Overtraining in hooded rats under unilateral spreading depression

Physiology and Behavior, Vol. 14, pp. 363--366. Brain Research Publications Inc., 1975. Printed in the U.S.A.

BRIEF COMMUNICATION Overtraining in Hooded Rats Under Unilateral Spreading Depression 1

G RETE MYHRE 2

Institute o f Neurophysiology, University o f Oslo, Oslo 1, Norway

(Received 20 May 1974)

MYHRE, G. Overtraining in hooded rats under unilateral spreading depression. PHYSIOL. BEHAV. 14(3) 363-366, 1975. - Rats were trained in an active avoidance pattern discrimination under unilateral cortical spreading depression produced by KC1. There was no transfer of training when the rats were trained to 90% performance. One hundred per cent overtraining, however, leads to transfer of training. Unilateral spreading depression

Overtraining

METHOD

BURES [1] has demonstrated that when rats are exposed to cortical spreading depression (SD) induced by KC1, no transfer of training to the depressed hemisphere takes place. However, even though there is no evidence of transfer of instrumental performance, it is still possible that some transfer of learning takes place. This was demonstrated by McCleary [5] who showed that even if there was no interhemispheric transfer of an instrumental avoidance response in goldfish, there was transfer of the conditioned heart rate response. In line with this, there is evidence for motivation specific transfer in cats with surgically sptit brains. Seehzer [6] found no transfer of a pattern discrimination when the cats were trained under food-approach motivation, but significant transfer with shock-avoidance motivation. Carlson [2] concluded that in rats treated with the spreading depression technique, complex m o t o r responses required cortical participation, while components of active and passive avoidance learning could be stored subcortically. Kukleta [4] demonstrated transfer of active avoidance training if the rats were given additional training after the criterion was reached. The present study examines if overtraining will help transfer of avoidance training also when the correct response depends on visual pattern discrimination.

Animals The animals were 18 experimentally naive female hooded rats, approximately 3 months of age and weighing 2 0 0 - 2 5 0 g. During pretraining three animals were housed in each cage. After the operation the animals were housed separately. Water and laboratory pellets were given ad lib.

Surgery Surgery was performed with the animal under sodium pentobarbital (Nembutal R) anesthesia (15 mg/kg, IP). With the head fixed in a stereotaxic apparatus, a midline incision was made and the skull was exposed. With a dental burr two trephine holes, 4 mm in dia., were made over the parieto-occipital cortex, 3 mm posterior to bregma and 2 mm bilateral to the midline. Care was taken not to damage the dura. Five stainless steel electrodes were then implanted for EEG-recordings. These electrodes rested on dura and were fixed with dental cement. One pair of electrodes was located symmetrically 4 mm anterior to bregma and 3 mm on either side of the midline; another pair was located 7 mm posterior of bregma, and 3 mm on either side of the midline. The fifth electrode served as ground. Surrounding the trephine holes, was a circular polyethylene

1The author is indebted to Holger Ursin for valuable criticism and suggestions concerning the manuscript. 2Requests for reprints should be sent to Grete Myhre, who is now at the Institute of Medical Biology, Physiological section, University of Tromso, N-9001 Norway. 363

364

MYHRE

B

A

c

D

RF

z z

...i

iv,

LF

LO

f

60rain

lOmin

180min

I7~/~ v I ~er..

FIG. 1. EEG recordings from one rat, monitored in 3 sec intervals at successive experimental stages. A; normal, awake. B; 10 rain after application of KC1 to the left hemisphere. C; after training or testing (60 min after application of KC1). D; return to normal hemispheric activity (180 min after SD was induced by KC1). The recordings are monitored from the right (RF) and left (LF) frontal region, and the right (RO) and left (LO) occipital region. ring, 2.5 mm thick, 15 mm in outer diameter, molded on to the skull with dental cement. The EEG-electrodes were connected to the ring with insulated copper thread. Between the experiments cotton soaked in saline was placed inside the ring to prevent infection, and a lid was screwed to the top of the ring. Recovery appeared uneventfully for all animals. Cortical spreading depression (SD) was produced by applying a filter paper, 4 mm in dia., soaked in 25% KC1 solution, to one of the trephine holes. Electroencephalogram was recorded on a Grass polygraph Model 7, 10 min before and after application of KC1. As a control, EEG recording was also performed after the testing procedure. The criterion for successful SD was a marked reduction of cortical electric activity in the affected hemisphere (Fig. 1). Cortical spreading depression was induced 10 min before testing.

Apparatus The active avoidance box [7] was 22.5 cm high, and consisted of a start box (12.5 cm X 17.5 cm), a V-shaped choice area (50 cm long), and a goal box of (35 cm X 35 cm). The start box was separated from the choice area by a guillotine door. The choice area had two doors (9 cm X 9 cm) leading into the goal area. Interchangeable stimulus cards were mounted on these doors. The doors were separated by a vertical partition extending 7.5 cm into the choice area. All walls were of black Masonite (R), the floor of the start box and the choice area consisted of brass rods (4 mm in dia.) spaced 1 cm apart. Shocks of 0.5 mA could be delivered manually to parts of this floor separately. The goal area floor was made of Masonite, and the ceiling was a Plexiglas (R) lid. A 60 W light bulb was mounted just above the box.

Pre training On the first day S was placed in the start box and the guillotine door was opened. The S was shocked if she did not leave the start box within 5 sec, or did not enter the goal box within 30 sec. Both the hanging doors to the goal box were gray, and kept open. When the rats ran from the start box 10 successive times without receiving shocks, the doors were gradually closed. Then the subjects were trained on a brightness discrimination problem. A light gray and a dark gray card were mounted on the doors. For half the animals the lighter card was the positive stimulus, for the other half, the darker card was the positive stimulus. The door with the positive card admitted S to the goal box, an approach to the negative, closed door was followed by a shock of two sec duration. The positive card was placed on the left or right door in a balanced order. The animal was given 30 trials a day with an intertrial interval of 20 sec.

Training and Test Procedure Group A (control, six rats) was trained only to the 9/10 criterion. Group B(six rats) underwent surgery after pretraining. The day after surgery they were started on a visual discrimination problem (horizontal vs. vertical stripes) while under unilateral SD. Training continued until the criterion, i.e., 9 correct responses out of 10, was reached for each rat. Group C (six rats) was given additional 100 percent overtraining after reaching the criterion under the SD condition. The stimulus cards consisted of black and white 1 cm broad stripes. Half of the S's within each group were trained to go to vertical stripes and the other half to horizontal stripes. In half of the S's of group B and C the right

O V E R T R A I N I N G UN D E R U N I L A T E R A L SPREADING DEPRESSION

365

TABLE 1 THE TABLE EXHIBITS THE NUMBER OF TRIALS IN TRAINING AND TESTING SESSION FOR ALL THE SUBJECTS. MEAN AND LEVEL OF SIGNIFICANCE ARE ALSO CALCULATED.

Group A

Treatment Control

Training Trials to Criterion

X

18 23 26 20 21 23

Test Trials to Criterion 12 12 15 12 10 10

21,8 B

Trained to criterion

30 15 14 21 30 20

100 % Overtraining

t = 7,35 p<0.0005

32 15 10 21 20 27

22 30 22 16 16 19

t = 0.19 p>0.45 (n.s.)

20,8 l0 10 15 10 13 15

20,8

hemisphere was first depressed while in the other half the left hemisphere was first depressed. Assignment to groups followed a randomized procedure. The results did not differ significantly between the right and the left subgroups, and they were therefore combined for the final analysis. Both hemispheres were left untreated for 24 hr. The day after the training SD was applied to the contralateral hemisphere for groups B and C, and all the animals were then returned to the discrimination box for testing. The discrimination problem was the same as during the training situation. The results were treated with Student's t-test. All probability levels are given for one-tailed tests. Results When normal rats were trained to the criterion (9 correct of 10 trials), they needed an average of 21.8 trials in the acquisition phase and 1 1.8 trials in the testing session (Group A:/9< 0.0005) (Table 1). When rats were trained to criterion under unilateral SD and tested under contralateral SD, an average of 21.7 trials were needed in the training period, and 20.8 in the test period, demonstrating no significant saving (Group B: p>0.45).

Level of Significance Between Test and Training

11,8

21,7 C

X

t = 3,6 p<0.005

12,2

When the animals were given 100 per cent overtraining under the same conditions as Group B they needed an average of 20.8 trials during the initial training period to reach criterion and 12.2 trials in the testing situation, d e m o n s t r a t i n g a highly significant saving (Group C: p<0.005).

DISCUSSION No interhemispheric transfer of a visual pattern discrimination situation was demonstrated when the rats were trained to a 9[10 criterion under unilateral SD, and tested under contralateral SD. However, when the animals were given 100 per cent overtrainingthey exhibited a highly significant savingin the testing situation. Since the normal pattern of electrical activity was absent in the hemisphere subjected to SD under the training period, the results are interpreted as interhemispherictransfer of the performance of the task. The results are in agreement with the observations of Kukleta [4] and Carlson [2] in that avoidance tasks are transferable if overtrained. The learning in the present study, however, was more complicated, since it involved visual pattern discrimination. Even in this task extended

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MYHRE

e x p o s u r e to t h e learning s i t u a t i o n c o m p e n s a t e s for t h e depressed h e m i s p h e r e d u r i n g testing. T h e errors c o m m i t t e d b y t h e rats in t h e t r a n s f e r test were exclusively o f t h e c o m m i s s i o n type. T h e m o t i v a t i o n and t h e avoidance h a b i t , t h e r e f o r e , seem u n c h a n g e d . W h e n

o v e r t r a i n e d , t h e d i s c r i m i n a t i o n itself also b e c o m e s i n d e p e n d e n t o f the s t r u c t u r e s a t t a c k e d b y the SD. T h e relative i m p o r t a n c e of cortical vs. s u b c o r t i c a l s t r u c t u r e s for t h e tasks involved m a y change during overtraining.

REFERENCES 1.

2. 3. 4.

Buret, J. Reversible decortication and behavior. In: The Central Nervous System and Behavior 11, edited by M.A.B. Brazier, New York: Macy Foundation, 1959, pp. 207-248. Carlson, K. Cortical spreading depression and subcortical memory storage. J. comp. physiol. Psyehol. 64: 4 2 2 - 4 3 0 , 1967. Kohn, B. Spreading depression and stimulus control of interhemispheric transfer. Neuropsychologia 5: 2 7 5 - 2 8 6 , 1967. Kukleta, M. The use of unilateral cortical spreading depression in the study of subcortical storage of memory traces in rats. Physiol. Behav. 2: 301-304, 1967.

5. 6. 7.

McCleary, R.A. Type of response as a factor in interocular transfer in the fish. J. eomp. physiol. Psyehol. 5 3 : 3 1 1 - 3 2 1 , 1960. Sechzer, J.A. Successful interocular transfer of pattern discrimination in split-brain cats with shock-avoidance motivation. J. eomp. physiol. Psyehol. 58: 7 6 - 8 3 , 1964. Thompson, T. and J. H. Bryant. Memory as affected by the activity of the relevant receptor. Psychol. Rep. 1:393 -400, 1955.