Journal ofNeuroscience Mcthods, 1 ( 1 9 7 9 ) 3 4 3 - - 3 5 2
343
© E l s e v i e r / N o r t h - H o l l a n d B i o m e d i c a l Press
RADIAL MAZE TYPE AS A D E T E R M I N A N T OF THE CHOICE BEHAVIOR OF RATS
S. M A G N I *, I. K R E K U L E a n d J. B U R E S
Institute of Physiology, Czechoslovak Academy of Sciences, Prague (Czechoslovakia) (Received January 15th, 1979) (Accepted August 30th, 1979)
Spatial m e m o r y o f rats was e x a m i n e d in t w o t y p e s of radial m a z e s in w h i c h a n i m a l s h a d to o b t a i n f o o d f r o m 6-, 8-, or 1 2 - d i f f e r e n t a r m s w i t h o u t r e p e a t i n g choices. In Maze 1 w i t h t u b u l a r c h a n n e l s closed at t h e far e n d rats r e t u r n e d t o t h e c e n t r a l p l a t f o r m b y backing o u t f r o m t h e just visited alley. In Maze 2 t h e t u b u l a r c h a n n e l s were e q u i p p e d w i t h o n e - w a y d o o r s at b o t h e n d s a n d t h e c e n t r a l p l a t f o r m was raised 5 c m a b o v e t h e floor. R a t s leaving t h e far e n d of t h e alley r e t u r n e d t o t h e c e n t r a l p l a t f o r m t h r o u g h a circular hole in its c e n t e r . P e r f o r m a n c e was a l m o s t p e r f e c t in b o t h a p p a r a t u s e s , b u t rats p r e f e r r e d t o e n t e r a d j a c e n t alleys in Maze 1 a n d widely s e p a r a t e d alleys in Maze 2. T h e s e b e h a v i o r s were significantly d i f f e r e n t f r o m c o m p u t e r - s i m u l a t e d r a n d o m c h o i c e f r o m t h e set of c o r r e c t s o l u t i o n s o f t h e task. It is c o n c l u d e d t h a t Maze 2 is b e t t e r suited for r e p e a t e d e x p e r i m e n t s w i t h t h e same a n i m a l b e c a u s e t h e o b l i g a t o r y r e t u r n t o t h e c e n t e r o f t h e p l a t f o r m p r e c l u d e s t h e s i m p l i f i c a t i o n o f t h e task b y r e s p o n s e chaining.
INTRODUCTION
Olton and Samuelson (1976) and Olton et al. (1977) have recently d e m o n s t r a t e d th a t rats have a remarkable m e m o r y capacity for spatial i n f o r m a t i o n which can be revealed by testing paradigms regarding the ecology and eth ol ogy of this species. The task used in the above studies required th e animal to sample f r om a set of choices until all alternatives were chosen. F o o d pellets were placed one into each of 8 arms of an elevated radial maze. The rat was allowed to get all pellets by entering the arms and returning to the central pl at f or m after each choice. The animals rapidly learned n o t to revisit the already chosen alleys and succeeded in selecting within 8 choices 7.6 di f f e r ent arms, on the average. The authors analyzed the behavior of rats in the 8-arm maze and this convincingly showed t hat the animals are guided by the m e m o r y of choices already made. Their results were recently conf i r m ed by Zoladek and Roberts (1978). The spatial m e m o r y involved seems to be based on b o t h extra-maze and intra-maze cues * Visiting s c i e n t i s t f r o m t h e U n i v e r s i t y o f Pavia, Italy.
344
and the error probability slightly increases with the n u m b e r of intervening choices. Th e purpose o f the present study was to ext end the above observations to radial mazes with 6 and 12 arms, in order to assess the effect of maze c o n s t r u c t i o n on the behavior o f the rats, and to examine the possibility t h a t response chains c o n t r i b u t e to correct performance. EXPERIMENT 1
T h e elevated maze used in the study by Olton and Samuelson (1976) stresses the three-dimensional nature of the task. Movement of the rat along the physical cliffs of the maze arms m a y play an i m p o r t a n t role in t he formation o f the spatial memories. The use of an elevated maze precludes efficient restriction o f extra-maze cues and leaves little o p p o r t u n i t y for fine control o f the animal's m o v e m e n t s through the maze. E x p e r i m e n t 1 examines the spatial m e m o r y o f rats in a closed radial maze. At the same time an a t t e m p t was made to assess the capacity of the rat's m e m o r y by establishing the relationship between p e r f o r m a n c e and the n u m b e r o f maze arms.
Method Subjects. Nine male h o o d e d rats (Druckrey strain) aged 2 m ont hs at the beginning o f the e x p e r i m e n t were used. T h e y were reduced t o 80% of initial b o d y weight and maintained on a 24-h f o o d deprivation schedule with water freely available. T h e y were housed in group cages (4 and 5 animals per cage). Apparatus. The radial maze consisted of a central circular platform 45 cm in d iameter and arms made of a 20 cm length of plexiglass tubing 7 cm in diameter. A recessed f o o d c u p (15 mm in diameter and 20 m m deep) was made accessible through a hole in the floor 2 cm from the closed far end of the maze channel. Each arm was fitted to a perpendicular wall segment (11 cm wide and 38.5 cm high), t he b o t t o m of which was equipped with bolts fitting into the corresponding holes in the central platform. A 6-, 8or 12-arm maze could be rapidly assembled by inserting the wall segments into the appropriate holes in the central platform. The diameter of the central p latf o r m was 22.5, 29.5 or 43.5 cm, for the 6-, 8- or 12-arm mazes, respectively. Th e maze was placed in the center o f a r o o m , 5 m long and 4 m wide with one large window and one door. The r o o m contained a writing table, two chairs and several pieces o f l aborat ory furniture and e q u i p m e n t along th e walls. Illumination was provided with an overhead fluorescent light. Procedure. T h e animal was allowed to explore the e m p t y apparatus for 10 min on three successive days. In t he actual experiments each food cup was baited with a 100 mg pellet o f Larsen's diet. The rat was than placed on the central p l a t f o r m and left in the apparatus until all pellets were removed from the f o o d c u p s or until 10 min elapsed. The sequence o f choices was recorded, a choice being defined as the rat placing all four legs into an arm. T w o to
345 t h r e e trials w e r e given p e r d a y w i t h a m i n i m u m intertrial interval o f 2 h. A d d i t i o n a l f o o d was given t o t h e a n i m a l d u r i n g 30 m i n a f t e r c o n c l u s i o n o f t h e last trial. T h e a n i m a l s w e r e t r a i n e d f o r 10 d a y s in t h e 8 - a r m m a z e , 5 d a y s in t h e 6 - a r m m a z e , 3 d a y s in t h e 8 - a r m m a z e again, a n d finally 5 d a y s in t h e 12-arm maze.
Results and discussion T h e first 5 d a y s in t h e 8 - a r m m a z e w e r e n o t e v a l u a t e d q u a n t i t a t i v e l y b e c a u s e t h e animals o f t e n did n o t s u c c e e d in ~etting all pellets d u r i n g the a l l o t t e d 10-min interval. A f t e r t h e rats b e c a m e familiar w i t h t h e task, t h e y avidly c o n s u m e d t h e pellet e i t h e r d i r e c t l y at t h e f o o d c u p or a f t e r r e t u r n i n g to the central platform, and rapidly headed for another arm, sometimes h e s i t a t i n g at t h e e n t r a n c e a n d w i t h d r a w i n g b e f o r e m a k i n g t h e n e x t choice. F r o m t h e sixth d a y all a n i m a l s c o n s u m e d all pellets. T h e results are s u m m a r i z e d in T a b l e 1, p r e s e n t i n g (a) t h e m e a n n u m b e r o f d i f f e r e n t a r m s c h o s e n w i t h i n t h e first 6, 8 or 12 choices; {b) t h e m e a n n u m b e r o f choices r e q u i r e d t o o b t a i n all pellets; a n d (c) t h e p e r c e n t a g e o f errorless trials. T h e rats s u c c e e d e d in c h o o s i n g 7.7 d i f f e r e n t a r m s in t h e first 8 choices d u r i n g d a y s 6 t o 10 o f training in t h e 8 - a r m m a z e , and t h e i r p e r f o r m a n c e f u r t h e r i m p r o v e d t o 7.9 d i f f e r e n t a r m s d u r i n g t h e s e c o n d training p e r i o d in this m a z e . T h e i m p r o v e m e n t was still m o r e c o n s p i c u o u s in t h e n u m b e r o f c h o i c e s r e q u i r e d t o o b t a i n all pellets a n d in t h e p e r c e n t a g e o f errorless trials w h i c h increased f r o m 7 0 . 9 t o 85.2. E x c e l l e n t p e r f o r m a n c e was f o u n d also in t h e 6- a n d 1 2 - a r m m a z e . In fact, t h e i n c i d e n c e o f errors in t h e 1 2 - a r m m a z e was n o t significantly higher t h a n in t h e 8 - a r m m a z e . T h e high p e r c e n t a g e o f errorless trials in all m a z e s m a d e it possible to c o n c e n t r a t e f u r t h e r analysis o n this t y p e o f r e s p o n d i n g . O l t o n a n d S a m u e l s o n ( 1 9 7 6 ) c o n c l u d e d f r o m t h e i r results t h a t t h e rats d o n o t utilize a n y p a r t i c u l a r r e s p o n s e s e q u e n c e s in solving t h e task. T o t e s t f o r
TABLE 1 PERFORMANCE MEASURES OF RATS IN MAZE 1 AND MAZE 2 Type of maze
1 1 1 1
2 2 2
Number of trials 117 117 54 107 34 83 78
Number of arms 8 6 8 12 6 8 12
Measure a
b
e
7.7 5.9 7.9 11.7 5.8 7.7 11.7
8.6 6.0 8.1 12.3 6.2 8.5 13.1
70.9% 95.7% 85.2% 79.4% 82.3% 81.9% 60.2%
12
11
10
9
8
7
6
5
4
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3
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17.6
2
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2
Choice
0.0
9.4 23.4 1.2 6.4 1.2 2.1 1.2 17.0 9.4 8.5 8.2 6.4 12.9 8.5 12.9 4.2 23.5 14.9 9.4 8.5
10.6
3
4.7
8.5
4.7 19.1 7.0 10.6 1.2 4.2 11.7 8.5 11.7 4.2 9.4 12.7 7.0 4.2 24.7 6.4 9.4 14.9 8.2 6.4
4
7.0
0.0 6.4 6.4 8.2 14.9 11.7 21.3 14.1 4.2 9.4 6.4 5.9 12.7 24.7 10.6 8.2 10.6 4.7 0.0 5.9 6.4
5 2.3 10.6 9.4 0.0 5.9 4.2 17.6 21.3 10.6 2.1 5.9 10.6 24.7 12.7 4.7 8.5 2.3 10.6 7.0 14.9 9.4 4.2
6 2.3 34,0 3.5 0.0 17.6 2.1 11.7 14.9 11.7 0.0 25.9 10.6 4.7 9.5 2.3 12.7 4.7 4.2 8.2 6.4 7.0 6.4
7 1.2 27.6 7.0 2.1 18.8 4.2 14.1 0.0 23.5 6.4 7.0 19.1 5.9 2.1 2.3 2.1 3.5 17.0 7.0 8.5 9.4 10.6
8 2.3 10.6 16.5 6.4 11.7 4.2 29.4 14.9 7.0 4.2 2.3 10.6 2.3 2.1 7.3 10.6 4.7 10.6 5.9 8.5 10.6 17.0
9
2.1 8.2 10.6 21.2 31.9 10.6 10.6 5.9 4.2 3.5 14.9 4.7 2.1 3.5 6.4 11.7 6.4 10.6 6.4 7.0 4.2
12.9
10
0.0 8.5 7.0 14.9 0.0 4.2 10.6 17.0 10.6 10.6 2.3 8.5 7.0 10.6 5.9 8.5 9.4 6.4 4.7 10.6
28.2
14.1
11
0.0 4.7 12.7 0.0 6.4 0.0 2.1 1.2 10,6 9.4 2.1 15.3 19.1 9.4 10.6 7.0 6.4 7.0 10.6 9.4 19.1
31.7
12
D I S T R I B U T I O N O F A R M S S E L E C T E D ON S U B S E Q U E N T C H O I C E S IN T H E 1 2 - A R M M A Z E 1 ( N O R M A L T Y P E , n = 8 5 ) A N D M A Z E 2 ( B O L D TYPE, n = 47)
TABLE 2
347
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Fig. l . D i s t r i b u t i o n of t h e l o n g e s t s e q u e n c e s o f successively s e l e c t e d a d j a c e n t m a z e c h a n n e l s in Maze 1 (M1), Maze 2 (M2) a n d in c o m p u t e r - s i m u l a t e d r a n d o m b e h a v i o r (CSRB). Abscissa: s e q u e n c e l e n g t h r. O r d i n a t e : p e r c e n t a g e o f trials w i t h c o r r e s p o n d i n g r value. T h e full a n d d a s h e d curves in M l l , 12 c o r r e s p o n d t o t h e first a n d s e c o n d t r a i n i n g p e r i o d in t h e 8 - a r m Maze 1. N o t e t h a t i n c i d e n c e o f long s e q u e n c e s o f a d j a c e n t choices is s i g n i f i c a n t l y a b o v e c h a n c e in Maze 1 a n d s i g n i f i c a n t l y b e l o w c h a n c e in Maze 2.
348 in shown in Table 2. For each trial was also not ed the longest sequence, r, of contiguous choices, i.e. when the animal selected the arm adjacent to the previously chosen one. (For instance, r = 4 for the sequence 1 2 3 4 6 7 5 8 and r = 1 f o r the sequence 1 5 8 3 7 2 4 6.) Th e radial maze task can be solved w i t h o u t error in ( n - - 1 ) ! ways. R a n d o m sampling f r om the set of errorless runs was simulated by a comp u t er program written in BASIC and i m pl e m ent ed on a PDP 8 minicomputer. Comparison of the observed and c o m p u t e d data (Fig. 1) shows that the rats t e n d e d to choose the contiguous arms much more frequent l y than corresponds to r a n d o m choice of errorless strategies. In the 12-arm maze the value o f r = 12 was e n c o u n t e r e d in 21.2% of trials, although the theoretically predicted incidence was only 5 × 10 -s. Similar t e n d e n c y was seen in the 6-arm maze and, more clearly, in the 8-arm maze where it increased with overtraining. Whereas there was only 1% o f trials with r ~> 5 in the com put ersimulated distribution, t he y were f o u n d in 14.4% of all errorless runs in the first series o f the 8-arm maze experiments and this p r o p o r t i o n increased to 32.5% in the second series. Conversely, the incidence of runs with low r values (1, 2) was significantly lower than in the distribution of simulated r a n d o m choices. The chi 2 test indicated highly significant difference between t h e observed and c o m p u t e d distributions for the 6-, 8- and 12-arm mazes. It seems th at with increasing familiarity with the task the rats start to use m o r e and more the parsimonious strategy of contiguous choices, which in t h e e x t r e m e case simplifies the task to r e m em beri ng the initial position and direction o f turning. This t e n d e n c y is also reflected in the distribution of arms chosen in successive choices. T he preference for contiguous arms leads t o a higher than e x p e c t e d selection of arms 2 and 12 on choice 2 and to an increased probability o f choices along t he diagonal in Table 2. This nonr a n d o m distribution is b e t t e r expressed in the first half of the choice field, however, and is significantly di f f er e nt from the values predicted by the r a n d o m choice model only when all diagonal cells are added. In this case we obtain 29.2% for t he first and 34.4% for the second period of the 8-arm maze training, respectively ( e x p e c t e d value 26.5%), 54% for the 6-arm maze ( e x p e c t e d value 36.0%) and 34.2% for the 12-arm maze (expected value 17.4%). The differences bet w e e n the observed and predicted values are significant at the P ~ 0.05 level (binomial comparison). EXPERIMENT 2 T he results o f E x p e r i m e n t 1 suggested t hat overtrained rats solved the radial maze task b y using a strategy which m a y considerably reduce demands o n spatial m e m o r y . Contiguous choices were facilitated because the animal backing o u t f r o m t h e last selected channel almost faced the adjacent arm, which usually r epr e s ent ed t he nearest cor r ect choice. T he rats did n o t return t o t he c e n t e r o f t he pl a t f or m b e t w e e n choices and the e x p l o r a t o r y activity was strikingly reduced. In o r d e r t o prevent the d e v e l o p m e n t of such stereo-
349 t y p e d behavior, t h e a p p a r a t u s was m o d i f i e d in a way which f o r c e d the animal t o r e t u r n t o the c e n t e r o f the p l a t f o r m a f t e r each c h o i c e and which negatively biased c o n t i g u o u s choices.
Method Subjects. T h e 9 rats used in E x p e r i m e n t 1 were also e m p l o y e d in Experim e n t 2.
Apparatus. T h e m a z e used in E x p e r i m e n t 1 was m o d i f i e d . A hole 8 cm in d i a m e t e r was m a d e in the c e n t e r o f the p l a t f o r m , which was raised b y 3 adjustable legs 5 cm above the floor. T h e arms o f the maze were m a d e longer (31 cm) and were e q u i p p e d o n b o t h ends with o n e - w a y swing doors. A 5 cm high p l a t f o r m b e l o w t h e far end o f each arm facilitated t h e d e s c e n t o f the animal to t h e floor. A 50 cm high plexiglass wall c o n s t r u c t e d o f 33.5 × 50.0 c m plexiglass sheets a r o u n d the a p p a r a t u s separated the m a z e f r o m the rest o f the r o o m . Procedure. T h e rats were allowed to e x p l o r e the new a p p a r a t u s with the swing d o o r s raised and t h e f o o d cups e m p t y for 10 min. T h e animals were t h e n t r a i n e d for 2 d a y s t o o p e n t h e swing doors, t o d e s c e n d f r o m the far end o f t h e arms on the floor, t o r e t u r n b e l o w the p l a t f o r m and t o climb o n t o it t h r o u g h t h e central hole. F r o m d a y 3 all animals m a s t e r e d the new task and their b e h a v i o r was q u a n t i t a t i v e l y evaluated. T h e y were trained for 2 days (34 trials) in t h e 6-arm maze, 4 days (83 trials) in the 8-arm maze and 3 d a y s (78 trials) in t h e 12-arm maze. Otherwise the p r o c e d u r e was the same as in E x p e r i m e n t 1. Results and discussion Switching f r o m A p p a r a t u s 1 t o A p p a r a t u s 2 did n o t r e d u c e t h e perform a n c e o f the animals. As s h o w n in Table 1, m o s t p e r f o r m a n c e measures were slightly worse b u t n o n e o f these d i f f e r e n c e s r e a c h e d the level o f statistical significance. Analysis o f the errorless trials s h o w e d , h o w e v e r , t h a t the b e h a v i o r o f the animals in A p p a r a t u s 2 was strikingly d i f f e r e n t f r o m the b e h a v i o r o f t h e same animals in A p p a r a t u s 1. D i s t r i b u t i o n o f c o n t i g u o u s choices (Fig. 1) revealed a m a r k e d a v o i d a n c e o f a d j a c e n t arms, m a n i f e s t e d b y a c o n s i d e r a b l y greater t h a n e x p e c t e d incidence o f sequences c o r r e s p o n d ing to r = 1 and r = 2. A f t e r r e t u r n i n g t o the central p l a t f o r m f r o m the previously c h o s e n arm, t h e animals usually faced t h e o p p o s i t e arm, and selected it in m o s t eases. This is r e f l e c t e d in t h e d i s t r i b u t i o n o f arms c h o s e n o n the s e c o n d c h o i c e (Table 2). T h e r e was a m a r k e d p r e f e r e n c e for arms o p p o s i t e t o arm 1, t h a t is, for arms 4, 5 and 7 in t h e 6-, 8- and 12-arm mazes, respectively. On t h e third c h o i c e p r e f e r e n c e shifted t o the vicinity o f c h a n n e l 1, b u t the choices were r a n d o m i z e d m o r e r a p i d l y t h a n in A p p a r a t u s 1. T h e m o d i f i c a t i o n s w h i c h changed A p p a r a t u s 1 t o A p p a r a t u s 2 achieved
350 the purposes of preventing contiguous choices and forcing the animal to rely on spatial m e m o r y . On the other hand, preference for non-contiguous choices indicated that responding in Apparatus 2 was also non-random. The latter bias does not lead, however, to a simpler solution of the task, since starting with the third choice the animal had always to choose between closely adjacent e m p t y and baited channels. Apparatus 2 seems to be better suited, therefore, for a prolonged experimentation with the same animal. GENERAL DISCUSSION The present results confirm previous reports (Olton and Samuelson, 1976; Olton et al., 1977; Zoladek and Roberts, 1978} describing the behavior of rats in the radial maze. Although rats of a different strain were used and the apparatus was considerably modified, the overall performance measures were almost the same. Restricted access to visual extra-maze cues did not reduce the animal's performance in the closed maze. This is in accordance with the findings by Zoladek and Roberts (1978) who showed t h a t blind rats are capable of accurate orientation in the elevated 8-arm maze (7.5 different alleys entered in first 8 choices). When other cues are not available, spatial orientation depends on vestibular, kinesthetic and tactile cues (Beritoff, 1965), which are probably more abundant in the closed maze than in the elevated maze. The main advantage of the mazes used in the present study is that they provide for better control of the animal's movement through the maze. In Maze 1 the animal moves into and backs out from the individual alleys and thus assumes the same body--maze relationship when entering and leaving a maze arm. In Maze 2 the rat must pass through the alleys in one direction only but can return to the choice platform through a multitude of routes {ranging from a straight run towards the central opening to prolonged exploration of the outside of the maze) which all converge on the center of the choice platform. The obligatory return to the same starting point seems to be a particularly desirable feature of Maze 2 which practically eliminates solution of the task by response chaining. The significance of the maze type is indicated by the quantitative characteristic of the choice behavior of the rats. Whereas Olton and Samuelson {1976) concentrated their analysis on the distribution of errors, the present paper attempts to elucidate the behavioral strategies underlying errorless solution of the task. This is especially important when incidence of errors rapidly decreases with overtraining. Olton and Samuelson {1976) reported that they analyzed the order of choices made by their rats, but they do not give the details of this analysis. They state that 'rats tended not to choose the adjacent arms although occasionally there were sequences in which 2, 3, or 4 adjacent arms were chosen in order'. They also feel that the rats preferred 'the arm 90 ° from the
351 arm just chosen'. Several reasons m ay a c c o u n t for the difference between the above claims and the present results. (1) Olton and Samuelson (1976) tested their animals at a lower level of overtraining a n d / o r performance. This is indicated by the lower percentage o f errorless trials in their experiments (46.7% in their Table A2 as compared with 70.9% in the first series of the present 8-arm maze experiments). (2) An i m p o r t a n t difference between Apparatus 1 and the apparatus used by Olton and Samuelson (1976) is that the latter was an elevated maze. Although the authors do not state it explicitly, the rats probably turned around after reaching the food cup and reentered the central platform with the head first, i.e. similarly to the present Apparatus 2. On the cont rary, rats could n o t turn in the tubular arms of Apparatus 1 and had to back to the central platform. One would expect that the rats of Olton and Samuelson (1976} would behave m or e like rats in Apparatus 2, but since t hey were not obliged to return to the center of the platform, t h e y developed an intermediate behavior which may well a c c o u n t for preference of 90 ° choices. (3) Closer analysis of Table A1 of Olton and Samuelson (1976) shows that the sequences o f contiguous choices with r ~< 4 were e n c o u n t e r e d in 16.1% o f errorless trials, i.e. a b o u t 4.6 times more frequent l y than would correspond to r an d o m incidence. The difference is reliable at the P < 0.05 level and shows that the rats actually chose adjacent arms more frequently than would co r r es p o n d to r a ndom selection from the set of correct strategies. The t e n d e n c y to turn in one direction and to choose the adjacent arms was clearly expressed in a later study (Olton et al., 1977) using a 17-arm elevated maze. Rats returning to the central platform entered an adjacent alley in 30% cases and the second ne xt alley in 42% cases. Olton and Werz (1978) f o u n d in the same 17-arm maze t hat adjacent alleys were chosen most frequently (35%}. When, after return of the animal to the central platform, all doors to arms were closed for 10--15 sec, the rat engaged in e x p l o r a t o r y activity which disrupted the above t e n d e n c y w i t h o u t affecting choice accuracy. T he animal usually entered the nearest correct alley, close t o the entrance of which it was standing at the time when the doors were raised again. The brief c o n f i n e m e n t on the central platform seems to have a similar ef f ect on the rat's behavior as the use of the present Maze 2. Since the possibility that rats recognize the baited alleys by the smell of f o o d , or th at t h e y use scent marking of the already entered alleys was ruled o u t by careful control experiments (Olton and Samuelson, 1976; Zoladek and Roberts, 1978}, the spectacular p e r f o r m a n c e of rats can only be explained b y a memorial process. Comparison of the behavior of rats in the 6-, 8- and 12-arm mazes indicates that even the 12-arm maze was easily mastered by the experienced animals. Olton et al. {1977) found that rats overtrained in a 17-arm elevated maze made 14.6 correct responses in the 17 first choices. Although accuracy started to decline after the first 10 choices, even the 17th choice was considerably b e t t e r than chance. Extrapolation o f the decreasing probability of correct responding with the increasing
352 n u m b e r o f c h o i c e s indicated t h a t c h o i c e a c c u r a c y m a y reach c h a n c e levels at 25 t o 30 choices. A l t h o u g h f u r t h e r research is n e e d e d t o d e t e r m i n e the e x t e n t o f r a t s ' w o r k i n g m e m o r y m o r e precisely, it is o b v i o u s t h a t the 'magical n u m b e r 7 -+ 2 ' (Miller, 1956} typical for h u m a n m e m o r y span does n o t limit t h e c a p a c i t y o f rats to process spatial i n f o r m a t i o n in this particular task. REFERENCES Beritoff, I.S. (1965) Neural Mechanisms of Higher Vertebrate Behavior (W.T. Liberson, Ed. and trans.), Little, Brown, Boston, Mass. Miller, G.A. (1956) The magical number seven plus or minus two: some limits on our capacity for processing information. Psyehol. Rev., 63: 81--97. Olton, D.S. and Samuelson, R.J. (1976) Remembrance of places passed: spatial memory in rats. J. exp. Psychol. (Animal Behavior Processes), 2: 97--116. Olton, D.S. and Werz, M.A. (1978) Hippocampai function and behavior: spatial discrimination and response inhibition. Physiol. Behav., 20: 597--605. Otton, D.S., Collison, C. and Werz, M.A. (1977) Spatial memory and radial arm maze performance of rats. Learn. Motivation, 8: 289--314. Zoladek, L. and Roberts, W.A. (1978) The sensory basis of spatial memory in the rat. Anim. Learn. Behav., 6: 77--81.
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RADIAL MAZE TYPE AS A D E T E R M I N A N T OF THE CHOICE BEHAVIOR OF RATS
S. M A G N I *, I. K R E K U L E a n d J. B U R E S
Institute of Physiology, Czechoslovak Academy of Sciences, Prague (Czechoslovakia) (Received January 15th, 1979) (Accepted August 30th, 1979)
Spatial m e m o r y o f rats was e x a m i n e d in t w o t y p e s of radial m a z e s in w h i c h a n i m a l s h a d to o b t a i n f o o d f r o m 6-, 8-, or 1 2 - d i f f e r e n t a r m s w i t h o u t r e p e a t i n g choices. In Maze 1 w i t h t u b u l a r c h a n n e l s closed at t h e far e n d rats r e t u r n e d t o t h e c e n t r a l p l a t f o r m b y backing o u t f r o m t h e just visited alley. In Maze 2 t h e t u b u l a r c h a n n e l s were e q u i p p e d w i t h o n e - w a y d o o r s at b o t h e n d s a n d t h e c e n t r a l p l a t f o r m was raised 5 c m a b o v e t h e floor. R a t s leaving t h e far e n d of t h e alley r e t u r n e d t o t h e c e n t r a l p l a t f o r m t h r o u g h a circular hole in its c e n t e r . P e r f o r m a n c e was a l m o s t p e r f e c t in b o t h a p p a r a t u s e s , b u t rats p r e f e r r e d t o e n t e r a d j a c e n t alleys in Maze 1 a n d widely s e p a r a t e d alleys in Maze 2. T h e s e b e h a v i o r s were significantly d i f f e r e n t f r o m c o m p u t e r - s i m u l a t e d r a n d o m c h o i c e f r o m t h e set of c o r r e c t s o l u t i o n s o f t h e task. It is c o n c l u d e d t h a t Maze 2 is b e t t e r suited for r e p e a t e d e x p e r i m e n t s w i t h t h e same a n i m a l b e c a u s e t h e o b l i g a t o r y r e t u r n t o t h e c e n t e r o f t h e p l a t f o r m p r e c l u d e s t h e s i m p l i f i c a t i o n o f t h e task b y r e s p o n s e chaining.
INTRODUCTION
Olton and Samuelson (1976) and Olton et al. (1977) have recently d e m o n s t r a t e d th a t rats have a remarkable m e m o r y capacity for spatial i n f o r m a t i o n which can be revealed by testing paradigms regarding the ecology and eth ol ogy of this species. The task used in the above studies required th e animal to sample f r om a set of choices until all alternatives were chosen. F o o d pellets were placed one into each of 8 arms of an elevated radial maze. The rat was allowed to get all pellets by entering the arms and returning to the central pl at f or m after each choice. The animals rapidly learned n o t to revisit the already chosen alleys and succeeded in selecting within 8 choices 7.6 di f f e r ent arms, on the average. The authors analyzed the behavior of rats in the 8-arm maze and this convincingly showed t hat the animals are guided by the m e m o r y of choices already made. Their results were recently conf i r m ed by Zoladek and Roberts (1978). The spatial m e m o r y involved seems to be based on b o t h extra-maze and intra-maze cues * Visiting s c i e n t i s t f r o m t h e U n i v e r s i t y o f Pavia, Italy.
344
and the error probability slightly increases with the n u m b e r of intervening choices. Th e purpose o f the present study was to ext end the above observations to radial mazes with 6 and 12 arms, in order to assess the effect of maze c o n s t r u c t i o n on the behavior o f the rats, and to examine the possibility t h a t response chains c o n t r i b u t e to correct performance. EXPERIMENT 1
T h e elevated maze used in the study by Olton and Samuelson (1976) stresses the three-dimensional nature of the task. Movement of the rat along the physical cliffs of the maze arms m a y play an i m p o r t a n t role in t he formation o f the spatial memories. The use of an elevated maze precludes efficient restriction o f extra-maze cues and leaves little o p p o r t u n i t y for fine control o f the animal's m o v e m e n t s through the maze. E x p e r i m e n t 1 examines the spatial m e m o r y o f rats in a closed radial maze. At the same time an a t t e m p t was made to assess the capacity of the rat's m e m o r y by establishing the relationship between p e r f o r m a n c e and the n u m b e r o f maze arms.
Method Subjects. Nine male h o o d e d rats (Druckrey strain) aged 2 m ont hs at the beginning o f the e x p e r i m e n t were used. T h e y were reduced t o 80% of initial b o d y weight and maintained on a 24-h f o o d deprivation schedule with water freely available. T h e y were housed in group cages (4 and 5 animals per cage). Apparatus. The radial maze consisted of a central circular platform 45 cm in d iameter and arms made of a 20 cm length of plexiglass tubing 7 cm in diameter. A recessed f o o d c u p (15 mm in diameter and 20 m m deep) was made accessible through a hole in the floor 2 cm from the closed far end of the maze channel. Each arm was fitted to a perpendicular wall segment (11 cm wide and 38.5 cm high), t he b o t t o m of which was equipped with bolts fitting into the corresponding holes in the central platform. A 6-, 8or 12-arm maze could be rapidly assembled by inserting the wall segments into the appropriate holes in the central platform. The diameter of the central p latf o r m was 22.5, 29.5 or 43.5 cm, for the 6-, 8- or 12-arm mazes, respectively. Th e maze was placed in the center o f a r o o m , 5 m long and 4 m wide with one large window and one door. The r o o m contained a writing table, two chairs and several pieces o f l aborat ory furniture and e q u i p m e n t along th e walls. Illumination was provided with an overhead fluorescent light. Procedure. T h e animal was allowed to explore the e m p t y apparatus for 10 min on three successive days. In t he actual experiments each food cup was baited with a 100 mg pellet o f Larsen's diet. The rat was than placed on the central p l a t f o r m and left in the apparatus until all pellets were removed from the f o o d c u p s or until 10 min elapsed. The sequence o f choices was recorded, a choice being defined as the rat placing all four legs into an arm. T w o to
345 t h r e e trials w e r e given p e r d a y w i t h a m i n i m u m intertrial interval o f 2 h. A d d i t i o n a l f o o d was given t o t h e a n i m a l d u r i n g 30 m i n a f t e r c o n c l u s i o n o f t h e last trial. T h e a n i m a l s w e r e t r a i n e d f o r 10 d a y s in t h e 8 - a r m m a z e , 5 d a y s in t h e 6 - a r m m a z e , 3 d a y s in t h e 8 - a r m m a z e again, a n d finally 5 d a y s in t h e 12-arm maze.
Results and discussion T h e first 5 d a y s in t h e 8 - a r m m a z e w e r e n o t e v a l u a t e d q u a n t i t a t i v e l y b e c a u s e t h e animals o f t e n did n o t s u c c e e d in ~etting all pellets d u r i n g the a l l o t t e d 10-min interval. A f t e r t h e rats b e c a m e familiar w i t h t h e task, t h e y avidly c o n s u m e d t h e pellet e i t h e r d i r e c t l y at t h e f o o d c u p or a f t e r r e t u r n i n g to the central platform, and rapidly headed for another arm, sometimes h e s i t a t i n g at t h e e n t r a n c e a n d w i t h d r a w i n g b e f o r e m a k i n g t h e n e x t choice. F r o m t h e sixth d a y all a n i m a l s c o n s u m e d all pellets. T h e results are s u m m a r i z e d in T a b l e 1, p r e s e n t i n g (a) t h e m e a n n u m b e r o f d i f f e r e n t a r m s c h o s e n w i t h i n t h e first 6, 8 or 12 choices; {b) t h e m e a n n u m b e r o f choices r e q u i r e d t o o b t a i n all pellets; a n d (c) t h e p e r c e n t a g e o f errorless trials. T h e rats s u c c e e d e d in c h o o s i n g 7.7 d i f f e r e n t a r m s in t h e first 8 choices d u r i n g d a y s 6 t o 10 o f training in t h e 8 - a r m m a z e , and t h e i r p e r f o r m a n c e f u r t h e r i m p r o v e d t o 7.9 d i f f e r e n t a r m s d u r i n g t h e s e c o n d training p e r i o d in this m a z e . T h e i m p r o v e m e n t was still m o r e c o n s p i c u o u s in t h e n u m b e r o f c h o i c e s r e q u i r e d t o o b t a i n all pellets a n d in t h e p e r c e n t a g e o f errorless trials w h i c h increased f r o m 7 0 . 9 t o 85.2. E x c e l l e n t p e r f o r m a n c e was f o u n d also in t h e 6- a n d 1 2 - a r m m a z e . In fact, t h e i n c i d e n c e o f errors in t h e 1 2 - a r m m a z e was n o t significantly higher t h a n in t h e 8 - a r m m a z e . T h e high p e r c e n t a g e o f errorless trials in all m a z e s m a d e it possible to c o n c e n t r a t e f u r t h e r analysis o n this t y p e o f r e s p o n d i n g . O l t o n a n d S a m u e l s o n ( 1 9 7 6 ) c o n c l u d e d f r o m t h e i r results t h a t t h e rats d o n o t utilize a n y p a r t i c u l a r r e s p o n s e s e q u e n c e s in solving t h e task. T o t e s t f o r
TABLE 1 PERFORMANCE MEASURES OF RATS IN MAZE 1 AND MAZE 2 Type of maze
1 1 1 1
2 2 2
Number of trials 117 117 54 107 34 83 78
Number of arms 8 6 8 12 6 8 12
Measure a
b
e
7.7 5.9 7.9 11.7 5.8 7.7 11.7
8.6 6.0 8.1 12.3 6.2 8.5 13.1
70.9% 95.7% 85.2% 79.4% 82.3% 81.9% 60.2%
12
11
10
9
8
7
6
5
4
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3
0.0
17.6
2
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2
Choice
0.0
9.4 23.4 1.2 6.4 1.2 2.1 1.2 17.0 9.4 8.5 8.2 6.4 12.9 8.5 12.9 4.2 23.5 14.9 9.4 8.5
10.6
3
4.7
8.5
4.7 19.1 7.0 10.6 1.2 4.2 11.7 8.5 11.7 4.2 9.4 12.7 7.0 4.2 24.7 6.4 9.4 14.9 8.2 6.4
4
7.0
0.0 6.4 6.4 8.2 14.9 11.7 21.3 14.1 4.2 9.4 6.4 5.9 12.7 24.7 10.6 8.2 10.6 4.7 0.0 5.9 6.4
5 2.3 10.6 9.4 0.0 5.9 4.2 17.6 21.3 10.6 2.1 5.9 10.6 24.7 12.7 4.7 8.5 2.3 10.6 7.0 14.9 9.4 4.2
6 2.3 34,0 3.5 0.0 17.6 2.1 11.7 14.9 11.7 0.0 25.9 10.6 4.7 9.5 2.3 12.7 4.7 4.2 8.2 6.4 7.0 6.4
7 1.2 27.6 7.0 2.1 18.8 4.2 14.1 0.0 23.5 6.4 7.0 19.1 5.9 2.1 2.3 2.1 3.5 17.0 7.0 8.5 9.4 10.6
8 2.3 10.6 16.5 6.4 11.7 4.2 29.4 14.9 7.0 4.2 2.3 10.6 2.3 2.1 7.3 10.6 4.7 10.6 5.9 8.5 10.6 17.0
9
2.1 8.2 10.6 21.2 31.9 10.6 10.6 5.9 4.2 3.5 14.9 4.7 2.1 3.5 6.4 11.7 6.4 10.6 6.4 7.0 4.2
12.9
10
0.0 8.5 7.0 14.9 0.0 4.2 10.6 17.0 10.6 10.6 2.3 8.5 7.0 10.6 5.9 8.5 9.4 6.4 4.7 10.6
28.2
14.1
11
0.0 4.7 12.7 0.0 6.4 0.0 2.1 1.2 10,6 9.4 2.1 15.3 19.1 9.4 10.6 7.0 6.4 7.0 10.6 9.4 19.1
31.7
12
D I S T R I B U T I O N O F A R M S S E L E C T E D ON S U B S E Q U E N T C H O I C E S IN T H E 1 2 - A R M M A Z E 1 ( N O R M A L T Y P E , n = 8 5 ) A N D M A Z E 2 ( B O L D TYPE, n = 47)
TABLE 2
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7
8
Fig. l . D i s t r i b u t i o n of t h e l o n g e s t s e q u e n c e s o f successively s e l e c t e d a d j a c e n t m a z e c h a n n e l s in Maze 1 (M1), Maze 2 (M2) a n d in c o m p u t e r - s i m u l a t e d r a n d o m b e h a v i o r (CSRB). Abscissa: s e q u e n c e l e n g t h r. O r d i n a t e : p e r c e n t a g e o f trials w i t h c o r r e s p o n d i n g r value. T h e full a n d d a s h e d curves in M l l , 12 c o r r e s p o n d t o t h e first a n d s e c o n d t r a i n i n g p e r i o d in t h e 8 - a r m Maze 1. N o t e t h a t i n c i d e n c e o f long s e q u e n c e s o f a d j a c e n t choices is s i g n i f i c a n t l y a b o v e c h a n c e in Maze 1 a n d s i g n i f i c a n t l y b e l o w c h a n c e in Maze 2.
348 in shown in Table 2. For each trial was also not ed the longest sequence, r, of contiguous choices, i.e. when the animal selected the arm adjacent to the previously chosen one. (For instance, r = 4 for the sequence 1 2 3 4 6 7 5 8 and r = 1 f o r the sequence 1 5 8 3 7 2 4 6.) Th e radial maze task can be solved w i t h o u t error in ( n - - 1 ) ! ways. R a n d o m sampling f r om the set of errorless runs was simulated by a comp u t er program written in BASIC and i m pl e m ent ed on a PDP 8 minicomputer. Comparison of the observed and c o m p u t e d data (Fig. 1) shows that the rats t e n d e d to choose the contiguous arms much more frequent l y than corresponds to r a n d o m choice of errorless strategies. In the 12-arm maze the value o f r = 12 was e n c o u n t e r e d in 21.2% of trials, although the theoretically predicted incidence was only 5 × 10 -s. Similar t e n d e n c y was seen in the 6-arm maze and, more clearly, in the 8-arm maze where it increased with overtraining. Whereas there was only 1% o f trials with r ~> 5 in the com put ersimulated distribution, t he y were f o u n d in 14.4% of all errorless runs in the first series o f the 8-arm maze experiments and this p r o p o r t i o n increased to 32.5% in the second series. Conversely, the incidence of runs with low r values (1, 2) was significantly lower than in the distribution of simulated r a n d o m choices. The chi 2 test indicated highly significant difference between t h e observed and c o m p u t e d distributions for the 6-, 8- and 12-arm mazes. It seems th at with increasing familiarity with the task the rats start to use m o r e and more the parsimonious strategy of contiguous choices, which in t h e e x t r e m e case simplifies the task to r e m em beri ng the initial position and direction o f turning. This t e n d e n c y is also reflected in the distribution of arms chosen in successive choices. T he preference for contiguous arms leads t o a higher than e x p e c t e d selection of arms 2 and 12 on choice 2 and to an increased probability o f choices along t he diagonal in Table 2. This nonr a n d o m distribution is b e t t e r expressed in the first half of the choice field, however, and is significantly di f f er e nt from the values predicted by the r a n d o m choice model only when all diagonal cells are added. In this case we obtain 29.2% for t he first and 34.4% for the second period of the 8-arm maze training, respectively ( e x p e c t e d value 26.5%), 54% for the 6-arm maze ( e x p e c t e d value 36.0%) and 34.2% for the 12-arm maze (expected value 17.4%). The differences bet w e e n the observed and predicted values are significant at the P ~ 0.05 level (binomial comparison). EXPERIMENT 2 T he results o f E x p e r i m e n t 1 suggested t hat overtrained rats solved the radial maze task b y using a strategy which m a y considerably reduce demands o n spatial m e m o r y . Contiguous choices were facilitated because the animal backing o u t f r o m t h e last selected channel almost faced the adjacent arm, which usually r epr e s ent ed t he nearest cor r ect choice. T he rats did n o t return t o t he c e n t e r o f t he pl a t f or m b e t w e e n choices and the e x p l o r a t o r y activity was strikingly reduced. In o r d e r t o prevent the d e v e l o p m e n t of such stereo-
349 t y p e d behavior, t h e a p p a r a t u s was m o d i f i e d in a way which f o r c e d the animal t o r e t u r n t o the c e n t e r o f the p l a t f o r m a f t e r each c h o i c e and which negatively biased c o n t i g u o u s choices.
Method Subjects. T h e 9 rats used in E x p e r i m e n t 1 were also e m p l o y e d in Experim e n t 2.
Apparatus. T h e m a z e used in E x p e r i m e n t 1 was m o d i f i e d . A hole 8 cm in d i a m e t e r was m a d e in the c e n t e r o f the p l a t f o r m , which was raised b y 3 adjustable legs 5 cm above the floor. T h e arms o f the maze were m a d e longer (31 cm) and were e q u i p p e d o n b o t h ends with o n e - w a y swing doors. A 5 cm high p l a t f o r m b e l o w t h e far end o f each arm facilitated t h e d e s c e n t o f the animal to t h e floor. A 50 cm high plexiglass wall c o n s t r u c t e d o f 33.5 × 50.0 c m plexiglass sheets a r o u n d the a p p a r a t u s separated the m a z e f r o m the rest o f the r o o m . Procedure. T h e rats were allowed to e x p l o r e the new a p p a r a t u s with the swing d o o r s raised and t h e f o o d cups e m p t y for 10 min. T h e animals were t h e n t r a i n e d for 2 d a y s t o o p e n t h e swing doors, t o d e s c e n d f r o m the far end o f t h e arms on the floor, t o r e t u r n b e l o w the p l a t f o r m and t o climb o n t o it t h r o u g h t h e central hole. F r o m d a y 3 all animals m a s t e r e d the new task and their b e h a v i o r was q u a n t i t a t i v e l y evaluated. T h e y were trained for 2 days (34 trials) in t h e 6-arm maze, 4 days (83 trials) in the 8-arm maze and 3 d a y s (78 trials) in t h e 12-arm maze. Otherwise the p r o c e d u r e was the same as in E x p e r i m e n t 1. Results and discussion Switching f r o m A p p a r a t u s 1 t o A p p a r a t u s 2 did n o t r e d u c e t h e perform a n c e o f the animals. As s h o w n in Table 1, m o s t p e r f o r m a n c e measures were slightly worse b u t n o n e o f these d i f f e r e n c e s r e a c h e d the level o f statistical significance. Analysis o f the errorless trials s h o w e d , h o w e v e r , t h a t the b e h a v i o r o f the animals in A p p a r a t u s 2 was strikingly d i f f e r e n t f r o m the b e h a v i o r o f t h e same animals in A p p a r a t u s 1. D i s t r i b u t i o n o f c o n t i g u o u s choices (Fig. 1) revealed a m a r k e d a v o i d a n c e o f a d j a c e n t arms, m a n i f e s t e d b y a c o n s i d e r a b l y greater t h a n e x p e c t e d incidence o f sequences c o r r e s p o n d ing to r = 1 and r = 2. A f t e r r e t u r n i n g t o the central p l a t f o r m f r o m the previously c h o s e n arm, t h e animals usually faced t h e o p p o s i t e arm, and selected it in m o s t eases. This is r e f l e c t e d in t h e d i s t r i b u t i o n o f arms c h o s e n o n the s e c o n d c h o i c e (Table 2). T h e r e was a m a r k e d p r e f e r e n c e for arms o p p o s i t e t o arm 1, t h a t is, for arms 4, 5 and 7 in t h e 6-, 8- and 12-arm mazes, respectively. On t h e third c h o i c e p r e f e r e n c e shifted t o the vicinity o f c h a n n e l 1, b u t the choices were r a n d o m i z e d m o r e r a p i d l y t h a n in A p p a r a t u s 1. T h e m o d i f i c a t i o n s w h i c h changed A p p a r a t u s 1 t o A p p a r a t u s 2 achieved
350 the purposes of preventing contiguous choices and forcing the animal to rely on spatial m e m o r y . On the other hand, preference for non-contiguous choices indicated that responding in Apparatus 2 was also non-random. The latter bias does not lead, however, to a simpler solution of the task, since starting with the third choice the animal had always to choose between closely adjacent e m p t y and baited channels. Apparatus 2 seems to be better suited, therefore, for a prolonged experimentation with the same animal. GENERAL DISCUSSION The present results confirm previous reports (Olton and Samuelson, 1976; Olton et al., 1977; Zoladek and Roberts, 1978} describing the behavior of rats in the radial maze. Although rats of a different strain were used and the apparatus was considerably modified, the overall performance measures were almost the same. Restricted access to visual extra-maze cues did not reduce the animal's performance in the closed maze. This is in accordance with the findings by Zoladek and Roberts (1978) who showed t h a t blind rats are capable of accurate orientation in the elevated 8-arm maze (7.5 different alleys entered in first 8 choices). When other cues are not available, spatial orientation depends on vestibular, kinesthetic and tactile cues (Beritoff, 1965), which are probably more abundant in the closed maze than in the elevated maze. The main advantage of the mazes used in the present study is that they provide for better control of the animal's movement through the maze. In Maze 1 the animal moves into and backs out from the individual alleys and thus assumes the same body--maze relationship when entering and leaving a maze arm. In Maze 2 the rat must pass through the alleys in one direction only but can return to the choice platform through a multitude of routes {ranging from a straight run towards the central opening to prolonged exploration of the outside of the maze) which all converge on the center of the choice platform. The obligatory return to the same starting point seems to be a particularly desirable feature of Maze 2 which practically eliminates solution of the task by response chaining. The significance of the maze type is indicated by the quantitative characteristic of the choice behavior of the rats. Whereas Olton and Samuelson {1976) concentrated their analysis on the distribution of errors, the present paper attempts to elucidate the behavioral strategies underlying errorless solution of the task. This is especially important when incidence of errors rapidly decreases with overtraining. Olton and Samuelson {1976) reported that they analyzed the order of choices made by their rats, but they do not give the details of this analysis. They state that 'rats tended not to choose the adjacent arms although occasionally there were sequences in which 2, 3, or 4 adjacent arms were chosen in order'. They also feel that the rats preferred 'the arm 90 ° from the
351 arm just chosen'. Several reasons m ay a c c o u n t for the difference between the above claims and the present results. (1) Olton and Samuelson (1976) tested their animals at a lower level of overtraining a n d / o r performance. This is indicated by the lower percentage o f errorless trials in their experiments (46.7% in their Table A2 as compared with 70.9% in the first series of the present 8-arm maze experiments). (2) An i m p o r t a n t difference between Apparatus 1 and the apparatus used by Olton and Samuelson (1976) is that the latter was an elevated maze. Although the authors do not state it explicitly, the rats probably turned around after reaching the food cup and reentered the central platform with the head first, i.e. similarly to the present Apparatus 2. On the cont rary, rats could n o t turn in the tubular arms of Apparatus 1 and had to back to the central platform. One would expect that the rats of Olton and Samuelson (1976} would behave m or e like rats in Apparatus 2, but since t hey were not obliged to return to the center of the platform, t h e y developed an intermediate behavior which may well a c c o u n t for preference of 90 ° choices. (3) Closer analysis of Table A1 of Olton and Samuelson (1976) shows that the sequences o f contiguous choices with r ~< 4 were e n c o u n t e r e d in 16.1% o f errorless trials, i.e. a b o u t 4.6 times more frequent l y than would correspond to r an d o m incidence. The difference is reliable at the P < 0.05 level and shows that the rats actually chose adjacent arms more frequently than would co r r es p o n d to r a ndom selection from the set of correct strategies. The t e n d e n c y to turn in one direction and to choose the adjacent arms was clearly expressed in a later study (Olton et al., 1977) using a 17-arm elevated maze. Rats returning to the central platform entered an adjacent alley in 30% cases and the second ne xt alley in 42% cases. Olton and Werz (1978) f o u n d in the same 17-arm maze t hat adjacent alleys were chosen most frequently (35%}. When, after return of the animal to the central platform, all doors to arms were closed for 10--15 sec, the rat engaged in e x p l o r a t o r y activity which disrupted the above t e n d e n c y w i t h o u t affecting choice accuracy. T he animal usually entered the nearest correct alley, close t o the entrance of which it was standing at the time when the doors were raised again. The brief c o n f i n e m e n t on the central platform seems to have a similar ef f ect on the rat's behavior as the use of the present Maze 2. Since the possibility that rats recognize the baited alleys by the smell of f o o d , or th at t h e y use scent marking of the already entered alleys was ruled o u t by careful control experiments (Olton and Samuelson, 1976; Zoladek and Roberts, 1978}, the spectacular p e r f o r m a n c e of rats can only be explained b y a memorial process. Comparison of the behavior of rats in the 6-, 8- and 12-arm mazes indicates that even the 12-arm maze was easily mastered by the experienced animals. Olton et al. {1977) found that rats overtrained in a 17-arm elevated maze made 14.6 correct responses in the 17 first choices. Although accuracy started to decline after the first 10 choices, even the 17th choice was considerably b e t t e r than chance. Extrapolation o f the decreasing probability of correct responding with the increasing
352 n u m b e r o f c h o i c e s indicated t h a t c h o i c e a c c u r a c y m a y reach c h a n c e levels at 25 t o 30 choices. A l t h o u g h f u r t h e r research is n e e d e d t o d e t e r m i n e the e x t e n t o f r a t s ' w o r k i n g m e m o r y m o r e precisely, it is o b v i o u s t h a t the 'magical n u m b e r 7 -+ 2 ' (Miller, 1956} typical for h u m a n m e m o r y span does n o t limit t h e c a p a c i t y o f rats to process spatial i n f o r m a t i o n in this particular task. REFERENCES Beritoff, I.S. (1965) Neural Mechanisms of Higher Vertebrate Behavior (W.T. Liberson, Ed. and trans.), Little, Brown, Boston, Mass. Miller, G.A. (1956) The magical number seven plus or minus two: some limits on our capacity for processing information. Psyehol. Rev., 63: 81--97. Olton, D.S. and Samuelson, R.J. (1976) Remembrance of places passed: spatial memory in rats. J. exp. Psychol. (Animal Behavior Processes), 2: 97--116. Olton, D.S. and Werz, M.A. (1978) Hippocampai function and behavior: spatial discrimination and response inhibition. Physiol. Behav., 20: 597--605. Otton, D.S., Collison, C. and Werz, M.A. (1977) Spatial memory and radial arm maze performance of rats. Learn. Motivation, 8: 289--314. Zoladek, L. and Roberts, W.A. (1978) The sensory basis of spatial memory in the rat. Anim. Learn. Behav., 6: 77--81.
