0.7 Duration (secs) _+SEM _+0.1 ±0.1 ±0.1 +0.1 _+0.1 _+0.1 _+0.0 ±0.1 +0.1 _+0.1 per contact

Mean 2.0 2.4 4.6 5.1 3.6 4.5 6.4 3.3 6.6 2.3 Duration (secs) _+SEM _+0.5 _+0.9 _+0.2 _+0.7 _+0.3 _+0.8 _+1.1 _+0.2 -+0.6 _+0.4 per trial

Mean (and SEM) CS contact and tray-entry rates, and CS duration per contact and trial are shown.

nose around the lever with movements of the vibrissae char- acteristic of sniffing an object. Each category was scored in terms of the percentage trials on which at least one instance of the behavior occurred. Two observers independently scored the responses for the first recorded session with 81% agreement between them, suggesting that the selected categories were reliable and objectively definable (see [9]).

When all the observation sessions had been scored, a profile was built up of chow-fed and complan-fed subjects ' CS-directed responses under conditions of liquid or solid reinforcement.

RESULTS

The mean body-weights of both Chow-fed and Complan- fed rats were compared before, during and after the experi- ment. There was no significant difference between the weights of the two groups over all conditions, F(1,8)=0.116, p>0.05 and similarly there was no evidence for any weight change across conditions, F(2,16) =0.107, p >0.05.

Table 1 summarizes all the quantitative CS-contact data for the Complan-fed and Chow-fed rats during each of the three autoshaping phases and the differential reversal phase of the experiment. Tray entries, contact duration and number of trials were collected separately for each experi- mental session. Mean response rates per trial were obtained for tray entry, contact, and duration of contact measures. In addition, a mean duration of individual contacts was also cal- culated and was referred to as the mean individual contact duration.

None of the between groups comparisons (Chow vs. Complan) were significant over the last 3 sessions of each UCS phase on any of the dependent measures, F(9.96)=0.577, p>0.05. Comparisons within groups how- ever showed that both Complan-fed and Chow-fed subjects spent more time contacting the lever per trial when it was paired with a solid UCS, F(9,96)=11.526; Tukey Q(2,96)=17.46, p<0.01, (see Table l , Row 4) and had a higher contact rate per trial (i.e., made more contacts (see Table l, Row l) on the solid-paired lever, Q(2,96)=19.27, p <0.01. In addition, the mean individual contact durations for both Complan-fed and Chow-fed subjects were longer during solid UCS-paired lever presentations, than liquid

UCS-paired presentations, Q(2,96)=17.39, p<0.01, (see Table 1, Row 3).

During the differential reversal phase of the experiment all subjects from both groups switched from contacting the left lever to contacting the right lever (Complan-fed t(3)=8.7009, p<0.005; Chow-fed t(3)=2.475, p<0.05, 1-tail), suggesting that lever-directed responding was controlled by its pairing with food.

Table 2 summarizes the data on signal-directed response topographies for the first observation session of each UCS phase. The earliest observations (session 3, liquid UCS !) showed that both Complan-fed and Chow-fed rats licked the CS predicting the liquid UCS. However the Complan-fed rats licked on significantly more trials than the Chow-fed sub- jects, F(8,288)=4.24, p<0.05; Q(2,288)=4.237, p<0.05. Al- though Chow-fed rats bit the CS on a greater number of trials than the Complan-fed rats in session 3, this difference was not quite significant at the 5% level, Q(2,288)=1.614, 0.05<p<0.1. Two out of the five Complan-fed rats bit the CS in this first observation session. There were no significant differences between groups on the level of CS mouthing, pawing and sniffing. However, later observations made on sessions 8 and 10 during the liquid UCS 1 phase (not included in the table) showed no significant differences between groups in the level of any of the behavioral topographies.

On the first observation session of the solid UCS 1 phase (see Table 2) neither Chow-fed nor Complan-fed subjects exhibited CS-directed licking, but both groups of subjects bit the CS. The only significant difference between groups in this phase was in the level of CS biting during observation session 8: Complan-fed rats bit the lever more often than Chow-fed rats, Q(2,288)=4.035, p<0.05.

Figure 1 shows the asymptotic topography data for both groups in relation to solid and liquid reinforcers (means are calculated from the final observation session of both liquid UCS phases (S10) and both solid UCS phase; (SI0 and $8). Both Complan-fed and Chow-fed rats directed more licking than biting at the CS paired with the liquid UCS and con- versely more biting than licking at the CS when it was paired with a solid UCS, F(16,288)=2.401, 0.05<p<0.1. Levels of mouthing, pawing and sniffing did not vary significantly with either UCS type or the homecage feeding regime of the sub- ject.

14 I )AVEY/: ) A I

TABLE 2 GROUP OBSERVATION DATA SHOWING MEAN (AND SEM) PERCENTAGE OF TRIALS DURING WHICH EACH RESPONSE TOPOGRAPHY OCCURRED FOR THE FIRST OBSERVATION SESSION

OF EACH OF THE FIRST 3 EXPERIMENTAL CONDFFIONS

Liquid UCS I Solid UCS I Liquid LiCS .-"

CO CH CO CH CO CH $3 $3 $3 $3 $4 $4 Behaviors

Mean 34 13 0.0 0.0 6.6 0.0 Licking +SEM +7.0 +4. I ~0.0 ~_- 0.0 +2.9 ±0.0

Mean 3.0 11.0 19.8 10.2 21.0 4.2 Biting ±SEM ±2.0 ±4.0 ±3.7 ±4.5 +2.4 +1.7

Mean 83.0 58.0 98.6 100.0 89.2 82.6 Mouthing ±SEM +_4.6 + 15.6 + 1.4 +0.0 +5.7 ~ 6. I

Mean 94.0 65.0 98.6 100.0 95.8 97.2 Pawing +SEM ±4.8 ± 18.6 ± 1.4 +0.0 ±2.8 :~: 1.7

Mean 93.0 63.0 98.6 100.0 91.6 100.0 Sniffing +SEM +4.9 + 17.6 ± 1.4 .~_0.0 +2.6 ~0.0

CO=Complan-fed group, CH=Chow-fed group.

DISCUSSION

The results from the present study strongly indicate that, while there is a mild effect of home-cage feeding regime on signal-directed response topography early in training, in gen- eral, signal-directed behavior is more likely to be influenced by the nature of the reinforcer in the autoshaping situation than by the nature of home-cage food or exclusive experience with specific food types during early development. From session 3 of the experiment onwards both groups of subjects exhibited signal-centered responding which reflected the al- ready documented influence of UCS type on response topog- raphy [6,9]. While both groups exhibited a tendency to bite rather than lick solid paired CSs and lick rather than bite liquid-paired CSs, they also exhibited longer duration CS contacts with a solid reinforcer than with a liquid reinforcer. This latter result is consistent with the findings of Hull [12] who found longer duration instrumental lever contacts with a solid rather than liquid reinforcer. Taken in conjunction with the topography data from the present experiment it would seem that this differential contact duration effect reflects different CS-contact topographies determined by reinforcer type.

The relative rapidity with which Complan-fed rats ac- quired signal-directed biting to the solid-paired CS does suggest that this activity existed in a relatively organized form prior to conditioning, even though these animals had no previous experience at biting food. This might suggest (1) that the behavior is a phylogenetically preorganized re- sponse to oral cues associated with solid food (cf. [4, 19, 20]) which requires little practice prior to initial experience with solid food, or (2) that biting has been established for a func- tion other than feeding and that this response can readily be transferred to serve other functions. For example, Bolles and Wood [2] report that while nibbling and chewing solid food does not occur until around 16 days of age in the young rat, pups of only 2-3 days of age will chew and mouth their own forepaws. The results of the present experiment could be interpreted to suggest that a response such as biting, which in the Complan-fed rats was observed to have been established to serve functions such as grooming, can readily

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LICKING BITING MOUTHING PAWING FIG. 1. Percentage of trials during which each response topography was directed at signals for liquid (LIQ) and solid (SOL) reinforcers. Open bars=Complan-fed subjects. Hatched bars=Chow-fed sub- jects. Data are compounded from the last observation session of liquid UCS 1, solid UCS 2, liquid UCS 2 and Differential Reversal, solid UCS 2.

be transferred to function as a feeding response, and fur- thermore, to become a Pavlovian conditioned response when the appropriate food-type is used as a reinforcer.

Finally, the results of the present experiment indicate that signal-directed behavior is not solely characteristic of gen- eralized species-specific foraging behaviors as some recent accounts of signal-centered behavior have suggested (e.g. [13,16]). Signal-centered topography is partly determined by the nature of the specific food UCS used in conditioning-- even when this food type is quite different from the food type experienced in the animal's home-cage. That is, signal- centered response topography does not necessarily repre- sent a transfer of home-cage feeding responses to the exper- imental situation and while these results do not imply that the biting response is pre-wired (if, indeed, that could ever be experimentally proven) they do suggest that certain novel consummatory activities which are not reflected in the animal's previous feeding experiences can be acquired with some rapidity.

S I G N A L - D I R E C T E D R E S P O N S E T O P O G R A P H Y 15

REFERENCES

I. Barnett, S. A. Behaviour components in the feeding of wild and laboratory rats. Behaviour 9: 24--43, 1956.

2. Bolles, R. C. and P. J. Woods. The ontogeny of behaviour in the albino rat. Anita Behav 12: 427--441, 1963.

3. Brown, P. L. and H. M. Jenkins. Autoshaping of the pigeon's key-peck. J Exp Anal Behav 11: 1-8, 1968.

4. Cleland, G. G. and G. C. L. Davey. The effects of satiation and reinforcer devaluation on signal-centered behavior in the rat. Learn Motiv 13: 343-360, 1982.

5. Cleland, G. G. and G. C. L. Davey. Autoshaping in the rat: the effects of localizable visual and auditory signals for food. J Exp Anal Behav 40: 47-56, 1983.

6. Davey, G. C. L. and G. G. Cleland. Topography of signal- centered behavior in the rat: effects of deprivation state and reinforcer type. J Exp Anal Behav 38: 291-304, 1982.

7. Davey, G. C. L. and G. G. Cleland. Nonassociative determi- nants of signal-directed behavior in rats. 1RCS Med Sci 10: 763--764, 1982.

8. Davey, G. C. L., D. A. Oakley and G. G. Cleland. Autoshaping in the rat: effects of omission on the form of the response. J Exp Anal Behav 36: 75-91, 1981.

9. Davey, G. C. L., S. Phillips and G. G. Cleland. The topography of signal-centered behaviour in the rat: the effects of solid and liquid food reinforcers. Behav Anal Lett 36" 75-91, 1981.

10. Ewer, R. F. The biology and behavior of a free-living population of black rats (Rattus rattus). Anim Behav Monogr 4: 127-174, 1971.

11. Hearst, E. and H. M. Jenkins. Sign Tracking: The Stimulus- Reinforcer Relation and Directed Action. Austin, TX: Mono- graphy of the Psychonomic Society, 1974.

12. Hull, J. H. Instrumental response topographies of rats. Anim Learn Behav 5: 207-212, 1977.

13. Jenkins, H. M., F. J. Barrera, C. Ireland and B. Woodside. Signal-centered action patterns of dogs in appetitive classical conditioning. Learn Motiv 9: 272-296, 1978.

14. Jenkins, H. M. and B. R. Moore. The form of the autoshaped response with food or water reinforcers. J Exp Anal Behav 20: 163-181, 1973.

15. Peterson, G. B. Response selection properties of food and brain stimulation reinforcers in rats. Physiol Behav 14: 681--688, 1975.

16. Timberlake, W. The functional organization of appetitive behav- iors: behavior systems and learning. In: Advances in Analysis of Behavior (vol 3): Biological Factors in Learning, edited by M. D. Zeiler and P. Harzem. Chichester: Wiley, 1983.

17. Timberlake, W. and D. L. Grant. Autoshaping in rats to the presentation of another rat prediciting food. Science 190: 690- 692, 1975.

18. Wasserman, E. A., H. B. Hunter, K. A. Gutowski and S. A. Bader. Autoshaping chicks with heat reinforcement: the role of stimulus-reinforcer and response-reinforcer relations. J Exp Psychol: Anim Behav Proc 104: 158-169, 1975.

19. Williams, D. R. Bioconditional behavior: conditioning without constraint. In: Autoshaping and Conditioning Theory, edited by C. M. Locurto, H. S. Terrace and J. Gibbon. New York: Aca- demic Press, 1981.

20. Woodruff, G. and D. R. Williams. The associative relation un- derlying autoshaping in the pigeon. J Exp Anal Behav 26: 1-13, 1976.