Instrumental avoidance conditioning in the Spinal rat

E&sin Researcfz Buileti~, Vat. 1, pp. 171-183, 1976. Copyright 0 ANKHO tnternationai ALI rights of reproduction in any form reserved. Printed in the U.S.A.

Inc.

Instrumental Avoidance Conditioning in the Spinal Rat

(Received 8 September

1975)

CHOPIN, S. F. AND A. A. BUERGER. Instrumental avoidance conditioning in the spinal rut. BRAIN RES. BULL. I(2) 177483, 1976. ~ Spinal rats exposed to an instrumental avoidance routine in a counterbalanced Horridge paradigm were able to achieve successively higher criteria. Both experimental and yoked control animals were capable of instrumental avoidance conditioning to incremental criteria; experimental animals exhibited retention of the task when tested. During acquisition, naive experimental animals were superior in performance to previous control animals. Due to the use of a counterbalanced Horridge paradigm, the effects of sensitization and response variability are probably not sufficient to expIain all of the results of this experiment, The data suggest that both graded acquisition and retention occur at the spinal

level. Instrumental avoidance

Spinal rat

Learning

Retention

LEARNING at the spinal level has been a subject of discussion for over thirty years 112, 13, 14, 163. The Horridge paradigm [ 111, which includes the use of a yoked animal as a control during acquisition and retention periods, has been successfully applied to spinal rats [ 3 I and spinal frogs [9]. Spinal rats and toads have also been instrumentally conditioned [4,1 Cl], although not according to a Horridge paradigm. The present report is additional evidence of learning by spinal vertebrates. The use of the Horridge paradigm, in conjunction with a counterbalanced design, reduces or eliminates possible effects due to sensitization or response variability (see ]2, 3, 6, 8] for reviews). fn an earlier experiment f4] , instrumental acquisition of successive criteria was demonstrated, but no attempt was made to test retention. In the present experiment, instrumental avoidance conditioning was attempted at the spinal level, establishing the ability of the spinal rat to achieve incrementally increasing criteria during acquisition of an instrumental shock avoidance task, and exhibit retention of this learned behavior. METHOD Animals

Eighteen female Sprague-Dawley rats were subjected to spinal cord transection under pentobarbital anesthesia (35 mg/kg, I% Procedure

Because the experimental techniques were similar to those described previously [2, 3, 4, 91, only a brief description, emphasizing the differences, is included here. Two to four days after surgery, the animals were paired and trained (Table 1). They were placed in a restraining ~_II

apparatus and allowed a 5 min shockless baseline period. If necessary at the end of this period, animals were repositioned in the restraining apparatus and a new baseline period begun. The experimental procedure was divided into 2 runs, each run consisting of 2 phases. In the first, or training phase of Run 1, both an experimental animal and its yoked control animal were subjected to electrical stimulation contingent on the leg position of the right leg of the experimental animal. A Teflon-coated, baretipped touch electrode was inserted through the plantar surface of both animals. In the resting leg position, this electrode was submerged I mm in an aqueous solution. Whenever this electrode made contact with the solution, a speciaily designed electronic switch ]9] defivered a 60 Hz, 0.8 A current to 2 platinum shock electrodes, one located in the dorsum of the foot, the other between the greater trochanter and the ilium. When the experimental anirnal’s leg was in a relaxed extended position which allowed the touch electrode to enter the solution, both animals received a shock. Leg flexion by the experimental animal operated the electronic switch, terminating the shock. Thus, shock to the yoked control animal was independent of the control animal’s leg position, but dependent on the leg position of the experimental animal. The electronic switch also produced a digital signal indicating whether the electrode was in or out of the solution. This signal was recorded on a Grass polygraph (Model 7B). With time, the experimental animal tended to maintain a flexed leg position; this animal was considered trained at the end of 2 consecutive minutes without shock, i.e., at the end of 2 min of maintained flexion. Attainment of this criterion was followed by raising the solution level to submerge the tip of the touch electrode an additional millimeter. This procedure was repeated four times; however, the experiment was ter-

’ Present address: Department of Anatomy, School of Medicine, University of South Florida, Tampa, FL 33620

CHOPIN

178 minated if a 30 min period occurred during which the experimental animal failed to reach criterion. At the end of the training phase of each run: the animals were made electrically independent. During this testing phase, delivery of shock to the leg of each animal was contigent on the leg position of that animal. During the testing phase of each run, the training procedure described above was repeated with the yoked control animal; the criterion was raised 4 times, such that the electrode tip was submerged a distance of 1 mm each time, or until a 30 min period occurred during which the animal failed to reach criterion. Thus, during the testing period, the experimental animal was tested for retention of this shock avoidance task and the yoked control animal was tested for acquisition of this task. At the end of the first run, the animals began a 1 hr rest period before beginning Run 2. Arr exception to this schedule occurred in Animals 5 and 6 in which one day elapsed between the first and second runs. Run 2 was identical to Run I, except that the contralateral legs were used, and the animal which had served as the yoked control animal in Run 1 served as the experimental animal in Run 2 and vice versa. KESULTS Because of the considerable variability in the behavior of individual animals, the performance of an above-average and below-average pair of animals is shown in Figs. 1 and 2; please see figure legends for a detailed description of the results of these representation individuals. The performance of all pairs is summarized in Table 1; the figure legends may aid in understanding this table.

fn Table 1, the leg position of the control animal during the 2 min criterion periods (in which the experimental animal’s leg was fiexed with the electrode out of the solution) is indicated by an Y’ if the control’s electrode were in the solution, or an “0” if the electrode were out of the solution. A sign test [ 171 based on the data obtained in the training period of Run 1 was significant at the p
When the number of successful criteria achieved by the experimental animals during training in Run 1 is compared to that same number in Run 2, no significant difference is observed (p>O.OS); likewise, when the training of the control animals during testing in Runs 1 and 2 is compared, IlCl significant difference between runs is observed (r~>O.05). All probability levels in this and the following sections were calculated according to the Mann-Whitney I_I Test. Since both of these calculations were not significant, the data on the number of successful criteria achieved by the

ANI) BUERGER

TABLE I MINUTES ‘TO SUCCESSIVE CRITERIA FOR NINE PAIRS OE: SPINAL RATS TRAINED BY INSTRUMENTAL AVOIDANCE CONDITIONING Animal NW&r

Vertebral Level of Tronrecrlon

6

R&z 1

&

._k!Z!X.

2

3~ 7 4, 7’

from

3t

‘, *_ ‘_ 1s

0.12.32. Ki

2

$, 1. s cc

5, 7’

4,8, IQ 11’

21.

2

6,28

3

2

//

NT

2

8,26, 7

2

0

i, I

20 NT

Ttrt

NT” 4

4

tot,,

NT

3, 3 3, 9

Id

2

11, 6, 4, 3

2

i

,

5

8

2

0, 0, i, i

3, 8, 3, 6

7, I8

5

9

3

3,34,5

2

1, I 0

NT

IO

3

b i 8,0

17

3, 6.13

3

1.,/t

Nl

3

NT

I2

3

13

3

Id

3

I5

4

16

d

a,i_bt

63

4

9, 4, 13~

113

d

‘, i,

I8

4,4. kra

3. 3 3, 4

9~15. 4. 3

3

/ i

NT

6

3

1. 1,

1. 8

17, 6, 11,5

NT

I

2 3. 3, 3, 3 It

1

5, IO, 3

2

-

14

NT

6 9 3,lO. 3 6 3

“These times represent the number of minutes to successive criteria during acquisition by the experimental animal in the training period or by the control animal in the testing period. tThis is the minute at the end of the initial 2 min period dtiring which the experimental animal maintained the flexed leg position in the testing session. $The letters designate the leg position of the control anu~la] during the 2 min periods when the Ieg of the experimentaI ankaI was flexed in the training period. The i indicates that the leg of the control animal was extended into the solution; the o indicates that it was flexed out of the solution. ONT designates an animal that did not reach the first criterion after 30 min of training. experimental animals during both training sessions can be pooled and compared with the similarly pooled data of the control animals during both testing sessions. According to this comparison, the performance of the control animals was inferior to that of the experimental animals (p~O.05). The calculations described above were repeated using the time in minutes to the first successful criterion rather than the number of successful criteria achieved. A comparison between the training of the e~pe~menta1 animals and the training of the control animals in Run 1 was not significant (p>O.OS). When this same comparison was calculated for Run 2, there was no significant difference observed (p>O.O5). A comparison of the performance of the experimental animals during training in Run I versus that in Run 2 was also not significant (p>O.OS). Similarly, a comparison af the performance of the control animals during testing in Run I versus that in Run 2 was not significant (p>Q.OS). Therefore, a comparison of the data pooled for both runs and based on the performance of the experimental animals during training versus that of the control animals during testing could be made. This compari-

AVUI~AN~E

~~N~ITIUN~~e

IN SPINAL RAT TRAIN

300

i

Y

2 fE

O TEST

TRAIN im tIII

7lwJwnw \ *

TIME-MINUTES FIG. 1. The performance of an above-average pair of animals (Numbers 1 and 2 in Table 1i. Animal 1 was the experimental animal and Animal 2, the yoked control animal. In Run 1 the right leg was trained (upper portion, between vertical axis and line). Animal 1 reached the first criterion at the end of Min 3; at this time, the animal’s leg was flexed and the tip of the touch electrode was 1-2 mm above the surface of the solution. The solution level was raised to submerge this tip 1 mm, a total rise of 2-3 mm, and training was reinstated. The second criterion was reached at the end of Min 7, and the procedure described above was repeated. The third and fourth criteria were reached at the end of Mins 4 and 7, respectively. Immediately after reaching the fourth criterion, the animals were made electri~ily independent and the testing phase begun. During the first min of the testing period of the first run {upper portion, to the right of the verticai line), the experimental animal received a few shocks, but thereafter spent most of the time with the Ieg flexed out of the solution. The first 2 min shockless period for the experimental animal had elapsed at the end of Min 3; therefore, this number is listed as such in Table 1. The yoked control animal reached the first criterion at the end of Min 5, whereupon the solution level was raised and training reinstated as previously described. The second criterion was reached at the end of Min 7; the animal failed to reach the third criterion after 30 min of training; hence, this portion of the experiment was terminated and the animals began a 1 hr rest period prior to Run 2. In the second run for this pair, Animal 2 served as the experimental animal and Animal 1 as the yoked controf animal; the left leg was trained. During the training session, Animal 2 reached criterion at the end of Mins 4,8,10, and 11 flower portion, between vertical axis and line). During testing, the experimental animal sustained the flexed leg position assumed during the fourth criterion for some time, eventually relaxing its leg and receiving some shocks before Min 36 of the testing period. Between Mins 36 and 51, even more shocks were received intermittently, but after Min 51 of the testing period, a flexed leg position was maintained. ‘Fhe yoked control animal reached criterion at Mins 6, i2,32, and 10.

son was significant at the ~~0.015 level, indicating the superior training performance of the experimental animals over the testing performance of the control animals. In conclusion, the data strongly suggest that previous experience as a yoked control subject interferes with the acquisition of this task by the contro1 animals during testing.

Achievement of Successive Criteria. There are three possible ways in which the animals could have achieved successive criteria: (a) each subsequent criteria may have been attained with increasing difficulty; (b) subsequent criteria may have been achieved with increasing ease; or (c) achievement of earlier criterion may not in any way affect attainment of subsequent criteria. These possibilities were

CHOPIN

1x0

1 TRAIN

TEST

4)

cd

2 -

AND BUERGER

6t

161

TEST

c_ 300

FIG. 2. The performance of a relatively poor pair of animals (Numbers 11 and 12 in Table 1). In Run 1, animal 11 was the experimental animal, the Animal 12, its yoked control. Animal 11 failed to reach the fist criterion after 30 min and this portion of the experiment was terminated (upper portion). Testing began immediately and Animal 11 maintained a flexed leg position from Mins 2-4, but thereafter continued to receive shock. Because Animal I1 did not reach criterion within 30 mm, NT (for not trained) is entered on Table 1. During testing, the control animal (Number 12) reached criterion at the end of Mins 4, 4, 6, and 16. Following attainment of the fourth criterion, this portion of the experiment was terminated and the animal were allowed to rest one hour before beginning Run 2. In Run 2, Animal 12 served as the experimental animal and Animal 11 as the yoked control animal; the left leg was trained. Animal 12 reached criterion at the end of Mins 3, 3, 3, and 4. Once the fourth criterion was reached, testing was initiated (lower portion, to right of vertical line). During testing, the experimental animal maintained the flexed leg position assumed during the fourth criterion. Thus, in terms of the testing period, the first 2 min without shock occurred at the end of Mm 2, and this number was entered in Table 1. The control animal, Animal 11, failed to reach the first criterion after 30 mm, and the experiment was terminated.

investigated by calculating the linear least mean squares regression line for the times to criterion for each animal; a sign test was applied to the slopes of these lines. No significant difference was obtained in Run 1 (p = 0.274); however, a significant difference was seen in Run 2 (p = 0.035), and in the pooled data from both Runs 1 and 2 (p = 0.032). The significant results suggest that each subsequent criterion tended to be achieved with increasing difficulty. Retention. The probabilities reported below were calculated using the Mann-Whitney U test [ 171 and the times in minutes to the first criterion. A comparison of experimental and control animals

during the testing period of Run 1 revealed a superior performance by the experimental animals (pO.OS). When the performance of both groups of experimental animals during testing was compared for Run 1 versus Run 2, no significant difference was observed (p>O.OS). Likewise, a comparison of the performance of the two groups of control animals during testing in Run 1 versus Run 2 yielded no significant difference (p>O.O5). Since these last two calculations revealed no significant differences between the runs, the data could be pooled and further analyzed. Comparison of

AVOIDANCE

CONDITIONING

181

IN SPINAL RAT

the testing performance of the experimental animals with that of the control animals yielded a significant difference at the pO.OS). When the performance of the two groups of experimental animals was compared during training, there was no significant difference (p>O.OS). difference was established by Likewise, no significant comparing the performance of the two groups of experimental animals during testing (p>O.O5). Since there was no significant difference established by the last two comparisons, the data obtained in Runs 1 and 2 could be pooled and recalculated. When the performance of both groups of experimental animals during training was compared with that during testing, a significant difference was observed @<0.0322). Thus, the combined data indicate that the experimental animals exhibited an improved performance during testing when contrasted with that observed during training. DISCUSSION

Acquisition Sensitization cannot account for all of the observed results because there was a significant difference between the experimental and control animals during the training sessions. When the legs of the experimental animals were flexed during the 2 min criterion period, the legs of the control animals tended to be extended. (Except as noted, all statistical results noted in this section may be found in the Results section.) If sensitization were involved, the legs of the control animals should have tended toward the same flexed position as those of the experimental animals, because both groups had received the same intensity and pattern of shock. The data therefore suggest that the leg withdrawal response of the experimental animals resulted from acquisition of instrumental avoidance conditioning rather than from sensitization. When the performance of the experimental and control animals was compared using the pooled training and testing data, the experimental animals were superior to their control animals, both in number of criteria achieved and in the time required to reach the first criterion. Acting as a yoked control animal apparently inhibited the subsequent acquisition of the task. This inhibition was evident only for the leg which the animal used when it was a yoked control, since there was no significant difference between the two runs of experimental animals in training nor between the two runs of control animals in testing. Thus, an animal was inferior only with the leg it has previously used as a control. Acquisition by the contralateral leg was not impaired. During the testing period, the experimental animals were superior to the control animals; this strongly suggests retention of a previously acquired instrumental avoidance task. If sensitization were responsible for performance during the training period, there should have been no difference between the two groups of animals during the testing period. In summary, the lines of evidence suggesting that this phenomenon is acquisition of instrumental avoidance learning rather that sensitization are: (a) the control animals tended to an extended leg position during the

periods when the legs of the experimental animals were flexed; (b) the experimental animals during training were superior to the control animals during testing; and, (c) the experimental animils were superior to the control animals during testing: Retention As mentioned above, during testing the experiments animals attained the flexed leg position more rapidly than did the control animals. Also, the performance of the experimental animals during testing was superior to that during training. This finding is evidence for retention of the task during testing. A significant difference in acquisition was observed in the training of experimental animals and the testing of their control animals. Acquisition by the control animals tended to be inferior to acquisition by the experimental animals. There are at least two explanation for this observation: response variability and learned helplessness. Response

Variability

Response variability has been suggested [5, 6, 8 1 as a possible explanation for the inferior performance of the yoked control animals; two types may be distinguished: inter-animal variability, and variability in responsiveness between the neutral and noxious zones above and below the fluid. (See [2] for a more extensive discussion.) If inter-animal response variability were a factor and experimental animals were more sensitive to stimulation than were the control animals, the experimental animals might demonstrate a superior performance caused by variations in animal responsiveness rather than by instrumental conditioning. To eliminate the possibility of interanimal variability, enormous numbers of animals would have to be run both as randomly yoked pairs and in all relevant patterns and intensities of stimulation. This is not feasible at the present time. However, the counterbalanced design used in this experiment minimizes the significance of inter-animal variability; animals which had previously performed as yoked control animals performed successfully as experimental animals, Indeed, there was no significant difference in the performance of the experimental animals of Run 1, for whom the experiment was novel, and the performance of the exper~ental animals of Run 2, which, using the contralateral foot, had previously served as yoked control animals. Variability in responsiveness within the neutral and noxious zones above and below the fluid can also be used to explain some but not all of the observed results 12, 5, 6, 81. In essence, the argument is that the zone above the fluid is neutral for the experimental animal; hence this animal might tend to not move from there. This zone is not neutral for the control animal during training; therefore, it might have less tendency to remain there than the experimental animal. However, this approach fails to account for at least two features of the available data. First, the control animals during training have a significant tendency to maintain their legs below the fluid level t&O.00003 according to a sign test [ 171). If this type of response variability were a significant factor, one would expect the control leg position to be randomly distributed with respect to fluid level. Second, the evidence for retention as presented above and for interference as discussed below both suggest that past experience is

182

CHOPIN AND BUERCEK

influencing present behavior. It is very difficult for arguments based on either type of response variability to account for these observations. Yoked control headless cockroaches @‘eripiuneta sp.) have beer, ,‘eported to take longer to acquire the avoidance response ‘ir the testing phase than did the experimental cockroaches in the training phase [ 11 J . Similar results have been tentatively suggested for spinal rats [3]. The data presented here strongly suggest that previous experience as yoked controls interferes with the acquisition of this instrumental avoidance response by the control animals during testing. (Learned helplessness as proposed by Seligman ef at. [ 151 is one explanation for this interference, although alternatives have recently been proposed 11,181). Most explanations of the present results based on the phenomena of response variability predict a tendency toward (1) a fixed effect of shock delivery on the foot position of the control animal, and/or (2) a tendency toward a fixed relation between the foot position of the control animals and the delivery of shocks to that animal. For example, one could ascribe the differences between the experimental and control animals during testing to the fact that the experimental animals made many more responses than the control animals during training. The validation of this hypothesis would require that the control animals of pairs showing relatively poor retention should tend to respond at levels roughly equal to those of their associated experimental animals. As will be documented below, this is not the case. In fact, pairs of animals responding at almost identical levels show excellent retention. (See for example Run 1 of Animals 1 and 2 in the paragraph below and in Fig. 1.) Data from the present experiment suggest that there is no tendency toward a stable relation between the foot position of and shock delivery to the control animals. Shock can be delivered to the control animals with one of four effects in relation to fluid level: (1) the control animal’s foot may remain entirely within the fluid; (2) the foot may move from within the fluid to out of it; (3) the foot may move from out of the fluid to within it; and, (4) the foot may remain entirely out of the fluid. These relations for all the control animals in this experiment are summarized below; the data are subdivided by animal pair, by experimental animal number and by criterion for the associated experimental animal; the first number is the total number of shocks received during each criterion and the next 4 numbers are the shocks received in the in, m-to-out, out-to-in, and out positions as described in (1) through (4) in the preceding sentence. If the experimental animal was not trained, its number is followed by NT. To aid in identifying inferior training sessions, those experimental animals whose time to criterion during testing was less than or equal to that of their associated control animals are designated by an asterisk (*I. First pair: Animal 1 - Criterion 1: 1, 0, 1, 0, 0; Criterion 2: 25, 2, 23, 0, 0; Criterion 3: 17, 0, 17, 0, 0; Criterion 4: 42, 0,40, 2, 0; Animal 2 - Criterion 1: 4, 0,4,

0, 0; Criterion 2: 56, 56, 0, 0, 0; Criterion 3: 1141, I 141, 0, 0, 0; Criterion 4: 1961, 1961, 0, 0, 0. Secoffd puir: Animal 3 --- Criterion 1: 1108. 6, 0, 0, 1102; Criterion 2: 2492, 0, 2295, 391, 6; Animal 4 Criterion 1: 55,O. 2, 5,48. Third pair: Animal 5 -- Criterion 1: 72, 0, 0, 0, 72; Criterion 2: 895, 104, 390, 0, 401; Criterion 3: 860, 398, 121, 101, 240; Animal 6 - Criterion 1: 2, 0, 2, 0, 0; Criterion 2 : 24, 24, 0, 0, 0; Criterion 3 : 18, 0, 9, 0, 9; Criterion 4: 15, 4,4, 0, 7. Fourth pair: Animal 7 -- Criterion 1: 93, 0, 0, 0, 93; Criterion 2: 32, 0, 0, 0, 32; Criterion 3: 56, 0, 1, 3, 52; Criterion 4: 1, 1, 0, 0,O; Animal 8(*) - Criterion 1: 66, 59, 5, 2,O; Criterion 2: 213,204, 2, 2, 5. Fifth puir: Animal 9 - Criterion 1: 4, 2, 2, 0, 0; Criterion 2: 2, 0, 2, 0, 0; Criterion 3: 8, 0, 8, 0, 0; Animal 10 ~- Criterion 1: 4, 0, 4, 0, 0; Criterion 2: 252, 0, 63, 0, 189;Criterion 3: 1742,0, 177, 7, 1558. Sixth pair: Animal 11 - (NT); Animal 12 ~ Criterion 1: 1, 1, 0, 0, 0; Criterion 2: 1, 1, 0, 0, 0;Criterion 3: 2, 1, 1, 0,O; Criterion 4: 52, 0, 52,0, 0. Seventh pair: Animal 13 -- Criterion 1: 18, 17, 1, 0, 0; Criterion 2: 76, 47, 29, 0, 0; Criterion 3: 12, i2, 0, 0, 0; Criterion 4: 2, 2, 0, 0, 0; Animal 14 (*) --- Criterion 1: 16. 0, 3, 2, 11. Eighth pair: Animal 15 - Criterion 1: 981,769, 212,0, 0; Criterion 2: 32, 0, 32, 0, 0; Criterion 3: 336, 0,336,0, 0; Criterion 4: 14, 0, 14, 0, 0; Animal 16(*) - Criterion 1: 92, 0, 92, 0, 0; Criterion 2: 1807, 75, 48, 9, 1675; Criterion 3: 9, 0,9, 0, 0. ~~~~i~hpair: Animal 17 -- Criterion 1: 55, 3, 28, 0, 24; Criterion 2: 98, 0, 98, 0, 0; Criterion 3: 89, 34> 55, 0, 0; Animal 18 (NT). The phenomena of sensitization and response variability do not explain all the results of this experiment. The success of the counterbalanced design in this and other experiments [9], together with the use of the yoked control paradigm, is especially difficult to interpret as sensitization and/or response variability [ 2 ] . To explain the superior performance of the experimental animal during the testing period, one must postulate that previous experience has influenced present behavior. These data suggest that the lumbosacral spinal cord of the rat is capable of acquiring and retaining learned behavior. The experiments demonstrate that the anatomically isolated spinal cord can associate leg position with shock delivery and retain that information. Analysis of the neuronal parameters involved in this response will be complex, but wilJ certainly be no more difficult than similar experiments in higher centers. ACKNOWLEDGEMEN’I We wish to thank Peggy Glenn, Sharon Jonker, and Sharon Liddle for their help during the preparation of this manuscript. This research was partially supported by the general research funds of the California College of Medicine, Department of Rehabilitation of the State of California (Grant No. SA 3542) and Sociai Rehabilitation Service (Grant No. 44-P-45064/9-12).

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