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Response suppression in normal and septal rats
Physiology & Behavior, Vol. 15, pp. 417--421. Pergamon Press and Brain Research Publ., 1975.
Printed in the U.S.A
Response Suppression in Normal and Septal Rats 1 GILBERT ATNIP AND DAVID HOTHERSALL
Laboratory o f Comparative and Physiological Psychology, The Ohio State University, 1314 Kinnear Road Columbus, OH 43212 (Received 7 February 1975) ATNIP, G. AND D. HOTHERSALL. Response suppression in normal and septal rats. PHYSIOL. BEHAV. 15(4) 4 1 7 - 4 2 1 , 1975 - After 11 days of lever press training on a variable interval 30 sec schedule, separate groups of septal and normal rats were exposed fi3r 10 days to one of two response-suppression procedures: extinction or differential reinforcement of other behaviors. Both procedures were effective in suppressing responding, with the septal rats taking longer to reach the suppression asymptote than did the normals. All rats then received 5 days of non-contingent food presentation. Under these conditions responding recovered to a much greater degree in rats previously given extinction than in those given DRO. Further, septal rats given extinction showed more recovery than did normals given extinction. Septal lesion
Response suppression
VI and DRO vs. extinction
R A T S with lesions of the septum show greater than normal response rates on a variety of reinforcement schedules, e.g., continuous [15], fixed interval [20], variable interval [2, 3, 6], and fixed ratio [12]. They also show unusually high response rates, relative to normal rats, on differential reinforcement of low rates (DRL) schedules [7]. Since D R L schedules make reinforcers contingent on responses that are spaced in time, septal rats typically receive few reinforcers, while emitting large numbers of responses. This ineffective behavior has been termed the "septal D R L deficit" [7]. It is both severe and long-lasting,and is not affected by either pretraining [4], or extensive D R L training [ l I ]. However, it can be attenuated by providing a stimulus which signals the availabilityof a reinforcer at the end of the D R L interval [5,14], or by training septal rats in environments which facilitatethe development of collateral mediating behaviors [21 ]. In addition to the high rates of responding noted above, septal rats show response persistence when shifted from an acquisition schedule to extinction. In extinction, septal rats typically continue to respond at greater than normal rates in spite of non-reinforcement [8, l0, 17, 19]. These results, together with the D R L data, suggest that rats with lesions of the septum are not as sensitive as normal rats to the effects of response-suppressing reinforcement contingencies. In the present experiment, we first compared the performance of normal and septal rats working for food reinforcement on a variable interval (VI) 30 sec schedule of reinforcement. In the second phase of the experiment, half of each group of rats was subjected to one of two response-suppression proccdures: extinction, in which rein-
forcement was discontinued, or differential-reinforcementof-other behaviors (DRO). In the D R O procedure, first described by Reynolds [18], a scheduled reinforcer is omitted if a response occurs during a specified interval,and so the animal must learn not to respond, possibly by developing some 'other' behaviors which are consistently reinforced. This procedure has been found to be comparable to extinction in the suppression of previouslyreinforced responding in normal rats [22,23]. W e were interested in whether D R O would prove to be effective in suppressing responding in septal rats, and if so, h o w it would compare in effectiveness with extinction. The stability of any effects produced by the two response-suppression procedures was assessed in the third phase of the experiment. All groups were exposed to a variable time (VT) 30 sec schedule, in which food was delivered on the average every 30 sec, but there was no contingency between responses and reinforcers. Uhl and Garcia [23] used this procedure with normal rats, and found that animals previously given D R O training responded less than those given extinction. That is, D R O produced a more durable response-suppression effect. W e investigated whether a similar effect would be found in rats with lesions of the septum. METHOD
Animals T w e n t y - f o u r naive male h o o d e d rats were used.
Apparatus Training
was
conducted
in
4
conventional
operant
aT his research was supported in part by grant number MH 06211 awarded to Drs. D. R. and P. M. Meyer. We gratefully acknowledge their support and assistance. 417
418 conditioning chambers. Two of the chambers were housed in sound-attenuating research chests equipped with exhaust fans. The other two were left unhoused so that the rats in them could be observed via closed-circuit television. Half of the animals in each group were trained in the closed chambers and half in the open ones. Electro-mechanical equipment for programming events and recording data was located in an adjoining room.
Procedure Surgery. Prior to the beginning of training, half of the rats received l~llateral lesions of the septum by means of stereotaxic surgery. Surgery was carried out under a combination of pentobarbital and metofane anesthesia, as described by Meyer and Meyer [16]. Coordinates for the lesions were determined from the DeGroot [1] atlas of the rat's brain. The electrolytic lesions were produced by a unipolar anode, insulated except for 0.5 mm at the tip, and an anal cathode. Current parameters were 2 mA for a duration of 15 sec. After surgery the rats were given an antibiotic and returned to their home cages. Training. Before training began, normal (i.e., nonoperated) animals were reduced to 80 percent -+ 15 g of their free-feeding body weights; rats with lesions were given a 7 - I 0 day postoperative recovery period, by which time their body weights had stabilized at approximately 80 percent of preoperative levels. They were maintained at these weight levels for the duration of the experiment by feeding them a daily ration of food after each experimental session. During the first training session, which was 45 min long, magazine training was accomplished by the delivery of 45 mg Noyes food pellets on an average of once every 30 sec. The response lever was not available during this session. The lever was available at the beginning of the next session and each rat was allowed to press it 100 times on a schedule of continuous reinforcement (CRF), with a single food pellet as the reinforcer for each response. If a rat did not begin to lever-press within 20 min, his behavior was observed using closed-circuit T.V. and the response shaped using the method of successive approximations. The animal was then allowed to obtain 100 reinforcers on CRF. All subsequent sessions were one hour long. During the first 11 of these, all rats pressed the lever for food on a VI 30 sec schedule. In the next phase of the experiment, both the normal and septal rats were divided into 2 groups. One normal and one septal group received 10 sessions of conventional extinction (Groups NE and SE). The other two groups were exposed to the DRO schedule for 10 sessions (Groups ND and SD). The DRO schedule we used had a parameter of 30 sees., .i.e., when 30 sec passed without a lever-press a food pellet was delivered; when a response occurred during the 30 sec interval, delivery of the reinforcer was delayed another 30 sec. In the final phase of the experiment, all the groups were exposed to a VT 30 sec schedule, in which food pellets were delivered, on the average, once every 30 sec, whether or not the rat pressed the lever. This phase lasted for 5 sessions. Experimental sessions were run 5 days per week and the rats were fed maintenance rations on weekends. Histology. After completion of training, rats with septai lesions were deeply anesthetized and were perfused intracardially with saline solution followed by i0 percent Formalin. The brains were removed, embedded in celloidin,
ATNIP AND HOTHERSALL and sectioned at 30 u. The sections were mounted on slides, stained with cresyl violet, and examined to determine the extent of the lesions. RESULTS
Histology Lesions of the septum were large and in all cases included the medial and lateral septai nuclei, the diagonal tract, the nucleus of the diagonal tract, the stria terminalis, the inter-stitial nucleus, of the stria terminalis, the septohypothalarnic tract, the precommissural fornix, the triangular septal nucleus, and the superior fornix. Other structures which sustained damage in some of the lesions included the anterior commissure, median and medial preoptic nuclei, stria medullaris, medial corticohypothalamic tract, medial corticohabenular tract, septohabenular tract, fimbrial nucleus of the septum, and, in one extreme case, the anterior nuclei of the thalamus. There was no apparent relation between variations in lesion placements and variations in observed behavior.
VI 30 Training The left panel of Fig. 1 shows the mean numbers of responses per session for each experimental group during VI training. Responding increased in all groups early in training. By the end of the eleventh day of training, responding in the septal rats appears to have reached an asymptote, while responding in the normal rats was still increasing. Throughout VI training, the rats with septai lesions showed response outputs consistently greater than those of the normal rats. An analysis of variance, with repeated measures, showed significant effects due to Lesion, F(1,20) = 10.71, p<0.05, and to Days, F(10,200) -6.13, p<0.05. There were no differences between the subgroups, which is to be expected since they had not yet received differential treatment.
Response Suppression DRO vs. exn'nction. The middle panel of Fig. 1 depicts the results of imposing the two response-suppression procedures, in terms of the mean numbers of responses per session, for each group. It is clear that all groups reached the same low level of responding by the sixth day of this phase. The two response-suppression procedures, DRO and extinction, effectively suppressed previously-reinforced responding in both normal and septal rats. However, the key question is whether rate of response-suppression was affected by the septal lesion and/or the type of suppression treatment. Since the normal and septal rats had different levels of response output at the end of VI training, it is most appropriate to compare their suppression performance in relative rather than in absolute terms. Therefore a baseline level of responding was computed for each rat by taking its mean number of responses for the last 3 days of VI training. Then the ratios of the number of responses on each of the 10 days of the suppression phase to this beseline level was calculated. The results of suppression training, shown in terms of these ratios, averaged for each group, are shown in Fig. 2. This figure shows that, in addition to having higher absolute levels of responding, the rats with septal lesions had higher ratios of suppression to baseline responding than did normal rats. This difference
RESPONSE SUPPRESSION IN NORMAL AND SEPTAL RATS
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FIG. 2 Response expressed in terms of group me~.n ratios of numbers of responses per day of suppression training to numbers of base line response~ decreased over days as the suppression ratios for both groups of rats decreased to asymptote. The normal rats reached the asymptote first. An analysis of variance on the ratio data for the first 5 days of suppression training
showed significant effects due to Lesion, F(4,80) = 77.0, p < 0 . 0 5 , and the Lesion x Days interaction, F(4,80) - 3.00, p<0.05. There were no statistically s i ~ i f i c a n t effects associated with the type of suppression treatment.
420
ATNIP AND HOTHERSALL
VT 30 testing. Due to equipment failure at the beginning of this phase of the experiment, one animal in each of the groups of rats with lesions of the septum had to be discarded. The data from the remaining rats, expressed in terms o f group mean numbers of responses per session, are shown in the right panel of Fig. 1. The two groups (SE and NE) trained using extinction during the suppression phase showed more responding under VT than did the two groups given DRO. Analysis of variance showed a significant effect due to type of suppression treatment, F(1,18) = 7.12, p<0.05. It also appears that Group SE recovered responding to a greater degree than did Group NE, but this lesion difference did not attain statistical significance. During successive days of VT training, the amount of responding in the two extinction groups declined systematicaily, while responding in the two DRO groups remained constant at a very low level. Analysis of variance showed a significant Days effect, F(4,72) = 2.68, p<0.05, and a significant Treatment x Days interaction, F(4,72) = 2.80, p<0.05. DISCUSSION Response rates of both groups o f rats working on the variable interval schedule o f reinforcement increased with training, but the response output of the rats with lesions o f the septum was significantly greater than that of the normal rats. Similar results showing increased respondin8 on VI schedules by septal rats have been reported by a number of previous investigators [2, 3, 6]. Both response-suppression procedures - extinction and DRO - quickly and effectively suppressed responding during the suppression phase and there was no difference in their relative effectiveness. In addition, both suppression procedures were effective in suppressing responding in both the normal rats and those with lesions of the septum. In view of the markedly increased response rates of rats with w,ptal lesions working for positive reinforcers found in this and many previous experiments [2, 3, 6, 9, 12, 13, 15], the increased resistance to extinction of previously reinforced responses
shown by septal rats [ 10, 17, 19], and the many reports of the ineffectiveness of DRL schedules in reducing response rate in septal rats [6, 11 ], the absence of any differences between the normal and septal rats at the end of suppression training is of interest. While the normal rats attained the suppression asymptote first, both groups of animals were making only small numbers of responses at the end o f suppression training. It would appear that it is possible to produce clear response suppression in septal rats by using appropriate contingencies of positive reinforcement. The results from the third phase of the experiment, in which a VT procedure was used, show that although the groups were very similar in their behaviors at the end of the suppression phase, they differed in their reactions to non-contingent reinforcement. Responding by the normal rats previously given DRO training, remained at a low level, while responding increased in the rats previously given extinction. Similar results have been reported by Uhl and Garcia [23], Uhl and Sherman [24], and Zeiler [25]. The two groups of rats with septal lesions showed the same pattern of differences due to the type of suppression treatment to which they were previously exposed. The magnitude of the difference between the two septal groups given extinction and DRO appeared to be greater than the difference between the two normal groups, but this proved to be only marginally ~lnificant statistically. It appears likely that both the normal and septal rats were able to develop some behavior other than lever pre4ming during suppression training and that they continued to emit this behavior during the non-contingent phase. Rats given extinction were not reinforced for such other behaviors during the suppression phase and so resumed lever-pressing when reinforcers were once again delivered. However this suggestion is speculative since systematic observations of the rats' behaviors were not made. The long-lasting suppression of responding produced by DRO i n both normal and septal rats is however clear evidence of the effectiveness of this response-suppression procedure.
REFERENCES
1. DeGroot, J. The rat forebrain in stereotaxic coordinates. Proc. k. hed. Akad Wet., C 52: 1-40, 1959. 2. Dickinson, A. Disruption of free-operant successive discriminations by septal damage in rats. Q. £ exp. P~ychol. 24: 524-535, 1972. 3. Dickinson, A. Septal lesions in rats and the acquisition of free-operant successive discriminations. Physiol. Behav. 10: 305-313, 1973. 4. Ellen, P., W. C. Aitken Jr. and J. M. StahL Pretrainizt, effects on the DRL performance of rats with septal lesions. Physiol. l~'ychol. 1: 380-384, 1973. 5. Ellen, P. and J. Butter. External cue control of DRL performance in rats with septal lesions. Phys/ol. Behav. 4: 1-6, 1969. 6. Ellen, P. and E. W. PoweH. Effects of septal lesions on behavior generated by positive reinforcement. Expl Neuroi. 6: 1 - I 1 , 1962. 7. Fried, P. A. Septum and behavior: A review. Psychol. Bull. 78: 292-310, 1972. 8. Gray, J. A., L. Quintao and M. T. Araujo-SUva. The partial reinforcement, extinction effect in rats with medial septal lesions. Physiol. Behav. 8: 491-496, 1972.
9. Harvey, J. A. and H. F. Hunt. Effect of septal lesions on thirst in the rat as indicated by water consumption and operant responding for water reward. £ comp. physiol. P~ychol. 59: 49-56, 1965. 10. Henke, P. G. Persistence of runway performance after septal lesions in rats. £ comp. physiol. Psychol. 86: 760-767, 1974. 11. HothermU, D., D. Alexander and R. Slonaker. The DRL deficit of rats with septal lesions: Effects of extended training in a mediated environment. Psyehon. SoL 29: 34-36, 1972. 12. HothersalL D., D. A. Johnson and A. Collen. Fixed-ratio responding following septal lesions in the rat. J. eomp. physiol. PsyehoL 73: 470-476, 1970. 13. JohMon, D. A. and K. Thatcher. Differential effects of food deprivation on the fixed ratio behavior of normal rats and rats with septal lesions. Psychon. $ci. 26: 45-46, 1972. 14. Kelsey, J. E. and S. P. Grossman. Non-perseverative disruption of behavioral inhibition following septal lesions in rats. J. corap, phyaol. Psychol. 75: 302-311, 1971. 15. Lorens, S. A. and C. Y. Kondo. Effects of septal lesions on food and water intake and operant responding for food. Physiol. Behav. 4: 729-732, 1969.
R E S P O N S E S U P P R E S S I O N IN N O R M A L A N D S E P T A L R A T S 16. Meyer, P. M. and D. R. Meyer. Neurosurgical procedures with special reference to aspiration lesiong In: Methods in Psychobiology, Vol. 1, edited by D. R. Myers New York: Academic Press, 1971. 17. Raphelson, A. C., R. L. Isaacson and R. 1. Douglas. The effect of limbic damage on the retention and performance of a runway response. Neuropsychologia 4: 243-264, 1966. 18. Reynolds, G. S. Behavioral contrast. J. exp. AnalysisBehav. 4: 57-71, 1961. 19. Schwartzbaum, J. A., M. H. Kellicutt, T. M. Spieth and J. B. Thompson. Effects of septal lesions in rats on response inhibition associated with food reinforced behavior. J. comp. physiol. Psychol. 58: 217-224, 1964. 20. Schwartzbaum, J. S. and P. Gay. Interacting behavioral effects of septal and anygdaloid lesions in the rat. J. comp. physiol. Psyehol. 61: 59-65, 1966.
421
21. Slonaker, R. L. and D. Hothersall. Collateral behaviors and the DRL deficit of rats With septal lesions. J. comp. physiol. PsychoL 80: 91-96, 1972. 22. Uhl, C. N. Eliminating behavior with omission and extinction after varying amounts of training. Anita. Learn. Behav. 1: 237-240, 1973. 23. Uhl, C. N. and E. E. Gareia. Comparison of omission with extinction in response elimination in rat~ J. comp. physiol. Psychol. 69: 554-562, 1969. 24. Uhl, C. N. and W. O. Sherman. Comparison of combinations of omission, punishment and extinction methods in response elimination in rats. J. comp. physiol. Psychol. 74: 59-65, 1971. 25. Zefler, M. D. Eliminating behavior with reinforcement. J. exp. Analysis Behav. 16: 401-405, 1971.
Printed in the U.S.A
Response Suppression in Normal and Septal Rats 1 GILBERT ATNIP AND DAVID HOTHERSALL
Laboratory o f Comparative and Physiological Psychology, The Ohio State University, 1314 Kinnear Road Columbus, OH 43212 (Received 7 February 1975) ATNIP, G. AND D. HOTHERSALL. Response suppression in normal and septal rats. PHYSIOL. BEHAV. 15(4) 4 1 7 - 4 2 1 , 1975 - After 11 days of lever press training on a variable interval 30 sec schedule, separate groups of septal and normal rats were exposed fi3r 10 days to one of two response-suppression procedures: extinction or differential reinforcement of other behaviors. Both procedures were effective in suppressing responding, with the septal rats taking longer to reach the suppression asymptote than did the normals. All rats then received 5 days of non-contingent food presentation. Under these conditions responding recovered to a much greater degree in rats previously given extinction than in those given DRO. Further, septal rats given extinction showed more recovery than did normals given extinction. Septal lesion
Response suppression
VI and DRO vs. extinction
R A T S with lesions of the septum show greater than normal response rates on a variety of reinforcement schedules, e.g., continuous [15], fixed interval [20], variable interval [2, 3, 6], and fixed ratio [12]. They also show unusually high response rates, relative to normal rats, on differential reinforcement of low rates (DRL) schedules [7]. Since D R L schedules make reinforcers contingent on responses that are spaced in time, septal rats typically receive few reinforcers, while emitting large numbers of responses. This ineffective behavior has been termed the "septal D R L deficit" [7]. It is both severe and long-lasting,and is not affected by either pretraining [4], or extensive D R L training [ l I ]. However, it can be attenuated by providing a stimulus which signals the availabilityof a reinforcer at the end of the D R L interval [5,14], or by training septal rats in environments which facilitatethe development of collateral mediating behaviors [21 ]. In addition to the high rates of responding noted above, septal rats show response persistence when shifted from an acquisition schedule to extinction. In extinction, septal rats typically continue to respond at greater than normal rates in spite of non-reinforcement [8, l0, 17, 19]. These results, together with the D R L data, suggest that rats with lesions of the septum are not as sensitive as normal rats to the effects of response-suppressing reinforcement contingencies. In the present experiment, we first compared the performance of normal and septal rats working for food reinforcement on a variable interval (VI) 30 sec schedule of reinforcement. In the second phase of the experiment, half of each group of rats was subjected to one of two response-suppression proccdures: extinction, in which rein-
forcement was discontinued, or differential-reinforcementof-other behaviors (DRO). In the D R O procedure, first described by Reynolds [18], a scheduled reinforcer is omitted if a response occurs during a specified interval,and so the animal must learn not to respond, possibly by developing some 'other' behaviors which are consistently reinforced. This procedure has been found to be comparable to extinction in the suppression of previouslyreinforced responding in normal rats [22,23]. W e were interested in whether D R O would prove to be effective in suppressing responding in septal rats, and if so, h o w it would compare in effectiveness with extinction. The stability of any effects produced by the two response-suppression procedures was assessed in the third phase of the experiment. All groups were exposed to a variable time (VT) 30 sec schedule, in which food was delivered on the average every 30 sec, but there was no contingency between responses and reinforcers. Uhl and Garcia [23] used this procedure with normal rats, and found that animals previously given D R O training responded less than those given extinction. That is, D R O produced a more durable response-suppression effect. W e investigated whether a similar effect would be found in rats with lesions of the septum. METHOD
Animals T w e n t y - f o u r naive male h o o d e d rats were used.
Apparatus Training
was
conducted
in
4
conventional
operant
aT his research was supported in part by grant number MH 06211 awarded to Drs. D. R. and P. M. Meyer. We gratefully acknowledge their support and assistance. 417
418 conditioning chambers. Two of the chambers were housed in sound-attenuating research chests equipped with exhaust fans. The other two were left unhoused so that the rats in them could be observed via closed-circuit television. Half of the animals in each group were trained in the closed chambers and half in the open ones. Electro-mechanical equipment for programming events and recording data was located in an adjoining room.
Procedure Surgery. Prior to the beginning of training, half of the rats received l~llateral lesions of the septum by means of stereotaxic surgery. Surgery was carried out under a combination of pentobarbital and metofane anesthesia, as described by Meyer and Meyer [16]. Coordinates for the lesions were determined from the DeGroot [1] atlas of the rat's brain. The electrolytic lesions were produced by a unipolar anode, insulated except for 0.5 mm at the tip, and an anal cathode. Current parameters were 2 mA for a duration of 15 sec. After surgery the rats were given an antibiotic and returned to their home cages. Training. Before training began, normal (i.e., nonoperated) animals were reduced to 80 percent -+ 15 g of their free-feeding body weights; rats with lesions were given a 7 - I 0 day postoperative recovery period, by which time their body weights had stabilized at approximately 80 percent of preoperative levels. They were maintained at these weight levels for the duration of the experiment by feeding them a daily ration of food after each experimental session. During the first training session, which was 45 min long, magazine training was accomplished by the delivery of 45 mg Noyes food pellets on an average of once every 30 sec. The response lever was not available during this session. The lever was available at the beginning of the next session and each rat was allowed to press it 100 times on a schedule of continuous reinforcement (CRF), with a single food pellet as the reinforcer for each response. If a rat did not begin to lever-press within 20 min, his behavior was observed using closed-circuit T.V. and the response shaped using the method of successive approximations. The animal was then allowed to obtain 100 reinforcers on CRF. All subsequent sessions were one hour long. During the first 11 of these, all rats pressed the lever for food on a VI 30 sec schedule. In the next phase of the experiment, both the normal and septal rats were divided into 2 groups. One normal and one septal group received 10 sessions of conventional extinction (Groups NE and SE). The other two groups were exposed to the DRO schedule for 10 sessions (Groups ND and SD). The DRO schedule we used had a parameter of 30 sees., .i.e., when 30 sec passed without a lever-press a food pellet was delivered; when a response occurred during the 30 sec interval, delivery of the reinforcer was delayed another 30 sec. In the final phase of the experiment, all the groups were exposed to a VT 30 sec schedule, in which food pellets were delivered, on the average, once every 30 sec, whether or not the rat pressed the lever. This phase lasted for 5 sessions. Experimental sessions were run 5 days per week and the rats were fed maintenance rations on weekends. Histology. After completion of training, rats with septai lesions were deeply anesthetized and were perfused intracardially with saline solution followed by i0 percent Formalin. The brains were removed, embedded in celloidin,
ATNIP AND HOTHERSALL and sectioned at 30 u. The sections were mounted on slides, stained with cresyl violet, and examined to determine the extent of the lesions. RESULTS
Histology Lesions of the septum were large and in all cases included the medial and lateral septai nuclei, the diagonal tract, the nucleus of the diagonal tract, the stria terminalis, the inter-stitial nucleus, of the stria terminalis, the septohypothalarnic tract, the precommissural fornix, the triangular septal nucleus, and the superior fornix. Other structures which sustained damage in some of the lesions included the anterior commissure, median and medial preoptic nuclei, stria medullaris, medial corticohypothalamic tract, medial corticohabenular tract, septohabenular tract, fimbrial nucleus of the septum, and, in one extreme case, the anterior nuclei of the thalamus. There was no apparent relation between variations in lesion placements and variations in observed behavior.
VI 30 Training The left panel of Fig. 1 shows the mean numbers of responses per session for each experimental group during VI training. Responding increased in all groups early in training. By the end of the eleventh day of training, responding in the septal rats appears to have reached an asymptote, while responding in the normal rats was still increasing. Throughout VI training, the rats with septai lesions showed response outputs consistently greater than those of the normal rats. An analysis of variance, with repeated measures, showed significant effects due to Lesion, F(1,20) = 10.71, p<0.05, and to Days, F(10,200) -6.13, p<0.05. There were no differences between the subgroups, which is to be expected since they had not yet received differential treatment.
Response Suppression DRO vs. exn'nction. The middle panel of Fig. 1 depicts the results of imposing the two response-suppression procedures, in terms of the mean numbers of responses per session, for each group. It is clear that all groups reached the same low level of responding by the sixth day of this phase. The two response-suppression procedures, DRO and extinction, effectively suppressed previously-reinforced responding in both normal and septal rats. However, the key question is whether rate of response-suppression was affected by the septal lesion and/or the type of suppression treatment. Since the normal and septal rats had different levels of response output at the end of VI training, it is most appropriate to compare their suppression performance in relative rather than in absolute terms. Therefore a baseline level of responding was computed for each rat by taking its mean number of responses for the last 3 days of VI training. Then the ratios of the number of responses on each of the 10 days of the suppression phase to this beseline level was calculated. The results of suppression training, shown in terms of these ratios, averaged for each group, are shown in Fig. 2. This figure shows that, in addition to having higher absolute levels of responding, the rats with septal lesions had higher ratios of suppression to baseline responding than did normal rats. This difference
RESPONSE SUPPRESSION IN NORMAL AND SEPTAL RATS
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FIG. 2 Response expressed in terms of group me~.n ratios of numbers of responses per day of suppression training to numbers of base line response~ decreased over days as the suppression ratios for both groups of rats decreased to asymptote. The normal rats reached the asymptote first. An analysis of variance on the ratio data for the first 5 days of suppression training
showed significant effects due to Lesion, F(4,80) = 77.0, p < 0 . 0 5 , and the Lesion x Days interaction, F(4,80) - 3.00, p<0.05. There were no statistically s i ~ i f i c a n t effects associated with the type of suppression treatment.
420
ATNIP AND HOTHERSALL
VT 30 testing. Due to equipment failure at the beginning of this phase of the experiment, one animal in each of the groups of rats with lesions of the septum had to be discarded. The data from the remaining rats, expressed in terms o f group mean numbers of responses per session, are shown in the right panel of Fig. 1. The two groups (SE and NE) trained using extinction during the suppression phase showed more responding under VT than did the two groups given DRO. Analysis of variance showed a significant effect due to type of suppression treatment, F(1,18) = 7.12, p<0.05. It also appears that Group SE recovered responding to a greater degree than did Group NE, but this lesion difference did not attain statistical significance. During successive days of VT training, the amount of responding in the two extinction groups declined systematicaily, while responding in the two DRO groups remained constant at a very low level. Analysis of variance showed a significant Days effect, F(4,72) = 2.68, p<0.05, and a significant Treatment x Days interaction, F(4,72) = 2.80, p<0.05. DISCUSSION Response rates of both groups o f rats working on the variable interval schedule o f reinforcement increased with training, but the response output of the rats with lesions o f the septum was significantly greater than that of the normal rats. Similar results showing increased respondin8 on VI schedules by septal rats have been reported by a number of previous investigators [2, 3, 6]. Both response-suppression procedures - extinction and DRO - quickly and effectively suppressed responding during the suppression phase and there was no difference in their relative effectiveness. In addition, both suppression procedures were effective in suppressing responding in both the normal rats and those with lesions of the septum. In view of the markedly increased response rates of rats with w,ptal lesions working for positive reinforcers found in this and many previous experiments [2, 3, 6, 9, 12, 13, 15], the increased resistance to extinction of previously reinforced responses
shown by septal rats [ 10, 17, 19], and the many reports of the ineffectiveness of DRL schedules in reducing response rate in septal rats [6, 11 ], the absence of any differences between the normal and septal rats at the end of suppression training is of interest. While the normal rats attained the suppression asymptote first, both groups of animals were making only small numbers of responses at the end o f suppression training. It would appear that it is possible to produce clear response suppression in septal rats by using appropriate contingencies of positive reinforcement. The results from the third phase of the experiment, in which a VT procedure was used, show that although the groups were very similar in their behaviors at the end of the suppression phase, they differed in their reactions to non-contingent reinforcement. Responding by the normal rats previously given DRO training, remained at a low level, while responding increased in the rats previously given extinction. Similar results have been reported by Uhl and Garcia [23], Uhl and Sherman [24], and Zeiler [25]. The two groups of rats with septal lesions showed the same pattern of differences due to the type of suppression treatment to which they were previously exposed. The magnitude of the difference between the two septal groups given extinction and DRO appeared to be greater than the difference between the two normal groups, but this proved to be only marginally ~lnificant statistically. It appears likely that both the normal and septal rats were able to develop some behavior other than lever pre4ming during suppression training and that they continued to emit this behavior during the non-contingent phase. Rats given extinction were not reinforced for such other behaviors during the suppression phase and so resumed lever-pressing when reinforcers were once again delivered. However this suggestion is speculative since systematic observations of the rats' behaviors were not made. The long-lasting suppression of responding produced by DRO i n both normal and septal rats is however clear evidence of the effectiveness of this response-suppression procedure.
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