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Differential recovery of delayed response function following prefrontal ablation
EXPERIME?U‘TAL
NEUROLOG\-
Differential S.
Recovery Following
PI;. CIANCI,
Laboratory
.381-38X
17.
P.
of Delayed Prefrontal
BLACK,
of Neztrologiral
(1967)
I’.
Sciences, Baltimore.
Received
Response Ablation
SPYROPOULOS,
AND
Friends of Psyzhiatrir Maryland
November
Function
J.
MASER’
RPsenrrh,
Iw..
22, 1966
Twelve monkeys, divided into three groups, were trained to perform a 6-set spatial delayed response task by the direct method, the indirect method, and by both methods. As each animal achieved stable performance at the criterion level, the banks and depths of the principal cerebral sulcus were removed by aspiration bilaterally in one stage. While the performance of al1 animals dropped to chance immediately after surgery, all trained by the indirect method were able to reachieve preoperative performance levels. By contrast; animals trained by the direct method showed greater variability and lower performance levels. Those trained by both methods relearned both tasks most readily and showed the least variability. We speculate that the superior postoperative performance of animals trained by the indirect method was due to the secondary rcinforccment value of the cue used to determine the side to be rewarded,
Introduction .I monkey that has been trained to perform delay-type tasks no longer is capable of consistent high level performance after bilateral prefrontal lobe ablation. In the spatial delayed response task the deficit is defined by the subject’s inability to consistently choose the correct side. Although the monkey with ablated lobes fails in this regard, it does continue to respond appropriately and can readily be seen to be a trained animal. Thus, if in order to obtain a reward it must push a food-well lid, it continues to push the lid postoperatively and does not attempt to pull or lift it as a naive animal might. This shows that only a portion of the total behavior necessary for appropriate and correct performance is disrupted by the lesions. Furthermore, the portion of the behavioral chain which deteriorates corresponds to that portion of the problem situation which is variable, i.e., the side of baiting. It appears that integrity of ablated tissue is necessary for the t This research was supported by funds from grant number NB 05178-02 from the Sational Institute of Neuroloaical Diseases and Blindness of the USPHS. A preliminary report of this work appeared in the Proceedings of the XVIIIth International Congress of Psychology. Moscow, 1966. .IS 1
382
CIANCI,
BLACK,
SPYROPOULOS.
AND
AMASER
animal to process information in the environment that is not constant or has only short-term value. The deficit has been attributed to diverse causes and is generally thought to be permanent; however, the behavioral deficit can be compensated for by appropriate alteration of the behavioral situation (3-S). Pribram and Mishkin (5) concluded that on the delay-type problem the successful performance of animals with frontal lobe operations could be accounted for in terms of “distinctiveness” which predelay cues acquired from contiguity with distinctive responses and differential reward. We think it possible that the process by which distinctiveness is imparted to a cue will act to add stability to that portion of the spatial delayed response task which is variable-the side to be rewarded. The present study was undertaken to test this hypothesis by comparing the traditional direct and indirect methods in a spatial delayed response task. Material
and
Methods
Twelve experimentally naive sexually immature monkeys (Macaca mu&?a) were assigned to three groups: Four were tested by the traditional direct method, four by the traditional indirect method, and four by both methods. All training and testing took place in a covered unlit cage in which the monkey was separated from the experimenter by transparent and opaque guillotine doors that could be raised or lowered separately or together. The experimenter’s portion of the chamber was lit and contained a work tray with two food-wells mounted on it 16.5 cm apart. The wells were covered by identical circular gray lids. Traditional Direct Method. The opaque barrier was raised while the animal observed the experimenter hold a food reward over one uncovered food-well, drop the bait into it and replace the cover. The opaque barrier was then lowered, the tray pushed forward and at the end of 6 set both barriers raised to permit the animal to respond. Noncorrection technique was used throughout but the delay period was gradually increased from 0 to 6 set until criterion performance was reached. Animals received 100 trials daily. Traditional Indirect A4ethod. Animals were first trained to discriminate between a food-well lid with a circular white patch, which signified the site of the reward, and a uniformly gray lid. After this simple visual discrimination task had been mastered the animal was allowed to view, but not respond to the covered food-wells one of which had the white cue. The opaque barrier was then lowered and the lid with the cue replaced by one identical to the other lid. At the end of 6 set both barriers were raised and the animal allowed to respond in the absence of any overt cue. As with the direct-method group, noncorrection technique was used and the delay period
FRONTAL
383
LOBE
was gradually increased to 6 sec. Animals received 100 trials daily. Upon reaching high level performance postoperatively, animals in this group were tested by the direct method. Direct and Indirect Methods. After the subject had mastered one method it began training with the other while continuing with the first. Animals received fifty trials daily with each method. Surgery. As each animal reached the criterion of 90% correct responses on 5 consecutive days it was anesthetized and subjected to surgery. A large IOO-
80-
TO-
60-
I
I 100
f 500
I 1000
I
I
I
I
I
I500
2000
2500
3000
3500
FIG. 1. Differential recovery of function between animals trained by the indirect and direct methods. Points plotted represent group means. Vertical axis, percentage correct; horizontal axis, number of trials.
frontal bone flap was removed and the banks and depths of the principal cerebral sulcus were extirpated by aspiration bilaterally in one stage. .4n otological microscope permitted good visualization in the depths during surgery. The bone flap was replaced and the wound was sutured in anatomical layers. Postoperative testing began 48-72 hours after surgery, Upon completion of postoperative testing, the animals were anesthetized again and perfused with physiological saline solution followed by lOr/r, formalin. The lesionswere reconstructed from frozen S0l.rsections. Results
Animals trained by either method alone were equally affected by the surgery and showed comparable deficits initially (Fig. 1) . \Vith continued testing, however, a consistent difference between the two groups appeared after about 1500 trials and continued to increase, with all those in the
383
CIANCI,
BLACK.
SPYROPOULOS,
ATU’D
MASER
indirect-method group reaching preoperative performance levels (p < 0.05, one-tailed t tests). Those trained by the direct method also showed improvement, but did so at a much slower rate (p < 0.10, Mann-Whitney U test, one-tailed) and with greater variability. However, two animals of the direct group reattained preoperative levels, and did so more rapidly than the poorest animal on the indirect method. As each animal trained by the indirect method reached its preoperative level, it was tested by the direct method. The monkeys trained by the
FIG.
tested
2. Rapid transfer of by direct method. Points
ability plotted
to delay by recovered indirect are means. Axes as in Fig. 1.
method
animals
indirect method were capable of performing the direct method delayed response task at high levels readily and performed better than animals trained by the direct method alone (Fig. 2). 4nimals in the third group, those trained by both methods prior to surgery, demonstrated the most notable postoperative gain. They reached levels, for each method, comparable to their preoperation performance, in about 600 trials. While there was no clear-cut difference between the two methods in rate of recovery, the indirect method did appear to be slightly superior (Fig. 3). Discussion
Diverse studies have demonstrated an apparent recovery of ability of animals with frontal lobe lesions to perform delay-type tasks. The recovery, typically, has been achieved by either altering the bodily state of the animal (7) or the experimental conditions (3, 4). There have been no reports of animals with frontal ablations achieving stable high-level performance on the traditional spatial delayed response task by either the direct or
FROXTAL
LOBI’
385
indirect method; however, occasional successes by individual animals were noted in 1945 by Campbell and Harlow ( 1) . The present study demonstrated that animals with frontal lobes ablated, trained by the traditional indirect method, could reachieve performance levels comparable to their preoperative ones. These animals: which had no prior experience with the direct method, when tested by it, achieved high performance levels more rapidly than those trained solely by the direct method. LVhile the study demonstrated that
100 200 300 FIG.
Group
3. Rapid recovery means are plotted.
400
500
of function by animals Axes as in Fig. 1.
600
700
trained
600
initially
900
by
1000
both
methods.
animals trained by the indirect method were capable of reachieving criterion performance levels postoperatively, many trials were required. In contrast, those animals trained initially by both the direct and indirect methods very quickly reattained high performance levels postoperatively in both situations. While the success of the animals with frontal ablations on the traditional direct and indirect methods is in contradistinction to the findings of Mishkin and Pribram (4) with the traditional methods, it is interpreted as being essentially consistent with the interpretation of cue distinctiveness as a factor in maintaining delayed response behavior. The difference in the findings of this study and that of Mishkin and Pribram can be that they trained their animals on delayed response postoperatively, stopped training after 500 trials, and produced larger lesions in their animals. While the comparatively small-sized lesions (Fig. 4) used in our study may account for some recovery, lesion size alone cannot account for the differential recovery. As can be seen in Fig. 5, there was little difference in lesion size within or between groups; therefore, the type of training must have been responsible for the differential recovery.
386
CIANCI,
BLACK;
SPYROPOULOS,
AND
MASER
We believe the success of the animals in this study on the traditional methods was achieved for the same reason that Mishkin and Pribram’s animals (4) succeeded on the delay-variations, i.e., the acquisition of distinctiveness by the cue through contiguity with differentially rewarded responses. The cue thus had acquired the characteristics of a secondary reinforcer. In this regard, the study parallels that of Finan (3) with the difference that Finan used a predelay primary reinforcer to sustain the behavior. In our study the cue used in the indirect method, acting as a secondary reinforcer, sustained the behavior.
# FIG.
pared
561
4. Surface view of lesion along the principal sulcus. to that usually made in delayed-response studies.
Note
smallness
of lesion
com-
Presumably, the sight of food, which is the cue in the direct method, also has secondary reinforcement value and should also sustain the behavior in the situation. However, empirically, it does not, and we believe this failure to be due to the complexity and instability of the cuing situation. The animal is not cued by food alone but by food being held in the trainer’s hand; different amounts of the bait may be exposed on any given trial, and the hand may be held in various positions. Furthermore, the animal has experienced the bait in contexts other than the training situation and the reaching hand may have been experienced in situations where it had a threatening or aversive quality. For these reasons we believe that the cuing process in the direct method does not readily assume the stabilizing property of distinctiveness or secondary reinforcement. Probably the characteristic of “cue distinctiveness” or secondary reinforcement acted to give stability to that portion of the spatial delayed
FRONTAL
LOBE
387
response task which is inconstant, i.e., the shifting of the side of baiting. On the basis of the continued failure of animals trained by the direct method, one may argue that the brain tissue removed (the banks and depths of the principal sulcus) is necessary for short-term associative processes. (This is considered as short term because place of reward on each trial is independent of the preceding trials.) If these assumptions are accepted, then one may
FIG. 5. Cross-sectional view of the lesion in the best (left) and worst (right) behavioral animals in each group. Section is approximately at the midpoint of the lesion along the longitudinal axis. Monkeys 534 and 566, by direct method; 561 and 563. b> indirect method: 557 and 544, by both methods.
argue that the success of the indirect group, with the same lesions, was due to the subsuming of the short term processes by some other area. In this instance it may be speculated that the neural mechanism mediated a Iong training by which the cue term process, i.e., the visual discrimination acquired its secondary reinforcement characteristics, is also capable of mediating short term processes if appropriate associations have been made. This interpretation would account for the rapid reachievement of high level performance of the operates trained by both the direct and indirect methods
388
CIANCI,
BLACK,
SPYROPOULOS,
AND
MASER
and would be consistent with findings implicating areas other than the prefrontal lobes in delayed response behavior (2, 6). References 1.
2. 3 4.
5. 6.
7.
CAMPBELL, R. J., and H. R. H~RLOW. 1945. Problem solution by monkeys following removal of the prefrontal areas: V. Spatial delayed reactions. J. Exptl. Psycho!35: 110-126. CIANCI, S. 1965. Effects of cortical and subcortical stimulation on delayed response in monkeys, Exptl. Neural. 11: 104-114. FINAN, J. L. 1942. Delayed response with pre-delay reinforcement in monkeys after removal of the frontal lobes. Am. J. Phychol. 5: 295-308. MISHKIN, M., and K. H. PRIBRALZ. 1956. Analysis of the effects of frontal lesions in monkey: II. Variations of delayed response. J. Camp. Physiol. Psychol. 49: 36-40. PRIBRAM, K. H., and M. MISHXIN. 1956. Analysis of the effects of frontal lesions in monkey: III. Object alternation. J. Comp. Physiol. Psychol. 49: 41-45. ROSVOLD, H. E., and J. M. R. DELG~DO. 1956. The effect on delayed alternation test performance of stimulating or destroying electrically structures within the frontal lobes of the monkeys brain. J. Camp. Physiol. Psychol. 49: 365-372. 1947. The effect of sedatives upon delayed response in monkeys followWADE, M. ing removal of the prefrontal lobes. 1. Neurophysiol. 10: 57-61.
NEUROLOG\-
Differential S.
Recovery Following
PI;. CIANCI,
Laboratory
.381-38X
17.
P.
of Delayed Prefrontal
BLACK,
of Neztrologiral
(1967)
I’.
Sciences, Baltimore.
Received
Response Ablation
SPYROPOULOS,
AND
Friends of Psyzhiatrir Maryland
November
Function
J.
MASER’
RPsenrrh,
Iw..
22, 1966
Twelve monkeys, divided into three groups, were trained to perform a 6-set spatial delayed response task by the direct method, the indirect method, and by both methods. As each animal achieved stable performance at the criterion level, the banks and depths of the principal cerebral sulcus were removed by aspiration bilaterally in one stage. While the performance of al1 animals dropped to chance immediately after surgery, all trained by the indirect method were able to reachieve preoperative performance levels. By contrast; animals trained by the direct method showed greater variability and lower performance levels. Those trained by both methods relearned both tasks most readily and showed the least variability. We speculate that the superior postoperative performance of animals trained by the indirect method was due to the secondary rcinforccment value of the cue used to determine the side to be rewarded,
Introduction .I monkey that has been trained to perform delay-type tasks no longer is capable of consistent high level performance after bilateral prefrontal lobe ablation. In the spatial delayed response task the deficit is defined by the subject’s inability to consistently choose the correct side. Although the monkey with ablated lobes fails in this regard, it does continue to respond appropriately and can readily be seen to be a trained animal. Thus, if in order to obtain a reward it must push a food-well lid, it continues to push the lid postoperatively and does not attempt to pull or lift it as a naive animal might. This shows that only a portion of the total behavior necessary for appropriate and correct performance is disrupted by the lesions. Furthermore, the portion of the behavioral chain which deteriorates corresponds to that portion of the problem situation which is variable, i.e., the side of baiting. It appears that integrity of ablated tissue is necessary for the t This research was supported by funds from grant number NB 05178-02 from the Sational Institute of Neuroloaical Diseases and Blindness of the USPHS. A preliminary report of this work appeared in the Proceedings of the XVIIIth International Congress of Psychology. Moscow, 1966. .IS 1
382
CIANCI,
BLACK,
SPYROPOULOS.
AND
AMASER
animal to process information in the environment that is not constant or has only short-term value. The deficit has been attributed to diverse causes and is generally thought to be permanent; however, the behavioral deficit can be compensated for by appropriate alteration of the behavioral situation (3-S). Pribram and Mishkin (5) concluded that on the delay-type problem the successful performance of animals with frontal lobe operations could be accounted for in terms of “distinctiveness” which predelay cues acquired from contiguity with distinctive responses and differential reward. We think it possible that the process by which distinctiveness is imparted to a cue will act to add stability to that portion of the spatial delayed response task which is variable-the side to be rewarded. The present study was undertaken to test this hypothesis by comparing the traditional direct and indirect methods in a spatial delayed response task. Material
and
Methods
Twelve experimentally naive sexually immature monkeys (Macaca mu&?a) were assigned to three groups: Four were tested by the traditional direct method, four by the traditional indirect method, and four by both methods. All training and testing took place in a covered unlit cage in which the monkey was separated from the experimenter by transparent and opaque guillotine doors that could be raised or lowered separately or together. The experimenter’s portion of the chamber was lit and contained a work tray with two food-wells mounted on it 16.5 cm apart. The wells were covered by identical circular gray lids. Traditional Direct Method. The opaque barrier was raised while the animal observed the experimenter hold a food reward over one uncovered food-well, drop the bait into it and replace the cover. The opaque barrier was then lowered, the tray pushed forward and at the end of 6 set both barriers raised to permit the animal to respond. Noncorrection technique was used throughout but the delay period was gradually increased from 0 to 6 set until criterion performance was reached. Animals received 100 trials daily. Traditional Indirect A4ethod. Animals were first trained to discriminate between a food-well lid with a circular white patch, which signified the site of the reward, and a uniformly gray lid. After this simple visual discrimination task had been mastered the animal was allowed to view, but not respond to the covered food-wells one of which had the white cue. The opaque barrier was then lowered and the lid with the cue replaced by one identical to the other lid. At the end of 6 set both barriers were raised and the animal allowed to respond in the absence of any overt cue. As with the direct-method group, noncorrection technique was used and the delay period
FRONTAL
383
LOBE
was gradually increased to 6 sec. Animals received 100 trials daily. Upon reaching high level performance postoperatively, animals in this group were tested by the direct method. Direct and Indirect Methods. After the subject had mastered one method it began training with the other while continuing with the first. Animals received fifty trials daily with each method. Surgery. As each animal reached the criterion of 90% correct responses on 5 consecutive days it was anesthetized and subjected to surgery. A large IOO-
80-
TO-
60-
I
I 100
f 500
I 1000
I
I
I
I
I
I500
2000
2500
3000
3500
FIG. 1. Differential recovery of function between animals trained by the indirect and direct methods. Points plotted represent group means. Vertical axis, percentage correct; horizontal axis, number of trials.
frontal bone flap was removed and the banks and depths of the principal cerebral sulcus were extirpated by aspiration bilaterally in one stage. .4n otological microscope permitted good visualization in the depths during surgery. The bone flap was replaced and the wound was sutured in anatomical layers. Postoperative testing began 48-72 hours after surgery, Upon completion of postoperative testing, the animals were anesthetized again and perfused with physiological saline solution followed by lOr/r, formalin. The lesionswere reconstructed from frozen S0l.rsections. Results
Animals trained by either method alone were equally affected by the surgery and showed comparable deficits initially (Fig. 1) . \Vith continued testing, however, a consistent difference between the two groups appeared after about 1500 trials and continued to increase, with all those in the
383
CIANCI,
BLACK.
SPYROPOULOS,
ATU’D
MASER
indirect-method group reaching preoperative performance levels (p < 0.05, one-tailed t tests). Those trained by the direct method also showed improvement, but did so at a much slower rate (p < 0.10, Mann-Whitney U test, one-tailed) and with greater variability. However, two animals of the direct group reattained preoperative levels, and did so more rapidly than the poorest animal on the indirect method. As each animal trained by the indirect method reached its preoperative level, it was tested by the direct method. The monkeys trained by the
FIG.
tested
2. Rapid transfer of by direct method. Points
ability plotted
to delay by recovered indirect are means. Axes as in Fig. 1.
method
animals
indirect method were capable of performing the direct method delayed response task at high levels readily and performed better than animals trained by the direct method alone (Fig. 2). 4nimals in the third group, those trained by both methods prior to surgery, demonstrated the most notable postoperative gain. They reached levels, for each method, comparable to their preoperation performance, in about 600 trials. While there was no clear-cut difference between the two methods in rate of recovery, the indirect method did appear to be slightly superior (Fig. 3). Discussion
Diverse studies have demonstrated an apparent recovery of ability of animals with frontal lobe lesions to perform delay-type tasks. The recovery, typically, has been achieved by either altering the bodily state of the animal (7) or the experimental conditions (3, 4). There have been no reports of animals with frontal ablations achieving stable high-level performance on the traditional spatial delayed response task by either the direct or
FROXTAL
LOBI’
385
indirect method; however, occasional successes by individual animals were noted in 1945 by Campbell and Harlow ( 1) . The present study demonstrated that animals with frontal lobes ablated, trained by the traditional indirect method, could reachieve performance levels comparable to their preoperative ones. These animals: which had no prior experience with the direct method, when tested by it, achieved high performance levels more rapidly than those trained solely by the direct method. LVhile the study demonstrated that
100 200 300 FIG.
Group
3. Rapid recovery means are plotted.
400
500
of function by animals Axes as in Fig. 1.
600
700
trained
600
initially
900
by
1000
both
methods.
animals trained by the indirect method were capable of reachieving criterion performance levels postoperatively, many trials were required. In contrast, those animals trained initially by both the direct and indirect methods very quickly reattained high performance levels postoperatively in both situations. While the success of the animals with frontal ablations on the traditional direct and indirect methods is in contradistinction to the findings of Mishkin and Pribram (4) with the traditional methods, it is interpreted as being essentially consistent with the interpretation of cue distinctiveness as a factor in maintaining delayed response behavior. The difference in the findings of this study and that of Mishkin and Pribram can be that they trained their animals on delayed response postoperatively, stopped training after 500 trials, and produced larger lesions in their animals. While the comparatively small-sized lesions (Fig. 4) used in our study may account for some recovery, lesion size alone cannot account for the differential recovery. As can be seen in Fig. 5, there was little difference in lesion size within or between groups; therefore, the type of training must have been responsible for the differential recovery.
386
CIANCI,
BLACK;
SPYROPOULOS,
AND
MASER
We believe the success of the animals in this study on the traditional methods was achieved for the same reason that Mishkin and Pribram’s animals (4) succeeded on the delay-variations, i.e., the acquisition of distinctiveness by the cue through contiguity with differentially rewarded responses. The cue thus had acquired the characteristics of a secondary reinforcer. In this regard, the study parallels that of Finan (3) with the difference that Finan used a predelay primary reinforcer to sustain the behavior. In our study the cue used in the indirect method, acting as a secondary reinforcer, sustained the behavior.
# FIG.
pared
561
4. Surface view of lesion along the principal sulcus. to that usually made in delayed-response studies.
Note
smallness
of lesion
com-
Presumably, the sight of food, which is the cue in the direct method, also has secondary reinforcement value and should also sustain the behavior in the situation. However, empirically, it does not, and we believe this failure to be due to the complexity and instability of the cuing situation. The animal is not cued by food alone but by food being held in the trainer’s hand; different amounts of the bait may be exposed on any given trial, and the hand may be held in various positions. Furthermore, the animal has experienced the bait in contexts other than the training situation and the reaching hand may have been experienced in situations where it had a threatening or aversive quality. For these reasons we believe that the cuing process in the direct method does not readily assume the stabilizing property of distinctiveness or secondary reinforcement. Probably the characteristic of “cue distinctiveness” or secondary reinforcement acted to give stability to that portion of the spatial delayed
FRONTAL
LOBE
387
response task which is inconstant, i.e., the shifting of the side of baiting. On the basis of the continued failure of animals trained by the direct method, one may argue that the brain tissue removed (the banks and depths of the principal sulcus) is necessary for short-term associative processes. (This is considered as short term because place of reward on each trial is independent of the preceding trials.) If these assumptions are accepted, then one may
FIG. 5. Cross-sectional view of the lesion in the best (left) and worst (right) behavioral animals in each group. Section is approximately at the midpoint of the lesion along the longitudinal axis. Monkeys 534 and 566, by direct method; 561 and 563. b> indirect method: 557 and 544, by both methods.
argue that the success of the indirect group, with the same lesions, was due to the subsuming of the short term processes by some other area. In this instance it may be speculated that the neural mechanism mediated a Iong training by which the cue term process, i.e., the visual discrimination acquired its secondary reinforcement characteristics, is also capable of mediating short term processes if appropriate associations have been made. This interpretation would account for the rapid reachievement of high level performance of the operates trained by both the direct and indirect methods
388
CIANCI,
BLACK,
SPYROPOULOS,
AND
MASER
and would be consistent with findings implicating areas other than the prefrontal lobes in delayed response behavior (2, 6). References 1.
2. 3 4.
5. 6.
7.
CAMPBELL, R. J., and H. R. H~RLOW. 1945. Problem solution by monkeys following removal of the prefrontal areas: V. Spatial delayed reactions. J. Exptl. Psycho!35: 110-126. CIANCI, S. 1965. Effects of cortical and subcortical stimulation on delayed response in monkeys, Exptl. Neural. 11: 104-114. FINAN, J. L. 1942. Delayed response with pre-delay reinforcement in monkeys after removal of the frontal lobes. Am. J. Phychol. 5: 295-308. MISHKIN, M., and K. H. PRIBRALZ. 1956. Analysis of the effects of frontal lesions in monkey: II. Variations of delayed response. J. Camp. Physiol. Psychol. 49: 36-40. PRIBRAM, K. H., and M. MISHXIN. 1956. Analysis of the effects of frontal lesions in monkey: III. Object alternation. J. Comp. Physiol. Psychol. 49: 41-45. ROSVOLD, H. E., and J. M. R. DELG~DO. 1956. The effect on delayed alternation test performance of stimulating or destroying electrically structures within the frontal lobes of the monkeys brain. J. Camp. Physiol. Psychol. 49: 365-372. 1947. The effect of sedatives upon delayed response in monkeys followWADE, M. ing removal of the prefrontal lobes. 1. Neurophysiol. 10: 57-61.