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Department of Behavioral and Brain Sciences, Primate Research Institute, Kyoto University, Aichi, Japan
Submitted 26 August 2006; accepted in final form 21 July 2007
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ABSTRACT |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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), we recorded single neuronal activities from the ventrolateral prefrontal cortex (VLPFC) during an SPR task, and found responses that depended on both the color and target object (CT response), the target object (T responses), and the color (C responses) during the color cue period. Because no object was presented during this period, these object-selective responses (CT and T responses) must be correlated with the retrieval process of an object from memorized multiple objects. This result suggests that VLPFC combines color information with object and temporal order information in the working memory, retrieves a particular object, and encodes the retrieved information as shown in the schema of Fig. 1B (Hypothesis 1). If this Hypothesis 1 is true, there will be no inferior temporal cortex (IT) neurons with CT or T response during the color cue period of the SPR task. However, several researchers have reported that neurons in IT showed object-selective activities during the delay period of working memory tasks (Colombo and Gross 1994; Fuster 1990; Fuster and Jervey 1981, 1982; Mikami 1995; Miller et al. 1991, 1993). IT sends dense projections to VLPFC (Barbas 1988; Seltzer and Pandya 1989; Ungerleider et al. 1989; Webster et al. 1994). Previous human neuroimaging studies have indicated that not only the prefrontal cortex but also the parietal cortex and IT were activated in the retrieval process from the working memory (Kostopoulos and Petrides 2003; Rowe and Passingham 2001; Rowe et al. 2000). Therefore IT could possibly contribute to combine color information with object and temporal order information in the working memory and send the retrieved object information to VLPFC as shown in the schema of Fig. 1C (Hypothesis 2). If this Hypothesis 2 is true, there will be IT neurons with CT or T response during the color cue period of the SPR task, and IT neurons with CT or T response will be activated earlier than VLPFC neurons with CT or T response. To test these two hypotheses shown in Fig. 1, B and C, we recorded single neuronal activities from IT during the SPR task and compared the properties of IT neurons with those of VLPFC neurons. The present data were reported elsewhere in an abstract form (Inoue and Mikami 2005). |
METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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The general methods used were the same as those described previously (Inoue and Mikami 2006). We used two adult monkeys (Macaca mulatta), one male (weight, 9.7 kg) and one female (weight, 5.7 kg), which were cared for according to the Guide for the Care and Use of Laboratory Primates by the Primate Research Institute, Kyoto University (1986, 2002). These monkeys were the same monkeys in which prefrontal neuronal activities were recorded in our previous study (Inoue and Mikami 2006). Each monkey was seated in a primate chair in a dark room, with a head-restraining device fixed to its head. The monkey focused on a 17-in. CRT monitor (FlexScan T565, Nanao), which was placed 40 cm away from its eyes. A computer (PC-9821Xa200, NEC) presented the fixation point and stimuli on the CRT monitor. The monkey's horizontal and vertical eye positions were sampled at 250 Hz by a monitoring system using an infrared camera (R-21C-AC, RMS Hirosaki). Sampled eye positions were fed into a computer (PC-9801BX, NEC) by an A/D converter to determine whether the monkey maintained a fixation and made a correct saccade.
Monkeys were trained to perform a serial probe reproduction (SPR) task (Fig. 1A). In this task, after 1 s of an intertrial interval (ITI), a fixation spot (white circle; diameter, 0.1°) was presented at the center of the monitor. After the monkey maintained its fixation for 1.5 s, the first object cue, consisting of one of three objects (a double cone, a cross, and a circle, 3 x 3°), was presented at the center of the monitor for 0.5 s. After 1 s of the first delay period, a second object cue, consisting of one of the remaining two objects, was presented at the center of the monitor for 0.5 s. After 1 s of the second delay period, a color cue (a red or a green rectangle, 3 x 3°) was presented at the center of the monitor for 0.5 s. The presentation of a color cue was followed by a third delay period of 1–1.5 s. Then, three objects were presented at the upper, lower left, and lower right positions at 9° of eccentricity from the center of the monitor. The fixation spot was distinguished simultaneously. The available options were 1) when the color cue was red, the monkey had to make a saccade to the object presented as the first cue; and 2) when the color cue was green, the monkey had to make a saccade to the object presented as the second cue. We prepared three patterns of arrangement of the target objects, and one of the patterns was randomly selected during the response period. Therefore until the appearance of objects during the response period, the monkeys could not determine the spatial location to which a saccade should be performed. When the monkey broke its fixation (the size of fixation window was ±2.5°) during the cue or the delay periods, the task was terminated and 3 s of ITI was introduced to initiate the next trial.
The monkeys also performed a fixation task to examine neuronal activity when selection was not required. While the monkey maintained its fixation, we presented a color stimulus the same as that used in the SPR task.
Recording method and data analysis
Neuronal activity was recorded using glass-coated elgiloy microelectrodes (1–2 M
). Single neuronal activity was isolated and converted to pulses by a window discriminator (DIS-1, BAK) and stored with task events as a data file on a hard disk. We first searched neuronal activity while the monkey performed the SPR task and, if a neuron responded during the object cue period, object delay period, and/or color cue periods, then we recorded the neuronal activity in the control tasks (fixation task and delayed matching to sample task). Recording sites were determined by magnetic resonance imaging (MRI). IT recordings were conducted from the lower bank of the superior temporal sulcus and ventral surface between A6.5 and A15.5 in monkey G and between A5.5 and A16 in monkey H.
In this study, we analyzed neuronal activity during the color cue period. Six averaged perievent time histograms (two colors and three objects as the future targets of saccade) triggered by the onset of the color cue were computed (bin width, 10 ms). From these histograms, when a neuron exhibited an excitatory response, the histogram with the highest peak value was chosen for determining the response window and, when a neuron exhibited an inhibitory response, the histogram with the lowest peak value was chosen. In this peak histogram, the starting point of the activity (the time at which the first three consecutive bins differed from the discharge rates for 1 s before the color cue presentation by >2 SD or <2 SD) and the endpoint (the time of the last bin) of the response window were determined. The time from the onset of the color cue to the starting point of the response window was taken as the onset latency. When its discharge rate during the response window of the color cue period differed significantly (Mann–Whitney U test) from the fixation period (for 1-s period before the first cue period), we inferred that the neuron exhibited a response. We performed two-way ANOVA of the responses during the color cue period, in terms of the color (red or green) and target (object to make a saccade; double cone, cross, or circle) factors. When the difference in response in terms of the color factor was significant and the difference in terms of the target factor was not significant, we classified the response as a C response. When the difference in response in terms of the color factor was not significant and the difference in terms of the target factor was significant, we classified the response as a T response. When the differences in terms of both the color factor and the target factor were significant or the difference of interaction was significant, we classified the response as a CT response. We also determined preferred object and preferred color for each neuron. The preferred object was the object that elicited the largest activity in each neuron and the nonpreferred object was the object that elicited the smallest activity in each neuron. The preferred color was the color that elicited the largest activity in each neuron and the nonpreferred color was the color that elicited the smallest activity in each neuron.
When we analyzed neuronal activity during the fixation task, two perievent time histograms triggered by the onset of the color cue were constructed for each color (bin width, 10 ms). From these histograms, when a neuron exhibited an excitatory response during the color cue period, the histogram with the highest peak for the color cue periods was chosen for determining the response window. In this histogram, the starting point of a response (the time at which the first three consecutive bins differed from the discharge rates for 1 s before the cue presentation by >2 SD or <2 SD) and the endpoint of a response (the time of the last bin) were determined. The time from the onset of the cue to the starting point of the response was taken as onset latency. When the discharge rate during the response window of the cue period differed significantly (Mann–Whitney U test) from that of the fixation period (for 1 s before the cue period), we concluded that the neuron exhibited a response. When the starting point and endpoint of a response could not be determined, we determined the discharge rate to compare the response during the SPR task. In those cases, we calculated mean discharge rate from 100 ms after the cue onset to the end of the cue period.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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We observed two types of error in the SPR task: 1) breaking the fixation during the cue or delay period (fixation error) and 2) making a saccade to an incorrect target object during the response period. To calculate performance rates, we excluded fixation error trials [monkey G, 15 ± 8 (SD) %; monkey H, 3 ± 4%]. In >85% correct trials (monkey G, 85.4%; monkey H, 89.7%), eye positions were restricted within ±1° of the fixation point, and there were no systematic differences in the eye movements among the trial conditions. The performance rates during the recording sessions were well above the chance level in both monkeys. We recorded from IT after we had recorded from the lateral prefrontal cortex (LPFC). Performance rate was not different between the sessions for the LPFC recording (monkey G, 75 ± 5%; monkey H, 70 ± 3%) and the sessions for the IT recording (monkey G, 76 ± 6%; monkey H, 71 ± 4%). The mean latencies for saccades from the onset of target object presentation were slightly longer during the IT recording sessions (monkey G, 293 ± 147 ms; monkey H, 277 ± 104 ms) than during the LPFC recording sessions (monkey G, 284 ± 105 ms; monkey H, 260 ± 89 ms).
Neuronal database
In this study, we recorded 287 single neuronal activities from IT during the SPR task. During the color cue period, 107 neurons exhibited significantly different activities compared with the baseline activity during the fixation period. ANOVA was performed on this population. In our previous study, we recorded 438 single neuronal activities from VLPFC during the SPR task, and 125 neurons exhibited significantly different activities compared with the baseline activity during the fixation period. We described the details of neuronal activity in VLPFC in our previous paper (Inoue and Mikami 2006).
Target- and/or color-selective response during color cue period
We could find responses that are selective to the target objects (CT and T responses) during the color cue period in IT. Figure 2A shows an example of a CT response of an IT neuron. This neuron was activated during the color cue period when the color cue was green and the target object was the double cone. The activity was not significant when the target object was a circle even if the color cue was green. The response of this neuron was significantly different in both the color factor [F(1,138) = 4.71, P < 0.05] and the target factor [F(2,138) = 3.86, P < 0.05]. Thus this response during the color cue period depended on both the color cue and the target object, and we defined green as the preferred color and red as the nonpreferred color of this neuron, and the double cone as the preferred target object and the circle as the nonpreferred target object of this neuron. Because two objects were presented before the color cue period in the SPR task, one object was the target and the other object was the nontarget. In other words, when the color was red, the first object was the target and the second object was the nontarget and vice versa when the color was green. We tested the effects of the nontarget object in the preferred color and preferred target object trials. We could not find any significant effects of the nontarget object (Mann–Whitney U test, z = 1.06, P > 0.05). We could find 24 IT neurons with a CT response and, in almost all (n = 23) of these neurons, we could not find any significant effects of the nontarget object.
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Figure 2C shows an example of a C response of an IT neuron. The neuron exhibited response when the color cue was green regardless of the target objects. The response was significantly different in terms of the color factor [F(1,111) = 105.59, P < 0.0001] but not significant in terms of the target factor [F(2,111) = 2.86, P > 0.05]. We defined green as the preferred color and red as the nonpreferred color of this neuron.
The CT, T, and C responses were recorded from both VLPFC and IT (Table 1). In VLPFC, about half of the neurons showed a CT response. In IT, about half of the neurons showed a C response. The proportion of the neurons with CT and C response was significantly different (CT response: 
2 = 9.39, df = 1, P < 0.005; C response: 2 = 6.87, df = 1, P < 0.01) between VLPFC and IT.
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Latency of target- and/or color-selective response
Figure 3A shows the cumulative summation of the onset latencies of the CT, T, and C responses recorded from VLPFC and IT. The latencies of the CT responses (mean = 260 ms) and T responses (mean = 290 ms) in IT were longer than those in VLPFC (CT responses, mean = 162 ms; T responses, mean = 171 ms). These differences were statistically significant (CT responses, z = 4.10, P < 0.0001; T responses, z = 2.61, P < 0.01). In contrast, the latencies of the C responses in IT (mean = 126 ms) were shorter than those in VLPFC (mean = 157 ms). The difference was also statistically significant (Mann–Whitney U test, z = 2.64, z <0.01).
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Comparison of response with color cue response in fixation task
When selection is not required, do neurons with a CT response or a T response respond to color stimuli? To answer this question, we examined neuronal activities during a fixation task. While the monkey maintained its fixation, we presented a color stimulus used in the SPR task. Figure 4A shows the histograms of the CT response of an IT neuron during the SPR and fixation tasks. The neuron showed no clear response during the fixation task. All tested IT neurons with a CT response (n = 8) or a T response (n = 1) showed no clear response during the fixation task. On the other hand, all tested IT neurons with a C response (n = 14) showed a response during the fixation task (Fig. 4B).
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Do the object-selective responses (CT and T responses) contribute to the retrieval process from the working memory? The behavioral relevance of these responses can be explored by analysis of error trials. We analyzed responses in error trials in which the monkey made a saccade to an incorrect object. When the number of error trials was small (n < 3), we did not analyze the data for these error trials.
Figure 5A shows the discharge rate during the correct and error trials of an IT neuron with a CT response shown in Fig. 2A. During the preferred color and target trials, mean discharge rates were not significantly different (Mann–Whitney U test, z = 0.34, P > 0.05) between the correct trials [22.25 ± 5.17 (SE) spikes/s] and the error trials (23.96 ± 7.91 spikes/s). During the preferred color and nonpreferred target trials, we did not analyze neuronal activity in the error trials of this trial condition because the number of error trials was small (n = 1).
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The mean response magnitudes of five neurons with a T response during the error trials were smaller than those during the correct preferred target trials, but the difference was not statistically significant (Wilcoxon signed-rank test, z = –1.48, P > 0.05, mean = 25.69 spikes/s in correct trials, mean = 18.56 spikes/s in error trials). At the individual neuronal level, only one neuron (a filled circle in Fig. 5C) showed a significantly different response between the correct and error trials. In the nonpreferred target trials, the mean magnitudes of the T responses during the error trials were not significantly different from those during the correct trials (Wilcoxon signed-rank test, z = –1.78, P > 0.05, mean = 13.48 spikes/s in correct trials, mean = 17.18 spikes/s in error trials).
Response stability of target-object–selective response in IT
The neuron shown in Fig. 5A did not always respond in the preferred trials. In other words, the response was variable. To identify how many trials showed a response that was different from the distribution of the discharge rate during the control period, we used a receiver operating characteristic (ROC)–based analysis. An area under an ROC curve (aROC) of 0.5 indicates no difference in distribution. A value of 1 indicates that two distributions were completely different. For example, the aROC of the neuron with a CT response shown in Figs. 2A and 5A was 0.83. Figure 6 shows the values of aROC of CT and T responses in IT and VLPFC. The aROC values of IT neurons with the CT and T responses were smaller (CT responses, mean = 0.83; T responses, mean = 0.75) than those of VLPFC neurons with CT and T responses (CT responses, mean = 0.89; T response, mean = 0.82). These results indicate that the CT and T responses in IT are relatively variable.
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Recording locations were determined according to the sterotaxic coordinates, MRI, and white–gray matter transitions encountered during the electrode penetrations. Our recording probably covered the areas TEa, TEm, TE3, TE2, and TE1 (Paxinos et al. 2000). Figure 7 shows the distribution of neurons with CT, T, and C responses. We could not find any systematic differences in the distributions among neurons with CT, T, and C responses.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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Object-selective response during color cue period
In our SPR task, two objects were presented sequentially and the target object was determined later when the color cue was presented. We used a color cue during this period because we intended to use different attributes from the object cue. During the color cue period, a target object was selected from the memorized two objects based on the color cue. During this period, the objects were not visible and the monkey was required to retrieve a memorized object from the working memory. Thus if a neuron was activated when a particular object was correct and not when the other object was correct even if the color cue itself was the same, the object-selective activity must be correlated with the retrieved and selected object. We recorded the target-object–selective response (CT and T responses) from both VLPFC and IT during the color cue period. Because no object image was presented during this period, the object-selective activity must be related to the retrieved object. In other words, VLPFC and IT must contribute to the retrieval process from the working memory. We compared the target-selective activities in IT with those in VLPFC. The proportion of object-selective activities in IT was smaller than that in VLPFC. The latencies of the object-selective activity in IT were longer than those in VLPFC. These facts suggest the relative importance of VLPFC in the retrieval process from the working memory. We further examined whether the object-selective responses (CT and T responses) contribute to the behavior of the retrieval process from the working memory. For this purpose, we analyzed the responses in error trials. Although the mean response magnitudes during the error trials of the preferred target trials were slightly decreased, in almost all neurons with CT and T responses the discharge rates did not decrease significantly in the error trials in IT. These results suggest that the CT and T responses in IT fluctuate depending on some other factors and the target-selective activities in IT are not directly involved in the performance of the current behavioral task. IT neurons with CT and T responses did not always respond in the preferred target trials. In other words, the response during the correct trials in the preferred target condition was somewhat variable in IT. Therefore although the IT neurons with a CT or T response did not respond in about half of the error trials, we detected no significant differences in the discharge rates between the correct and error trials in IT. These results are different from the results obtained from VLPFC. VLPFC neurons with a CT or T response were activated in most of the correct preferred target trials. These observations also correspond with results showing that the aROC values of the VLPFC neurons were higher than those of the IT neurons. During the error trials of the nonpreferred target trials, about one third of the VLPFC neurons with CT and T responses increased their discharge rate significantly, but most of IT neurons with CT and T responses did not increase their discharge rate. From these results, we considered that CT and T responses in IT were not critical for performance of the SPR task.
We found that 28% of the IT neurons showed object selectivity during the color cue period. This percentage was lower than that of the VLPFC neurons (59%). Because it is well known that IT neurons are selectively activated by complex visual stimuli (Bruce et al. 1981; Desimone et al. 1984; Perrett et al. 1982), the restricted number of stimuli used in our SPR task could be the reason that the percentage of object-selective neurons was low in IT during the color cue period. However, during the C1 and C2 periods of the SPR task, 100 IT neurons showed a visual response and, of these, 73 neurons (73%) showed an object-selective response. This percentage was similar to that of the VLPFC neurons (68%; Inoue and Mikami 2006). Thus the percentage of object-selective responses during the color cue period was low in IT relative to the object-selective visual responses during the object cue period. This result suggests that the small number of visual stimuli could not be the reason for the low percentage of object-selective responses during the color cue period in IT.
Color-selective response during the color cue period
We also found color-selective responses (CT and C responses) in VLPFC and IT during the color cue period. In our SPR task, the red cue requires the monkey to retrieve the first cue and the green cue requires the monkey to retrieve the second cue. Therefore the color-selective responses could be caused by the sensory property (color) or the rule for retrieval. The results during the fixation task can help us to interpret the properties of these responses. The C response must reflect the sensory property because all the tested IT neurons with a C response also responded to the same color stimulus during the fixation task. On the other hand, the CT response must be independent from the pure sensory property because all the tested IT neurons with a CT response did not respond to the same color stimulus during the fixation task. Therefore the color dependence of the CT response must be caused by the rule for retrieval. In contrast to these results in IT, the neurons with a CT response in VLPFC showed a color-selective response during the fixation task, but the neurons with a T response in VLPFC showed no response during the fixation task (Inoue and Mikami 2006). These differences also suggest that the integration process with color and object information occurs mainly in VLPFC and that IT is just receiving the integrated information. The CT responses started at 260 ms after the color cue onset in IT. This timing was later than the onset of the retrieval-related response in VLPFC. Therefore the color dependence of the CT response in IT could not play an important role in performance of the SPR task.
Comparison of latencies in IT and VLPFC
To reveal the flow of information between IT and VLPFC, we compared the onset latencies and latencies of the color- and object selectivity of the CT, T, and C responses of IT in this study with those of VLPFC recorded in our previous study (Inoue and Mikami 2006). The onset latencies and latencies of the color selectivity of the C responses in IT were significantly shorter than those in VLPFC. The onset latencies and latencies of the object selectivity of the CT and T responses in IT were significantly longer than those in VLPFC. These results suggest the following flow of information during the color cue period of the SPR task as shown in Fig. 8. Neurons with a C response in IT send color information to the neurons with C and CT responses in VLPFC. VLPFC neurons with a CT response receive object and temporal order information from the neurons with object- and order-selective delay-period activities, and combine the color and object information. Then, VLPFC neurons with a T response integrate the combined information into object information. These VLPFC neurons with CT and T responses could participate in the retrieval process because VLPFC has a larger neuronal population with this type of activity and, in addition, about half of the VLPFC neurons with a CT or T response showed a smaller response during the error trials. The IT neurons with CT and T responses receive the retrieved object information from VLPFC. These suggestions are also supported by the results of the activities during the fixation task discussed in the previous section.
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Top-down modulation from the prefrontal cortex
Top-down modulation from the prefrontal cortex has been suggested in some previous studies. Suppression of the function of the prefrontal cortex by cooling leads to the excitation and inhibition of the discharge rate of IT neurons (Fuster et al. 1985). Tomita et al. (1999) recorded the activities of IT neurons in posterior-split-brain monkeys under the top-down and bottom-up conditions of the paired-association task. They found that IT neurons were activated by the top-down signal from the prefrontal cortex in the absence of bottom-up visual inputs. These results were similar to our results because some of our IT neurons showed a retrieved-object–selective response during the color cue period and this must be the feedback signal from PFC. However, we concluded that the functional roles of the CT and T responses in IT could be relatively small in the SPR task because CT and T responses were not decreased in the error trials. Tomita et al. (1999) did not analyze the top-down signals in error trials because the number of error trials was not sufficient in their task for such analyses (Tomita et al. 1999). Therefore the behavioral relevance of their top-down signal has not yet been explored. Moore and Armstrong (2003) showed that, at 70 ms after frontal eye field stimulation, neurons in V4 with corresponding retinotopy exhibit enhanced response to a visual target within their receptive field. In human patients with unilateral lesion of the prefrontal cortex, attentional modulation of event-related potentials recorded from the extrastriate cortex during attention task were dramatically reduced in ipsilateral hemisphere compared with contralateral hemisphere at 150–350 ms poststimulus onset that related to the selection of object features (Barceló et al. 2000; Yago et al. 2004). These findings as well as our results indicate that the prefrontal cortex can manage information processing in other cortical areas using top-down signal pathways. However, in the SPR task, the functional roles of the CT and T responses in IT could be relatively small because the proportion of IT neurons with a CT or T response was small; the CT and T responses in IT were relatively variable, and these responses were not significantly different between the correct and error trials. Although our monkeys were extensively trained to do the SPR task, even during the performance of a routine task, some other events could occur. We suspect that the brain is always preparing for those unexpected events and top-down signals reported here might be useful on such an occasion.
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FOOTNOTES |
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Address for reprint requests and other correspondence: A. Mikami, Department of Behavioral and Brain Sciences, Primate Research Institute, Kyoto University, Kanrin, Inuyama, Aichi 484-8506, Japan (E-mail: mikami{at}pri.kyoto-u.ac.jp)
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REFERENCES |
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