Journal of Comparative and Physiological Psychology 1075, Vol. 88, No. 1, 147-154
CLASSICAL DIFFERENTIAL AND OPERANT CONDITIONING IN RABBITS (ORYCTOLAGUS CUNICULUS) WITH SEPTAL LESIONS1 MEL LOCKHART AND JOHN W. MOORE2 University of Massachusetts—Amherst Normal rabbits and rabbits with septal lesions received classical differential conditioning of the nictitating membrane response (NMR), followed by auditory generalization tests run in extinction. Although rate of acquisition and asymptotic responding to positive conditioned stimuli did not differ, septals responded more than normals to nonreinforced stimuli. Resultant decrements in differential conditioning could not be attributed to changes in auditory or shock thresholds or to increased spontaneous NMRs. Septals also responded at higher rates in both operant conditioning (bar presses reinforced with food pellets on a variable interval schedule) and extinction sessions. No difference in suppression in a passive avoidance task was found. Results are discussed in relation to McCleary's response disinhibition analysis of septal function, and an habituation hypothesis is considered.
Deficits in classical conditioning have been hypothesized to account for the observations that animals with septal lesions show (a) decrements in performance of passive avoidance (Thomas, 1972) and conditioned emotional response (CER) tasks (Duncan, 1971), (b) lower rates or responding in free operant avoidance (Kelsey & Grossman, 1971), and (c) less differential responding in shuttle avoidance when compartments differ in shock intensity (Garber & Simmons, 1968). However, there is presently little direct evidence that septal lesions impair acquisition of a classically conditioned response. Holdstock (1970) and Duncan (1972) have reported attenuation of conditioned heart rate responses in animals with septal lesions, but this effect could be attributed to reduced autonomic reactivity (Holdstock, 1970) or to a reduction in the magnitude of the unconditioned response (UR; Duncan, 1972). Nor is there
any direct evidence on the role of the septum in classical differential conditioning. The present study investigated classical differential conditioning of the nictitating membrane response (NMR) in rabbits with septal lesions. Two tones of equal intensity but differing in frequency served as CS+ and CS— (positive and negative conditioned stimulus, respectively), and an infraorbital (eye) shock served as the unconditioned stimulus (US). Differential conditioning was followed by tests for stimulus generalization run in extinction. Assuming that septal rabbits are able to acquire a conditioned NMR to CS+, withholding the CR to CS— in differential conditioning or in extinction-generalization testing would require either response inhibition (cf. McCleary, 1966) or selective gating ("tuning out") of the CS- through, e.g., habituation of the orienting response. Several authors have noted deficits in habituation resulting from lesions in the septo-hippocampal com1 This research was based upon a Master's thesis plex (e.g., Douglas, 1972). Higher than norsubmitted to the graduate school of the Univer- mal levels of responding to CS— by rabbits sity of Massachusetts by Mel Lockhart. The with septal lesions may indicate that the authors would like to acknowledge the contribution of N. R. Carlson, whose advice on design and septum plays a role in either one or both technique was most helpful. This research was mechanisms of differential conditioning. supported by National Science Foundation Grant The present study also investigated the GB 24557 to the second author. effects of septal lesions on operant condi2 Requests for reprints should be sent to John W. Moore, Department of Psychology, Middlesex tioning in rabbits. In the operant portion of House, University of Massachusetts, Amherst, Mas- the experiment, rabbits were trained to bar sachusetts 01002. press for food reinforcement available on a 147
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variable interval (VI) schedule. Potential stant masking noise of 76 db. 8PL. An 18 X 7 X differences in rate of responding associated 4.5 in. Plexiglas box with adjustable front stock back plate was used to restrain each subject with passive avoidance and extinction con- and (Gormezano, 1966). A rotary Minitorque potentitingencies were also determined. If the be- ometer (Giannini No. 85153), attached to an earhavior of rabbits with septal lesions is com- bar style head mount and connected to a nylon parable with that of other species, rabbits suture, monitored lateral movement of the nicitimembrane. Amplification and recording of with septal lesions would be expected to re- tating the NMR was done on a 4-channel Grass 5D inkspond more than normal animals in each writing oscillograph at a paper speed of 100 mm/ phase of the task (Fried, 1972). sec. A conditioned response was defined as a posiMETHOD Subjects Subjects were experimentally naive male and female New Zealand white rabbits (Oryctolagus cuniculus), 90-120 days old at the outset of the experiment and randomly assigned to 2 groups, 12 nonoperates and 20 operates. Throughout the experiment animals were housed individually. Rabbits were maintained on ad-lib Purina Rabbit Chow and water during recovery from surgery and classical conditioning phases of the experiment. One week prior to the onset of operant conditioning, animals were placed on food deprivation; food was available for only 3 hr. a day for the first 2 days of deprivation, then 1 hr. a day for the next 2 days. For the 3 days immediately preceding shaping, no food was given. During the operant phase, food was made available in the home cage for 15 min. after the operant conditioning session.
Surgery Animals in the septal group were anesthetized with Thorazine (12.5 mg/kg body weight) injected intramuscularly, and 10 min. later with Nembutal (20 mg/kg body weight) injected intravenously. Brain lesions were made by passing current generated by a Grass LM4 radio frequency lesion maker set at 18 v. for 20 sec. through an electrode made of a Clay Adams E80 stainless steel insect pin, with diameter approximately .3 mm., Size 00 insulated with Epoxylite except for about 1.5 mm. at the tip. Coagulations were produced at the stereotactic coordinates ± 0.5 mm. lateral, 2.0 mm. anterior, and 10.3 mm. ventral to bregma (lambda 1.5 mm. inferior to bregma) in a 1-stage operation. Following the operation, animals were allowed 7-14 days recovery before beginning the next phase of the experiment.
Conditioning Apparatus Classical. Four animals were run concurrently in individual sound-proofed ventilated drawers of a fireproof filing cabinet. The stimulating and recording components for each drawer were identical. Each drawer was illuminated with 2 6-v. incandescent lights situated in the front portion of the drawer. Three speakers, also in the front portion of the drawer, delivered tonal stimuli and a con-
tive deflection of the oscillograph pen greater than 1 mm. (corresponding to a movement of the nictitating membrane of less than 1 mm.) occurring within the CS-US interval. Operant. Two identical operant conditioning chambers consisted of Plexiglas Skinner boxes, 24 X 15 X 20 in., with 2 audio speakers in the rear wall of each to deliver masking noise. The front wall contained a food magazine (3.5 X 4 X 1.25 in.) and a Lehigh Valley Model 1405 M retractable lever, the lever being 2.5 in. from the floor of the chamber. The floor of the operant chambers consisted of .25-in. stainless steel rods .75 in. apart (center to center). A Scientific Prototype Model D-100 feeder delivered single 97-mg. Noyes alfalfa pellets for reinforced responses. A Grason-Stadler model E1064GS shock generator capable of delivering shocks of intensities ranging .5-4.0 ma. for durations of between .5 sec. and 3.0 sec. was connected with the floor rods of one operant chamber, while a Lehigh Valley Electronics shock generator model 1531 with range from 0-10 ma. was attached to the grid of the other chamber. House lighting was provided by a 25-w. incandescent light source suspended centrally above the test chambers. A continuous background of white noise (75 db. SPL) masked extraneous auditory stimuli throughout each session. Reinforcement and shocks were programmed through standard relay circuits located in a separate room from the operant chambers.
Classical Conditioning Adaptation. Following insertion of the nylon loop in the right nictitating membrane, rabbits were habituated to restraint and the file-drawer apparatus for 20 min. 2 days before the beginning of differential conditioning. On the day before conditioning, a spontaneous NMR rate was determined for each animal during a 15-min. period in the conditioning chamber. No tones or shocks were presented during the adaptation period, although background noise was present. Differential conditioning. Following adaptation, animals were given 11 sessions of differential conditioning. For half the subjects a 1,600-Hz. tone was used as CS+ and a 700-Hz. tone was used as CS—; for the remaining subjects, CS+ was a 700Hz. tone and CS— a 1,600-Hz. tone. The intensity of all tones was 76 db. SPL. The CS duration was 450 msec. The US was a 2-ma. ac shock of 50-msec.
SEPTAL LESIONS AND CONDITIONING duration delivered to the subject via 2 stainless steel wound clips attached posterior and inferior to the right eye. The interstimulus interval was 400 msec. Each daily session consisted of 50 CS+ trials and 50 CS— trials in a pseudorandom order, with the constraint that there were no more than 2 trials of the same type in sequence. The intertrial interval was 30 sec. Generalization test. After differential conditioning was completed, a generalization test was given in 2 extinction sessions. The tones used for testing were 400 Hz., 700 Hz., 1,000 Hz., 1,300 Hz., 1,600 Hz., 1,900 Hz., 2,200 Hz., and 2,500 Hz., each at 76 db. SPL. Again, the intertrial interval was 30 sec., and each tone was presented 12 times per session, 3 times within each block of 24 trials. Thresholds. Finally, both CS+ and former CS— were paired with the US for 1 100-trial session. After each animal had reacquired the CR to both tones, tone intensity was varied systematically to determine auditory threshold for the CR. Threshold for the US was then determined by presenting shocks of varying intensity unsignaled by a CS.
Operant Conditioning Baseline and shaping. The operant training sequence followed the classical conditioning sequence for all animals. On the seventh day of 23-hr, food deprivation, each animal was placed in the operant chamber with the bar extended, but no reinforcement available. The number of bar-press responses made in a 15-min. period was recorded. On the following day, rabbits were magazine trained and shaped according to the method of successive approximations. Each bar press was reinforced during shaping. Shaping was considered complete and the session was ended when the animal had made 50 reinforced bar presses. The following day, variable interval training began. Variable interval training. For the next 7 days, animals were reinforced on a variable interval 30sec. (VI 30-sec.) schedule, with intervals generated according to the method of Fleshier and Hoffman (1962). Sessions were 30-min. long. Passive avoidance. For the next 2 days, all bar presses were punished by footshocks of gradually increasing intensity and duration, while responding continued to be reinforced on a VI 30-sec. schedule. On the first day of punishment, hind paws were shaved approximately 1 hr. before the session. The sequence of shock durations and intensities ranged .5-3 sec. and .5-4 ma. Punishment was followed by 2 sessions of reacquisition with reinforcement available on a VI 30-sec. schedule. No responses were punished. Extinction. During the next 3-day period, no reinforcement was scheduled for bar pressing. Shock threshold. On the final day of the experiment, jump-flinch responses to grid shocks of varying intensity were determined for each rabbit, using the Lehigh Valley shock generator. Threshold determination was based on increments of roughly .2 ma.
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Histology Following the conclusion of experimental training, animals with lesions were anesthesized with an overdose of Nembutal and perfused percardially with physiological saline followed by 10% Formol saline. Following fixation, serial frozen sections were taken at 30 tan. Every fourth section through the lesion was mounted and stained with cresyl violet. The extent of damage to each brain was estimated by eye and drawn on pages traced from the Urban and Richard stereotaxic atlas (1972).
RESULTS Histology Examinations of the brain sections revealed that 10 animals in the septal lesion group had sustained bilateral damage to the septal area. Reconstructions of representative lesions showing maximal and minimal extent of damage in rabbits with bilateral septal damage are shown in Figure 1. All lesions in this group produced extensive bilateral damage to the medial septal nuclei and interrupted the precommissural fornix.
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FIQUEE 1. Reconstructions of minimal (S19) and maximal (S22) extent of septal lesions superimposed on plates taken from the stereotaxic atlas of Urban and Richard (1972).
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Most lesions destroyed varying portions of the lateral septal nuclei. Five animals sustained some unilateral damage to the lateral septal nuclei, and four animals had some bilateral damage to those nuclei. Some animals sustained damage to dorsal portions of the nucleus of the diagonal band of Broca (7), corpus callosum (2), caudate nucleus (2), or columns of the fornix (1), as well as the septum. Eight additional septal subjects all sustained varying degrees of damage confined to one hemisphere. These unilateral lesions differed widely in both size and location, and the data of the animals with unilateral lesions were considerably more variable than those of either the normal or animals with bilateral septal lesions. Consequently, only data from normal and animals with bilateral septal lesions are described here. One normal animal and 2 operates died during the classical conditioning phase. These incomplete data were discarded. Classical Conditioning Normal animals and those with septal lesions showed distinct differences in response patterns in the classical conditioning phase of the study. Although rates of acquisition
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and asymptotic performance to CS+ were virtually the same for both groups, animals NORMAl CS I with septal lesions showed higher rates of responding to the CS— than would ordinarily be expected under conditions that typi100 cally produce low response rates in normal 90 animals. Figure 2 summarizes group results of classical differential conditioning. The 80 figure indicates that animals with septal •70 lesions made more conditioned responses to 60 CS— resulting in poorer overall discrimination than that of normals. Animals with 50 septal lesions had significantly lower aver40 age difference scores (t = 2.18, df = 19, 30 p < .05) and discrimination indices, de20 fined as percent CR to CS+/ (percent CRs to CS+ plus percent CRs to CS-), (t = 10 2.43, df = 19, p < .05) than normals. These 3 4 5 6 7 9 10 11 and all subsequent t tests are 2-tailed. Results of the generalization test, shown SESSION in Figure 3, indicate higher resistance to FIGURE 2. Classical differential conditioning performance for septal and normal groups expressed as extinction among rabbits with septal lesions. percent conditioned responses (CRs) to CS+ and This provides further evidence that septal lesions inflate response rates on a variety of CS-. +
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tasks. Animals with septal lesions responded significantly more on the first day of extinction than normals (t = 2.79, df = 19, p < .02), although the shape of the generalization gradients were roughly the same for both groups. The 2 groups did not differ on the second day, since conditioned responding was completely extinguished for both groups. Several explanations of the increased responding found among animals with septal lesions were considered. The differences found in differential conditioning and resistance to extinction might have been due to differences in sensitivity to the auditory CSs. Threshold tests suggest that this was not the case. While mean threshold for CR to the CS+ in animals with septal lesions (57.8 db. SPL) was significantly lower than that of normals (61.5 db. SPL) (t = 2.45, df — 19, p < .05), there was no significant difference between groups in CS— threshold (t = .48, df = 19). Similarly, differences in reactivity to the US might have mediated the difference in differentiation between the groups, but eye-shock threshold tests of unconditioned NMR revealed no significant difference between the 2 groups (t = .77, df = 19). Finally, animals with septal lesions might have exhibited a higher spontaneous NMR rate at the outset of the experiment making inhibition of CS— responses more difficult. Two pieces of evidence argue against this possibility. First, baseline rate of NMRs was not significantly different between the 2 groups (t = .26, df = 19). Further, as indicated by the graph in Figure 2, during the first 4 days of training there was no significant difference between the number of conditioned responses emitted to the CS— by normal subjects and those with septal lesions (t = .38, df = 19). Taken together, these results support the interpretation that rabbits with bilateral septal lesions are less able to learn to withhold responses to nonreinforced stimuli in the classically conditioned differentiation and extinction tasks. Operant Conditioning Two animals in each group failed to acquire the bar-press response within 3 shap-
ing sessions. Acquisition of the bar press under VI 30-sec. reinforcement for the remaining rabbits (8 operates and 9 normals) is shown in Figure 4. Animals with septal lesions had higher rates of responding on an appetitively reinforced schedule than normals, as indicated by a comparison of mean response rate across all days of VI acquisition (t = 2.42, df = 15, p < .05). However, the mean rates of response found in the Vl-reacquisition period (t = .50, df = 15) suggest that normal rabbits and rabbits with septal lesions may approach the same asymptotic levels of responding but that operates reach asymptote sooner. Also, as has been found by other authors, animals with septal lesions responded more through a period of operant extinction than did normals (t = 2.34, df = 15, p < .05). Passive avoidance response rates weresimilar for septal and normal animals (t' — .54, df = 15). However, when rate of response during passive avoidance was divided by the average rate for the 2 days of acquisition preceding passive avoidance, rabbits with septal lesions suppressed better than normal subjects (t — 2.65, df = 15, p < .05). With the gradations in shock intensity employed in this experiment, there was no difference in jump-flinch thresholds between normal animals and those with septal lesions, nor was there any observable difference in behavior at suprathreshold levels of grid shock. If the close correspond60
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FIGURE 4. Operant conditioning performance during variable interval (VI) acquisition, passive avoidance, VI reacquisition, and extinction for septal and normal groups, expressed as mean rate of response in each session.
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ence between rates of response on the last day of acquisition and during reacquisition are taken as evidence that animals with septal lesions reached asymptotic level before passive avoidance training began, the difference in suppression may be attributed to the fact that actual baseline rate of VI responding for normal animals continued to increase during the passive avoidance session. Thus, the 2 days prior to avoidance training may not be the best basis for comparison. If rate of response during passive avoidance is divided by average rate during reacquisition, instead of the last 2 days of acquisition, there is no significant difference between groups (t = .88, df — 15). DISCUSSION The principal result of the present investigation is the finding that rabbits with septal lesions are less able to inhibit conditioned responding to CS— during classical differential conditioning and extinction than normals. This higher level of responding was not due to lower auditory thresholds for the CR or eye-shock thresholds for the UR, nor was it due to differences in spontaneous NMRs. Rabbits with septal lesions did not differ significantly from normal rabbits on any of these indices. Rate of acquisition and asymptotic responding to CS+ was also virtually the same for both groups. This result is clearly inconsistent with the notion that behavioral deficits produced by septal lesions are mediated by deficits in classical conditioning. However, behavioral deficits may be mediated by failure to form a differentiated CR (e.g., Garber & Simmons, 1968). In the operant conditioning phase of the study, rabbits with septal lesions bar pressed at higher rates than normals during both VI acquisition and extinction, but they suppressed responding as well as normals under a passive avoidance contingency. The 2 groups did not differ in their jump-flinch threshold or in their observable motor reactions to grid shock. Relation to Other Species The finding that normal rabbits and those with septal lesions had similar auditory
thresholds is consistent with studies involving rats that have reported no difference between animals with septal lesions and normal animals in response to auditory stimulation when startle reflex (Kemble & Ison, 1971) or escape latency (Brown & Remley, 1971) are compared. The finding that normal rabbits and rabbits with septal lesions had similar shock thresholds seems inconsistent with studies that have reported increased reactivity to shock among rats with septal lesions. However, several recent articles have shown that differences in reactivity are mitigated if (a) lesions are small and medially placed (Carey, 1972), (b) postoperative handling is delayed by several days (Gotsick & Marshall, 1972), or (c) extended periods intervene between operation and testing (e.g., Hammond & Thomas, 1971). All of these conditions obtained in the present study, and this might account for failure to observe differences in jumpflinch or NMR thresholds to shock between the 2 groups. Increased responding among rats with septal lesions has been found on a variety of appetitively reinforced tasks (Fried, 1972; Sodetz & Koppell, 1972). These findings were replicated here: Rabbits with septal lesions reinforced with food pellets on a VI 30-sec. schedule responded at higher rates than normals. Furthermore, rabbits with septal lesions made more responses in operant extinction than did normals, which is consistent with the results of similar studies involving rhesus monkeys (Butter & Rosvold, 1968) and rats (Gray, Quintao, & Araujo-Silva, 1972). When passive avoidance contingencies were superimposed on an appetitively reinforced continuous reinforcement (CRF) baseline, decrements in suppression are often found among animals with septal lesions (e.g., Kaada, Rasmussen, & Kveim, 1962). However, Sodetz and Koppell (1972) failed to observe suppression decrements in rats with septal lesions when reinforcement is available on a VI schedule, as it was in this study. Here, rabbits with septal lesions suppressed bar-press responses as well as normals during passive avoidance when asymptotic levels of response are taken into
SEPTAL LESIONS AND CONDITIONING
account. A possible explanation is that, although suppression on a CRF task necessarily reduces the number of reinforcements, on a VI schedule even large changes in response rate may not alter the number of reinforcements obtained in a session. Furthermore, Miczek, Kelsey, and Grossman (1972) have demonstrated that punishment effects are strongly dependent on time since lesion. In their study, rats with septal lesions suppressed as well as normals when the passive avoidance contingency was introduced at least 10 days postoperatively. In the present study, passive avoidance training occurred approximately 1 mo. postoperatively. Theoretical Relevance McCleary's (1966) hypothesis that septal lesions result in the loss of motoric inhibition and perseveration of dominant responses can account for the increased responding to CS— and greater resistance to extinction observed in the classical conditioning phase of this study, as well as the higher than normal bar-press rates observed in the operant phase. Whereas the increased responding to the CS— in differential conditioning found in rabbits with septal lesions is consistent with hypothesized decreases in learned response or motoric inhibition, possible changes in attention to or salience of the CS— might also account for the observed results, since if the CS— is not attended to or is "gated out", it is unlikely to elicit a conditioned response. That is, the elevated level of responding to the CS— by rabbits with septal lesions might be attributed to failure to habituate the orienting response instead of (or in addition to) a decrease in learned response inhibition. Electrophysiological results provide indirect support for the notion that septal lesions interfere with habituation. Gray (1972) has shown that stimulation of the medial septal nuclei alters hippocampal theta rhythm. Douglas (1967) cites evidence that septal lesions modify hippocampal theta pacemakers, disrupting the theta rhythm (Green & Arduini, 1954). Although the exact role of the hippocampal theta rhythm is not clear, several authors have
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noted a correlation between arousal or attention and absence of the theta rhythm (e.g., Jarrard, 1973). Routtenberg (1968) reports that theta decreases occur during repeated exposure to novel stimuli, presumably correlating with stimulus habituation. Furthermore, Bremner (1970) found that hippocampal theta rhythm was present during classical conditioning and habituation trials but that there was a statistically significant difference between the dominant frequency in each situation. Presumably, lesions of the septum, which clearly influence the theta rhythm, would disrupt this relationship, possibly interfering with the habituation process. Carlton (1969) has proposed a central cholinergic inhibitory system anatomically located in the septo-hippocampal complex. This hypothesis was based on similarities found between effects of hippocampal lesions and direct hippocampal injections of anticholinergic drugs. Several recent studies that have evaluated the effects of direct injections of cholinergic blocking agents into the medial septal nuclei have reported behavior patterns similar to those produced by hippocampal lesions (e.g., Hamilton & Grossman, 1969). Leaton and Rech (1972) interpret their finding that both intraseptal and intrahippocampal injections of atropine heighten locomotive activity as strong evidence in support of the role of septo-hippocampal cholinergic neurons in mediating behavioral inhibition. The increased responding to the nonreinforced stimulus in classical differential conditioning produced by lesions confined primarily to the medial septal nuclei in rabbits is consistent with the existence of a cholinergic inhibitory loop involving septum and hippocampus. REFERENCES Bremner, F. J. The effect of habituation and conditioning trials on hippocampal EEG. Psychonomic Science, 1970,18,181-183. Brown, G. E., & Remley, N. R. The effects of septal and olfactory bulb lesions on stimulus reactivity. Physiology and Behavior, 1971, 6, 497-502. Butter, N., & Rosvold, H. E. Effect of caudate and septal nuclei lesions on resistance to extinction and delayed alternation. Journal of
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Comparative and Physiological Psychology, 1968, 65,397-403. Carey, R. J. A neuronanatomical investigation of enhanced cutaneous and gustatory responsivity associated with septal forebrain injury. Journal oj Comparative and Physiological Psychology, 1972, 80, 449-457. Carlton, P. L. Brain acetylcholine and inhibition. In J. T. Tapp (Ed.), Reinforcement and behavior. New York: Academic Press, 1969. Douglas, R. J. The hippocampus and behavior. Psychological Bulletin, 1967, 67, 416-442. Douglas, R. J. Pavlovian conditioning and the brain. In R. A. Boakes & M. S. Halliday (Eds.), Inhibition and learning. New York: Academic Press, 1972. Duncan, P. M. Effect of temporary septal dysfunction on conditioning and performance of fear responses in rats. Journal of Comparative and Physiological Psychology, 1971, 74, 340-348. Duncan, P. M. Effect of septal area damage and base-line activity levels of conditioned heartrate response in rats. Journal oj Comparative and Physiological Psychology, 1972, 81,131-142. Flesher, M., & Hoffman, H. S. A progression for generating variable interval schedules. Journal of the Experimental Analysis of Behavior, 1962, 5, 529-530. Fried, P. A. Septum and behavior: A review. Psychological Bulletin, 1972, 78, 292-310. Garber, E. E., & Simmons, H. J. Facilitation of two-way avoidance performance by septal lesions in rats. Journal of Comparative and Physiological Psychology, 1968, 66, 559-562. Gormezano, I. Classical conditioning. In J. B. Sidowski (Ed.), Experimental methods and instrumentation in psychology. New York: McGraw-Hill, 1966. Gotsick, J. E., & Marshall, R. C. Time course of the septal rage syndrome. Physiology and Behavior, 1972, 9, 685-688. Gray, J. A. Effects of septal driving of the hippocampal theta rhythm on resistance to extinction. Physiology and Behavior, 1972, 8,481-490. Gray, J. A., Quintao, L., & Araujo-Silva, M. T. The partial reinforcement extinction effect in rats with medial septal lesion. Physiology and Behavior, 1972, 8, 491-496. Green, J. D., & Arduini, A. Hippocampal electrical activity in arousal. Journal of Ne'urophysiology, 1954, 17, 533-557. Hamilton, L. W., & Grossman, S. P. Behavioral changes following disruption of central cholin-
ergic pathways. Journal of Comparative and Physiological Psychology, 1969,69, 76-82. Hammond, G. R., & Thomas, G. J. Failure to reactivate the septal syndrome in rats. Physiology and Behavior, 1971, 6,599-602. Holdstock, T. L. Plasticity of autonomic functions in rats with septal lesions.- Neuropsychologia, 1970, 8,147-160. Jarrard, L. E. The hippocampus and motivation. Psychological Bulletin, 1973, 79,1-12. Kaada, B. R., Rasmussen, E. W., & Kveim, 0. Impaired acquisition of passive avoidance behavior by subcallosal, septal, hypothalamic, and insular lesions in rats. Journal of Comparative and Physiological Psychology, 1962, 55,661-670. Kelsey, J. E., & Grossman, S. P. Nonperseverative disruption of behavioral inhibition following septal lesions in the rat. Journal of Comparative and Physiological Psychology, 1971, 75, 302312. Kemble, E. D., & Ison, J. R. Limbie lesions and inhibition of startle reactions in the r'at by conditions of preliminary stimulation. Physiology and Behavior, 1971, 7, 925-928. Leaton, R. N., & Rech, R. H. Locomotor activity increases produced by intrahippocampal and intraseptal atropine in rats. Physiology and Behavior, 1972, 8,539-541. McCleary, R. A. Response modulating functions of the limbic system: Initiation and suppression. In E. Stellar & J. M. Sprague (Eds.), Progress in physiological psychology. Vol. 1. New York: Academic Press, 1966. Miczek, K. A., Kelsey, J. E., & Grossman, S. P. Time course of effects of septal lesions on avoidance, response suppression, and reactivity to shock. Journal of Comparative and Physiological Psychology, 1972, 79, 318-327. Routtenberg, A. Hippocampal correlates of consummatory and observed behavior. Physiology and Behavior, 1968, 3, 533-535. Sodetz, F. J., & Koppell, S. Suppressive effects of punishment of operant responding in rats with septal lesions. Physiology and Behavior, 1972, 8, 837-840. Thomas, J. B. Non-appetitive passive avoidance in rats with septal lasions. Physiology and Behavior, 1972, 8,1087-1092. Urban, I., & Richard, P. A stereotaxic atlas of the New Zealand rabbit's brain. Springfield, 111.: Charles C Thomas, 1972. (Received September 14, 1973)
CLASSICAL DIFFERENTIAL AND OPERANT CONDITIONING IN RABBITS (ORYCTOLAGUS CUNICULUS) WITH SEPTAL LESIONS1 MEL LOCKHART AND JOHN W. MOORE2 University of Massachusetts—Amherst Normal rabbits and rabbits with septal lesions received classical differential conditioning of the nictitating membrane response (NMR), followed by auditory generalization tests run in extinction. Although rate of acquisition and asymptotic responding to positive conditioned stimuli did not differ, septals responded more than normals to nonreinforced stimuli. Resultant decrements in differential conditioning could not be attributed to changes in auditory or shock thresholds or to increased spontaneous NMRs. Septals also responded at higher rates in both operant conditioning (bar presses reinforced with food pellets on a variable interval schedule) and extinction sessions. No difference in suppression in a passive avoidance task was found. Results are discussed in relation to McCleary's response disinhibition analysis of septal function, and an habituation hypothesis is considered.
Deficits in classical conditioning have been hypothesized to account for the observations that animals with septal lesions show (a) decrements in performance of passive avoidance (Thomas, 1972) and conditioned emotional response (CER) tasks (Duncan, 1971), (b) lower rates or responding in free operant avoidance (Kelsey & Grossman, 1971), and (c) less differential responding in shuttle avoidance when compartments differ in shock intensity (Garber & Simmons, 1968). However, there is presently little direct evidence that septal lesions impair acquisition of a classically conditioned response. Holdstock (1970) and Duncan (1972) have reported attenuation of conditioned heart rate responses in animals with septal lesions, but this effect could be attributed to reduced autonomic reactivity (Holdstock, 1970) or to a reduction in the magnitude of the unconditioned response (UR; Duncan, 1972). Nor is there
any direct evidence on the role of the septum in classical differential conditioning. The present study investigated classical differential conditioning of the nictitating membrane response (NMR) in rabbits with septal lesions. Two tones of equal intensity but differing in frequency served as CS+ and CS— (positive and negative conditioned stimulus, respectively), and an infraorbital (eye) shock served as the unconditioned stimulus (US). Differential conditioning was followed by tests for stimulus generalization run in extinction. Assuming that septal rabbits are able to acquire a conditioned NMR to CS+, withholding the CR to CS— in differential conditioning or in extinction-generalization testing would require either response inhibition (cf. McCleary, 1966) or selective gating ("tuning out") of the CS- through, e.g., habituation of the orienting response. Several authors have noted deficits in habituation resulting from lesions in the septo-hippocampal com1 This research was based upon a Master's thesis plex (e.g., Douglas, 1972). Higher than norsubmitted to the graduate school of the Univer- mal levels of responding to CS— by rabbits sity of Massachusetts by Mel Lockhart. The with septal lesions may indicate that the authors would like to acknowledge the contribution of N. R. Carlson, whose advice on design and septum plays a role in either one or both technique was most helpful. This research was mechanisms of differential conditioning. supported by National Science Foundation Grant The present study also investigated the GB 24557 to the second author. effects of septal lesions on operant condi2 Requests for reprints should be sent to John W. Moore, Department of Psychology, Middlesex tioning in rabbits. In the operant portion of House, University of Massachusetts, Amherst, Mas- the experiment, rabbits were trained to bar sachusetts 01002. press for food reinforcement available on a 147
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variable interval (VI) schedule. Potential stant masking noise of 76 db. 8PL. An 18 X 7 X differences in rate of responding associated 4.5 in. Plexiglas box with adjustable front stock back plate was used to restrain each subject with passive avoidance and extinction con- and (Gormezano, 1966). A rotary Minitorque potentitingencies were also determined. If the be- ometer (Giannini No. 85153), attached to an earhavior of rabbits with septal lesions is com- bar style head mount and connected to a nylon parable with that of other species, rabbits suture, monitored lateral movement of the nicitimembrane. Amplification and recording of with septal lesions would be expected to re- tating the NMR was done on a 4-channel Grass 5D inkspond more than normal animals in each writing oscillograph at a paper speed of 100 mm/ phase of the task (Fried, 1972). sec. A conditioned response was defined as a posiMETHOD Subjects Subjects were experimentally naive male and female New Zealand white rabbits (Oryctolagus cuniculus), 90-120 days old at the outset of the experiment and randomly assigned to 2 groups, 12 nonoperates and 20 operates. Throughout the experiment animals were housed individually. Rabbits were maintained on ad-lib Purina Rabbit Chow and water during recovery from surgery and classical conditioning phases of the experiment. One week prior to the onset of operant conditioning, animals were placed on food deprivation; food was available for only 3 hr. a day for the first 2 days of deprivation, then 1 hr. a day for the next 2 days. For the 3 days immediately preceding shaping, no food was given. During the operant phase, food was made available in the home cage for 15 min. after the operant conditioning session.
Surgery Animals in the septal group were anesthetized with Thorazine (12.5 mg/kg body weight) injected intramuscularly, and 10 min. later with Nembutal (20 mg/kg body weight) injected intravenously. Brain lesions were made by passing current generated by a Grass LM4 radio frequency lesion maker set at 18 v. for 20 sec. through an electrode made of a Clay Adams E80 stainless steel insect pin, with diameter approximately .3 mm., Size 00 insulated with Epoxylite except for about 1.5 mm. at the tip. Coagulations were produced at the stereotactic coordinates ± 0.5 mm. lateral, 2.0 mm. anterior, and 10.3 mm. ventral to bregma (lambda 1.5 mm. inferior to bregma) in a 1-stage operation. Following the operation, animals were allowed 7-14 days recovery before beginning the next phase of the experiment.
Conditioning Apparatus Classical. Four animals were run concurrently in individual sound-proofed ventilated drawers of a fireproof filing cabinet. The stimulating and recording components for each drawer were identical. Each drawer was illuminated with 2 6-v. incandescent lights situated in the front portion of the drawer. Three speakers, also in the front portion of the drawer, delivered tonal stimuli and a con-
tive deflection of the oscillograph pen greater than 1 mm. (corresponding to a movement of the nictitating membrane of less than 1 mm.) occurring within the CS-US interval. Operant. Two identical operant conditioning chambers consisted of Plexiglas Skinner boxes, 24 X 15 X 20 in., with 2 audio speakers in the rear wall of each to deliver masking noise. The front wall contained a food magazine (3.5 X 4 X 1.25 in.) and a Lehigh Valley Model 1405 M retractable lever, the lever being 2.5 in. from the floor of the chamber. The floor of the operant chambers consisted of .25-in. stainless steel rods .75 in. apart (center to center). A Scientific Prototype Model D-100 feeder delivered single 97-mg. Noyes alfalfa pellets for reinforced responses. A Grason-Stadler model E1064GS shock generator capable of delivering shocks of intensities ranging .5-4.0 ma. for durations of between .5 sec. and 3.0 sec. was connected with the floor rods of one operant chamber, while a Lehigh Valley Electronics shock generator model 1531 with range from 0-10 ma. was attached to the grid of the other chamber. House lighting was provided by a 25-w. incandescent light source suspended centrally above the test chambers. A continuous background of white noise (75 db. SPL) masked extraneous auditory stimuli throughout each session. Reinforcement and shocks were programmed through standard relay circuits located in a separate room from the operant chambers.
Classical Conditioning Adaptation. Following insertion of the nylon loop in the right nictitating membrane, rabbits were habituated to restraint and the file-drawer apparatus for 20 min. 2 days before the beginning of differential conditioning. On the day before conditioning, a spontaneous NMR rate was determined for each animal during a 15-min. period in the conditioning chamber. No tones or shocks were presented during the adaptation period, although background noise was present. Differential conditioning. Following adaptation, animals were given 11 sessions of differential conditioning. For half the subjects a 1,600-Hz. tone was used as CS+ and a 700-Hz. tone was used as CS—; for the remaining subjects, CS+ was a 700Hz. tone and CS— a 1,600-Hz. tone. The intensity of all tones was 76 db. SPL. The CS duration was 450 msec. The US was a 2-ma. ac shock of 50-msec.
SEPTAL LESIONS AND CONDITIONING duration delivered to the subject via 2 stainless steel wound clips attached posterior and inferior to the right eye. The interstimulus interval was 400 msec. Each daily session consisted of 50 CS+ trials and 50 CS— trials in a pseudorandom order, with the constraint that there were no more than 2 trials of the same type in sequence. The intertrial interval was 30 sec. Generalization test. After differential conditioning was completed, a generalization test was given in 2 extinction sessions. The tones used for testing were 400 Hz., 700 Hz., 1,000 Hz., 1,300 Hz., 1,600 Hz., 1,900 Hz., 2,200 Hz., and 2,500 Hz., each at 76 db. SPL. Again, the intertrial interval was 30 sec., and each tone was presented 12 times per session, 3 times within each block of 24 trials. Thresholds. Finally, both CS+ and former CS— were paired with the US for 1 100-trial session. After each animal had reacquired the CR to both tones, tone intensity was varied systematically to determine auditory threshold for the CR. Threshold for the US was then determined by presenting shocks of varying intensity unsignaled by a CS.
Operant Conditioning Baseline and shaping. The operant training sequence followed the classical conditioning sequence for all animals. On the seventh day of 23-hr, food deprivation, each animal was placed in the operant chamber with the bar extended, but no reinforcement available. The number of bar-press responses made in a 15-min. period was recorded. On the following day, rabbits were magazine trained and shaped according to the method of successive approximations. Each bar press was reinforced during shaping. Shaping was considered complete and the session was ended when the animal had made 50 reinforced bar presses. The following day, variable interval training began. Variable interval training. For the next 7 days, animals were reinforced on a variable interval 30sec. (VI 30-sec.) schedule, with intervals generated according to the method of Fleshier and Hoffman (1962). Sessions were 30-min. long. Passive avoidance. For the next 2 days, all bar presses were punished by footshocks of gradually increasing intensity and duration, while responding continued to be reinforced on a VI 30-sec. schedule. On the first day of punishment, hind paws were shaved approximately 1 hr. before the session. The sequence of shock durations and intensities ranged .5-3 sec. and .5-4 ma. Punishment was followed by 2 sessions of reacquisition with reinforcement available on a VI 30-sec. schedule. No responses were punished. Extinction. During the next 3-day period, no reinforcement was scheduled for bar pressing. Shock threshold. On the final day of the experiment, jump-flinch responses to grid shocks of varying intensity were determined for each rabbit, using the Lehigh Valley shock generator. Threshold determination was based on increments of roughly .2 ma.
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Histology Following the conclusion of experimental training, animals with lesions were anesthesized with an overdose of Nembutal and perfused percardially with physiological saline followed by 10% Formol saline. Following fixation, serial frozen sections were taken at 30 tan. Every fourth section through the lesion was mounted and stained with cresyl violet. The extent of damage to each brain was estimated by eye and drawn on pages traced from the Urban and Richard stereotaxic atlas (1972).
RESULTS Histology Examinations of the brain sections revealed that 10 animals in the septal lesion group had sustained bilateral damage to the septal area. Reconstructions of representative lesions showing maximal and minimal extent of damage in rabbits with bilateral septal damage are shown in Figure 1. All lesions in this group produced extensive bilateral damage to the medial septal nuclei and interrupted the precommissural fornix.
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FIQUEE 1. Reconstructions of minimal (S19) and maximal (S22) extent of septal lesions superimposed on plates taken from the stereotaxic atlas of Urban and Richard (1972).
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Most lesions destroyed varying portions of the lateral septal nuclei. Five animals sustained some unilateral damage to the lateral septal nuclei, and four animals had some bilateral damage to those nuclei. Some animals sustained damage to dorsal portions of the nucleus of the diagonal band of Broca (7), corpus callosum (2), caudate nucleus (2), or columns of the fornix (1), as well as the septum. Eight additional septal subjects all sustained varying degrees of damage confined to one hemisphere. These unilateral lesions differed widely in both size and location, and the data of the animals with unilateral lesions were considerably more variable than those of either the normal or animals with bilateral septal lesions. Consequently, only data from normal and animals with bilateral septal lesions are described here. One normal animal and 2 operates died during the classical conditioning phase. These incomplete data were discarded. Classical Conditioning Normal animals and those with septal lesions showed distinct differences in response patterns in the classical conditioning phase of the study. Although rates of acquisition
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and asymptotic performance to CS+ were virtually the same for both groups, animals NORMAl CS I with septal lesions showed higher rates of responding to the CS— than would ordinarily be expected under conditions that typi100 cally produce low response rates in normal 90 animals. Figure 2 summarizes group results of classical differential conditioning. The 80 figure indicates that animals with septal •70 lesions made more conditioned responses to 60 CS— resulting in poorer overall discrimination than that of normals. Animals with 50 septal lesions had significantly lower aver40 age difference scores (t = 2.18, df = 19, 30 p < .05) and discrimination indices, de20 fined as percent CR to CS+/ (percent CRs to CS+ plus percent CRs to CS-), (t = 10 2.43, df = 19, p < .05) than normals. These 3 4 5 6 7 9 10 11 and all subsequent t tests are 2-tailed. Results of the generalization test, shown SESSION in Figure 3, indicate higher resistance to FIGURE 2. Classical differential conditioning performance for septal and normal groups expressed as extinction among rabbits with septal lesions. percent conditioned responses (CRs) to CS+ and This provides further evidence that septal lesions inflate response rates on a variety of CS-. +
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tasks. Animals with septal lesions responded significantly more on the first day of extinction than normals (t = 2.79, df = 19, p < .02), although the shape of the generalization gradients were roughly the same for both groups. The 2 groups did not differ on the second day, since conditioned responding was completely extinguished for both groups. Several explanations of the increased responding found among animals with septal lesions were considered. The differences found in differential conditioning and resistance to extinction might have been due to differences in sensitivity to the auditory CSs. Threshold tests suggest that this was not the case. While mean threshold for CR to the CS+ in animals with septal lesions (57.8 db. SPL) was significantly lower than that of normals (61.5 db. SPL) (t = 2.45, df — 19, p < .05), there was no significant difference between groups in CS— threshold (t = .48, df = 19). Similarly, differences in reactivity to the US might have mediated the difference in differentiation between the groups, but eye-shock threshold tests of unconditioned NMR revealed no significant difference between the 2 groups (t = .77, df = 19). Finally, animals with septal lesions might have exhibited a higher spontaneous NMR rate at the outset of the experiment making inhibition of CS— responses more difficult. Two pieces of evidence argue against this possibility. First, baseline rate of NMRs was not significantly different between the 2 groups (t = .26, df = 19). Further, as indicated by the graph in Figure 2, during the first 4 days of training there was no significant difference between the number of conditioned responses emitted to the CS— by normal subjects and those with septal lesions (t = .38, df = 19). Taken together, these results support the interpretation that rabbits with bilateral septal lesions are less able to learn to withhold responses to nonreinforced stimuli in the classically conditioned differentiation and extinction tasks. Operant Conditioning Two animals in each group failed to acquire the bar-press response within 3 shap-
ing sessions. Acquisition of the bar press under VI 30-sec. reinforcement for the remaining rabbits (8 operates and 9 normals) is shown in Figure 4. Animals with septal lesions had higher rates of responding on an appetitively reinforced schedule than normals, as indicated by a comparison of mean response rate across all days of VI acquisition (t = 2.42, df = 15, p < .05). However, the mean rates of response found in the Vl-reacquisition period (t = .50, df = 15) suggest that normal rabbits and rabbits with septal lesions may approach the same asymptotic levels of responding but that operates reach asymptote sooner. Also, as has been found by other authors, animals with septal lesions responded more through a period of operant extinction than did normals (t = 2.34, df = 15, p < .05). Passive avoidance response rates weresimilar for septal and normal animals (t' — .54, df = 15). However, when rate of response during passive avoidance was divided by the average rate for the 2 days of acquisition preceding passive avoidance, rabbits with septal lesions suppressed better than normal subjects (t — 2.65, df = 15, p < .05). With the gradations in shock intensity employed in this experiment, there was no difference in jump-flinch thresholds between normal animals and those with septal lesions, nor was there any observable difference in behavior at suprathreshold levels of grid shock. If the close correspond60
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FIGURE 4. Operant conditioning performance during variable interval (VI) acquisition, passive avoidance, VI reacquisition, and extinction for septal and normal groups, expressed as mean rate of response in each session.
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ence between rates of response on the last day of acquisition and during reacquisition are taken as evidence that animals with septal lesions reached asymptotic level before passive avoidance training began, the difference in suppression may be attributed to the fact that actual baseline rate of VI responding for normal animals continued to increase during the passive avoidance session. Thus, the 2 days prior to avoidance training may not be the best basis for comparison. If rate of response during passive avoidance is divided by average rate during reacquisition, instead of the last 2 days of acquisition, there is no significant difference between groups (t = .88, df — 15). DISCUSSION The principal result of the present investigation is the finding that rabbits with septal lesions are less able to inhibit conditioned responding to CS— during classical differential conditioning and extinction than normals. This higher level of responding was not due to lower auditory thresholds for the CR or eye-shock thresholds for the UR, nor was it due to differences in spontaneous NMRs. Rabbits with septal lesions did not differ significantly from normal rabbits on any of these indices. Rate of acquisition and asymptotic responding to CS+ was also virtually the same for both groups. This result is clearly inconsistent with the notion that behavioral deficits produced by septal lesions are mediated by deficits in classical conditioning. However, behavioral deficits may be mediated by failure to form a differentiated CR (e.g., Garber & Simmons, 1968). In the operant conditioning phase of the study, rabbits with septal lesions bar pressed at higher rates than normals during both VI acquisition and extinction, but they suppressed responding as well as normals under a passive avoidance contingency. The 2 groups did not differ in their jump-flinch threshold or in their observable motor reactions to grid shock. Relation to Other Species The finding that normal rabbits and those with septal lesions had similar auditory
thresholds is consistent with studies involving rats that have reported no difference between animals with septal lesions and normal animals in response to auditory stimulation when startle reflex (Kemble & Ison, 1971) or escape latency (Brown & Remley, 1971) are compared. The finding that normal rabbits and rabbits with septal lesions had similar shock thresholds seems inconsistent with studies that have reported increased reactivity to shock among rats with septal lesions. However, several recent articles have shown that differences in reactivity are mitigated if (a) lesions are small and medially placed (Carey, 1972), (b) postoperative handling is delayed by several days (Gotsick & Marshall, 1972), or (c) extended periods intervene between operation and testing (e.g., Hammond & Thomas, 1971). All of these conditions obtained in the present study, and this might account for failure to observe differences in jumpflinch or NMR thresholds to shock between the 2 groups. Increased responding among rats with septal lesions has been found on a variety of appetitively reinforced tasks (Fried, 1972; Sodetz & Koppell, 1972). These findings were replicated here: Rabbits with septal lesions reinforced with food pellets on a VI 30-sec. schedule responded at higher rates than normals. Furthermore, rabbits with septal lesions made more responses in operant extinction than did normals, which is consistent with the results of similar studies involving rhesus monkeys (Butter & Rosvold, 1968) and rats (Gray, Quintao, & Araujo-Silva, 1972). When passive avoidance contingencies were superimposed on an appetitively reinforced continuous reinforcement (CRF) baseline, decrements in suppression are often found among animals with septal lesions (e.g., Kaada, Rasmussen, & Kveim, 1962). However, Sodetz and Koppell (1972) failed to observe suppression decrements in rats with septal lesions when reinforcement is available on a VI schedule, as it was in this study. Here, rabbits with septal lesions suppressed bar-press responses as well as normals during passive avoidance when asymptotic levels of response are taken into
SEPTAL LESIONS AND CONDITIONING
account. A possible explanation is that, although suppression on a CRF task necessarily reduces the number of reinforcements, on a VI schedule even large changes in response rate may not alter the number of reinforcements obtained in a session. Furthermore, Miczek, Kelsey, and Grossman (1972) have demonstrated that punishment effects are strongly dependent on time since lesion. In their study, rats with septal lesions suppressed as well as normals when the passive avoidance contingency was introduced at least 10 days postoperatively. In the present study, passive avoidance training occurred approximately 1 mo. postoperatively. Theoretical Relevance McCleary's (1966) hypothesis that septal lesions result in the loss of motoric inhibition and perseveration of dominant responses can account for the increased responding to CS— and greater resistance to extinction observed in the classical conditioning phase of this study, as well as the higher than normal bar-press rates observed in the operant phase. Whereas the increased responding to the CS— in differential conditioning found in rabbits with septal lesions is consistent with hypothesized decreases in learned response or motoric inhibition, possible changes in attention to or salience of the CS— might also account for the observed results, since if the CS— is not attended to or is "gated out", it is unlikely to elicit a conditioned response. That is, the elevated level of responding to the CS— by rabbits with septal lesions might be attributed to failure to habituate the orienting response instead of (or in addition to) a decrease in learned response inhibition. Electrophysiological results provide indirect support for the notion that septal lesions interfere with habituation. Gray (1972) has shown that stimulation of the medial septal nuclei alters hippocampal theta rhythm. Douglas (1967) cites evidence that septal lesions modify hippocampal theta pacemakers, disrupting the theta rhythm (Green & Arduini, 1954). Although the exact role of the hippocampal theta rhythm is not clear, several authors have
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noted a correlation between arousal or attention and absence of the theta rhythm (e.g., Jarrard, 1973). Routtenberg (1968) reports that theta decreases occur during repeated exposure to novel stimuli, presumably correlating with stimulus habituation. Furthermore, Bremner (1970) found that hippocampal theta rhythm was present during classical conditioning and habituation trials but that there was a statistically significant difference between the dominant frequency in each situation. Presumably, lesions of the septum, which clearly influence the theta rhythm, would disrupt this relationship, possibly interfering with the habituation process. Carlton (1969) has proposed a central cholinergic inhibitory system anatomically located in the septo-hippocampal complex. This hypothesis was based on similarities found between effects of hippocampal lesions and direct hippocampal injections of anticholinergic drugs. Several recent studies that have evaluated the effects of direct injections of cholinergic blocking agents into the medial septal nuclei have reported behavior patterns similar to those produced by hippocampal lesions (e.g., Hamilton & Grossman, 1969). Leaton and Rech (1972) interpret their finding that both intraseptal and intrahippocampal injections of atropine heighten locomotive activity as strong evidence in support of the role of septo-hippocampal cholinergic neurons in mediating behavioral inhibition. The increased responding to the nonreinforced stimulus in classical differential conditioning produced by lesions confined primarily to the medial septal nuclei in rabbits is consistent with the existence of a cholinergic inhibitory loop involving septum and hippocampus. REFERENCES Bremner, F. J. The effect of habituation and conditioning trials on hippocampal EEG. Psychonomic Science, 1970,18,181-183. Brown, G. E., & Remley, N. R. The effects of septal and olfactory bulb lesions on stimulus reactivity. Physiology and Behavior, 1971, 6, 497-502. Butter, N., & Rosvold, H. E. Effect of caudate and septal nuclei lesions on resistance to extinction and delayed alternation. Journal of
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Comparative and Physiological Psychology, 1968, 65,397-403. Carey, R. J. A neuronanatomical investigation of enhanced cutaneous and gustatory responsivity associated with septal forebrain injury. Journal oj Comparative and Physiological Psychology, 1972, 80, 449-457. Carlton, P. L. Brain acetylcholine and inhibition. In J. T. Tapp (Ed.), Reinforcement and behavior. New York: Academic Press, 1969. Douglas, R. J. The hippocampus and behavior. Psychological Bulletin, 1967, 67, 416-442. Douglas, R. J. Pavlovian conditioning and the brain. In R. A. Boakes & M. S. Halliday (Eds.), Inhibition and learning. New York: Academic Press, 1972. Duncan, P. M. Effect of temporary septal dysfunction on conditioning and performance of fear responses in rats. Journal of Comparative and Physiological Psychology, 1971, 74, 340-348. Duncan, P. M. Effect of septal area damage and base-line activity levels of conditioned heartrate response in rats. Journal oj Comparative and Physiological Psychology, 1972, 81,131-142. Flesher, M., & Hoffman, H. S. A progression for generating variable interval schedules. Journal of the Experimental Analysis of Behavior, 1962, 5, 529-530. Fried, P. A. Septum and behavior: A review. Psychological Bulletin, 1972, 78, 292-310. Garber, E. E., & Simmons, H. J. Facilitation of two-way avoidance performance by septal lesions in rats. Journal of Comparative and Physiological Psychology, 1968, 66, 559-562. Gormezano, I. Classical conditioning. In J. B. Sidowski (Ed.), Experimental methods and instrumentation in psychology. New York: McGraw-Hill, 1966. Gotsick, J. E., & Marshall, R. C. Time course of the septal rage syndrome. Physiology and Behavior, 1972, 9, 685-688. Gray, J. A. Effects of septal driving of the hippocampal theta rhythm on resistance to extinction. Physiology and Behavior, 1972, 8,481-490. Gray, J. A., Quintao, L., & Araujo-Silva, M. T. The partial reinforcement extinction effect in rats with medial septal lesion. Physiology and Behavior, 1972, 8, 491-496. Green, J. D., & Arduini, A. Hippocampal electrical activity in arousal. Journal of Ne'urophysiology, 1954, 17, 533-557. Hamilton, L. W., & Grossman, S. P. Behavioral changes following disruption of central cholin-
ergic pathways. Journal of Comparative and Physiological Psychology, 1969,69, 76-82. Hammond, G. R., & Thomas, G. J. Failure to reactivate the septal syndrome in rats. Physiology and Behavior, 1971, 6,599-602. Holdstock, T. L. Plasticity of autonomic functions in rats with septal lesions.- Neuropsychologia, 1970, 8,147-160. Jarrard, L. E. The hippocampus and motivation. Psychological Bulletin, 1973, 79,1-12. Kaada, B. R., Rasmussen, E. W., & Kveim, 0. Impaired acquisition of passive avoidance behavior by subcallosal, septal, hypothalamic, and insular lesions in rats. Journal of Comparative and Physiological Psychology, 1962, 55,661-670. Kelsey, J. E., & Grossman, S. P. Nonperseverative disruption of behavioral inhibition following septal lesions in the rat. Journal of Comparative and Physiological Psychology, 1971, 75, 302312. Kemble, E. D., & Ison, J. R. Limbie lesions and inhibition of startle reactions in the r'at by conditions of preliminary stimulation. Physiology and Behavior, 1971, 7, 925-928. Leaton, R. N., & Rech, R. H. Locomotor activity increases produced by intrahippocampal and intraseptal atropine in rats. Physiology and Behavior, 1972, 8,539-541. McCleary, R. A. Response modulating functions of the limbic system: Initiation and suppression. In E. Stellar & J. M. Sprague (Eds.), Progress in physiological psychology. Vol. 1. New York: Academic Press, 1966. Miczek, K. A., Kelsey, J. E., & Grossman, S. P. Time course of effects of septal lesions on avoidance, response suppression, and reactivity to shock. Journal of Comparative and Physiological Psychology, 1972, 79, 318-327. Routtenberg, A. Hippocampal correlates of consummatory and observed behavior. Physiology and Behavior, 1968, 3, 533-535. Sodetz, F. J., & Koppell, S. Suppressive effects of punishment of operant responding in rats with septal lesions. Physiology and Behavior, 1972, 8, 837-840. Thomas, J. B. Non-appetitive passive avoidance in rats with septal lasions. Physiology and Behavior, 1972, 8,1087-1092. Urban, I., & Richard, P. A stereotaxic atlas of the New Zealand rabbit's brain. Springfield, 111.: Charles C Thomas, 1972. (Received September 14, 1973)
