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On functions of the mamillary bodies in the squirrel monkey
EXPERIMEKTAL
NEUROLOGY
On
7,,76-85
Functions
of
in the DETLEV Laboratory Institutes
W.
(1963)
the
Mamillary”
Squirrel
PLOOG
AND
Monkey PAUL
D.
of Neurophysiology, National Institute of Health, Public Health Service, Department Welfare, Bethesda, Maryland Received
Bodies
October
MACLEAN~ of
Mental Health, National of Health, Education, and
1, 1962
This study was undertaken to learn the effects of destruction of the mamillary bodies on the retention of conditioned avoidance responses. Squirrel monkeys were trained in a conditioned avoidance test which required the animal to jump at the sound of a buzzer to the illuminated one of two pedestals in order to avoid shock. The test provided for introducing a progressive delay between the termination of the visual cue and the beginning of the auditory signal. When trained to criterion, the animals were submitted to electrocoagulation in the region of the mamillary bodies and retested. The results allow the conclusion that extensive to virtually complete destruction of the mamillary bodies does not appreciably interfere with the animal’s ability to perform a complex conditioned avoidance test, including one which requires a short delay in the response. As previously reported, and also shown in the present study, stimulation in respective parts of the mamillary bodies results in penile erection. This finding indicates that these nuclei comprise part of a system involved in genital function and would suggest that a further investigation of functions be planned accordingly. Introduction
In previous studies it was found that propagated hippocampal “seizure” discharges abolished conditioned avoidance responses (6)) as well as conditioned cardiac and respiratory responses (2, 6). Such discharges * AUTHORS' NOTE: Spelling of mamillary follows that of Nomina Anatomica; most American dictionaries have not yet made this change and still empIoy the doubIe m. 1 We owe particular thanks to Mrs. Jean Cobb Blitz and Dr. Frauke Ploog for their invaluable part in testing the animals and to Mrs. Martha Oliphant and Mr. George Creswell for their help in preparing the histological material and the illustrations. We would also like to express our appreciation to Drs. Mortimer Mishkin and Stefan Brutkowski for their helpful discussions. Dr. Detlev W. Ploog’s address is: Abteilung fiir Verhaltensforschung, Deutsche Forschungsanstalt fiir Psychiatric, Max-Planck-Institut, Munich, Germany.
76
MAMILLARY
BODIES
77
were interpreted as resulting in what was tantamount to a “functional ablation” of the limbic system. The mamillary bodies receive a large proportion of hippocampal projections. It has been estimated that about half of the postcommissural fornix fibers project to the mamillary nuclei (4, 9). The possibility exists that disruption of function of these nuclei during hippocampal discharges might contribute significantly to the loss of conditioned responses. In order to test this hypothesis, the present experiments on squirrel monkeys were undertaken to learn whether or not isolated lesions of the mamillary bodies would have an effect comparable to that produced by hippocampal discharges. Another purpose was to devise a combined delayed response and conditioned avoidance test for squirrel monkeys which would be useful in evaluating the clinical hypothesis that the mamillary bodies are involved in memory processes. Methods
Six male squirrel monkeys (Saimiri sciureus) of adult size were used for these studies. A conditioned avoidance test (see below) was devised in which the animal was required to jump at the sound of a buzzer to the illuminated one of two pedestals in order to avoid a shock. The test provided for introducing a progressive delay between the termination of the visual cue and the beginning of the auditory signal. When trained to criterion (see below) the animals were anesthetized and two electrodes were successively placed stereotaxically about 1.5 mm apart in the region of the mamillary bodies. At each electrode site the effects of electrical stimulation were tested and then a small electrocoagulation of about 0.75 mm in radius was made with a Wyss high-frequency coagulator (10). Afterwards the effect of electrical stimulation was again tried at the site of coagulation and sometimes at points 1.0 mm above. Details of the stimulation technique have been given (7), After recovery from operation the animals were retested on their ability to perform the conditioned avoidance test. When a plateau level of performance was reached, they were killed, and the brains were examined histologically in order to reconstruct the extent of the lesions. Conditioned Avoidance Test. Conditioned avoidance testing is conducted in a special viewing stage for taking motion pictures. This stage, built in the form of an arc and having a concave window of hard-surface Lucite, allows motion pictures to be obtained with a constant source of fluorescent light and with the animal kept in the focal range of the
78
PLOOG
AND
MAC
LEAN
camera. A diagram of the testing situation is shown in Fig. 1. Three translucent pedestals, each containing a white electric light bulb, are placed equidistant on an electrified grid forming the floor of the stage. On each pedestal is also an electrified grid. The center pedestal serves as a home pedestal (HP) from which the monkey can jump either to pedestal Pl or P2. A partial partition prevents jumping back and forth
TO
PiorP2
TO
HP
FIG. 1. The drawing of the experimental setting shows viewing stage with Lucite window in which testing was conducted. By means of electrified grids on floor of stage and pedestals, monkey is trained to remain on home pedestal (HP). In test situation Pl or P2 is randomly illuminated and animal must jump at sound of buzzer to the illuminated pedestal in order to avoid shock. As indicated in schematic of formal training procedure, buzzer is also signal to return to HP. A partial partition prevents jumping back and forth between PI and Pz.
MAMILLARY
BODIES
79
between Pl and P2. In the test situation either Pl or P2 is illuminated and the animal must jump to the illuminated pedestal at the sound of a buzzer located under the stage in order to avoid shock. When the buzzer is sounded again, it must jump back to HP. In the test involving a delay, the pedestal is first illuminated for several seconds and then after an interval the buzzer is sounded. Timers and electrical relays are used for presenting the conditioned and unconditioned stimuli. A scrambling device is employed to ensure against short-circuiting by feces of the current applied to the pedestals. The respective durations of the light, buzzer, and shock are usually 5, 2, and 1 sec. The difficult training procedure involves a preliminary phase and a formal phase. In the preliminary phase the monkey is first trained to remain on one of the three pedestals and thus avoid shock from the continuously electrified grid of the stage. Then it is required to remain on HP and so avoid shock at Pl or P2. Finally, it must learn to jump at the sound of a buzzer to Pl or P2 and then back to HP when the buzzer is sounded again. During the formal phase of training 20 trials per day are conducted 5 days a week. The three steps involved in this training are schematized in Fig. 1: (i) Pedestals Pl or P2 are illuminated in random order and the animal must learn to jump to the illuminated pedestal at the sound of a buzzer in order to avoid a shock; the sound of the same buzzer serves as a cue to return to HP, (ii) The next step of training is the same except that the light is terminated immediately at the sound of the buzzer. (iii) Finally, after the animal reaches a plateau performance of 80% correct trials, a delay is introduced between the termination of the visual cue and the presentation of the auditory signal. The delay is increased when criterion is again reached. Results
Preoperative Testing. One purpose of this study was to devise a combined delayed response and conditioned avoidance test for squirrel monkeys. It was previously found in this laboratory that these monkeys could rapidly achieve a high level of performance in a shuttle-type test which involved jumping at the sound of a buzzer from one pedestal to another in order to avoid shock. For example, eight monkeys achieved 90 to 1000/o correct performance within 120 to 240 trials, with a mean of 175 trials. On the basis of the foregoing experience, electrified pedestals were
80
PLO%
AND
MAC
LEAN
used in the test devised for the present study. As evident in Fig. 1, the arrangement of the three pedestals provided a situation somewhat comparable to that used in a T-shaped maze pattern. In testing the first four animals it was found that the use of a partial partition which prevented jumping between Pl and P2 accelerated the rate at which the test was
0
200
ONE SECOND
DELAY
CNE SECOND
DELAY
400
FIG. 2. Learning curves of the five the animal’s initial at the lower left of number of antecedent trials. Graphs performance obtained with and ~ithozit entation of the visual cue and the achieved after coagulation in mamillary period. Percentage of correct responses
600
600
1000
loo
monkeys referred to in text and identified by each row. Numerals in parentheses represent for Andy, Marvin, and Ptvcy show level of the introduction of delay between the presonset of the auditory signal. Performance region compares to that of preoperative on ordinate; trials on abscissa.
learned. At the same time it became apparent that it was impractical in this kind of a test to introduce a delay of more than 2 set between the termination of the visual cue and the beginning of the auditory signal. Only one animal, Otto, was able to resist jumping for as long as 2 set to the previously lighted pedestal. This animal failed to survive electrocoagulation. The other three animals, Andy, Marvin, and Percy, were tested with a I-set delay before and following surgery, and their learning curves are shown in Fig. 2. The curves are identified by the initials of the animals’ names at the left. The monkey, Andy, seldom exceeded a
MAMILLARY
81
BODIES
score of 75% correct responses, whereas Marvin and Percy achieved a plateau performance of close to 80%. The top two graphs of Fig. 2 show the learning curves of the other two animals, Dewey and Louie, which were subsequently tested without the
A8.5
A80
A75
A7.0
A6.5
FIG, 3. Reconstructions of brain lesions in same animals referred to in preceding figure and similarly identified by animal’s initial at left. Shaded areas show extent of destruction projected on drawings made from a stereotaxic atlas of squirrel monkey’s brain (3). Drawings from left to right show successively OS-mm levels from anterior frontal plane A 8.5 to A 6.5. Scale at bottom center is in millimeters. Abbreviations: III, ventriculus tertius; AL, ansa lenticularis; H, area tegmentalis (Forel) ; M, corpus mamillare; MT, tractus mamillothalamicus; NS, nucleus subthalamicus; CM, nervus oculomotorius; PC, pedunculus cerebri; SN, substantia nigra; TO, tractus opticus; and VM, nucleus ventromedialis hypothalami.
82
PLOOG
AND
MAC
LEAN
introduction of the delay. The curves indicate that in the test without the delay the squirrel monkey is capable of achieving consistent high level performance in about 1,000 trials. Postoperative Testing. Five of the six animals survived electrocoagulation in the region of the mamillary bodies and were submitted to postoperative testing. Figure 3 shows a reconstruction of the lesions in these
FIG.
in which
4.
Nissl-stained section showing transverse extent of lesion destruction of mamillary bodies was virtually complete.
in animal
Dewey,
animals projected on drawings of various levels of the hypothalamus made from a Horsley-Clarke atlas of the squirrel monkey brain (3). Each reconstruction is identified by the initial of the animal’s name at the left. The shaded areas show that the lesions in every case extended somewhat rostra1 to the mamillary bodies and frequently impinged on the ventromedial nucleus of the hypothalamus. It is also evident that, with the exception of the animal Louie, the mamillary bodies were almost completely destroyed in all animals. The destruction appeared to be virtually complete in Dewey, whereas there remained only very small, and questionably functioning, fragments in Andy, Marvin and Percy. Figure 4 shows histologically the transverse extent of the lesion in Dewey.
MAMILLARY
BODIES
83
Following surgery, four animals showed transient slowness and awkwardness of movement which was attributed to generalized weakness rather than to a specific motor deficit. Two monkeys had partial bilateral ptosis of the eyelids, and the two others had unequal (but active) pupils owing to involvement of the oculomotor nerve. Otherwise there were no signs during the period of postoperative observation that the behavior or disposition of any animal had been changed by surgery. The animal with the smallest lesion, Lo&, appeared so unchanged by surgery that testing was resumed on the next day. As illustrated in Fig. 2, he was able to achieve a perfect performance on the second day of testing. The other animals were tested after a recovery of five days. The animal (Dewey) with apparent complete destruction of mamillary bodies achieved close to perfect performance on the second day of testing. Of the three animals tested with the 1-set delay, only iMarvin showed significant temporary deterioration in his performance. After 6 days of testing, however, he obtained a perfect score, a record. that had not been achieved during preoperative testing. Effects of Brain Stimulation. As we have previously reported (7), electrical stimulation in respective parts of the mamillary bodies results in penile erection. Anesthesia does not prevent the response, although a greater stimulus intensity is required. In five animals of the present study stimulation was applied under Nembutal anesthesia at each point to be coagulated. In every case one electrode appeared to be well within the mamillary bodies, while the others involved part of it or came to rest largely within the neighboring medial forebrain bundle. In all but one instance electrical stimulation resulted in partial (2 to 3f) erection. Following coagulation, stimulation at the same point with the same intensity elicited little or no response, indicating that the previous positive findings were not owing to spread of current to structures beyond those involved in destruction. Further details are given in the original paper (7). Discussion
From the foregoing findings it may be concluded that animals are able to perform a complex conditioned avoidance test following complete or extensive destruction of the mamillary bodies. In regard to the question posed in the introduction the results would indicate that an animal’s failure in conditioned avoidance testing during propagated hippocampal discharges is not attributable to interference with previously learned performance because of disruption of function of the mamillary nuclei.
84
PLOOG
AND
MAC
LEAN
The findings of the present study have the added significance that the avoidance test which was used involved a visual cue as well as the auditory signal employed as the sole conditioned stimulus in the investigations on the hippocampal discharges. The delayed response conditioned avoidance test which was devised and tried out in the present investigation proved inadequate for evaluating the role of the mamillary bodies in immediate recall. As mentioned, only one animal was able to resist jumping to the previously lighted pedestal for as long as 2 sec. A practical method for testing delays of more than 2 set would require some additional device for restraining the monkey until it was time to jump, such as is done in Lawicka’s delayed response test for dogs (5). The squirrel monkey’s difficulty in learning to defer jumping for more than 1 to 2 set after the preliminary visual signal may in part reflect its m&e
nf
life
in
itc
nnt11ra1
hnhitgt
rxrhmm
fnr
~~-a~mnl~
cdftnncc
nf
-ot:nn
MAMILLARY
BODIES
85
and would suggest that further investigation of their functions be planned accordingly. In studies on the social and sexual behavior of squirrel monkeys we have observed certain forms of sexual display by this monkey that lend themselves to experimental manipulation (7, 8). An investigation is being undertaken in which the effects of various brain lesions on these forms of display will be observed. This will provide an opportunity to re-evaluate the effects of bilateral mamillary destruction. References 1. AKERT, K., and 0. J. ANDY. 1955. Experimental studies on corpus mammiilare and tegmento-mammiilary system in the cat. Am. J. Physiol. 183: 591. 2. FLYNN, J. P., P. D. MACLEAN, and CHUL IM. 1961. Effects of hippocampal afterdischarges on conditioned responses, pp. 382-386. In “Electrical stimulation of the Brain,” D. E. Sheer [ed.]. University of Texas Press, Austin. 3. GERGEN,J. A., and P. D. MACLEAN. 1962. “A Stereo&& Atlas of the Squirrel Monkey’s Brain (Saimiri sciureus),” 91 pp. Government Printing Office, Washton, D.C. 4. GUILLERY, R. W. 1955. A quantitative study of the mamillary bodies and their connexions. J. Anat. 39: 19-32. 5. LA~ICKA, W. 1959. Physiological mechanism of delayed reactions. II. Delayed reactions in dogs and cats to directional stimuli. Acta Biol. Exptl. Lodz 19: 199-219. 6. MACLEAN, P. D., S. FLANIGAN, J. P. FLYNN, CHUL KIM, and J. R. STEVENS. 1956. Hippocampal function: Tentative correlations of conditioning, EEG, drug, and radioautographic studies. Yale J. Biol. Med. 33: 380-395. 7. MACLEAN, P. D., and D. W. PLOOC. 1962. Cerebral representation of penile erection. J. Neurophysiol. 36: 29-5.5. 8. PLOOG, D. W., and P. D. MACLEAN. 1963. Display of penile erection in squirrel monkey (Saimiri scizrreus). Animal Behaviour (In press). 9. Powxu, T. P. S., R. W. GUULERY, and W. M. COWAN. 1957. A quantitative study of the fomix-mamillo-thalamic system. 1. Amt. 91: 419-437. 10. WYSS, 0. A. M. 1945. Ein Hochfrequenz-Koagulationsgerlt xur reizlosen Ausschaltung. Helv. Physiol. et Pharmacol. Acta 3: 437-443.
NEUROLOGY
On
7,,76-85
Functions
of
in the DETLEV Laboratory Institutes
W.
(1963)
the
Mamillary”
Squirrel
PLOOG
AND
Monkey PAUL
D.
of Neurophysiology, National Institute of Health, Public Health Service, Department Welfare, Bethesda, Maryland Received
Bodies
October
MACLEAN~ of
Mental Health, National of Health, Education, and
1, 1962
This study was undertaken to learn the effects of destruction of the mamillary bodies on the retention of conditioned avoidance responses. Squirrel monkeys were trained in a conditioned avoidance test which required the animal to jump at the sound of a buzzer to the illuminated one of two pedestals in order to avoid shock. The test provided for introducing a progressive delay between the termination of the visual cue and the beginning of the auditory signal. When trained to criterion, the animals were submitted to electrocoagulation in the region of the mamillary bodies and retested. The results allow the conclusion that extensive to virtually complete destruction of the mamillary bodies does not appreciably interfere with the animal’s ability to perform a complex conditioned avoidance test, including one which requires a short delay in the response. As previously reported, and also shown in the present study, stimulation in respective parts of the mamillary bodies results in penile erection. This finding indicates that these nuclei comprise part of a system involved in genital function and would suggest that a further investigation of functions be planned accordingly. Introduction
In previous studies it was found that propagated hippocampal “seizure” discharges abolished conditioned avoidance responses (6)) as well as conditioned cardiac and respiratory responses (2, 6). Such discharges * AUTHORS' NOTE: Spelling of mamillary follows that of Nomina Anatomica; most American dictionaries have not yet made this change and still empIoy the doubIe m. 1 We owe particular thanks to Mrs. Jean Cobb Blitz and Dr. Frauke Ploog for their invaluable part in testing the animals and to Mrs. Martha Oliphant and Mr. George Creswell for their help in preparing the histological material and the illustrations. We would also like to express our appreciation to Drs. Mortimer Mishkin and Stefan Brutkowski for their helpful discussions. Dr. Detlev W. Ploog’s address is: Abteilung fiir Verhaltensforschung, Deutsche Forschungsanstalt fiir Psychiatric, Max-Planck-Institut, Munich, Germany.
76
MAMILLARY
BODIES
77
were interpreted as resulting in what was tantamount to a “functional ablation” of the limbic system. The mamillary bodies receive a large proportion of hippocampal projections. It has been estimated that about half of the postcommissural fornix fibers project to the mamillary nuclei (4, 9). The possibility exists that disruption of function of these nuclei during hippocampal discharges might contribute significantly to the loss of conditioned responses. In order to test this hypothesis, the present experiments on squirrel monkeys were undertaken to learn whether or not isolated lesions of the mamillary bodies would have an effect comparable to that produced by hippocampal discharges. Another purpose was to devise a combined delayed response and conditioned avoidance test for squirrel monkeys which would be useful in evaluating the clinical hypothesis that the mamillary bodies are involved in memory processes. Methods
Six male squirrel monkeys (Saimiri sciureus) of adult size were used for these studies. A conditioned avoidance test (see below) was devised in which the animal was required to jump at the sound of a buzzer to the illuminated one of two pedestals in order to avoid a shock. The test provided for introducing a progressive delay between the termination of the visual cue and the beginning of the auditory signal. When trained to criterion (see below) the animals were anesthetized and two electrodes were successively placed stereotaxically about 1.5 mm apart in the region of the mamillary bodies. At each electrode site the effects of electrical stimulation were tested and then a small electrocoagulation of about 0.75 mm in radius was made with a Wyss high-frequency coagulator (10). Afterwards the effect of electrical stimulation was again tried at the site of coagulation and sometimes at points 1.0 mm above. Details of the stimulation technique have been given (7), After recovery from operation the animals were retested on their ability to perform the conditioned avoidance test. When a plateau level of performance was reached, they were killed, and the brains were examined histologically in order to reconstruct the extent of the lesions. Conditioned Avoidance Test. Conditioned avoidance testing is conducted in a special viewing stage for taking motion pictures. This stage, built in the form of an arc and having a concave window of hard-surface Lucite, allows motion pictures to be obtained with a constant source of fluorescent light and with the animal kept in the focal range of the
78
PLOOG
AND
MAC
LEAN
camera. A diagram of the testing situation is shown in Fig. 1. Three translucent pedestals, each containing a white electric light bulb, are placed equidistant on an electrified grid forming the floor of the stage. On each pedestal is also an electrified grid. The center pedestal serves as a home pedestal (HP) from which the monkey can jump either to pedestal Pl or P2. A partial partition prevents jumping back and forth
TO
PiorP2
TO
HP
FIG. 1. The drawing of the experimental setting shows viewing stage with Lucite window in which testing was conducted. By means of electrified grids on floor of stage and pedestals, monkey is trained to remain on home pedestal (HP). In test situation Pl or P2 is randomly illuminated and animal must jump at sound of buzzer to the illuminated pedestal in order to avoid shock. As indicated in schematic of formal training procedure, buzzer is also signal to return to HP. A partial partition prevents jumping back and forth between PI and Pz.
MAMILLARY
BODIES
79
between Pl and P2. In the test situation either Pl or P2 is illuminated and the animal must jump to the illuminated pedestal at the sound of a buzzer located under the stage in order to avoid shock. When the buzzer is sounded again, it must jump back to HP. In the test involving a delay, the pedestal is first illuminated for several seconds and then after an interval the buzzer is sounded. Timers and electrical relays are used for presenting the conditioned and unconditioned stimuli. A scrambling device is employed to ensure against short-circuiting by feces of the current applied to the pedestals. The respective durations of the light, buzzer, and shock are usually 5, 2, and 1 sec. The difficult training procedure involves a preliminary phase and a formal phase. In the preliminary phase the monkey is first trained to remain on one of the three pedestals and thus avoid shock from the continuously electrified grid of the stage. Then it is required to remain on HP and so avoid shock at Pl or P2. Finally, it must learn to jump at the sound of a buzzer to Pl or P2 and then back to HP when the buzzer is sounded again. During the formal phase of training 20 trials per day are conducted 5 days a week. The three steps involved in this training are schematized in Fig. 1: (i) Pedestals Pl or P2 are illuminated in random order and the animal must learn to jump to the illuminated pedestal at the sound of a buzzer in order to avoid a shock; the sound of the same buzzer serves as a cue to return to HP, (ii) The next step of training is the same except that the light is terminated immediately at the sound of the buzzer. (iii) Finally, after the animal reaches a plateau performance of 80% correct trials, a delay is introduced between the termination of the visual cue and the presentation of the auditory signal. The delay is increased when criterion is again reached. Results
Preoperative Testing. One purpose of this study was to devise a combined delayed response and conditioned avoidance test for squirrel monkeys. It was previously found in this laboratory that these monkeys could rapidly achieve a high level of performance in a shuttle-type test which involved jumping at the sound of a buzzer from one pedestal to another in order to avoid shock. For example, eight monkeys achieved 90 to 1000/o correct performance within 120 to 240 trials, with a mean of 175 trials. On the basis of the foregoing experience, electrified pedestals were
80
PLO%
AND
MAC
LEAN
used in the test devised for the present study. As evident in Fig. 1, the arrangement of the three pedestals provided a situation somewhat comparable to that used in a T-shaped maze pattern. In testing the first four animals it was found that the use of a partial partition which prevented jumping between Pl and P2 accelerated the rate at which the test was
0
200
ONE SECOND
DELAY
CNE SECOND
DELAY
400
FIG. 2. Learning curves of the five the animal’s initial at the lower left of number of antecedent trials. Graphs performance obtained with and ~ithozit entation of the visual cue and the achieved after coagulation in mamillary period. Percentage of correct responses
600
600
1000
loo
monkeys referred to in text and identified by each row. Numerals in parentheses represent for Andy, Marvin, and Ptvcy show level of the introduction of delay between the presonset of the auditory signal. Performance region compares to that of preoperative on ordinate; trials on abscissa.
learned. At the same time it became apparent that it was impractical in this kind of a test to introduce a delay of more than 2 set between the termination of the visual cue and the beginning of the auditory signal. Only one animal, Otto, was able to resist jumping for as long as 2 set to the previously lighted pedestal. This animal failed to survive electrocoagulation. The other three animals, Andy, Marvin, and Percy, were tested with a I-set delay before and following surgery, and their learning curves are shown in Fig. 2. The curves are identified by the initials of the animals’ names at the left. The monkey, Andy, seldom exceeded a
MAMILLARY
81
BODIES
score of 75% correct responses, whereas Marvin and Percy achieved a plateau performance of close to 80%. The top two graphs of Fig. 2 show the learning curves of the other two animals, Dewey and Louie, which were subsequently tested without the
A8.5
A80
A75
A7.0
A6.5
FIG, 3. Reconstructions of brain lesions in same animals referred to in preceding figure and similarly identified by animal’s initial at left. Shaded areas show extent of destruction projected on drawings made from a stereotaxic atlas of squirrel monkey’s brain (3). Drawings from left to right show successively OS-mm levels from anterior frontal plane A 8.5 to A 6.5. Scale at bottom center is in millimeters. Abbreviations: III, ventriculus tertius; AL, ansa lenticularis; H, area tegmentalis (Forel) ; M, corpus mamillare; MT, tractus mamillothalamicus; NS, nucleus subthalamicus; CM, nervus oculomotorius; PC, pedunculus cerebri; SN, substantia nigra; TO, tractus opticus; and VM, nucleus ventromedialis hypothalami.
82
PLOOG
AND
MAC
LEAN
introduction of the delay. The curves indicate that in the test without the delay the squirrel monkey is capable of achieving consistent high level performance in about 1,000 trials. Postoperative Testing. Five of the six animals survived electrocoagulation in the region of the mamillary bodies and were submitted to postoperative testing. Figure 3 shows a reconstruction of the lesions in these
FIG.
in which
4.
Nissl-stained section showing transverse extent of lesion destruction of mamillary bodies was virtually complete.
in animal
Dewey,
animals projected on drawings of various levels of the hypothalamus made from a Horsley-Clarke atlas of the squirrel monkey brain (3). Each reconstruction is identified by the initial of the animal’s name at the left. The shaded areas show that the lesions in every case extended somewhat rostra1 to the mamillary bodies and frequently impinged on the ventromedial nucleus of the hypothalamus. It is also evident that, with the exception of the animal Louie, the mamillary bodies were almost completely destroyed in all animals. The destruction appeared to be virtually complete in Dewey, whereas there remained only very small, and questionably functioning, fragments in Andy, Marvin and Percy. Figure 4 shows histologically the transverse extent of the lesion in Dewey.
MAMILLARY
BODIES
83
Following surgery, four animals showed transient slowness and awkwardness of movement which was attributed to generalized weakness rather than to a specific motor deficit. Two monkeys had partial bilateral ptosis of the eyelids, and the two others had unequal (but active) pupils owing to involvement of the oculomotor nerve. Otherwise there were no signs during the period of postoperative observation that the behavior or disposition of any animal had been changed by surgery. The animal with the smallest lesion, Lo&, appeared so unchanged by surgery that testing was resumed on the next day. As illustrated in Fig. 2, he was able to achieve a perfect performance on the second day of testing. The other animals were tested after a recovery of five days. The animal (Dewey) with apparent complete destruction of mamillary bodies achieved close to perfect performance on the second day of testing. Of the three animals tested with the 1-set delay, only iMarvin showed significant temporary deterioration in his performance. After 6 days of testing, however, he obtained a perfect score, a record. that had not been achieved during preoperative testing. Effects of Brain Stimulation. As we have previously reported (7), electrical stimulation in respective parts of the mamillary bodies results in penile erection. Anesthesia does not prevent the response, although a greater stimulus intensity is required. In five animals of the present study stimulation was applied under Nembutal anesthesia at each point to be coagulated. In every case one electrode appeared to be well within the mamillary bodies, while the others involved part of it or came to rest largely within the neighboring medial forebrain bundle. In all but one instance electrical stimulation resulted in partial (2 to 3f) erection. Following coagulation, stimulation at the same point with the same intensity elicited little or no response, indicating that the previous positive findings were not owing to spread of current to structures beyond those involved in destruction. Further details are given in the original paper (7). Discussion
From the foregoing findings it may be concluded that animals are able to perform a complex conditioned avoidance test following complete or extensive destruction of the mamillary bodies. In regard to the question posed in the introduction the results would indicate that an animal’s failure in conditioned avoidance testing during propagated hippocampal discharges is not attributable to interference with previously learned performance because of disruption of function of the mamillary nuclei.
84
PLOOG
AND
MAC
LEAN
The findings of the present study have the added significance that the avoidance test which was used involved a visual cue as well as the auditory signal employed as the sole conditioned stimulus in the investigations on the hippocampal discharges. The delayed response conditioned avoidance test which was devised and tried out in the present investigation proved inadequate for evaluating the role of the mamillary bodies in immediate recall. As mentioned, only one animal was able to resist jumping to the previously lighted pedestal for as long as 2 sec. A practical method for testing delays of more than 2 set would require some additional device for restraining the monkey until it was time to jump, such as is done in Lawicka’s delayed response test for dogs (5). The squirrel monkey’s difficulty in learning to defer jumping for more than 1 to 2 set after the preliminary visual signal may in part reflect its m&e
nf
life
in
itc
nnt11ra1
hnhitgt
rxrhmm
fnr
~~-a~mnl~
cdftnncc
nf
-ot:nn
MAMILLARY
BODIES
85
and would suggest that further investigation of their functions be planned accordingly. In studies on the social and sexual behavior of squirrel monkeys we have observed certain forms of sexual display by this monkey that lend themselves to experimental manipulation (7, 8). An investigation is being undertaken in which the effects of various brain lesions on these forms of display will be observed. This will provide an opportunity to re-evaluate the effects of bilateral mamillary destruction. References 1. AKERT, K., and 0. J. ANDY. 1955. Experimental studies on corpus mammiilare and tegmento-mammiilary system in the cat. Am. J. Physiol. 183: 591. 2. FLYNN, J. P., P. D. MACLEAN, and CHUL IM. 1961. Effects of hippocampal afterdischarges on conditioned responses, pp. 382-386. In “Electrical stimulation of the Brain,” D. E. Sheer [ed.]. University of Texas Press, Austin. 3. GERGEN,J. A., and P. D. MACLEAN. 1962. “A Stereo&& Atlas of the Squirrel Monkey’s Brain (Saimiri sciureus),” 91 pp. Government Printing Office, Washton, D.C. 4. GUILLERY, R. W. 1955. A quantitative study of the mamillary bodies and their connexions. J. Anat. 39: 19-32. 5. LA~ICKA, W. 1959. Physiological mechanism of delayed reactions. II. Delayed reactions in dogs and cats to directional stimuli. Acta Biol. Exptl. Lodz 19: 199-219. 6. MACLEAN, P. D., S. FLANIGAN, J. P. FLYNN, CHUL KIM, and J. R. STEVENS. 1956. Hippocampal function: Tentative correlations of conditioning, EEG, drug, and radioautographic studies. Yale J. Biol. Med. 33: 380-395. 7. MACLEAN, P. D., and D. W. PLOOC. 1962. Cerebral representation of penile erection. J. Neurophysiol. 36: 29-5.5. 8. PLOOG, D. W., and P. D. MACLEAN. 1963. Display of penile erection in squirrel monkey (Saimiri scizrreus). Animal Behaviour (In press). 9. Powxu, T. P. S., R. W. GUULERY, and W. M. COWAN. 1957. A quantitative study of the fomix-mamillo-thalamic system. 1. Amt. 91: 419-437. 10. WYSS, 0. A. M. 1945. Ein Hochfrequenz-Koagulationsgerlt xur reizlosen Ausschaltung. Helv. Physiol. et Pharmacol. Acta 3: 437-443.