Figure 1.

Apparatus, tasks and recording sites. A, Monkeys S and K controlled each an individual visual cursor (Monkey S, blue dot; Monkey K, green dot) on a screen, by applying a force pulse on an isometric joystick with one hand. B, Initially, each animal had to move its own cursor within a central target from an offset position, by exerting a static force for a variable CT, until a peripheral target was presented in 1 of 8 positions. In the SOLO condition, one monkey called into action individually by the color of the peripheral target (Monkey S, blue; Monkey K, green), within a subjective RT, had to exert a dynamic force pulse on the joystick (DFT). During Monkey S's SOLO trials, Monkey K observed on the screen the motion of the other monkey visual cursor (OBS-OTHER condition), and vice versa. In the SOLO and OBS-OTHER conditions, each animal was rewarded (green/blue drop) depending on its own successful performance, regardless of the behavior of its mate. When the color of the peripheral target was white (A, represented in grey in B), both monkeys had to act jointly (TOGETHER trial), to move together a large common object (yellow circle) toward the peripheral target, by coordinating their forces. To successfully perform the task and receive simultaneously a drop of fruit juice, the animals had to keep their cursors within a maximal inter-cursor distance for the entire DFT and until the end of the THT. Lack of intersubject coordination resulted in unsuccessful trials and neither animal was rewarded. C, Reconstruction of recording sites in areas F7/F2 of Monkey S and Monkey K.