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Position reversal learning in aged Japanese macaques
Behavioural Brain Research 129 (2002) 107– 112 www.elsevier.com/locate/bbr
Research report
Position reversal learning in aged Japanese macaques Junko Tsuchida a,*, Namiko Kubo b, Shozo Kojima a b
a Primate Research Institute, Kyoto Uni6ersity, Inuyama, Aichi 484 -8506, Japan Department of Psychology, Japan Women’s Uni6ersity, Kawasaki, Kanagawa 214 -8565, Japan
Received 11 April 2001; received in revised form 13 July 2001; accepted 13 July 2001
Abstract We examined aged and young monkeys using a multiple position reversal task to investigate declines in cognitive functions with aging. The task consisted of an original learning task (simple position discrimination task) and a reversal learning task. While the performance of the aged monkeys was not different from that of the young monkeys in the original learning task, the aged monkeys showed a poorer performance than the young monkeys in the reversal learning task. According to our response analysis, the poor performance of aged monkeys in the reversal learning was not caused mainly by repetition of error responses, but rather by the impairment of understanding of the association between stimulus and reward. These results suggest that the prefrontal cortex, particularly the medial orbital cortex, is impaired with aging. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Aging; Aged monkey; Multiple position reversal learning task; Prefrontal cortex; Inferior prefrontal convexity; Medial orbital cortex; Macaca fuscata
1. Introduction Many studies indicated that aged nonhuman primates show declines in various cognitive abilities, and these deficits are similar to those of elderly humans [1 – 3,8,10,14 –16,18]. Some structural changes in the human brain that occur with aging were also observed in the nonhuman primate brain [12,13]. These findings suggest that aged nonhuman primates are suited as models of human aging. The reversal task is one of the tasks through which behavioral impairments in aged monkeys are revealed. At the beginning of this task, an experimenter presents two choices (e.g. positions, objects, and patterns); one is correct and the other is incorrect. If a monkey chooses the correct choice, the animal receives a reward. This is the original discrimination phase. If the monkey satisfies a learning criterion, the reinforcement contingency is reversed. The choice that is rewarded in the * Corresponding author. Tel.: + 81-568-63-0567; Fax: +81-56862-9552. E-mail address: [email protected] (J. Tsuchida).
original discrimination phase becomes incorrect, and the choice that is incorrect in the original discrimination phase becomes correct and is rewarded. This reversal is usually repeated several times. The monkey is required to change his/her response from the previously correct choice to the currently correct one. The reversal learning involves the ability to change responses in compliance with the reversal of reinforcement contingencies. Although two studies that tested aged monkeys using a multiple position reversal task were reported previously [10,18], their results are not entirely in agreement. Lai et al. reported that aged monkeys showed poorer performances than young monkeys in the position reversal task [10]. On the other hand, Voytko reported that aged monkeys performed as well as the young monkeys [18]. From the brain lesion studies of nonhuman primates, it was suggested that the ability to correctly complete the position reversal task depends on the integrity of some subregions of the prefrontal cortex (PFC). The PFC can be divided into five subregions based on their functions: the sulcus principalis, the inferior prefrontal
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J. Tsuchida et al. / Beha6ioural Brain Research 129 (2002) 107–112
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convexity, the medial orbital cortex, the arcuate concavity, and the superior prefrontal convexity [6]. It was suggested that the performance in the position reversal task depends on the integrity of the inferior prefrontal convexity and medial orbital cortex. The pattern of deficits differed in terms of the lesioned subregion. Monkeys, in whom the inferior prefrontal convexity was removed, failed to inhibit their tendency to perseveration [4]. In other words, they repeated a specific response even if the response was not rewarded, and had difficulties in changing their behavior to the appropriate one. In contrast, monkeys with medial orbital cortex lesion did not show the tendency to perseveration. However, they failed to improve their performance in subsequent reversals in contrast with normal controls [4,11]. If an aged monkey exhibits some type of behavioral impairment in the multiple position reversal task, it is expected that the animal has a dysfunction in a brain region that affects behavior. In previous studies in which aged monkeys were tested using a position reversal task, the authors did not discuss the function of PFC subregions [10,18]. Moreover, they repeated the reversal only for a few times. Therefore, they could not discuss the performance improvement of aged monkeys for the case when the reversal is repeated many times. In this study, we repeated the reversal 15 times and analyzed the responses of the aged monkeys in detail. A learning stage analysis was carried out to confirm whether the tendency to perseveration of aged monkeys exists. This analysis was also carried out in previous studies [10,18]. Moreover, we analyzed transitional probabilities of monkeys to investigate their abilities to associate a stimulus with a reward.
2. Materials and methods
2.1. Subjects Twelve Japanese macaques (Macaca fuscata) were used (Table 1). At the beginning of this experiment, they were divided into three groups, namely aged, early aged, and young. All the aged and early aged monkeys were born in the wild or free-ranging troops. Most of them were infants when they were brought to the Primate Research Institute (PRI), Kyoto University. Therefore, their age was estimated based on dentition. Because two aged monkeys (A1, A2) were adults when they were delivered to PRI, their estimated age was not as accurate as that of the other aged monkeys. All the young monkeys were born in PRI and their dates of birth are known. All the monkeys were kept in open enclosures in PRI with other monkeys. Before the experiment, they were transferred to indoor, individual cages. They were weighed before the daily experiment. Veterinarians checked their physical condition every morning. No serious physical abnormality was reported during this experiment. The experiment was carried out in accordance with the ‘Guide for the Care and Use of Laboratory Primates’ of PRI.
2.2. Apparatus A Wisconsin general test apparatus (WGTA) was used. It was placed in a dark room. During the experiment, sounds from outside were masked by white noise. Monkeys were transferred from their home cages to the testing cage of WGTA before the daily experiment. There were three food wells (3 cm in diameter) on the test tray at equal intervals. In this experiment, the two
Table 1 Profiles and performance of the monkeysa Monkey
Nickname
Sex
Age (years)
Tasks performed
Number of errors in the OL
Young Y1 Y2 Y3 Y4
Shin Tama Okori Titi
m m f f
6 6 10 10
OD, OD, OD, OD,
GNGs GNGs ODR, PD, LS, GNG, GNGs ODR, PD, LS, GNG, GNGs
9 3 13 3
Early aged A1 A2 A3 A4
Take Tora Ume Sizu
f f f f
22 22 22 23
OD, OD, OD, OD,
ODR, PD, LS, GNG PD, LS, GNG, GNGs ODR, PD, CD, LS, GNG, GNGs PD, CD, LS, GNG
3 12 1 4
Aged A5 A6 A7 A8
Shika Gin Gome Yase
f f f f
25 27 28 28
OD, OD, OD, OD,
GNGs ODR, PD, LS CD GNGs
16 5 28 15
a CD, Concurrent object discrimination; GNG, Go/no go successive discrimination; GNGs, GNG object substitution test; LS, Learning set formation; OD, simultaneous object discrimination; ODR, multiple object discrimination reversals; PD, simultaneous position discrimination.
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side wells were used. Wooden circular plates (10 cm in diameter) were placed on the wells. There was an opaque screen between the testing cage and the test tray of WGTA. A one-way screen was placed between the test tray and the experimenter, and the experimenter could observe responses of the monkey during the experiment. Raisins or pieces of sweet potato were used as rewards.
2.3. Procedure The experiment was conducted on alternate days. The multiple position reversal task consisted of an original learning task (OL) and reversal learning tasks (REVs). The OL was a simple position discrimination learning task. In the OL, the correct position was the right well (which was baited). However, the correct position of subjects Y3 and A2 was the left well. At the beginning of a trial, the test tray was hidden from the monkey by an opaque screen. The experimenter baited a reward at the correct well and covered both wells with the plates. Then, the screen was raised and the monkey was required to displace one of the two plates. When the monkey displaced the correct plate, he/she was allowed to get the reward from the well. However, if the monkey chose the incorrect well, he/she could not get the reward and the screen was closed immediately. After an intertrial interval (about 20 s), the next trial was started. A session consisted of ten trials, and each monkey had one session daily. The learning criterion was 90% correct responses in one session (nine or ten trials correct in a day). When the monkey’s performance was higher than the criterion, the REV was started. In the first reversal (REV-1), the correct position, i.e. the baited well, was opposite that of the OL, except that the procedure and the learning criterion were the same as those in the OL. In the second reversal (REV-2), the baited well was opposite that in the REV-1 (and the same as that in the OL). The reversal was repeated 15 times.
2.4. Analysis The numbers of errors in the OL and REVs were compared among the three age groups. The data obtained in REVs were analyzed by a learning stage analysis [10]. We defined one daily session (ten trials) as a block. Stage I was defined as the block in which a monkey made more than nine errors (chose the incorrect position significantly). Stage II was defined as the block in which a monkey made two to eight errors (chose both of the two positions by chance statistically). Stage III was the block in which a monkey made less than two errors (chose the correct position significantly). However, the blocks defined as stage III
Fig. 1. Mean errors in each reversal: ( ) aged monkeys, ( ) early aged monkeys, ( ) young monkeys. Vertical lines show standard errors of the means.
were the sessions in which the monkeys have a correct response for nine or ten trials. In other words, stage III was the session in which the monkey satisfied the learning criterion. Therefore, for all monkeys, there was only one block of stage III in each reversal. The total numbers of blocks of stage III for all monkeys were the same. Therefore, we did not analyze data obtained at this stage. Transitional probability was analyzed. Responses in the two consecutive trials of one daily session (the first and second trials, the second and third trials, etc.) constituted a response sequence. We divided these sequences into two categories. One was the sequence in which the previous response was correct, and the other was the sequence in which the previous response was incorrect. We calculated the percentage of correct latter responses for each category in each REV.
3. Results The number of errors of each subject in the OL is shown in Table 1. It was not significantly different among the three groups (t-test: aged and early aged, t(6)= 2.08, not significant; aged and young, t(6)=1.70, not significant; early aged and young, t(6)= −0.58, not significant). Fig. 1 shows the mean errors in each REV. The main effect of age and the interaction between age and reversal were significant (repeated-measures ANOVA: F(2,9)=4.98, PB 0.05 and F(28,126)= 1.82, PB0.05, respectively), but the main effect of reversal was not statistically significant (F(14,126)= 1.25, not significant). Post-hoc comparisons (Tukey’s HSD test) indicated that the aged monkeys showed a significantly poorer performance than the young monkeys (PB 0.05). On the other hand, the performance of early aged monkeys did not differ from those of aged and young monkeys. In eighth reversals, two of the aged monkeys (A6 and A7) showed a remarkably poor performance compared
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Fig. 2. Mean percentage of sessions of each learning stage in reversal learning. Stage I (left) was defined as the session in which a monkey made more than nine errors (chose the incorrect position significantly). Stage II (right) was defined as the session in which a monkey made two to eight errors (chose both of the positions by chance). Black box, Aged monkeys; striped box, early aged monkeys; open box, young monkeys. Vertical lines show standard errors of the means.
with the performance of the other subjects or their own performance in other reversals. However, after the eighth reversal, the animals performed normally and stably. We divided the sessions based on the number of errors into three stages, namely stage I (nine or ten errors), stage II (two to eight errors), and stage III (zero to one errors). The mean percent sessions in stages I and II in REVs are shown in Fig. 2. The main effects of age and stage were significant (repeated-measures ANOVA: F(2,9)= 21.60, P B0.0005 and F(1,9)= 480.967, PB 0.0001, respectively). The interaction between age and stage was not statistically significant (F(2,9) = 3.294, not significant). The results of the transitional probability analysis are shown in Fig. 3. The main effects of age and sequence were significant (repeated-measures ANOVA: F(2,9)=
Fig. 3. Mean percentage of correct of latter responses in two consecutive trials. We defined responses in two consecutive trials of one daily session as a sequence. The sequences were divided into two response groups based on the former response, whether it is correct or incorrect. Then, we calculated the percentage of correct responses in the latter trials for these two response groups: ( ) aged monkeys, ( ) early aged monkeys, ( ) young monkeys. Vertical lines show standard errors of the means.
20.8, PB 0.05 and F(1,9)= 17.1, PB 0.05, respectively). However, the main effect of reversal was not significant (F(14,126)= 0.56, not significant).The interactions between age and sequence, between age and reversal, and between sequence and reversal were not significant (F(2,9)= 1.03, not significant; F(28,126)= 0.70, not significant and F(14,126)= 0.80, not significant, respectively). Moreover, the interaction among age, sequence and reversal did not reach statistical significance (F(28,126)=1.01, not significant). Post-hoc comparisons (Tukey’s HSD test) indicated that the aged and early aged monkeys had significant behavioral impairment in comparison with the young monkeys (PB 0.05).
4. Discussion The results suggest that the ability of simple position discrimination in monkeys is intact during aging, because the total number of errors in the OL was not different among the three age groups. This is consistent with the findings of previous investigations using position reversal tasks [10,18]. Previous investigations using other types of reversal tasks also indicated that aged monkeys showed no declines in the original discrimination phase [1,3,14]. Therefore, the ability of simple discrimination and the motivational state are intact during aging. However, most of the aged monkeys scored outside the range of young monkeys. Similar individual differences in aged monkeys were also pointed out in previous investigations [2,9,16]. On the other hand, the ability to correctly complete REVs declined with aging. The aged monkeys could not reduce their errors during the 15-times repetition of reversal. However, the performance of the early aged animals was not different from that of the young monkeys. These results suggest that the ability gradually declines during aging. Although the young monkeys also showed no improvement in their performance during REVs, this phenomenon could be explained by the floor effect. The number of errors of the young monkeys in early REVs was not different from that in the OL, and the young monkeys maintained a good performance throughout the entire REVs. Therefore, the REV was not difficult for young monkeys from the start. On the other hand, the performance of the aged monkeys in REVs was poorer than that in the OL and that of the young monkeys in REVs. Although the aged monkeys showed a poor performance in REVs, it is suggested that the tendency to perseveration in the aged monkeys was not very high. In the learning stage analysis, the percentage of sessions at stage I was much lower than that at stage II. Even after an error response, the aged and early aged monkeys could give a correct response by chance. More-
J. Tsuchida et al. / Beha6ioural Brain Research 129 (2002) 107–112
over, this tendency was consistent throughout the REVs. These results were consistent with those of previous reports. Voytko [18] reported the absence of age differences in the learning stage analysis. The aged monkeys used by Lai et al. [10] spent more sessions in stage I only in the first reversal, but the animals reduced the number of sessions in stage I dramatically during the repetition of the reversal. The absence of the tendency to perseveration in the aged monkeys was also observed in the other tasks. Itoh et al. tested some of the monkeys used in our experiment using a multiple object discrimination reversal task, and suggested that the aged monkeys made errors due to a mild tendency to perseveration (see Ref. [8], Fig. 10). It is suggested that the poor performance of the aged monkeys in this task is caused by the difficulty to form an association between stimulus and reward. The aged monkeys showed a poorer performance after giving error responses than the young monkeys. However, this result does not suggest the tendency to perseveration of the aged monkeys, because they could respond correctly by chance even after the error trials. Itoh et al. also indicated this phenomenon in a learning set formation task (see Ref. [8], Fig. 4). As we have already pointed out, the impairments in forming stimulus–reward association in the aged monkeys cannot be explained by their motivational state. They appeared only when the cognitive load is heavy. In some previous investigations, similar phenomena have been reported. Bachevalier et al. tested aged monkeys using a visuospatial orientation task [2]. While the aged monkeys performed normally in a simple version of this task, they showed impairments in the complex version. Rapp and Amaral tested aged monkeys using two types of a delayed nonmatching to sample task, namely delayed nonmatching to sample with trial unique objects (DNMS-TU) and delayed nonmatching to sample with repeated objects (DNMS-RO) [15]. In DNMS-TU, a new pair of objects was used in each trial. In DNMSRO, on the other hand, the same pair of objects was used in each trial. Therefore, the effect of a proactive interference of DNMS-RO was stronger than that of DNMS-TU. The aged monkeys showed impairments only in DNMS-RO. The aged monkeys showed the impairments in forming stimulus–reward association and difficulties to reduce error responses during the reversal repetition in this experiment. It is known that monkeys with a lesion in the medial orbital cortex, a subregion of the PFC, showed similar deficits in the position reversal task [4,11]. Butter reported that monkeys with a lesion in the medial orbital cortex showed a poor performance in the position reversal task [4]. The lesioned monkeys performed normally in the original position discrimination phase, but showed a tendency to make more errors in subsequent reversals than normal controls. Moreover,
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the medial orbital cortex is important to process reward information [7,17]. From the brain lesion studies, it was shown that the inferior prefrontal convexity, which is another subregion of the PFC, is also important to perform a position reversal task. Monkeys who were lesioned in this area showed a strong tendency to perseveration, particularly in early reversals in the position reversal task [4]. However, the aged monkeys showed only a mild tendency to perseveration throughout our experiment. These results imply that the medial orbital cortex is more sensitive to aging than the inferior prefrontal convexity. Eberling et al. examined aging effects on glucose metabolism by positron emission tomography [5]. They found that glucose metabolism declined with aging only in the medial orbital cortex. These findings are in accordance with our findings. Further studies to identify possible neuronal changes that are related to our results with the monkeys used in this experiment are necessary. These studies may contribute to the understanding of cognitive aging in nonhuman primates.
Acknowledgements This work was supported by a Research Fellowship from the Japan Society for the Promotion of Science for Young Scientists (Japan).
References [1] Anderson JR, de Monte M, Kempf J. Discrimination learning and multiple reversals in young adult and older monkeys (Macaca arctoides). Q J Exp Psychol 1996;49B:193 – 200. [2] Bachevalier J, Landis LS, Walker LC, Brickson M, Mishkin M, Price DL, Cork LC. Aged monkeys exhibit behavioral deficits indicative of widespread cerebral dysfunction. Neurobiol Aging 1991;12:99 – 111. [3] Bartus RT, Dean RL, Fleming DL. Aging in the rhesus monkey: effects on visual discrimination learning and reversal learning. J Gerontol 1979;34:209 – 19. [4] Butter CM. Perseveration in extinction and in discrimination reversal tasks following selective frontal ablations in Macaca mulatta. Physiol Behav 1969;4:163 – 71. [5] Eberling JL, Roberts JA, Rapp PR, Tuszynski MH, Jagust WJ. Cerebral glucose metabolism and memory in aged rhesus macaques. Neurobiol Aging 1997;18:437 – 43. [6] Fuster JM. Chapter 4: animal neuropsychology. In: Fuster JM, editor. The Prefrontal Cortex: Anatomy, Physiology, and Neuropsychology of the Frontal Lobe. Philadelphia, PA: LippincottRaven, 1997:66 – 101. [7] Gaffan D, Murray EA. Amygdalar interaction with the mediodorsal nucleus of the thalamus and the ventromedial prefrontal cortex in stimulus-reward associative learning in the monkey. J Neurosci 1990;10:3479 – 93. [8] Itoh K, Izumi A, Kojima S. Object discrimination learning in aged Japanese monkeys. Behav Neurosci 2001;115:259 –70. [9] Kubo N, Koyama T, Kawasaki K, Tsuchida J, Sankai T, Terao K, Yoshikawa Y. Behavioral complements in a positional learning and memory task by aged monkeys. Behav Process (in press).
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[10] Lai ZC, Moss MB, Killiany RJ, Rosene DL, Herndon JG. Executive system dysfunction in the aged monkey: spatial and object reversal learning. Neurobiol Aging 1995;16:947 – 54. [11] Mahut H. Spatial and object reversal learning in monkeys with partial temporal lobe ablations. Neuropsychologia 1971;9:409 – 24. [12] Martin LJ. Cellular dynamics of senile plaque formation and amyloid deposition in cerebral cortex: an ultrastructural study of aged nonhuman primates. Neurobiol Aging 1993;14:681 – 3. [13] Price DL, Martin LJ, Sisodia SS, Wagster MV, Koo EH, Walker LC, Koliatsos VE, Cork LC. Aged non-human primates: an animal model of age-associated neurodegenerative disease. Brain Pathol 1991;1:287 – 96.
[14] Rapp PR. Visual discrimination and reversal learning in the aged monkey (Macaca mulatta). Behav Neurosci 1990;104:876 –84. [15] Rapp PR, Amaral DG. Evidence for task-dependent memory dysfunction in the aged monkey. J Neurosci 1989;9:3568 –76. [16] Rapp PR, Amaral DG. Individual differences in the cognitive and neurobiological consequences of normal aging. TINS 1992;15:340 – 5. [17] Tremblay L, Schultz W. Reward-related neuronal activity during go-nogo task performance in primate orbitofrontal cortex. J Neurophysiol 2000;83:1864 – 76. [18] Voytko ML. Impairments in acquisition and reversals of twochoice discriminations by aged rhesus monkeys. Neurobiol Aging 1999;20:617 – 27.
Research report
Position reversal learning in aged Japanese macaques Junko Tsuchida a,*, Namiko Kubo b, Shozo Kojima a b
a Primate Research Institute, Kyoto Uni6ersity, Inuyama, Aichi 484 -8506, Japan Department of Psychology, Japan Women’s Uni6ersity, Kawasaki, Kanagawa 214 -8565, Japan
Received 11 April 2001; received in revised form 13 July 2001; accepted 13 July 2001
Abstract We examined aged and young monkeys using a multiple position reversal task to investigate declines in cognitive functions with aging. The task consisted of an original learning task (simple position discrimination task) and a reversal learning task. While the performance of the aged monkeys was not different from that of the young monkeys in the original learning task, the aged monkeys showed a poorer performance than the young monkeys in the reversal learning task. According to our response analysis, the poor performance of aged monkeys in the reversal learning was not caused mainly by repetition of error responses, but rather by the impairment of understanding of the association between stimulus and reward. These results suggest that the prefrontal cortex, particularly the medial orbital cortex, is impaired with aging. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Aging; Aged monkey; Multiple position reversal learning task; Prefrontal cortex; Inferior prefrontal convexity; Medial orbital cortex; Macaca fuscata
1. Introduction Many studies indicated that aged nonhuman primates show declines in various cognitive abilities, and these deficits are similar to those of elderly humans [1 – 3,8,10,14 –16,18]. Some structural changes in the human brain that occur with aging were also observed in the nonhuman primate brain [12,13]. These findings suggest that aged nonhuman primates are suited as models of human aging. The reversal task is one of the tasks through which behavioral impairments in aged monkeys are revealed. At the beginning of this task, an experimenter presents two choices (e.g. positions, objects, and patterns); one is correct and the other is incorrect. If a monkey chooses the correct choice, the animal receives a reward. This is the original discrimination phase. If the monkey satisfies a learning criterion, the reinforcement contingency is reversed. The choice that is rewarded in the * Corresponding author. Tel.: + 81-568-63-0567; Fax: +81-56862-9552. E-mail address: [email protected] (J. Tsuchida).
original discrimination phase becomes incorrect, and the choice that is incorrect in the original discrimination phase becomes correct and is rewarded. This reversal is usually repeated several times. The monkey is required to change his/her response from the previously correct choice to the currently correct one. The reversal learning involves the ability to change responses in compliance with the reversal of reinforcement contingencies. Although two studies that tested aged monkeys using a multiple position reversal task were reported previously [10,18], their results are not entirely in agreement. Lai et al. reported that aged monkeys showed poorer performances than young monkeys in the position reversal task [10]. On the other hand, Voytko reported that aged monkeys performed as well as the young monkeys [18]. From the brain lesion studies of nonhuman primates, it was suggested that the ability to correctly complete the position reversal task depends on the integrity of some subregions of the prefrontal cortex (PFC). The PFC can be divided into five subregions based on their functions: the sulcus principalis, the inferior prefrontal
0166-4328/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 6 - 4 3 2 8 ( 0 1 ) 0 0 3 3 6 - 9
J. Tsuchida et al. / Beha6ioural Brain Research 129 (2002) 107–112
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convexity, the medial orbital cortex, the arcuate concavity, and the superior prefrontal convexity [6]. It was suggested that the performance in the position reversal task depends on the integrity of the inferior prefrontal convexity and medial orbital cortex. The pattern of deficits differed in terms of the lesioned subregion. Monkeys, in whom the inferior prefrontal convexity was removed, failed to inhibit their tendency to perseveration [4]. In other words, they repeated a specific response even if the response was not rewarded, and had difficulties in changing their behavior to the appropriate one. In contrast, monkeys with medial orbital cortex lesion did not show the tendency to perseveration. However, they failed to improve their performance in subsequent reversals in contrast with normal controls [4,11]. If an aged monkey exhibits some type of behavioral impairment in the multiple position reversal task, it is expected that the animal has a dysfunction in a brain region that affects behavior. In previous studies in which aged monkeys were tested using a position reversal task, the authors did not discuss the function of PFC subregions [10,18]. Moreover, they repeated the reversal only for a few times. Therefore, they could not discuss the performance improvement of aged monkeys for the case when the reversal is repeated many times. In this study, we repeated the reversal 15 times and analyzed the responses of the aged monkeys in detail. A learning stage analysis was carried out to confirm whether the tendency to perseveration of aged monkeys exists. This analysis was also carried out in previous studies [10,18]. Moreover, we analyzed transitional probabilities of monkeys to investigate their abilities to associate a stimulus with a reward.
2. Materials and methods
2.1. Subjects Twelve Japanese macaques (Macaca fuscata) were used (Table 1). At the beginning of this experiment, they were divided into three groups, namely aged, early aged, and young. All the aged and early aged monkeys were born in the wild or free-ranging troops. Most of them were infants when they were brought to the Primate Research Institute (PRI), Kyoto University. Therefore, their age was estimated based on dentition. Because two aged monkeys (A1, A2) were adults when they were delivered to PRI, their estimated age was not as accurate as that of the other aged monkeys. All the young monkeys were born in PRI and their dates of birth are known. All the monkeys were kept in open enclosures in PRI with other monkeys. Before the experiment, they were transferred to indoor, individual cages. They were weighed before the daily experiment. Veterinarians checked their physical condition every morning. No serious physical abnormality was reported during this experiment. The experiment was carried out in accordance with the ‘Guide for the Care and Use of Laboratory Primates’ of PRI.
2.2. Apparatus A Wisconsin general test apparatus (WGTA) was used. It was placed in a dark room. During the experiment, sounds from outside were masked by white noise. Monkeys were transferred from their home cages to the testing cage of WGTA before the daily experiment. There were three food wells (3 cm in diameter) on the test tray at equal intervals. In this experiment, the two
Table 1 Profiles and performance of the monkeysa Monkey
Nickname
Sex
Age (years)
Tasks performed
Number of errors in the OL
Young Y1 Y2 Y3 Y4
Shin Tama Okori Titi
m m f f
6 6 10 10
OD, OD, OD, OD,
GNGs GNGs ODR, PD, LS, GNG, GNGs ODR, PD, LS, GNG, GNGs
9 3 13 3
Early aged A1 A2 A3 A4
Take Tora Ume Sizu
f f f f
22 22 22 23
OD, OD, OD, OD,
ODR, PD, LS, GNG PD, LS, GNG, GNGs ODR, PD, CD, LS, GNG, GNGs PD, CD, LS, GNG
3 12 1 4
Aged A5 A6 A7 A8
Shika Gin Gome Yase
f f f f
25 27 28 28
OD, OD, OD, OD,
GNGs ODR, PD, LS CD GNGs
16 5 28 15
a CD, Concurrent object discrimination; GNG, Go/no go successive discrimination; GNGs, GNG object substitution test; LS, Learning set formation; OD, simultaneous object discrimination; ODR, multiple object discrimination reversals; PD, simultaneous position discrimination.
J. Tsuchida et al. / Beha6ioural Brain Research 129 (2002) 107–112
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side wells were used. Wooden circular plates (10 cm in diameter) were placed on the wells. There was an opaque screen between the testing cage and the test tray of WGTA. A one-way screen was placed between the test tray and the experimenter, and the experimenter could observe responses of the monkey during the experiment. Raisins or pieces of sweet potato were used as rewards.
2.3. Procedure The experiment was conducted on alternate days. The multiple position reversal task consisted of an original learning task (OL) and reversal learning tasks (REVs). The OL was a simple position discrimination learning task. In the OL, the correct position was the right well (which was baited). However, the correct position of subjects Y3 and A2 was the left well. At the beginning of a trial, the test tray was hidden from the monkey by an opaque screen. The experimenter baited a reward at the correct well and covered both wells with the plates. Then, the screen was raised and the monkey was required to displace one of the two plates. When the monkey displaced the correct plate, he/she was allowed to get the reward from the well. However, if the monkey chose the incorrect well, he/she could not get the reward and the screen was closed immediately. After an intertrial interval (about 20 s), the next trial was started. A session consisted of ten trials, and each monkey had one session daily. The learning criterion was 90% correct responses in one session (nine or ten trials correct in a day). When the monkey’s performance was higher than the criterion, the REV was started. In the first reversal (REV-1), the correct position, i.e. the baited well, was opposite that of the OL, except that the procedure and the learning criterion were the same as those in the OL. In the second reversal (REV-2), the baited well was opposite that in the REV-1 (and the same as that in the OL). The reversal was repeated 15 times.
2.4. Analysis The numbers of errors in the OL and REVs were compared among the three age groups. The data obtained in REVs were analyzed by a learning stage analysis [10]. We defined one daily session (ten trials) as a block. Stage I was defined as the block in which a monkey made more than nine errors (chose the incorrect position significantly). Stage II was defined as the block in which a monkey made two to eight errors (chose both of the two positions by chance statistically). Stage III was the block in which a monkey made less than two errors (chose the correct position significantly). However, the blocks defined as stage III
Fig. 1. Mean errors in each reversal: ( ) aged monkeys, ( ) early aged monkeys, ( ) young monkeys. Vertical lines show standard errors of the means.
were the sessions in which the monkeys have a correct response for nine or ten trials. In other words, stage III was the session in which the monkey satisfied the learning criterion. Therefore, for all monkeys, there was only one block of stage III in each reversal. The total numbers of blocks of stage III for all monkeys were the same. Therefore, we did not analyze data obtained at this stage. Transitional probability was analyzed. Responses in the two consecutive trials of one daily session (the first and second trials, the second and third trials, etc.) constituted a response sequence. We divided these sequences into two categories. One was the sequence in which the previous response was correct, and the other was the sequence in which the previous response was incorrect. We calculated the percentage of correct latter responses for each category in each REV.
3. Results The number of errors of each subject in the OL is shown in Table 1. It was not significantly different among the three groups (t-test: aged and early aged, t(6)= 2.08, not significant; aged and young, t(6)=1.70, not significant; early aged and young, t(6)= −0.58, not significant). Fig. 1 shows the mean errors in each REV. The main effect of age and the interaction between age and reversal were significant (repeated-measures ANOVA: F(2,9)=4.98, PB 0.05 and F(28,126)= 1.82, PB0.05, respectively), but the main effect of reversal was not statistically significant (F(14,126)= 1.25, not significant). Post-hoc comparisons (Tukey’s HSD test) indicated that the aged monkeys showed a significantly poorer performance than the young monkeys (PB 0.05). On the other hand, the performance of early aged monkeys did not differ from those of aged and young monkeys. In eighth reversals, two of the aged monkeys (A6 and A7) showed a remarkably poor performance compared
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Fig. 2. Mean percentage of sessions of each learning stage in reversal learning. Stage I (left) was defined as the session in which a monkey made more than nine errors (chose the incorrect position significantly). Stage II (right) was defined as the session in which a monkey made two to eight errors (chose both of the positions by chance). Black box, Aged monkeys; striped box, early aged monkeys; open box, young monkeys. Vertical lines show standard errors of the means.
with the performance of the other subjects or their own performance in other reversals. However, after the eighth reversal, the animals performed normally and stably. We divided the sessions based on the number of errors into three stages, namely stage I (nine or ten errors), stage II (two to eight errors), and stage III (zero to one errors). The mean percent sessions in stages I and II in REVs are shown in Fig. 2. The main effects of age and stage were significant (repeated-measures ANOVA: F(2,9)= 21.60, P B0.0005 and F(1,9)= 480.967, PB 0.0001, respectively). The interaction between age and stage was not statistically significant (F(2,9) = 3.294, not significant). The results of the transitional probability analysis are shown in Fig. 3. The main effects of age and sequence were significant (repeated-measures ANOVA: F(2,9)=
Fig. 3. Mean percentage of correct of latter responses in two consecutive trials. We defined responses in two consecutive trials of one daily session as a sequence. The sequences were divided into two response groups based on the former response, whether it is correct or incorrect. Then, we calculated the percentage of correct responses in the latter trials for these two response groups: ( ) aged monkeys, ( ) early aged monkeys, ( ) young monkeys. Vertical lines show standard errors of the means.
20.8, PB 0.05 and F(1,9)= 17.1, PB 0.05, respectively). However, the main effect of reversal was not significant (F(14,126)= 0.56, not significant).The interactions between age and sequence, between age and reversal, and between sequence and reversal were not significant (F(2,9)= 1.03, not significant; F(28,126)= 0.70, not significant and F(14,126)= 0.80, not significant, respectively). Moreover, the interaction among age, sequence and reversal did not reach statistical significance (F(28,126)=1.01, not significant). Post-hoc comparisons (Tukey’s HSD test) indicated that the aged and early aged monkeys had significant behavioral impairment in comparison with the young monkeys (PB 0.05).
4. Discussion The results suggest that the ability of simple position discrimination in monkeys is intact during aging, because the total number of errors in the OL was not different among the three age groups. This is consistent with the findings of previous investigations using position reversal tasks [10,18]. Previous investigations using other types of reversal tasks also indicated that aged monkeys showed no declines in the original discrimination phase [1,3,14]. Therefore, the ability of simple discrimination and the motivational state are intact during aging. However, most of the aged monkeys scored outside the range of young monkeys. Similar individual differences in aged monkeys were also pointed out in previous investigations [2,9,16]. On the other hand, the ability to correctly complete REVs declined with aging. The aged monkeys could not reduce their errors during the 15-times repetition of reversal. However, the performance of the early aged animals was not different from that of the young monkeys. These results suggest that the ability gradually declines during aging. Although the young monkeys also showed no improvement in their performance during REVs, this phenomenon could be explained by the floor effect. The number of errors of the young monkeys in early REVs was not different from that in the OL, and the young monkeys maintained a good performance throughout the entire REVs. Therefore, the REV was not difficult for young monkeys from the start. On the other hand, the performance of the aged monkeys in REVs was poorer than that in the OL and that of the young monkeys in REVs. Although the aged monkeys showed a poor performance in REVs, it is suggested that the tendency to perseveration in the aged monkeys was not very high. In the learning stage analysis, the percentage of sessions at stage I was much lower than that at stage II. Even after an error response, the aged and early aged monkeys could give a correct response by chance. More-
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over, this tendency was consistent throughout the REVs. These results were consistent with those of previous reports. Voytko [18] reported the absence of age differences in the learning stage analysis. The aged monkeys used by Lai et al. [10] spent more sessions in stage I only in the first reversal, but the animals reduced the number of sessions in stage I dramatically during the repetition of the reversal. The absence of the tendency to perseveration in the aged monkeys was also observed in the other tasks. Itoh et al. tested some of the monkeys used in our experiment using a multiple object discrimination reversal task, and suggested that the aged monkeys made errors due to a mild tendency to perseveration (see Ref. [8], Fig. 10). It is suggested that the poor performance of the aged monkeys in this task is caused by the difficulty to form an association between stimulus and reward. The aged monkeys showed a poorer performance after giving error responses than the young monkeys. However, this result does not suggest the tendency to perseveration of the aged monkeys, because they could respond correctly by chance even after the error trials. Itoh et al. also indicated this phenomenon in a learning set formation task (see Ref. [8], Fig. 4). As we have already pointed out, the impairments in forming stimulus–reward association in the aged monkeys cannot be explained by their motivational state. They appeared only when the cognitive load is heavy. In some previous investigations, similar phenomena have been reported. Bachevalier et al. tested aged monkeys using a visuospatial orientation task [2]. While the aged monkeys performed normally in a simple version of this task, they showed impairments in the complex version. Rapp and Amaral tested aged monkeys using two types of a delayed nonmatching to sample task, namely delayed nonmatching to sample with trial unique objects (DNMS-TU) and delayed nonmatching to sample with repeated objects (DNMS-RO) [15]. In DNMS-TU, a new pair of objects was used in each trial. In DNMSRO, on the other hand, the same pair of objects was used in each trial. Therefore, the effect of a proactive interference of DNMS-RO was stronger than that of DNMS-TU. The aged monkeys showed impairments only in DNMS-RO. The aged monkeys showed the impairments in forming stimulus–reward association and difficulties to reduce error responses during the reversal repetition in this experiment. It is known that monkeys with a lesion in the medial orbital cortex, a subregion of the PFC, showed similar deficits in the position reversal task [4,11]. Butter reported that monkeys with a lesion in the medial orbital cortex showed a poor performance in the position reversal task [4]. The lesioned monkeys performed normally in the original position discrimination phase, but showed a tendency to make more errors in subsequent reversals than normal controls. Moreover,
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the medial orbital cortex is important to process reward information [7,17]. From the brain lesion studies, it was shown that the inferior prefrontal convexity, which is another subregion of the PFC, is also important to perform a position reversal task. Monkeys who were lesioned in this area showed a strong tendency to perseveration, particularly in early reversals in the position reversal task [4]. However, the aged monkeys showed only a mild tendency to perseveration throughout our experiment. These results imply that the medial orbital cortex is more sensitive to aging than the inferior prefrontal convexity. Eberling et al. examined aging effects on glucose metabolism by positron emission tomography [5]. They found that glucose metabolism declined with aging only in the medial orbital cortex. These findings are in accordance with our findings. Further studies to identify possible neuronal changes that are related to our results with the monkeys used in this experiment are necessary. These studies may contribute to the understanding of cognitive aging in nonhuman primates.
Acknowledgements This work was supported by a Research Fellowship from the Japan Society for the Promotion of Science for Young Scientists (Japan).
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