The Joint Simon task is not joint for capuchin monkeys

Abstract

Human cooperation can be facilitated by the ability to create a mental representation of one's own actions, as well as the actions of a partner, known as action co-representation. Even though other species also cooperate extensively, it is still unclear whether they have similar capacities. The Joint Simon task is a two-player task developed to investigate this action co-representation. We tested brown capuchin monkeys (Sapajus [Cebus] apella), a highly cooperative species, on a computerized Joint Simon task and found that, in line with previous research, the capuchins' performance was compatible with co-representation. However, a deeper exploration of the monkeys' responses showed that they, and potentially monkeys in previous studies, did not understand the control conditions, which precludes the interpretation of the results as a social phenomenon. Indeed, further testing to investigate alternative explanations demonstrated that our results were due to low-level cues, rather than action co-representation. This suggests that the Joint Simon task, at least in its current form, cannot determine whether non-human species co-represent their partner's role in joint tasks.

Figure 1

Examples of compatible (a) and incompatible trials (b). Note that when the orange circle is presented, the correct response is to move the cursor to the left, and when the purple triangle is presented, the correct response is to move the cursor to the right. The red dot represents the cursor on the screen and the gray rectangular shapes are the response boxes.

Figure 2

Schematic representation of the setup for the four conditions in Experiment 1. In the Simon task (a), subjects solved the task alone (i.e. they could move the cursor to either of the two response boxes in response to the stimuli). In the Half task (b), only one response box is available and thus, subjects performed half of the Simon task (i.e. selecting the response box when one stimulus is shown and not giving any response when the other stimulus is shown). If subjects successfully refrained from selecting a response when the correct response box was not available, the answer was coded as correct, but they did not receive a reward. In the Joint Simon task (c), subjects were tested in dyads, and each individual was responsible for half of the task (i.e., they could only move the cursor to one of the two response boxes). Subjects were rewarded for both their correct choices and their partner’s correct choices. In the Social facilitation task (d), subjects performed the Half task with their partner present but engaged in a different task on a separate computer. Shaded area represents the screen area in which subjects can not move the cursor. In addition, in (c) the areas in which first and second subject can move the cursor are delimited by a gray line and gridded area respectively (note that both overlap in the middle of the screen). In Experiment 2, the Half task with unrewarded correct-no answers was identical to the Half task in Experiment 1 (b), as was the Half task with rewarded correct answers, except that unlike (b), subjects were rewarded for correct no-answers.

Figure 3

Results of Experiment 1 and Experiment 2. (a) Compatibility effect (% of incorrect choices in incompatible trials minus compatible trials) and (b) Percentage of correct responses, in all the conditions of Experiment 1 and Experiment 2. Boxes and whiskers show the median value and the lower and upper quartile scores. Mean values are represented by a red circle. The dots display the average by subject. In (b) the dashed line shows the chance level (50%). Half-N half task with non-rewarded correct no-responses, Half-R half task with rewarded correct no-responses.