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Eating banana in cats for brain stimulation reward
Physiology and Behavior, Vol. 12, pp. 1063-1066. Brain Research Publications Inc., 1974. Printed in the U.S.A.
BRIEF COMMUNICATION Eating Banana in Cats for Brain Stimulation Reward 1 W A N D A WYRW1CKA
Department o f Anatomy, University o f California, School o f Medicine, Los Angeles, CA. 90024
( R e c e i v e d 13 N o v e m b e r 1 9 7 3 )
WYRWlCKA, W. Eating banana in cats for brain stimulation reward. PHYSIOL. BEHAV. 12(6) 1063-1066, 1974. -Hungry cats offered banana and meat pellets, preferred banana to meat pellets when electrical stimulation in the hypothalamus (ESB) was delivered for consumption of banana but never for consumption of meat pellets. Withholding ESB caused the cessation of banana consumption which was resumed with the reapplication of ESB. When, after a few weeks of training, both foods were presented to the hungry cats in their home compartment, without ESB, the animals approached banana and ignored meat pellets. Brain stimulation as a reward
Food selection
A N U M B E R of studies have s h o w n t h a t f o o d s e l e c t i o n or a c c e p t a n c e o f a given food s t r o n g l y d e p e n d s o n oral sensory i n p u t from t h e food. In early studies o f K o n [3] rats were allowed t o select t h e i r o w n diet f r o m a cafeteria of purified casein, sucrose a n d a salt m i x t u r e . It was f o u n d t h a t these rats ate m o s t l y sucrose (90%) and very little casein (6.5%), the o n l y source o f p r o t e i n . As a result, all a n i m a l s lost weight a n d some of t h e m died. Similar results were o b t a i n e d by S c o t t [7] w h o f o u n d t h a t cafeteria-fed rats ate only fat until all o f t h e m died. T h e a u t h o r s c o n c l u d e d t h a t the a n i m a l s refused t o eat casein because o f t h e bad taste. In e x p e r i m e n t s o f M c G i n t y et al. [4] h y p e r p h a g i c rats signif i c a n t l y increased t h e i r rate o f bar pressing for i n t r a g a s t r i c food w h e n t h e y were given a d r o p o f s a c c h a r i n s o l u t i o n to lick each time t h e n o u r i s h m e n t was p u m p e d i n t o t h e s t o m a c h ; in this case t h e desirable oral s t i m u l a t i o n a p p e a r e d crucial for t h e m a i n t e n a n c e of the feeding i n s t r u m e n t a l reaction. We have s h o w n t h a t cats p r e f e r r e d t o lick a m i l k / b r o t h m i x t u r e f r o m the feeder w h e r e the desired s t i m u l a t i o n o f t h e b r a i n was given as a reward a n d i g n o r e d t h e same m i x t u r e in t h e o t h e r feeder w h e r e s t i m u l a t i o n was n o t given [ 9 ] . Here, t h e p r o b a b l y desirable s e n s a t i o n o b t a i n e d f r o m b r a i n s t i m u l a t i o n was a critical f a c t o r in c h o o s i n g m i l k / b r o t h f r o m t h e feeder where s u c h s t i m u l a t i o n was delivered. A q u e s t i o n arises as t o w h e t h e r it w o u l d be possible to change t h e n a t u r a l p r e f e r e n c e for o n e f o o d over a n o t h e r b y associating an a d d i t i o n a l s e n s o r y i n p u t k n o w n t o be desired b y t h e a n i m a l w i t h t h e originally n o n - p r e f e r r e d food. T h e p r e s e n t s t u d y was designed t o e x p l o r e this area.
METHOD In 7 cats, m o n o p o l a r e l e c t r o d e s (0.2 m m of dia., w i t h 1 m m long b a r e d tip) were i m p l a n t e d in the lateral and p o s t e r i o r h y p o t h a l a m u s . A j e w e l e r ' s screw was placed i n t o the f r o n t a l b o n e to serve as a reference electrode. A f t e r recovery, a m o d e r a t e l y h u n g r y cat was placed in t h e experim e n t a l c o m p a r t m e n t w h i c h was p r o v i d e d w i t h a c o n t a i n e r w i t h m e a t pellets and a lever c o n n e c t e d w i t h a s t i m u l a t o r . Pressing t h e lever resulted in electrical s t i m u l a t i o n o f the b r a i n ( E S B ) t h r o u g h one of t h e h y p o t h a l a m i c electrodes. T h e ESB consisted of a 0.5 sec t r a i n of 100/sec, 2 - 3 V pulses o f 0.75 msec d u r a t i o n p e r pulse. Each of the cats learned to press t h e lever to o b t a i n ESB. T h e l o c a t i o n of the effective e l e c t r o d e has b e e n e x a m i n e d in 2 cats so far. In b o t h of t h e m t h e e l e c t r o d e tip was s i t u a t e d in the lateral h y p o t h a l a m u s , close t o t h e i n t e r n a l capsule (Fig. 1). After the establishment of the self-stimulation behavior, the cats ignored m e a t pellets a n d c o n c e n t r a t e d o n pressing the lever for ESB. However, t h e y a c c e p t e d t h e same m e a t pellets in the h o m e c o m p a r t m e n t w h e r e t h e y were fed once a day a f t e r t h e session. A f t e r this p r e l i m i n a r y training, t h e lever was r e m o v e d and 2 k i n d s o f food were p r e s e n t e d t o t h e h u n g r y cats in s e p a r a t e identical c o n t a i n e r s , n a m e l y m e a t pellets, a n a t u r a l l y p r e f e r r e d , familiar f o o d and pieces of b a n a n a , a n o n p r e f e r r e d food. The c o n t a i n e r s were m e t a l bowls, 12 cm i n dia., usually used for feeding the cats in t h e i r h o m e c o m p a r t m e n t s . T h e choice of b a n a n a as n o n p r e f e r r e d food was based o n t h e fact t h a t cats usually d o n o t eat fruits and t h a t all the cats ignored t h e b a n a n a w h e n it was
J This research was supported by USPHS Grant No. MH 13958. 1063
1064
WY R W I C K A
A.
B.
FIG. 1. Brain photomicrographs through the diencephalon of cat SSI (A) and cat SS7 (B). Arrows show the sites of electrical stimulation which served as a reward first for lever pressing and then for eating banana.
offered to t h e m . Previously it was also f o u n d t h a t cats refused to eat b a n a n a d u r i n g electrical s t i m u l a t i o n of the h y p o t h a l a m i c " f e e d i n g c e n t e r " [ 10]. T h e general view of the e x p e r i m e n t a l s i t u a t i o n is s h o w n in Fig. 2. F u r t h e r e x p e r i m e n t a l p r o c e d u r e s are described in the Results section. RESULTS
FIG. 2. A view of experimental situation. A food deprived cat eats banana in the presence of meat pellets when consumption of banana is rewarded by electrical stimulation of the hypothalamus.
When b a n a n a pieces and meat pellets were p r e s e n t e d to the cat in t h e e x p e r i m e n t a l c o m p a r t m e n t at the same time, the animal would start to eat pellets, ignoring the b a n a n a . S o o n e r or later, however, some investigation of the b a n a n a occurred, for w h i c h ESB was given. After a few such episodes, the cat m o v e d close to the c o n t a i n e r with b a n a n a and started to sniff the b a n a n a pieces, occasionally t o u c h i n g the fruit with the m o u t h , w h i c h was each time followed by ESB. Finally the animal started to lick the b a n a n a or to bite and take a piece i n t o the m o u t h . Once this h a p p e n e d , ESB was given only for c o n s u m i n g the b a n a n a . The rate of eating was usually increased and biting was m o r e energetic w h e n ESB was given r e p e a t e d l y every few seconds. In 3 cats, licking a n d occasional b i t i n g s t a r t e d d u r i n g the first session. In others, licking d e v e l o p e d gradually over a few sessions (see Table 1). In t h e first sessions, the cats sniffed t h e meat pellets from t i m e to t i m e and s o m e t i m e s ate t h e m , altern a t i n g this with m o m e n t s of a t t e n t i v e raising of the head as if they were waiting for s t i m u l a t i o n . W h e n n o ESB was given, t h e y r e t u r n e d t o t h e b a n a n a c o n t a i n e r . T h e t r a i n i n g
E A T I N G B A N A N A IN CATS
1065 TABLE 1
AVERAGE CONSUMPTION OF BANANA IN GRAMS DURING 200 TRIAL SESSIONS
Average Consumption* Consumption during the first session
Mean of 10 Successive sessions
Range
SS1
5.0
18.0
11.0 - 27.0
SS2
0.0
1.5
Cat No.
1.0
-
2.0
SS7
3.0
3.5
3.0-
SS8
0.0
9.0
5 . 0 - 17.5
SS9
7.0
8.0
6.0 - 12.0
SS10
5.0
8.0
6.5 - 11.5
SS11
2.0
26.5
15.0 - 42.0
6.5
*Based on the data obtained in the period between the fourth and thirteenth sessions.
sessions were c o n d u c t e d two or three times a week with 200 trials per session. Each trial consisted of a discrete act of licking or biting off a small morsel of banana and consuming it, which was i m m e d i a t e l y followed by the ESB reward. The intervals b e t w e e n trials usually lasted not less than 2 sec, depending on the individual rate of eating of the animal. Cessation o f eating resulted in the i m m e d i a t e withholding of the rewarding stimulation. Usually the session consisted of periods of 2 0 - 6 0 sec of c o n t i n u o u s eating of the bananas and obtaining ESB, which alternated with intervals from a few seconds to one minute (longer intervals were an exception), during which the cat was sitting quietly before resuming eating. Table 1 shows the c o n s u m p t i o n of banana during the first session and the average c o n s u m p t i o n of banana after the first three sessions. In 5 cats the banana c o n s u m p t i o n was low at first and gradually increased. In 2 o t h e r cats the c o n s u m p t i o n did not change significantly during the training. It should be noted that after the preliminary training period the cats entirely ignored meat pellets during the session. The only e x c e p t i o n was cat SS9 which usually consumed a few meat pellets ( 2 - 4 g) at the beginning of each session and then ate banana. During one session the ESB reward was withheld for 15 min. The reaction to the cessation of ESB was more or less the same in all cats. At first the animal c o n t i n u e d to consume banana as usual. Then after about 1 - 3 min, the animal's behavior changed; the cat would frequently interrupt the c o n s u m p t i o n , raise its head attentively, wait for a m o m e n t , then resume licking. A few minutes later on the cat moved to the container with meat pellets, sniffed them and sometimes ate some. Usually, 10 min after the withholding of ESB, the cat walked away from b o t h food containers and became engaged in different activities such as sniffing the floor or climbing the wall. Then, after 15 min of the withholding period, the ESB was given free when the cat happened to c o m e close to the banana container. A f t e r a few free stimulations, the cat resumed c o n s u m p t i o n of the banana pieces. ( S o m e experimental
sessions, including the extinction session, were filmed. The film can be seen on request.) A f t e r 3 weeks of training, a preference test was performed in the animals' h o m e c o m p a r t m e n t s w i t h o u t ESB. Just before the usual feeding time, the two containers used during training, one with banana pieces and the other with meat pellets, were put into the cage. The reaction of most cats was similar. After a brief orienting reaction lasting from a few seconds to 1 min, the animal approached the banana container, sniffed banana pieces and sometimes licked them for a few seconds, then walked away only to return again to sniff and lick. The sequence of reactions was repeated several times. The meat pellets were, in general, ignored, although eating pellets usually started immediately when presented alone to the cats at their usual feeding time. Only one cat, SS9, after a few licks of banana, began to eat meat pellets. After 3 m i n , however, she returned to sniff banana. The pattern of sniffing banana pieces resembled that observed during the experimental session with ESB. During the l 0 min test, however, there was little ( 1 - 3 g) or no c o n s u m p t i o n of banana. The behavior of most cats suggested a high degree of excitation. In only one cat, SS8, inhibitory behavior prevailed. After sniffing both the banana pieces and the meat pellets the cat moved to the shelf and remained there until the end of the test. DISCUSSION The results obtained suggest that it is possible to reverse the food preference of the animal by associating the c o n s u m p t i o n of an initially nonpreferred food with desirable intracranial stimulation. The approach to banana while ignoring meat pellets during the test in the h o m e c o m p a r t m e n t indicates that the sight of banana could acquire the p r o p e r t y of a conditioned stimulus. On the other hand, the extinction of banana c o n s u m p t i o n when ESB was withheld shows that the preference for banana could be maintained only when it was rewarded by the
1066
WY RWICKA
desired s e n s o r y i n p u t p r o v i d e d by ESB. It is possible t h a t ESB could also s t i m u l a t e eating as such, as suggested by an increase in t h e rate o f licking or biting b a n a n a with repetitive s t i m u l a t i o n . O t h e r a u t h o r s observed t h a t self-stimulation was f r e q u e n t l y associated with some eating or d r i n k i n g in rats; at t h e same time, availability o f f o o d or w a t e r significantly increased the rate of self-stimulation [1, 2, 5, 6 ] . Nevertheless, in t h e p r e s e n t e x p e r i m e n t s t h e cats ignored m e a t pellets in t h e p r e l i m i n a r y training w i t h lever pressing for ESB as well as later w h e n b a n a n a was i n t r o d u c e d . This suggests t h a t the facilitation in eating b a n a n a could be a s e c o n d a r y effect of ESB while the main effect was the p r o d u c t i o n of the desired sensory i n p u t .
T h e results o f the present e x p e r i m e n t s u p p o r t t h e idea t h a t the food preference m a y d e p e n d u p o n the sensory e x p e r i e n c e associated with c o n s u m p t i o n in t h a t animals will c h o o s e the kind of food w h i c h provides the highest sensory, t h o u g h n o t necessarily oral, satisfaction [ 8 ] . In fact, there are cases in real life w h e n s o m e i m p r o p e r and, perhaps, primarily u n t a s t y s u b s t a n c e s (e.g. alcohol) are c o n s u m e d because t h e y provide a specific satisfactory sensory experience. T h e results o b t a i n e d yield f u r t h e r s u p p o r t to the idea t h a t eating can be c o n s i d e r e d an i n s t r u m e n t a l act r e w a r d e d b y pleasurable sensations (cf. [8,9] ).
REFERENCES 1. Coons, E. E. and J. A. F. Cruce. Lateral hypothalamus: Food and current intensity in maintaining self-stimulation of hunger. Science 1 5 9 : 1 1 1 7 - 1 1 1 9 , 1968. 2. Hoebel, B. G., cited by B. G. Hoebel in Ann. Rev. Physiol. 33: 544, 1971. 3. Kon, S. K. The self-selection of food constituents by the rat. Biochem. J. 25: 473-481, 1931. 4. McGinty, D., A. N. Epstein and P. Teitelbaum. The contribution of oropharyngeal sensations to hypothalamic hyperphagia. Anim. Behav. 13:413 418, 1965. 5. Mendelson, J. Lateral hypothalamic stimulation in satiated rats: the rewarding effect of self-induced drinking. Science 157: 1077-1079, 1967.
6. Mogenson, G. J. and M. Kaplinski. Brain self-stimulation and mechanism of reinforcement. Learn. Motivat. 1: 186-198, 1970. 7. Scott, E. M. Self-selection of diet. I. Selection of purified components. J. Nutr. 31: 397-406, 1946. 8. Wyrwicka, W. The Mechanisms o f Conditioned Behavior. Springfield, 111.: C. C. Thomas, 1972. 9. Wyrwicka, W. and M. H. Chase. Eating as an instrumental reaction rewarded by electrical stimulation of the brain. Physiol. Behav. 9: 717-720, 1972. 10. Wyrwicka, W. and R. W. Doty. Feeding induced in cats by electrical stimulation of the brain stem. Expl Brain Res. 1: 152-160, 1966.
BRIEF COMMUNICATION Eating Banana in Cats for Brain Stimulation Reward 1 W A N D A WYRW1CKA
Department o f Anatomy, University o f California, School o f Medicine, Los Angeles, CA. 90024
( R e c e i v e d 13 N o v e m b e r 1 9 7 3 )
WYRWlCKA, W. Eating banana in cats for brain stimulation reward. PHYSIOL. BEHAV. 12(6) 1063-1066, 1974. -Hungry cats offered banana and meat pellets, preferred banana to meat pellets when electrical stimulation in the hypothalamus (ESB) was delivered for consumption of banana but never for consumption of meat pellets. Withholding ESB caused the cessation of banana consumption which was resumed with the reapplication of ESB. When, after a few weeks of training, both foods were presented to the hungry cats in their home compartment, without ESB, the animals approached banana and ignored meat pellets. Brain stimulation as a reward
Food selection
A N U M B E R of studies have s h o w n t h a t f o o d s e l e c t i o n or a c c e p t a n c e o f a given food s t r o n g l y d e p e n d s o n oral sensory i n p u t from t h e food. In early studies o f K o n [3] rats were allowed t o select t h e i r o w n diet f r o m a cafeteria of purified casein, sucrose a n d a salt m i x t u r e . It was f o u n d t h a t these rats ate m o s t l y sucrose (90%) and very little casein (6.5%), the o n l y source o f p r o t e i n . As a result, all a n i m a l s lost weight a n d some of t h e m died. Similar results were o b t a i n e d by S c o t t [7] w h o f o u n d t h a t cafeteria-fed rats ate only fat until all o f t h e m died. T h e a u t h o r s c o n c l u d e d t h a t the a n i m a l s refused t o eat casein because o f t h e bad taste. In e x p e r i m e n t s o f M c G i n t y et al. [4] h y p e r p h a g i c rats signif i c a n t l y increased t h e i r rate o f bar pressing for i n t r a g a s t r i c food w h e n t h e y were given a d r o p o f s a c c h a r i n s o l u t i o n to lick each time t h e n o u r i s h m e n t was p u m p e d i n t o t h e s t o m a c h ; in this case t h e desirable oral s t i m u l a t i o n a p p e a r e d crucial for t h e m a i n t e n a n c e of the feeding i n s t r u m e n t a l reaction. We have s h o w n t h a t cats p r e f e r r e d t o lick a m i l k / b r o t h m i x t u r e f r o m the feeder w h e r e the desired s t i m u l a t i o n o f t h e b r a i n was given as a reward a n d i g n o r e d t h e same m i x t u r e in t h e o t h e r feeder w h e r e s t i m u l a t i o n was n o t given [ 9 ] . Here, t h e p r o b a b l y desirable s e n s a t i o n o b t a i n e d f r o m b r a i n s t i m u l a t i o n was a critical f a c t o r in c h o o s i n g m i l k / b r o t h f r o m t h e feeder where s u c h s t i m u l a t i o n was delivered. A q u e s t i o n arises as t o w h e t h e r it w o u l d be possible to change t h e n a t u r a l p r e f e r e n c e for o n e f o o d over a n o t h e r b y associating an a d d i t i o n a l s e n s o r y i n p u t k n o w n t o be desired b y t h e a n i m a l w i t h t h e originally n o n - p r e f e r r e d food. T h e p r e s e n t s t u d y was designed t o e x p l o r e this area.
METHOD In 7 cats, m o n o p o l a r e l e c t r o d e s (0.2 m m of dia., w i t h 1 m m long b a r e d tip) were i m p l a n t e d in the lateral and p o s t e r i o r h y p o t h a l a m u s . A j e w e l e r ' s screw was placed i n t o the f r o n t a l b o n e to serve as a reference electrode. A f t e r recovery, a m o d e r a t e l y h u n g r y cat was placed in t h e experim e n t a l c o m p a r t m e n t w h i c h was p r o v i d e d w i t h a c o n t a i n e r w i t h m e a t pellets and a lever c o n n e c t e d w i t h a s t i m u l a t o r . Pressing t h e lever resulted in electrical s t i m u l a t i o n o f the b r a i n ( E S B ) t h r o u g h one of t h e h y p o t h a l a m i c electrodes. T h e ESB consisted of a 0.5 sec t r a i n of 100/sec, 2 - 3 V pulses o f 0.75 msec d u r a t i o n p e r pulse. Each of the cats learned to press t h e lever to o b t a i n ESB. T h e l o c a t i o n of the effective e l e c t r o d e has b e e n e x a m i n e d in 2 cats so far. In b o t h of t h e m t h e e l e c t r o d e tip was s i t u a t e d in the lateral h y p o t h a l a m u s , close t o t h e i n t e r n a l capsule (Fig. 1). After the establishment of the self-stimulation behavior, the cats ignored m e a t pellets a n d c o n c e n t r a t e d o n pressing the lever for ESB. However, t h e y a c c e p t e d t h e same m e a t pellets in the h o m e c o m p a r t m e n t w h e r e t h e y were fed once a day a f t e r t h e session. A f t e r this p r e l i m i n a r y training, t h e lever was r e m o v e d and 2 k i n d s o f food were p r e s e n t e d t o t h e h u n g r y cats in s e p a r a t e identical c o n t a i n e r s , n a m e l y m e a t pellets, a n a t u r a l l y p r e f e r r e d , familiar f o o d and pieces of b a n a n a , a n o n p r e f e r r e d food. The c o n t a i n e r s were m e t a l bowls, 12 cm i n dia., usually used for feeding the cats in t h e i r h o m e c o m p a r t m e n t s . T h e choice of b a n a n a as n o n p r e f e r r e d food was based o n t h e fact t h a t cats usually d o n o t eat fruits and t h a t all the cats ignored t h e b a n a n a w h e n it was
J This research was supported by USPHS Grant No. MH 13958. 1063
1064
WY R W I C K A
A.
B.
FIG. 1. Brain photomicrographs through the diencephalon of cat SSI (A) and cat SS7 (B). Arrows show the sites of electrical stimulation which served as a reward first for lever pressing and then for eating banana.
offered to t h e m . Previously it was also f o u n d t h a t cats refused to eat b a n a n a d u r i n g electrical s t i m u l a t i o n of the h y p o t h a l a m i c " f e e d i n g c e n t e r " [ 10]. T h e general view of the e x p e r i m e n t a l s i t u a t i o n is s h o w n in Fig. 2. F u r t h e r e x p e r i m e n t a l p r o c e d u r e s are described in the Results section. RESULTS
FIG. 2. A view of experimental situation. A food deprived cat eats banana in the presence of meat pellets when consumption of banana is rewarded by electrical stimulation of the hypothalamus.
When b a n a n a pieces and meat pellets were p r e s e n t e d to the cat in t h e e x p e r i m e n t a l c o m p a r t m e n t at the same time, the animal would start to eat pellets, ignoring the b a n a n a . S o o n e r or later, however, some investigation of the b a n a n a occurred, for w h i c h ESB was given. After a few such episodes, the cat m o v e d close to the c o n t a i n e r with b a n a n a and started to sniff the b a n a n a pieces, occasionally t o u c h i n g the fruit with the m o u t h , w h i c h was each time followed by ESB. Finally the animal started to lick the b a n a n a or to bite and take a piece i n t o the m o u t h . Once this h a p p e n e d , ESB was given only for c o n s u m i n g the b a n a n a . The rate of eating was usually increased and biting was m o r e energetic w h e n ESB was given r e p e a t e d l y every few seconds. In 3 cats, licking a n d occasional b i t i n g s t a r t e d d u r i n g the first session. In others, licking d e v e l o p e d gradually over a few sessions (see Table 1). In t h e first sessions, the cats sniffed t h e meat pellets from t i m e to t i m e and s o m e t i m e s ate t h e m , altern a t i n g this with m o m e n t s of a t t e n t i v e raising of the head as if they were waiting for s t i m u l a t i o n . W h e n n o ESB was given, t h e y r e t u r n e d t o t h e b a n a n a c o n t a i n e r . T h e t r a i n i n g
E A T I N G B A N A N A IN CATS
1065 TABLE 1
AVERAGE CONSUMPTION OF BANANA IN GRAMS DURING 200 TRIAL SESSIONS
Average Consumption* Consumption during the first session
Mean of 10 Successive sessions
Range
SS1
5.0
18.0
11.0 - 27.0
SS2
0.0
1.5
Cat No.
1.0
-
2.0
SS7
3.0
3.5
3.0-
SS8
0.0
9.0
5 . 0 - 17.5
SS9
7.0
8.0
6.0 - 12.0
SS10
5.0
8.0
6.5 - 11.5
SS11
2.0
26.5
15.0 - 42.0
6.5
*Based on the data obtained in the period between the fourth and thirteenth sessions.
sessions were c o n d u c t e d two or three times a week with 200 trials per session. Each trial consisted of a discrete act of licking or biting off a small morsel of banana and consuming it, which was i m m e d i a t e l y followed by the ESB reward. The intervals b e t w e e n trials usually lasted not less than 2 sec, depending on the individual rate of eating of the animal. Cessation o f eating resulted in the i m m e d i a t e withholding of the rewarding stimulation. Usually the session consisted of periods of 2 0 - 6 0 sec of c o n t i n u o u s eating of the bananas and obtaining ESB, which alternated with intervals from a few seconds to one minute (longer intervals were an exception), during which the cat was sitting quietly before resuming eating. Table 1 shows the c o n s u m p t i o n of banana during the first session and the average c o n s u m p t i o n of banana after the first three sessions. In 5 cats the banana c o n s u m p t i o n was low at first and gradually increased. In 2 o t h e r cats the c o n s u m p t i o n did not change significantly during the training. It should be noted that after the preliminary training period the cats entirely ignored meat pellets during the session. The only e x c e p t i o n was cat SS9 which usually consumed a few meat pellets ( 2 - 4 g) at the beginning of each session and then ate banana. During one session the ESB reward was withheld for 15 min. The reaction to the cessation of ESB was more or less the same in all cats. At first the animal c o n t i n u e d to consume banana as usual. Then after about 1 - 3 min, the animal's behavior changed; the cat would frequently interrupt the c o n s u m p t i o n , raise its head attentively, wait for a m o m e n t , then resume licking. A few minutes later on the cat moved to the container with meat pellets, sniffed them and sometimes ate some. Usually, 10 min after the withholding of ESB, the cat walked away from b o t h food containers and became engaged in different activities such as sniffing the floor or climbing the wall. Then, after 15 min of the withholding period, the ESB was given free when the cat happened to c o m e close to the banana container. A f t e r a few free stimulations, the cat resumed c o n s u m p t i o n of the banana pieces. ( S o m e experimental
sessions, including the extinction session, were filmed. The film can be seen on request.) A f t e r 3 weeks of training, a preference test was performed in the animals' h o m e c o m p a r t m e n t s w i t h o u t ESB. Just before the usual feeding time, the two containers used during training, one with banana pieces and the other with meat pellets, were put into the cage. The reaction of most cats was similar. After a brief orienting reaction lasting from a few seconds to 1 min, the animal approached the banana container, sniffed banana pieces and sometimes licked them for a few seconds, then walked away only to return again to sniff and lick. The sequence of reactions was repeated several times. The meat pellets were, in general, ignored, although eating pellets usually started immediately when presented alone to the cats at their usual feeding time. Only one cat, SS9, after a few licks of banana, began to eat meat pellets. After 3 m i n , however, she returned to sniff banana. The pattern of sniffing banana pieces resembled that observed during the experimental session with ESB. During the l 0 min test, however, there was little ( 1 - 3 g) or no c o n s u m p t i o n of banana. The behavior of most cats suggested a high degree of excitation. In only one cat, SS8, inhibitory behavior prevailed. After sniffing both the banana pieces and the meat pellets the cat moved to the shelf and remained there until the end of the test. DISCUSSION The results obtained suggest that it is possible to reverse the food preference of the animal by associating the c o n s u m p t i o n of an initially nonpreferred food with desirable intracranial stimulation. The approach to banana while ignoring meat pellets during the test in the h o m e c o m p a r t m e n t indicates that the sight of banana could acquire the p r o p e r t y of a conditioned stimulus. On the other hand, the extinction of banana c o n s u m p t i o n when ESB was withheld shows that the preference for banana could be maintained only when it was rewarded by the
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desired s e n s o r y i n p u t p r o v i d e d by ESB. It is possible t h a t ESB could also s t i m u l a t e eating as such, as suggested by an increase in t h e rate o f licking or biting b a n a n a with repetitive s t i m u l a t i o n . O t h e r a u t h o r s observed t h a t self-stimulation was f r e q u e n t l y associated with some eating or d r i n k i n g in rats; at t h e same time, availability o f f o o d or w a t e r significantly increased the rate of self-stimulation [1, 2, 5, 6 ] . Nevertheless, in t h e p r e s e n t e x p e r i m e n t s t h e cats ignored m e a t pellets in t h e p r e l i m i n a r y training w i t h lever pressing for ESB as well as later w h e n b a n a n a was i n t r o d u c e d . This suggests t h a t the facilitation in eating b a n a n a could be a s e c o n d a r y effect of ESB while the main effect was the p r o d u c t i o n of the desired sensory i n p u t .
T h e results o f the present e x p e r i m e n t s u p p o r t t h e idea t h a t the food preference m a y d e p e n d u p o n the sensory e x p e r i e n c e associated with c o n s u m p t i o n in t h a t animals will c h o o s e the kind of food w h i c h provides the highest sensory, t h o u g h n o t necessarily oral, satisfaction [ 8 ] . In fact, there are cases in real life w h e n s o m e i m p r o p e r and, perhaps, primarily u n t a s t y s u b s t a n c e s (e.g. alcohol) are c o n s u m e d because t h e y provide a specific satisfactory sensory experience. T h e results o b t a i n e d yield f u r t h e r s u p p o r t to the idea t h a t eating can be c o n s i d e r e d an i n s t r u m e n t a l act r e w a r d e d b y pleasurable sensations (cf. [8,9] ).
REFERENCES 1. Coons, E. E. and J. A. F. Cruce. Lateral hypothalamus: Food and current intensity in maintaining self-stimulation of hunger. Science 1 5 9 : 1 1 1 7 - 1 1 1 9 , 1968. 2. Hoebel, B. G., cited by B. G. Hoebel in Ann. Rev. Physiol. 33: 544, 1971. 3. Kon, S. K. The self-selection of food constituents by the rat. Biochem. J. 25: 473-481, 1931. 4. McGinty, D., A. N. Epstein and P. Teitelbaum. The contribution of oropharyngeal sensations to hypothalamic hyperphagia. Anim. Behav. 13:413 418, 1965. 5. Mendelson, J. Lateral hypothalamic stimulation in satiated rats: the rewarding effect of self-induced drinking. Science 157: 1077-1079, 1967.
6. Mogenson, G. J. and M. Kaplinski. Brain self-stimulation and mechanism of reinforcement. Learn. Motivat. 1: 186-198, 1970. 7. Scott, E. M. Self-selection of diet. I. Selection of purified components. J. Nutr. 31: 397-406, 1946. 8. Wyrwicka, W. The Mechanisms o f Conditioned Behavior. Springfield, 111.: C. C. Thomas, 1972. 9. Wyrwicka, W. and M. H. Chase. Eating as an instrumental reaction rewarded by electrical stimulation of the brain. Physiol. Behav. 9: 717-720, 1972. 10. Wyrwicka, W. and R. W. Doty. Feeding induced in cats by electrical stimulation of the brain stem. Expl Brain Res. 1: 152-160, 1966.