- Email: [email protected]
The effect of medial thalamic lesions on emotionality, activity, and discrimination learning in the rat
Physiology & Behavior, Vol. 17, pp. 181-186. Pergamon Press and Brain Research Publ., 1976. Printed in the U.S.A.
The Effect of Medial Thalamic Lesions on Emotionality, Activity, and Discrimination Learning in the Rat ANN E. WARING AND L A R R Y W. MEANS 2
East Carolina University, Greenville, N C 27834 (Received 8 September 1975)
WARING, A. E. AND L. W. MEANS. The effect o f medial thalamic lesions on emotionality, activity, and discrimination learning in the rat. PHYSIOL. BEHAV. 17(2) 181-186, 1976.- Three experiments were conducted to examine the effects of medial thalamic lesions on emotionality, open-field activity, and discrimination learning. The first study revealed that rats with large medial thalamie lesions are significantly less responsive to capture and to handling than are sham-operated rats. The second study failed to produce any significant differences in open-field activity. A third study revealed that rats with large medial thalamic lesions, and with both small anterior and posterior medial thalamic lesions are impaired on the acquisition of an appetitive visual-tactile discrimination task, with the deficit being greatest in the large lesion group. It was concluded that the medial thalamus plays a role in the neural mediation of certain emotional responses and is critical in the acquisition of complex appetitive tasks. Medial thalamus
Emotionality
Activity
Discrimination
CHANGES in emotionality, activity, and acquisition and retention have been observed to follow lesions of the dorsomedial nucleus of the thalamus (MD) in both humans and infrahuman species. Rats with MD lesions have been observed to be tamer than normal animals [12,21], to freeze less in novel exploration, and to display less emotionality, as evidenced by less defecation [10,21]. The animals, however, exhibit increased freezing in response to aversive stimuli, resulting in deficits in one-way [ 13, 18, 19] and two-way avoidance [1,13]. No deficits are noted in passive avoidance [7,18]. Inconsistent results in activity have been noted; increases have been noted in a bar press situation [8], in the open field [6], and in a nonaversive situation [ 18,20]. Decreases have been found in running wheel activity [5] and spontaneous activity [4]. Several studies have demonstrated no changes in maze exploration [5,12], cage activity [5], cage emergence, and closed field exploration [7]. Impairments in rats with MD lesions have been demonstrated on acquisition of visual discrimination [2,3], visual-tactile discrimination [ 12], visual discrimination reversal, tactile discrimination and reversal [16], and complex spatial discrimination [22]. The rats are also impaired on go, no-go alternation tasks [ 11 ] and go, no-go visual-auditory discrimination tasks [9].
The present study was designed to examine the effects of both large and small medial thalamic lesions on emotionality, activity, and acquisition of a visual-tactile discrimination task. Relatively large lesions were centered on the MD for one group, while small anterior or posterior medial thalamic lesions were produced in the other two groups. The small lesions were an attempt to destroy the anterior or posterior portions of tissue destroyed by the large lesions, and were included to permit a more precise determination of the medial thalamic structures necessary for normal behavior. G E N E R A L METHOD
Animals The same 39 male Long-Evans rats, approximately 110 days old at the time of surgery, were used in all 3 experiments. They were housed individually and maintained on a 14-hr-light, 10-hr-dark cycle. Water was continuously available.
Surgical and Histological Procedure Thirty-one rats received bilateral lesions of the medial thalamus, while 8 rats received sham-operations. The animals were anesthetized with 40 mg/kg pentobarbital
t This paper is based on an MA thesis submitted by AEW to East Carolina University. The authors wish to thank Larry Hayes and John Whisnant for their assistance and Karl Wuensch for his helpful comments. 2Requests for reprints should be sent to Larry W. Means, Department of Psychology, East Carolina University, Greenville, N. C. 27834. 181
182
(
WARING AND MEANS
.
) 1,
) FIG. 1. Reconstruction of the largest and smallest lesions of Group A (left column), Group P (center column), and Group C (right column). (Nembutal) and received 0.20 ml of Bicillin. Each animal was placed in the stereotaxic with the top of the incisor bar elevated 5.0 mm above the ear bar, and lesions were made by passing 2.0 rnA anodal current through a stainless steel electrode that was insulated with Epoxylite except for the tip. Ten of the rats received small bilateral lesions of the anterior medial thalamus (Group A). The lesions were made by passing current for 8 sec with the tip of the electrode placed at coordinates 0.6 mm behind bregma, 1.0 mm lateral to the midline, and 6.2 mm below the top surface of the skull. Eight rats received small bilateral lesions of the posterior medial thalamus (Group P). These lesions also were formed by passing current for 8 sec, with the coordinates being 1.8 mm behind bregma, 1.0 mm lateral to the midline, and 6.2 mm below the skull surface. Thirteen animals received large bilateral lesions of the medial thalamus (Group C). These animals had current passed for 15 sec with the coordinates being 1.2 mm behind bregma, 1.0 mm lateral to the midline, and 6.2 mm below the skull surface. Eight animals received sham-operations (Group S), which involved the same surgical procedure except the electrode was lowered only 5.0 mm below the surface of the skull and no current was applied. At the completion of the behavioral testing all rats were
sacrificed with an overdose of Nembutal, the animals were perfused with Formalin, and the brains were extracted and sectioned at 50 u while frozen. Every fourth section throughout the extent of the lesion was photographed using 35 mm Kodak High Contrast Copy Film. The extent of lesion damage was determined by projecting the negatives onto corresponding plates from Pellegrino and Cushman's [14] stereotaxic atlas. The lesions were traced onto copies of plates and a grid was used to determine the per cent damage to each structure. Figure 1 shows reconstructions of the largest and smallest lesions in each group. Group A lesions were centered on the anterior portion of the MD and destroyed bilaterally a mean of 38% of the MD. In most animals in Group A the lesions also invaded the midline nuclei, the anteroventral nucleus, the habenulointerpeduncular tract, and the stria medullaris. Group P lesions, which were on the posterior border of the MD, resulted in a mean bilateral destruction of the MD of only 6 per cent. Most of the animals in this group had extensive damage of the habenula and some damage to the pretectal area, parafascicular nucleus, paraventricular nucleus, and stria medullaris. The Group C lesions were centered on the MD and bilaterally destroyed a mean of 44 per cent of the MD. Lesions in this
MEDIAL THALAMUS AND BEHAVIOR
183
group also destroyed the midline nuclei and invaded the habenula, anterodorsal nucleus, pretectal area, and stria medullaris. Twelve animals received minimal bilateral damage to t h e hippocampus, averaging approximately 1.0 percent. Note, that as intended, there was considerable overlap of the small lesions of Groups A & P with the relatively large lesions of Group C.
Anterior
¢ "
:
Posterior
.~*
--"
Central
e---e
Sham
3.5
3.0
d
2.5
General Procedure 2.0
All animals were handled 15 min daily for 3 days prior to surgery, and then presurgery emotionality and activity testing was done. Following surgery they were allowed to recuperate for 14 days and then were once again handled 15 min daily for 3 days. At this time, postsurgery emotionality and activity testing were conducted. Immediately following these tests food deprivation was begun, with animals being maintained throughout the remainder of the experiment at 85% of their ad lib body weights. EXPERIMENT 1: EMOTIONALITY
1.5 IJJ
1.0
.5 I
!
Pre-operative
Post-operative
FIG. 2. Mean preoperative and postoperative emotionality scores for all groups (Experiment i).
Method Apparatus. A modified version of the emotionality rating scale constructed by Tryon, Tryon, and Kuznets [17] was used. The scale included t h e following 6 components: (a) attack or flight reaction to a lead pencil held directly in front of the animal, (b) startle or flight reaction to a light tap on the animal's back with a lead pencil, (c) resistance to capture, (d) muscular tension and resistance to handling, (e) vocalization reaction when captured and handled, (f) urination and/or defecation when captured and handled. Each component was given a rating from 0 - 2 , with 0 being no reaction, 1 being a moderate reaction, and 2 being an extreme reaction. Procedure. After 3 days of handling, both pre- and postoperatively, each animal was carried in his home cage to a table in a quiet room. Each rat then had a pencil placed in front of him, was tapped on the back, was picked up, and was held for 7 sec. Two independent observers rated each animal on all 6 components and a total preoperative and postoperative score were obtained for each animal. Correlations between the observers were calculated for both pre- and postoperative scores (preoperative r = .89; postoperative r = .97). Results and Discussion The results of Experiment I (see Fig. 2) reveal that the emotionality scores of all 3 brain-damaged groups were decreased following surgery, while the scores of Group S remained the same. A Kruskal-Wallis analysis of variance run on the difference scores between pre- and postoperative scores indicated that the groups differed significantly, and subsequent tests (all a posteriori tests reported in this paper were done with the ManmWhitney procedure) showed that the only significant difference was between Group C and Group S, (p<0.01). Neither small lesion group differed significantly from Group S, nor was any specific thalamic structure found to be significantly correlated with the postoperative emotionality score. Table 1 shows the median and range of each of the 6 components of the emotionality scale. Separate analyses of each of these components indicated that the groups differed significantly on resistance to capture and resistance to handling. Mann-Whitney tests
indicated that Group C differed significantly from Group S (p<0.001), Group A, and Group P (both p<0.025) on resistance to handling. On resistance to capture Group A differed significantly from Group S (p<0.025) and Group C (p<0.05). It should be noted that in Group C all animals except 2 received preoperative scores of 0 on this component. On the postoperative test all animals except 1 received scores of 0; this component did not seem to discriminate very well for this particular group of animals. These results suggest that medial thalamic lesions, particularly in the anterior and central portion of the medial thalamus, result in reduced responsivity to capture and handling, a finding consistent with previous observations [12,21]. EXPERIMENT 2: ACTIVITY
Method Apparatus. This experiment took place in a quiet room with lighting designed to cast a minimum of shadows, using a gray box measuring 60 cm x 60 cm x 46 cm. The floor of the box was divided by lines into 36 squares that measured 1 0 c m × 10cm. Procedure. The day following both pre- and postoperative emotionality testing each rat while in his cage was placed in the open-field activity box and the latency measure on emergence from the cage was taken. If the rat had not emerged in 2 min, he was removed from the cage and placed in the open field, and for the 5 min following either emergence into or placement in the open field the number of squares entered was recorded. In addition, the number of urinations and defecations, both in the cage and in the open field, was recorded. Between trials the floor of the activity box was wiped with a cloth. Results and Discussion No significant differences among the groups were demonstrated on latency into the open field, number of squares entered, or number of urinations and defecations. It was observed that all groups had decreased latency scores and decreased total number of urinations and defecations postoperatively and that all lesioned groups entered more
184
WARING AND MEANS
TABLE 1 MEAN DIFFERENCESCORES FOR THE SIX COMPONENTSOF THE EMOTIONALITYRATING SCALE
Group Anterior Posterior Central Sham
Pencil in face Med Range 0 0 0 0
0 0 0 0
Pencil tap Med Range .25 0 0 .5
-1-1.5 0-1 -.5-1 0-1
Dependent Variable Resist Resist to to capture handling Med Range Med Range .5 0 0 0
squares postoperatively, while the sham-operated group entered less squares postoperatively. This observation, although not significant, is consistent with previous studies [10,181. EXPERIMENT 3: DISCRIMINATION
Method Apparatus. The two-choice maze used in an earlier study [ 12], consisting of a startbox, runway, choicepoint, and 2 goalboxes was used for discrimination training. The maze was equipped with guillotine doors to prevent retracing between the startbox and runway and the choicepoint and goalboxes. The startbox, runway, choicepoint, and one goalbox were painted gray, while the other goalbox was painted with 2.5 cm wide black and white horizontal stripes. The gray goalbox was equipped with a slightly elevated wire mesh floor, while the striped goalbox had a smooth wooden floor. The goalboxes were moveable, so that the right or left position of either could be changed. The discrimination maze was placed in a quiet room. Procedure. Following the postoperative emotionality and activity testing, all animals were placed on food deprivation until they reached 85% of their ad lib body weights. At this point, 5 days of maze adaptation were begun, which consisted of placing each animal in the maze with four 45 mg Noyes reinforcement pellets in the reinforcement cup of each goalbox. The right-left position of the goalboxes was changed each day, and each rat remained in the maze until all reinforcement pellets were consumed or for 5 min, whichever occurred first. After maze adaptation, acquisition training was begun. Training consisted of 10 trials per day with an intertrial interval of approximately 10 min. One half of each group were reinforced with four 45 mg Noyes pellets for entering the rough floor, gray goalbox, and the other half of each group were reinforced for entering the smooth floor, striped goalbox, with the left-right position of the goalboxes being semirandomly switched with the restrictions that on any one day each goalbox was on each side 5 times and that a goalbox was on the same side o n no more than 3 consecutive trials. If an animal did not respond within 4 min, he was removed from the maze and an error was recorded. Each rat was run in the maze for 15 consecutive days, or 150 trials. Criterion for successful acquisition of
0-1.5 0-1 0-.5 -1-0
0 0 1 0
0-1 0-1 0-1 -.5-0
Vocalization Med/Range 0 0 0 0
-1-1 -1-2 -1-1.5 -1-1
Med
U/D Range
0 0 0 0
-1-1 0-1 -2-1 -2-1
the task was 9 out of 10 correct responses for 3 consecutive days.
Results and Discussion Table 2 shows the median and range of the total number of errors, errors to criterion, and sessions to criterion for all 4 groups. Kruskal-Wailis tests followed by Mann-Whitney comparisons revealed that all 3 experimental groups differ significantly from the sham-operated group on total number of errors (p<0.001 in each case), errors to criterion (p<0.001 in each case), and sessions to criterion (p<0.001 in e a c h case). Follow-up tests further showed that on total number of errors both Group A and Group P differed significantly from Group C (p<0.01 and p<0.025), but Groups A and P did not differ from each other. On errors to criterion Groups A and P once again did not differ, but both differed significantly from Group C (both p's<0.01). On sessions to criterion Group A differed significantly from Group C (p<0.01), but Group P did not differ from Group A or Group C. TABLE 2 MEAN TOTAL ERRORS, ERRORS TO CRITERION, AND SESSIONS TO CRITERIONFOR ALL GROUPS
Group
Dependent Variable Errors to Sessions to Total Errors Criterion Criterion Med Range Med Range Med Range
Anterior Posterior Central Sham
37.5 18-69 47.5 20-65 66 24-75 9 2-28
37.5 14-69 45 19-65 66 24-75 9 2-27
12 13 15 5
7-15 7-15 8-15 3-9
Correlations between the percent of bilateral damage to various structures and the errors to criterion were calculated. The only structures found to be reliably correlated with errors to criterion were the habenula (r = .47, p<0.01), the medial habenular nucleus (r = .40, p<0.05), and the lateral habenular nucleus (r = .48, p<0.01). This study suggests that both large and small lesions of the medial thalamus result in an acquisition deficit, with
MEDIAL THALAMUS AND BEHAVIOR
185
the large lesion group showing the severest impairment, the small posterior lesion group showing less impairment, and the small anterior lesion group showing the least impairment. This s t u d y also suggests that the habenula plays an important role in the acquisition deficit. GENERAL DISCUSSION The results of Experiment 1 are consistent with the hypothesis that the thalamus, and in particular the medial portion of the thalamus, plays a role in coordinating emotional response to provoking stimuli, Vanderwolf [21] hypothesized that lesions of the medial thalamus prevent the initiation of voluntary movement in response to painful stimuli. The present study suggests that possibly this same inhibition of voluntary movement also occurs with some nonpainful stimuli, such as capture and handling. Thus, perhaps one explanation of the observed reduction in emotional response is that the medial thalamus is involved in the initiation and/or coordination of motor response to certain aversive stimuli. An alternative explanation of the lessened response to capture and handling could be that these particular nonpainful stimuli are not perceived adequately by the animals, thus the response is minimal. This hypothesis is speculative at this point, however, because no effort was made to measure tactile sensitivity. Although Experiment 2 did not demonstrate any significant differences in activity level among the groups, the sham-operated animals did show a slight decrease while the brain-damaged animals showed a slight increase in activity. These data are consistent with the finding that habituation of exploratory responses is retarded in animals with medial thalamic damage [ 10]. The results of Experiment 3 indicate that both large and small lesions of the medial thalamus produce deficits in the acquisition of an appetitive discrimination task, with the larger lesion producing the most severe deficit. That the more posterior area of the medial thalamus, including the
habenula, is the portion most critical for acquisition was suggested, but not, however, conclusively demonstrated. This hypothesis is consistent with earlier results from our laboratory [9]. Several possible explanations for the acquisition deficits should be considered. The habenula and the habenular nuclei were the only specific structures found to be reliably correlated with the acquisition deficits. Rausch [15] hypothesized that the active avoidance deficits found with habenular lesions were the result of an inability to inhibit ongoing behavior, and discussed one role of the habenula as an inhibitor of motor activity. This could be an explanation for the present deficit because of the 9 animals who gave no evidence of acquiring the discrimination after 150 trials, 8 gave perseverative responses to either the right of the left throughout most of the experiment. It should be noted that the habenula is not the only important structure for performance of the discrimination, however, as the large lesion group had the most severe deficit. Apparently, damage to many of the medial thalamic nuclei contribute to the deficit, with damage to the habenula being indicated as most important by the present study. One hypothesis appears to be consistent with the results of all 3 experiments. The various results seem to indicate that rats with medial thalamic lesions are rendered somehow less responsive to environmental influences, particularly of a nonpainful nature. Perhaps the animals are unable to acquire the discrimination task because the reinforcement, visual cues, or tactile cues have a less powerful effect upon their behavior than upon the behavior of normal animals. In the same vein, perhaps certain nonpainful stimuli affect animals with medial thalamic damage to a lesser extent, thus they respond less. Finally, the slightly increased activity of the brain-damaged animals, which has been interpreted as a decrease in habituation [10,12], further supports the hypothesis that medial thalamic damage impairs the normal response of habituation to the environment.
REFERENCES
i. Bohus, B. and D. de Wied. Avoidance and escape behavior following medial thaiamic lesions in rats. J. comp. physiol. Psychol. 64: 26-29, 1967. 2. Broome, Belinda. The effects of dorsomedial thalamic lesions on acquisition and retention of a visual pattern discrimination in a water-Filled maze, 1974. Unpublished Master's Thesis at East Carolina University. 3. Brown, C. W. and E. E. Ghiselli. Subcortical mechanisms in learning: VI Pattern vision discrimination. J. comp. Psychol. 26: 287-300, 1938. 4. Delacour, J. Effects of medial thalamic lesions in the rat: a review and an interpretation. Neuropsychologia 9: 157-174, 1971. 5. Dirlam, D. K. The effects of septal, thalamic, and tegmental lesions on general activity in the hooded rat. Can. J. Psychol. 23: 304-314, 1969. 6. Eichelman, B. S. Jr. Effect of subcortical lesions on shockinduced aggression. J. comp. physiol. Psychol. 74: 331-339, 1971. 7. Gerbrandt, L. K. The effects of anteromedial and dorsomedial thalamic lesions on passive avoidance and activity. Psychon. Sci. 2: 39-40, 1965. 8. LeJeune, H. Lesions thalamiques medianes et regulation temporelle acquise in programme a intervaUe fixe chez le rat albinos. Physiol. Behav. l: 575-582, 1971.
9. Means, L. W., J. W. Harrington and G. T. Miller. The effects of medial thalamic lesions on acquisition of a go, no-go tone-light discrimination task. Bull. Psychon. Soc. 5: 495-497, 1975. 10. Means, L. W., T. H. Harrell, E. S. Mayo and G. B. Alexander. Effects of dorsomedial thalamic lesions on spontaneous alternation, maze activity, and runway performance in the rat. Physiol. Behav. 12: 973-979, 1974. 11. Means, L. W., M. W. Hunt, R. R. Whiteside and T. W. Bates. Deficient acquisition and retention of single-alternation go, no-go in rats with medial thalamic lesions. Physiol. Psychol. 1: 287-291, 1973. 12. Means, L. W., D. H. Huntley, H. P. Anderson and T. H. Harrell. Deficient acquisition and retention of a visual-tactile dis` crimination task in rats with medial thalamic lesions. Behav. Biol. 9: 435-447, 1973. 13. Olton, D. S. and R. L. Isaacson. Effects of lateral and dorsomedial thalamic lesions on retention of active avoidance tasks. J. eomp physiol. Psyehol. 64: 256-261, 1967. 14. Pellegrino, L. J. and A. J. Cushman. A Stereotaxic Atlas o f the Rat Brain. New York: Appleton-Century-Crofts, 1967. 15. Rausch, L. J. "Effects of habenular lesions on odor dis. crimination, avoidance conditioning, and activity in the albino rat. Diss. Abs. Int. 33: 4143, 1973.
186 16. Tigner, J. C. The effects of dorsomedial thalamic lesions on learning, reversal, and alternation behavior in the rat. Physiol. Behav. 12: 13-17, 1974. 17. Tryon, R. C., C. M. Tryon and G. Kuznets. Studies in individual differences in maze ability. X Ratings and other measures on initial emotional responses of rats to novel inanimate objects. J. comp. Psychol. 32: 4 0 7 - 4 3 5 , 1941. 18. Vanderwolf, C. H. Medial thalamic functions in voluntary behavior. Can. J. Psychol. 16: 318-330, 1962. 19. Vanderwolf, C. H. Effects of experimental diencephalic damage on food hoarding and shock avoidance behavior in the rat. Physiol. Behav. 2: 399-402, 1967.
WARING AND MEANS 20. Vanderwolf, C. H. Effects of medial thalamic damage on initiation of movement and learning. Psychon. Sci. 17: 2 3 - 3 5 , 1969. 21. Vanderwolf, C. H. Limbic-diencephalic mechanisms of voluntary movement. Psychol. Rev. 78: 83-113, 1971. 22. Winocur, G. and C. B. Breckenridge. Cue-dependent behavior of hippocampally damaged rats in a complex maze. J. comp. physiol. Psychol. 82: 512-522, 1973.
The Effect of Medial Thalamic Lesions on Emotionality, Activity, and Discrimination Learning in the Rat ANN E. WARING AND L A R R Y W. MEANS 2
East Carolina University, Greenville, N C 27834 (Received 8 September 1975)
WARING, A. E. AND L. W. MEANS. The effect o f medial thalamic lesions on emotionality, activity, and discrimination learning in the rat. PHYSIOL. BEHAV. 17(2) 181-186, 1976.- Three experiments were conducted to examine the effects of medial thalamic lesions on emotionality, open-field activity, and discrimination learning. The first study revealed that rats with large medial thalamie lesions are significantly less responsive to capture and to handling than are sham-operated rats. The second study failed to produce any significant differences in open-field activity. A third study revealed that rats with large medial thalamic lesions, and with both small anterior and posterior medial thalamic lesions are impaired on the acquisition of an appetitive visual-tactile discrimination task, with the deficit being greatest in the large lesion group. It was concluded that the medial thalamus plays a role in the neural mediation of certain emotional responses and is critical in the acquisition of complex appetitive tasks. Medial thalamus
Emotionality
Activity
Discrimination
CHANGES in emotionality, activity, and acquisition and retention have been observed to follow lesions of the dorsomedial nucleus of the thalamus (MD) in both humans and infrahuman species. Rats with MD lesions have been observed to be tamer than normal animals [12,21], to freeze less in novel exploration, and to display less emotionality, as evidenced by less defecation [10,21]. The animals, however, exhibit increased freezing in response to aversive stimuli, resulting in deficits in one-way [ 13, 18, 19] and two-way avoidance [1,13]. No deficits are noted in passive avoidance [7,18]. Inconsistent results in activity have been noted; increases have been noted in a bar press situation [8], in the open field [6], and in a nonaversive situation [ 18,20]. Decreases have been found in running wheel activity [5] and spontaneous activity [4]. Several studies have demonstrated no changes in maze exploration [5,12], cage activity [5], cage emergence, and closed field exploration [7]. Impairments in rats with MD lesions have been demonstrated on acquisition of visual discrimination [2,3], visual-tactile discrimination [ 12], visual discrimination reversal, tactile discrimination and reversal [16], and complex spatial discrimination [22]. The rats are also impaired on go, no-go alternation tasks [ 11 ] and go, no-go visual-auditory discrimination tasks [9].
The present study was designed to examine the effects of both large and small medial thalamic lesions on emotionality, activity, and acquisition of a visual-tactile discrimination task. Relatively large lesions were centered on the MD for one group, while small anterior or posterior medial thalamic lesions were produced in the other two groups. The small lesions were an attempt to destroy the anterior or posterior portions of tissue destroyed by the large lesions, and were included to permit a more precise determination of the medial thalamic structures necessary for normal behavior. G E N E R A L METHOD
Animals The same 39 male Long-Evans rats, approximately 110 days old at the time of surgery, were used in all 3 experiments. They were housed individually and maintained on a 14-hr-light, 10-hr-dark cycle. Water was continuously available.
Surgical and Histological Procedure Thirty-one rats received bilateral lesions of the medial thalamus, while 8 rats received sham-operations. The animals were anesthetized with 40 mg/kg pentobarbital
t This paper is based on an MA thesis submitted by AEW to East Carolina University. The authors wish to thank Larry Hayes and John Whisnant for their assistance and Karl Wuensch for his helpful comments. 2Requests for reprints should be sent to Larry W. Means, Department of Psychology, East Carolina University, Greenville, N. C. 27834. 181
182
(
WARING AND MEANS
.
) 1,
) FIG. 1. Reconstruction of the largest and smallest lesions of Group A (left column), Group P (center column), and Group C (right column). (Nembutal) and received 0.20 ml of Bicillin. Each animal was placed in the stereotaxic with the top of the incisor bar elevated 5.0 mm above the ear bar, and lesions were made by passing 2.0 rnA anodal current through a stainless steel electrode that was insulated with Epoxylite except for the tip. Ten of the rats received small bilateral lesions of the anterior medial thalamus (Group A). The lesions were made by passing current for 8 sec with the tip of the electrode placed at coordinates 0.6 mm behind bregma, 1.0 mm lateral to the midline, and 6.2 mm below the top surface of the skull. Eight rats received small bilateral lesions of the posterior medial thalamus (Group P). These lesions also were formed by passing current for 8 sec, with the coordinates being 1.8 mm behind bregma, 1.0 mm lateral to the midline, and 6.2 mm below the skull surface. Thirteen animals received large bilateral lesions of the medial thalamus (Group C). These animals had current passed for 15 sec with the coordinates being 1.2 mm behind bregma, 1.0 mm lateral to the midline, and 6.2 mm below the skull surface. Eight animals received sham-operations (Group S), which involved the same surgical procedure except the electrode was lowered only 5.0 mm below the surface of the skull and no current was applied. At the completion of the behavioral testing all rats were
sacrificed with an overdose of Nembutal, the animals were perfused with Formalin, and the brains were extracted and sectioned at 50 u while frozen. Every fourth section throughout the extent of the lesion was photographed using 35 mm Kodak High Contrast Copy Film. The extent of lesion damage was determined by projecting the negatives onto corresponding plates from Pellegrino and Cushman's [14] stereotaxic atlas. The lesions were traced onto copies of plates and a grid was used to determine the per cent damage to each structure. Figure 1 shows reconstructions of the largest and smallest lesions in each group. Group A lesions were centered on the anterior portion of the MD and destroyed bilaterally a mean of 38% of the MD. In most animals in Group A the lesions also invaded the midline nuclei, the anteroventral nucleus, the habenulointerpeduncular tract, and the stria medullaris. Group P lesions, which were on the posterior border of the MD, resulted in a mean bilateral destruction of the MD of only 6 per cent. Most of the animals in this group had extensive damage of the habenula and some damage to the pretectal area, parafascicular nucleus, paraventricular nucleus, and stria medullaris. The Group C lesions were centered on the MD and bilaterally destroyed a mean of 44 per cent of the MD. Lesions in this
MEDIAL THALAMUS AND BEHAVIOR
183
group also destroyed the midline nuclei and invaded the habenula, anterodorsal nucleus, pretectal area, and stria medullaris. Twelve animals received minimal bilateral damage to t h e hippocampus, averaging approximately 1.0 percent. Note, that as intended, there was considerable overlap of the small lesions of Groups A & P with the relatively large lesions of Group C.
Anterior
¢ "
:
Posterior
.~*
--"
Central
e---e
Sham
3.5
3.0
d
2.5
General Procedure 2.0
All animals were handled 15 min daily for 3 days prior to surgery, and then presurgery emotionality and activity testing was done. Following surgery they were allowed to recuperate for 14 days and then were once again handled 15 min daily for 3 days. At this time, postsurgery emotionality and activity testing were conducted. Immediately following these tests food deprivation was begun, with animals being maintained throughout the remainder of the experiment at 85% of their ad lib body weights. EXPERIMENT 1: EMOTIONALITY
1.5 IJJ
1.0
.5 I
!
Pre-operative
Post-operative
FIG. 2. Mean preoperative and postoperative emotionality scores for all groups (Experiment i).
Method Apparatus. A modified version of the emotionality rating scale constructed by Tryon, Tryon, and Kuznets [17] was used. The scale included t h e following 6 components: (a) attack or flight reaction to a lead pencil held directly in front of the animal, (b) startle or flight reaction to a light tap on the animal's back with a lead pencil, (c) resistance to capture, (d) muscular tension and resistance to handling, (e) vocalization reaction when captured and handled, (f) urination and/or defecation when captured and handled. Each component was given a rating from 0 - 2 , with 0 being no reaction, 1 being a moderate reaction, and 2 being an extreme reaction. Procedure. After 3 days of handling, both pre- and postoperatively, each animal was carried in his home cage to a table in a quiet room. Each rat then had a pencil placed in front of him, was tapped on the back, was picked up, and was held for 7 sec. Two independent observers rated each animal on all 6 components and a total preoperative and postoperative score were obtained for each animal. Correlations between the observers were calculated for both pre- and postoperative scores (preoperative r = .89; postoperative r = .97). Results and Discussion The results of Experiment I (see Fig. 2) reveal that the emotionality scores of all 3 brain-damaged groups were decreased following surgery, while the scores of Group S remained the same. A Kruskal-Wallis analysis of variance run on the difference scores between pre- and postoperative scores indicated that the groups differed significantly, and subsequent tests (all a posteriori tests reported in this paper were done with the ManmWhitney procedure) showed that the only significant difference was between Group C and Group S, (p<0.01). Neither small lesion group differed significantly from Group S, nor was any specific thalamic structure found to be significantly correlated with the postoperative emotionality score. Table 1 shows the median and range of each of the 6 components of the emotionality scale. Separate analyses of each of these components indicated that the groups differed significantly on resistance to capture and resistance to handling. Mann-Whitney tests
indicated that Group C differed significantly from Group S (p<0.001), Group A, and Group P (both p<0.025) on resistance to handling. On resistance to capture Group A differed significantly from Group S (p<0.025) and Group C (p<0.05). It should be noted that in Group C all animals except 2 received preoperative scores of 0 on this component. On the postoperative test all animals except 1 received scores of 0; this component did not seem to discriminate very well for this particular group of animals. These results suggest that medial thalamic lesions, particularly in the anterior and central portion of the medial thalamus, result in reduced responsivity to capture and handling, a finding consistent with previous observations [12,21]. EXPERIMENT 2: ACTIVITY
Method Apparatus. This experiment took place in a quiet room with lighting designed to cast a minimum of shadows, using a gray box measuring 60 cm x 60 cm x 46 cm. The floor of the box was divided by lines into 36 squares that measured 1 0 c m × 10cm. Procedure. The day following both pre- and postoperative emotionality testing each rat while in his cage was placed in the open-field activity box and the latency measure on emergence from the cage was taken. If the rat had not emerged in 2 min, he was removed from the cage and placed in the open field, and for the 5 min following either emergence into or placement in the open field the number of squares entered was recorded. In addition, the number of urinations and defecations, both in the cage and in the open field, was recorded. Between trials the floor of the activity box was wiped with a cloth. Results and Discussion No significant differences among the groups were demonstrated on latency into the open field, number of squares entered, or number of urinations and defecations. It was observed that all groups had decreased latency scores and decreased total number of urinations and defecations postoperatively and that all lesioned groups entered more
184
WARING AND MEANS
TABLE 1 MEAN DIFFERENCESCORES FOR THE SIX COMPONENTSOF THE EMOTIONALITYRATING SCALE
Group Anterior Posterior Central Sham
Pencil in face Med Range 0 0 0 0
0 0 0 0
Pencil tap Med Range .25 0 0 .5
-1-1.5 0-1 -.5-1 0-1
Dependent Variable Resist Resist to to capture handling Med Range Med Range .5 0 0 0
squares postoperatively, while the sham-operated group entered less squares postoperatively. This observation, although not significant, is consistent with previous studies [10,181. EXPERIMENT 3: DISCRIMINATION
Method Apparatus. The two-choice maze used in an earlier study [ 12], consisting of a startbox, runway, choicepoint, and 2 goalboxes was used for discrimination training. The maze was equipped with guillotine doors to prevent retracing between the startbox and runway and the choicepoint and goalboxes. The startbox, runway, choicepoint, and one goalbox were painted gray, while the other goalbox was painted with 2.5 cm wide black and white horizontal stripes. The gray goalbox was equipped with a slightly elevated wire mesh floor, while the striped goalbox had a smooth wooden floor. The goalboxes were moveable, so that the right or left position of either could be changed. The discrimination maze was placed in a quiet room. Procedure. Following the postoperative emotionality and activity testing, all animals were placed on food deprivation until they reached 85% of their ad lib body weights. At this point, 5 days of maze adaptation were begun, which consisted of placing each animal in the maze with four 45 mg Noyes reinforcement pellets in the reinforcement cup of each goalbox. The right-left position of the goalboxes was changed each day, and each rat remained in the maze until all reinforcement pellets were consumed or for 5 min, whichever occurred first. After maze adaptation, acquisition training was begun. Training consisted of 10 trials per day with an intertrial interval of approximately 10 min. One half of each group were reinforced with four 45 mg Noyes pellets for entering the rough floor, gray goalbox, and the other half of each group were reinforced for entering the smooth floor, striped goalbox, with the left-right position of the goalboxes being semirandomly switched with the restrictions that on any one day each goalbox was on each side 5 times and that a goalbox was on the same side o n no more than 3 consecutive trials. If an animal did not respond within 4 min, he was removed from the maze and an error was recorded. Each rat was run in the maze for 15 consecutive days, or 150 trials. Criterion for successful acquisition of
0-1.5 0-1 0-.5 -1-0
0 0 1 0
0-1 0-1 0-1 -.5-0
Vocalization Med/Range 0 0 0 0
-1-1 -1-2 -1-1.5 -1-1
Med
U/D Range
0 0 0 0
-1-1 0-1 -2-1 -2-1
the task was 9 out of 10 correct responses for 3 consecutive days.
Results and Discussion Table 2 shows the median and range of the total number of errors, errors to criterion, and sessions to criterion for all 4 groups. Kruskal-Wailis tests followed by Mann-Whitney comparisons revealed that all 3 experimental groups differ significantly from the sham-operated group on total number of errors (p<0.001 in each case), errors to criterion (p<0.001 in each case), and sessions to criterion (p<0.001 in e a c h case). Follow-up tests further showed that on total number of errors both Group A and Group P differed significantly from Group C (p<0.01 and p<0.025), but Groups A and P did not differ from each other. On errors to criterion Groups A and P once again did not differ, but both differed significantly from Group C (both p's<0.01). On sessions to criterion Group A differed significantly from Group C (p<0.01), but Group P did not differ from Group A or Group C. TABLE 2 MEAN TOTAL ERRORS, ERRORS TO CRITERION, AND SESSIONS TO CRITERIONFOR ALL GROUPS
Group
Dependent Variable Errors to Sessions to Total Errors Criterion Criterion Med Range Med Range Med Range
Anterior Posterior Central Sham
37.5 18-69 47.5 20-65 66 24-75 9 2-28
37.5 14-69 45 19-65 66 24-75 9 2-27
12 13 15 5
7-15 7-15 8-15 3-9
Correlations between the percent of bilateral damage to various structures and the errors to criterion were calculated. The only structures found to be reliably correlated with errors to criterion were the habenula (r = .47, p<0.01), the medial habenular nucleus (r = .40, p<0.05), and the lateral habenular nucleus (r = .48, p<0.01). This study suggests that both large and small lesions of the medial thalamus result in an acquisition deficit, with
MEDIAL THALAMUS AND BEHAVIOR
185
the large lesion group showing the severest impairment, the small posterior lesion group showing less impairment, and the small anterior lesion group showing the least impairment. This s t u d y also suggests that the habenula plays an important role in the acquisition deficit. GENERAL DISCUSSION The results of Experiment 1 are consistent with the hypothesis that the thalamus, and in particular the medial portion of the thalamus, plays a role in coordinating emotional response to provoking stimuli, Vanderwolf [21] hypothesized that lesions of the medial thalamus prevent the initiation of voluntary movement in response to painful stimuli. The present study suggests that possibly this same inhibition of voluntary movement also occurs with some nonpainful stimuli, such as capture and handling. Thus, perhaps one explanation of the observed reduction in emotional response is that the medial thalamus is involved in the initiation and/or coordination of motor response to certain aversive stimuli. An alternative explanation of the lessened response to capture and handling could be that these particular nonpainful stimuli are not perceived adequately by the animals, thus the response is minimal. This hypothesis is speculative at this point, however, because no effort was made to measure tactile sensitivity. Although Experiment 2 did not demonstrate any significant differences in activity level among the groups, the sham-operated animals did show a slight decrease while the brain-damaged animals showed a slight increase in activity. These data are consistent with the finding that habituation of exploratory responses is retarded in animals with medial thalamic damage [ 10]. The results of Experiment 3 indicate that both large and small lesions of the medial thalamus produce deficits in the acquisition of an appetitive discrimination task, with the larger lesion producing the most severe deficit. That the more posterior area of the medial thalamus, including the
habenula, is the portion most critical for acquisition was suggested, but not, however, conclusively demonstrated. This hypothesis is consistent with earlier results from our laboratory [9]. Several possible explanations for the acquisition deficits should be considered. The habenula and the habenular nuclei were the only specific structures found to be reliably correlated with the acquisition deficits. Rausch [15] hypothesized that the active avoidance deficits found with habenular lesions were the result of an inability to inhibit ongoing behavior, and discussed one role of the habenula as an inhibitor of motor activity. This could be an explanation for the present deficit because of the 9 animals who gave no evidence of acquiring the discrimination after 150 trials, 8 gave perseverative responses to either the right of the left throughout most of the experiment. It should be noted that the habenula is not the only important structure for performance of the discrimination, however, as the large lesion group had the most severe deficit. Apparently, damage to many of the medial thalamic nuclei contribute to the deficit, with damage to the habenula being indicated as most important by the present study. One hypothesis appears to be consistent with the results of all 3 experiments. The various results seem to indicate that rats with medial thalamic lesions are rendered somehow less responsive to environmental influences, particularly of a nonpainful nature. Perhaps the animals are unable to acquire the discrimination task because the reinforcement, visual cues, or tactile cues have a less powerful effect upon their behavior than upon the behavior of normal animals. In the same vein, perhaps certain nonpainful stimuli affect animals with medial thalamic damage to a lesser extent, thus they respond less. Finally, the slightly increased activity of the brain-damaged animals, which has been interpreted as a decrease in habituation [10,12], further supports the hypothesis that medial thalamic damage impairs the normal response of habituation to the environment.
REFERENCES
i. Bohus, B. and D. de Wied. Avoidance and escape behavior following medial thaiamic lesions in rats. J. comp. physiol. Psychol. 64: 26-29, 1967. 2. Broome, Belinda. The effects of dorsomedial thalamic lesions on acquisition and retention of a visual pattern discrimination in a water-Filled maze, 1974. Unpublished Master's Thesis at East Carolina University. 3. Brown, C. W. and E. E. Ghiselli. Subcortical mechanisms in learning: VI Pattern vision discrimination. J. comp. Psychol. 26: 287-300, 1938. 4. Delacour, J. Effects of medial thalamic lesions in the rat: a review and an interpretation. Neuropsychologia 9: 157-174, 1971. 5. Dirlam, D. K. The effects of septal, thalamic, and tegmental lesions on general activity in the hooded rat. Can. J. Psychol. 23: 304-314, 1969. 6. Eichelman, B. S. Jr. Effect of subcortical lesions on shockinduced aggression. J. comp. physiol. Psychol. 74: 331-339, 1971. 7. Gerbrandt, L. K. The effects of anteromedial and dorsomedial thalamic lesions on passive avoidance and activity. Psychon. Sci. 2: 39-40, 1965. 8. LeJeune, H. Lesions thalamiques medianes et regulation temporelle acquise in programme a intervaUe fixe chez le rat albinos. Physiol. Behav. l: 575-582, 1971.
9. Means, L. W., J. W. Harrington and G. T. Miller. The effects of medial thalamic lesions on acquisition of a go, no-go tone-light discrimination task. Bull. Psychon. Soc. 5: 495-497, 1975. 10. Means, L. W., T. H. Harrell, E. S. Mayo and G. B. Alexander. Effects of dorsomedial thalamic lesions on spontaneous alternation, maze activity, and runway performance in the rat. Physiol. Behav. 12: 973-979, 1974. 11. Means, L. W., M. W. Hunt, R. R. Whiteside and T. W. Bates. Deficient acquisition and retention of single-alternation go, no-go in rats with medial thalamic lesions. Physiol. Psychol. 1: 287-291, 1973. 12. Means, L. W., D. H. Huntley, H. P. Anderson and T. H. Harrell. Deficient acquisition and retention of a visual-tactile dis` crimination task in rats with medial thalamic lesions. Behav. Biol. 9: 435-447, 1973. 13. Olton, D. S. and R. L. Isaacson. Effects of lateral and dorsomedial thalamic lesions on retention of active avoidance tasks. J. eomp physiol. Psyehol. 64: 256-261, 1967. 14. Pellegrino, L. J. and A. J. Cushman. A Stereotaxic Atlas o f the Rat Brain. New York: Appleton-Century-Crofts, 1967. 15. Rausch, L. J. "Effects of habenular lesions on odor dis. crimination, avoidance conditioning, and activity in the albino rat. Diss. Abs. Int. 33: 4143, 1973.
186 16. Tigner, J. C. The effects of dorsomedial thalamic lesions on learning, reversal, and alternation behavior in the rat. Physiol. Behav. 12: 13-17, 1974. 17. Tryon, R. C., C. M. Tryon and G. Kuznets. Studies in individual differences in maze ability. X Ratings and other measures on initial emotional responses of rats to novel inanimate objects. J. comp. Psychol. 32: 4 0 7 - 4 3 5 , 1941. 18. Vanderwolf, C. H. Medial thalamic functions in voluntary behavior. Can. J. Psychol. 16: 318-330, 1962. 19. Vanderwolf, C. H. Effects of experimental diencephalic damage on food hoarding and shock avoidance behavior in the rat. Physiol. Behav. 2: 399-402, 1967.
WARING AND MEANS 20. Vanderwolf, C. H. Effects of medial thalamic damage on initiation of movement and learning. Psychon. Sci. 17: 2 3 - 3 5 , 1969. 21. Vanderwolf, C. H. Limbic-diencephalic mechanisms of voluntary movement. Psychol. Rev. 78: 83-113, 1971. 22. Winocur, G. and C. B. Breckenridge. Cue-dependent behavior of hippocampally damaged rats in a complex maze. J. comp. physiol. Psychol. 82: 512-522, 1973.