To ameliorate their condition of life we routinely introduced in the home cage environment toys (often containing items of food that they liked) to promote their exploratory behaviour. At the end of each experimental session, the researcher that tested the animals spent half an hour interacting with the monkeys directly, giving for example new objects to manipulate. We think that this interaction with humans, in addition to the interaction that was part of the task performed, can help to reduce potential stress related to the experiment. Recently, to increase the level of enrichment in the animal facility room, we have also showed movies in a monitor and we are currently starting to evaluate the benefits. Surgical techniques In monkey 1 the experiments were carried out while the monkey’ s head was fixed. For this purpose, a head-holder was implanted. The animal was preanesthetized with ketamine (10 mg/kg, i.m.) and anesthetized with isofluoran (Abbott Laboratories) through a constant flux of isofluoran/air mixture (1–3%, to effect). Antibiotics and analgesics were administered postoperatively. Behavioral testing The monkeys sat in a primate chair, monkey 1 with the head fixed and monkey 2 with the head free, facing a video touch screen (Microtouch, 19 inches, 8006600 pixel resolution) 40 cm away. For both monkeys, the human partner was standing upright, close to the animal, on the monkey’s right side. For this experiment, the human partner was always the same researcher with whom the monkeys were accustomed. Figure 1A shows the sequence of events in the NMTG task. The trial began when a white circle, the central stimulus, appeared at the center of the video screen (Fig. 1A). The monkeys had to touch the circle within 1 s, otherwise the trial was aborted and a new trial started. After the monkeys touched the central stimulus, a horizontal grey bar appeared. The monkeys were required to continue touching the central stimulus for 0.9–1.6 s. Next, two targets (called object goals) appeared, one to the right and one to the left of the central stimulus, after which the monkeys maintained their right hand on the central stimulus for a delay period of 0.4–1 s. The disappearance of the horizontal bar served as a go signal. The monkeys then made a reaching movement to one of the two goals, within a 3.5 s limit, and maintained the hand on the chosen goal for a hold time of 0.8– 1.3 s. For correctly performed trials, three drops of fluid reward were delivered. After both correct and error trials, the two goals and the central stimulus disappeared and a 0.1 s intertrial interval with a black screen began. We randomized all the durations of the epochs in 50 ms steps. The human partner could decide to begin a ‘human trial sequence’ (1, 2, 3, or 4 trials in a row) at any time during the session. The sequence of events for these trials was identical to that of trials performed by the animals; however, reward was delivered to the monkey in all cases. After we concluded the first experiment, to test whether the monkeys’ ability to follow the human partner’s trials was strictly dependent on the reward delivery, we studied the monkeys’ performance in a second experiment. We introduced a visual feedback at the end of each NMTG trial, to indicate correct and error responses. Two visual stimuli, an empty square or a circle around the chosen target, were used for correct and error trials, respectively. We presented the feedback 0.5 s after the monkeys/ subject touched the target for 0.8–1.3 s, followed by the reward in the case of correct trials. All the other events were the same as in the standard NMTG task. We omitted reward in a subset of Monkeys Monitor Human Goals correctly (one out of six) performed human trials and measured monkeys’ performance in these unrewarded interactive trials. Trial types Monkeys performed two classes of trials, classified based on who performed the trial immediately before the current trial. If the monkey performed the previous trial it was called a ‘noninteractive trial’, whereas if the human partner performed the previous trial it was called an ‘interactive trial’. Between two interactive trials, the monkeys performed a variable number of trials, typically three or more. For the first trial of a session, any chosen goal was accepted as correct and rewarded. This goal was then designated as the previous goal and this designation was used for the following trial. On each trial, the previous goal was presented on the screen together with another goal. In order to receive a reward, the monkeys had to reject the previous goal and choose the alternative (future goal). Figure 1B shows the 262 combinations of goals and positions for the current trial when the previous goal was a purple polygon. Goal positions were always assigned pseudorandomly unless the trial followed a mistake. Two types of trials were pseudorandomly interspersed, and they differed in terms of whether the correct goal on the current trial had appeared on the previous trial. When it had, we called this trial type a ‘familiar- goal’ trial, but when the correct future goal had not appeared on the previous trials, we called it an ‘unfamiliar-goal’ trial. The familiarity is defined only with respect to the previous goal; overall, familiar and unfamiliar future goals were presented with equal probability. If the monkeys made a mistake, by erroneously choosing the previous goal, they did not receive a reward and a correction trial followed. Correction trials consisted of a repetition of the same two goals in the same positions. The correction trial required the monkeys to choose the goal that should have been chosen during the previous trial. In the interactive trials the monkeys interacted with a human partner. When the human partner moved his arm toward the screen, the monkeys drew back their arm letting the human partner perform the trial. The human partner decided the number of trials to perform: one, two, three or four (Figure 1C). When the human partner drew back his arm at the end of a trial, the monkeys again approached the central stimulus, and in this way began their next trial. In the interactive trials the monkeys had to monitor the human partner’s choice, and later reject human’s previous goal choice in favor of the alternative goal, neglecting their own previous choice. Data collection A noncommercial software package, CORTEX (www.cortex. salk.edu), was used to control stimuli presentation and reward delivery and to record touches on the screen. A video camera recorded who performed each trial, the monkey or the human partner. An experimenter classified each trial offline, using the video recordings. Results We studied the performance of Monkey 1 in 9 sessions and of Monkey 2 in 5 sessions. Each session was run on different days. We examined the performance of both monkeys in the two conditions: interactive and noninteractive. We found that both monkeys performed the task at a high level of performance in both conditions. Figure 2 shows the results for the interactive and noninteractive trials. PLoS ONE | www.plosone.org 2 February 2012 | Volume 7 | Issue 2 | e32209