Figure 2. Behavioral performance. Bars show percent correct response of monkey 1 and monkey 2 for interactive (blue bars) and noninteractive trials (red bars). Numbers 1–4 indicate in the interactive trials the number of previous trials performed in sequence by the human partner. doi:10.1371/journal.pone.0032209.g002 to the monkeys’ ability to remember its own previous choice and applying one of the following rules: (1) choose own previous goal, (2) choose opposite of own previous goal. Each of these rules increases the probability of choosing the correct goal to above chance. However, the first rule works only if an odd number of trials are performed by the human partner, while the second works only after an even number of trials performed by the human partner. To reduce the possibility that the monkeys learned one of these rules, the human partner varied his number (from 1 to 4) of trials. To quantitatively assess whether the monkeys tended to follow one of the rules previously described, we divided the errors based on the number of trials performed by the human agent before an interactive trial. We found that both monkeys had a comparable performance in this regard. Monkey 1 performed at 92% (61/66), 93% (42/45), 93% (13/14) and 100% (20/20), respectively after sequences of 1, 2, 3, and 4 trials of the human partner, with no significant difference (p.0.05, Chi Square test). Similarly, monkey 2 performed at 77% (53/69), 81% (44/54), 83%, (29/35), and 76% (16/21), respectively, with no significant difference between sequences (p.0.05, Chi Square test). A similar success rate for 1 and 3 intervening human trials versus 2 and 4 of such trials shows that monkeys based their choice on what their partner did rather than the previous choices that the monkeys had made. The results for 3 and 4 intervening trials performed by the human partner show that monkeys were able to maintain a prolonged attention on the behavior of the human partner. Using the video recordings, we also examined the ability of monkeys to take turns and coordinate with the human agent. We found that both monkeys initiated trials immediately after seeing the human agent withdraw the hand from the screen, with a success of 100% in both monkeys. We studied the performance of the two monkeys in the NMTG tasks with visual feedbacks to test whether they could perform interactive trials without reward delivery after correctly performed human trials (four sessions with monkey 1 and three sessions with monkey 2). We found that the average correct performance in the interactive trials that followed a correct but not rewarded trial were respectively, 88% (71/81) for monkey 1, and 86% (95/111) for monkey 2, and this was above chance level for both monkeys (binomial test, p,0.0001). Discussion We tested two monkeys in a NMTG task, which required monkeys to interact with a human agent. Both monkeys showed Monkeys Monitor Human Goals an excellent performance with a very high success rate in both the noninteractive condition, after a self-acquired goal, and in the interactive condition, after the human partner-acquired goal. The second experiment showed that monkeys retained a high success rate of interactive trials when the reward delivery was replaced by visual feedback in human trials. We emphasize that our study is not one of social cooperation or social learning. Nevertheless, it has several implications for future neurophysiological and behavioral studies directed to studying the cognitive processes underlying such functions. To date, only a handful of studies have shown that monkeys can learn from other monkeys [10,15–18] as humans do so readily [19]. Other studies have shown that monkeys can learn the ordinal position of pictures watching other monkeys [16,18] and monitoring their behavior [6]. It is clear that we are only beginning to understand the potentials of using monkeys as a model for studying the cognitive and affective processes underlying social behavior at the neural level [5,6]. In this regard, it is important to mention the pioneering study of Yoshida et al. [6]. They have shown that medial prefrontal neurons are able to distinguish self from others by discharging specifically for another agent’s actions, but only in a social contest. Here we extend this line of investigation, showing that monkeys can monitor the behavior of their human partner for multiple trials, alternating the role of actor and observer with a human agent. We emphasize that, although several previous studies have shown monkeys responding to one another’s behavior, there were strong doubts as to whether they could do so when interacting with humans. Questions about the ability of monkeys to interact socially with humans occurred even in behaviors much simpler than the current one, such as following the gaze of others. While it has been shown that monkeys are able to follow the gaze of a conspecific [20,21], there has been more controversy on the ability of monkey to use the communicative gestures of humans, with some studies failing to find signs of this ability [22,23] while others showing some evidence of this ability in macaques [24,25]. Social learning in the form of imitating tool use by human agents has also generated controversy, with some studies finding some evidence in support of such imitation [26,27] and others failing to find such evidence [28,29], depending on the experimental design. When we go beyond gaze following or learning about tool use, to consider the ability of monkeys to monitor or learn more complex behaviors, previous studies have casted serious doubts on the capacity to learn from observing human behavior [17,10]. Meunier and colleagues [10] have shown that monkeys could learn, to a certain degree, which object was associated to a reward by observing another monkey, but not a human, performing the task. That result leads to the possibility that there could be a limit to the ability of monkeys to interact with individuals of another species. Here, we showed strong evidence that, at least in the context of our laboratory task, monkeys can engage in such interactions and do so extraordinary well. The differences between those previous results and ours might be explained by several differences in the experiment design. First, the imitative nature of the task used by Meunier et al. [10], is different from ours in that it might have required social identification. We think, however, that the main difference is the seminatural settings adopted by Meunier et al. [10], which did not require monkeys to pay attention to the other monkeys or to the human agents. In contrast, our laboratory setting posed greater demands on attention to the performance of the human model. Moreover, we showed that the human’s goals could be similarly monitored both by the monkey with the head free and by the monkey with the head fixed. PLoS ONE | www.plosone.org 4 February 2012 | Volume 7 | Issue 2 | e32209